Class 12th Chemistry Chapter 12 Aldehydes, Ketones and Carboxylic Acids Revision Notes & Important Question

www.free-education.in is a platform where you can get pdf notes from 6th to 12th class notes, General Knowledge post, Engineering post, Career Guidelines , English Speaking Trick , How to crack interview and lots more. ( Aldehydes Ketones and Carboxylic Acids )

Aldehydes, Ketones and Carboxylic Acids Notes

Introduction

Aldehydes and ketones

They are functional isomers of each other and collectively called as carbonyl compounds (C=O).

Class_12_Aldehydes_&_Ketones_Aldehydes_&_Ketones

Chemical properties of aldehydes and ketones

  • Structure of Carbonyl group
Class_12_Aldehydes_&_Ketones_Carbonyl_Group
  • If we look at structure ,multiple bonds are seen. Therefore ,they will show addition reactions .
  • Oxygen being electronegative,attract electrons .
  • As a result, partial negative charge is acquired on oxygen and partial postive on carbon .
  • Therefore ,nucleophilic attack occur on carbon .So, in them common reactions that is seen is nucleophilic reactions.
  • The electronegativity is same but due to approach of attacking agent temporary transfer of electrons occur .

Types of reactions shown

  1. Nucleophilic addition reactions
  2. Nucleophilic addition reactions followed by loss of water molecule
  3. Oxidation reactions
  4. Reduction reactions
  5. Miscellaneous reactions
  • Nucleophilic addition reaction

     In this given structure

Class_12_Aldehydes_&_Ketones_Carbonyl_Group

The nucleophile attack on the carbon atom and the reaction occur in a way as shown below:

Class_12_Aldehydes_&_Ketones_Attack_Of_Nucleophilic_Addition_Reaction

This reaction is acid catalysed reaction .Due to which hydroxyl group is formed from aldehyde.

Relative strength of Aldehydes and ketones

  • On the basis of Inductive effect

 If the magnitude of positive charge is more on carbon atom ,then more readily it shows nucleophilic addition reaction .So, out of all members HCHO shows this reaction more readily as it has no alkyl group( electron releasing group) present.

So, if we compare formaldehyde, ethanol and propanone, the order of their strength towards nucleophilic addition is:

Class_12_Aldehydes_&_Ketones_Inductive_Effect
  • It can also be explained on the basis of Steric effect

More the bulky groups attached, less space will be available for nucleophile to attack. Therefore, reactivity will be less. So, this can also be the reason to explain the trend.

Class_12_Aldehydes_&_Ketones_Steric_Effect
  • If we compare Benzaldehyde with any other aldehyde, its strength towards nucleophilic addition is less. The reason is due to delocalisation of electron that takes place. Therefore, the charge is not so intensified.

Let’s show some nucleophilic addition reactions:

(a)Addition of HCN (Cyanohydrins are formed)

If only HCN is used, the reaction is slow. So, the base is used that help in removing hydrogen ion from HCN and sets the nucleophile free. As a result, reaction becomes fast. The base reacts as shown below:

Class_12_Aldehydes_&_Ketones_Addition_Of_HCN

If we carry out hydrolysis of the above compound, 2-methyl 2-hydroxy propionitrile, then the lactic acid is produced due to hydrolysis of CN group as shown below:

Class_12_Aldehydes_&_Ketones_Hydrolysis_1

 If we heat lactic acid, in presence of sulphuric acid then the water molecule gets removed and acrylic acid is formed as shown:

Class_12_Aldehydes_&_Ketones_Heating_Of_Lactic_Acid
  •   Addition of Sodium bisulphite (this reaction is used in separation of aldehyde and ketone)

The reaction involved is given below:

Class_12_Aldehydes_&_Ketones_Addition_Of_Sodium_BiSulphite

For aldehyde, the reaction goes in forward reaction and for ketone backside is favoured (as two methyl groups increase bulkiness and form unstable products).

Note: diethyl ketone, acetophenone and Benzophenone will not undergo these reactions because of bulkiness.

  •  Addition of Grignard reagent

When Grignard reagent is added following reactions occur:

Class_12_Aldehydes_&_Ketones_Addition_Of_Grignard_Reaction

Please note: In this formaldehyde form primary alcohol, any other aldehyde form secondary alcohol and ketone forms tertiary alcohol.

(d)Addition of alcohols: In this we get two products that are hemi-acetals and acetals from aldehyde. From ketone we get, ketals.

The reaction occurs as shown:

Class_12_Aldehydes_&_Ketones_Addition_Of_Alcohols

 Please note in this we use dry HCl

  • So as, to protonate the carbonyl oxygen. Therefore, increasing the electrophilicity of carbonyl carbon to enhance the attack of alcohol.
  • It removes the water produced during the reaction and takes the reaction in forward direction.
Class_12_Aldehydes_&_Ketones_Use_Of_Dry_HCl

Nomenclature

Class_12_Aldehydes_&_Ketones_Nomenclature_Of_Aldehydes_&_Ketones

Aromatic compounds

Class_12_Aldehydes_&_Ketones_Nomenclature_Of_Aromatic_Compounds
  • Nucleophilic addition reaction with loss of water molecule

In these reactions, mainly the reaction occurs with ammonia and its derivatives. The general reaction involved is:

Class_12_Aldehydes_&_Ketones_Nucleophilic_Addition_Reaction
  •      Reaction with hydroxylamine (NH2OH): Oxime is formed
Class_12_Aldehydes_&_Ketones_Reaction_With_Hydroxyl_Amine
  • Reaction with hydrazine (NH2NH2):Hydrazones are formed
Class_12_Aldehydes_&_Ketones_Reaction_With_Hydrazine
  • Reaction with phenyl hydrazine: (phenyl hydrazone is formed )
Class_12_Aldehydes_&_Ketones_Reaction_With_PhenylHydrazine
  • Reaction with 2,4 dinitrophenylhydrazine: orange crystals are formed
Class_12_Aldehydes_&_Ketones_Reaction_With_DinitrophenylHydrazine
  • Reaction with semi carbazide (NH2NHCONH2):semi carbazone is formed
Class_12_Aldehydes_&_Ketones_Semi-Carbazide
  • Oxidation
  • Aldehyde and ketones on oxidation give carboxylic acid .But this happens only,in presence of Oxidising agents like HNO3 , KMnO4 , k2Cr2O7 , Na2Cr2O7Collin’s Fehling reagent and many more.

When we use KMnO4 or k2Cr2O7 ,the medium gets acidic. There is a trend, that when we carry oxidation of aldehyde or ketone it forms acid like –

Class_12_Aldehydes_&_Ketones_Oxidation_Reaction

We get acid, irrespective of the alkyl group used. Likewise, ketones also give acids but we get mixtures.

     Class_12_Aldehydes_&_Ketones_Oxidation_Reaction_1                     The product formed is in accordance with Popoff’s rule.

According to it : “ketones get oxidised in such a manner that the keto group remains with smaller alkyl group.

 Example:

  Class_12_Aldehydes_&_Ketones_Oxidation_Reaction_2                   Aldehyde

 They are easily oxidised even with mild oxidising agents like Collin’s, Fehling etc. They can be oxidised but in case of ketone we need to use strong oxidising agents .The use of weak and strong oxidising agents like Colin’s, Fehling’s can be used to distinguish between aldehydes and ketones.

 Mild Oxidising agent: Used for oxidising aldehyde to carboxylic acids.

        Mild Oxidising agents used are Tollen’s reagent.

For aldehydes

  • Tollen’s reagent :It is a solution of Ammonical silver nitrate and it has formula [ Ag (NH3)2 ]+ OH ] .Now ,Let us take example of any aldehyde like :
Class_12_Aldehydes_&_Ketones_Oxidation_Reaction_Tollen's_Agent

Therefore, it is called as “Silver mirror test”. This test  is known as silver mirror test and  is not given by ketones.

  • Fehling solution test
  • It is prepared by missing equal amount of Fehling’s Solution A and Fehling’s solution B.

Has to be changed

      (Aq. CuSO4) (Sodium potassium Carbonate ” Rochelle Salt”)     

  • We take test tube and add both solutions A & B equally.
  • The resulting solution is blue due to CuSO4.
  • Now when we add aldehyde this blue solution, it turns red. The medium is alkaline ketone due to Fehling solution B.
  • The following reaction occurs.

RCHO    +       Cu2+ +5OH –>  RCOO–       +       Cu2O + 3H2O

(Aldehyde) Copper ion   carboxylate ion  (Cuprous oxide) (Reddish Brown ppt)

  • Identification : Colour changes from blue to red .This test is not given by ketones.
  •  Benedict’s solution test– It is alkaline solution of cupric ion complexed with citrate ions. When we react with aldehyde the following reaction occur:

RCHO    +       Cu2+ +5OH –>  RCOO–       +       Cu2O + 3H2O

(Aldehyde) Copper ion   carboxylate ion  (Cuprous oxide) (Reddish Brown ppt)

for Ketones

  • Iodoform test: By performing this test, we get yellow precipitate due to formation of iodoform.
  • Limitation of this test: This is given only methyl ketones i.e. one of the R in ketone should be CH3 and also by ethanol as it contains the methyl group.

In this, aldehyde reacts with I2 in presence of NaOH. In this 3 hydrogen are substituted by Iodine as shown below:

Class_12_Aldehydes_&_Ketones_IodoForm_Test

Isomerism

  • Chain isomerism
Class_12_Aldehydes_&_Ketones_Chain_Isomerism
  • Position isomerism

It is not shown by aliphatic compounds because CHO group is given least number.

Class_12_Aldehydes_&_Ketones_Position_Isomerism

          Metamerism

Class_12_Aldehydes_&_Ketones_Metamerism_Isomerism
  • Functional isomerism
Class_12_Aldehydes_&_Ketones_Functional_Isomerism

Methods of preparation

There are different methods for their preparation

  1. Common Methods for both aldehyde and ketones
  2. Preparation for Only aldehydes
  3. Preparation for Only Ketones

1. From alcohols

  1. Oxidation of alcohols: It is prepared from oxidation of alcohols using Oxidising agents  like K2Cr2O7 , Na2Cr2O7 , PCC , collins reagent (Cro3 + pyridine). All of them are good oxidising agents. So, we need to poison in order to stop it at aldehydic stage.
Class_12_Aldehydes_&_Ketones_Preparation_From_Alcohols

 (From tertiary alcohol ) : We can’t get aldehyde from tertiary alcihols as there is no alpha hydrogen present. So ,in this case we get alkene.

  • If we have cyclohex – 2 ene –1ol
Class_12_Aldehydes_&_Ketones_Preparation_From_Alcohols_1

b.Dehydrogenation of alcohols –  Cu (573k)

Class_12_Aldehydes_&_Ketones_Dehydrogenation_Of_Alcohols
  1. FORM CARBOXYLIC ACIDS
Class_12_Aldehydes_&_Ketones_Catalytic_Decomposition_Of_Alcohols
  • Distillation of calcium salts of acids
Class_12_Aldehydes_&_Ketones_Distillation_Of_Calcium_Salts
  • From hydrocarbons :
  • By acid hydration of alkynes
Class_12_Aldehydes_&_Ketones_Acid_Hydration_Of_Alkynes
  •  Ozonolysis of alkenes
Class_12_Aldehydes_&_Ketones_Onzonolysis_of_Alkenes
  •   From gem dihalide
Class_12_Aldehydes_&_Ketones_From_Gem_Dihalide
  • Reduction of Aldehyde and Ketone

       The major product on reduction is alcohol.

  • Aldehyde gives primary alcohol.
  • Ketones give secondary alcohol.

Major Oxidising agents can be used like Ni , Pt , LiALH4, NaBH4 etc. Out of them , LiALH4is the strongest one .The reaction that occurs is given below:

Class_12_Aldehydes_&_Ketones_Reduction_Of_Aldehydes_&_Ketones
  • Reduction to HC’s
  •  Clemenson reduction – In this ,we take aldehyde and use Zinc in presence of HCl or Zn / Hg amalgam in HCl. The reaction involved is:
Class_12_Aldehydes_&_Ketones_Clemenson_Reduction
  • Wolf kishner reduction: In this aldehyde is reacted with hydrazine as shown below :
Class_12_Aldehydes_&_Ketones_Wolf_Kishner_Reaction
  • For aromatic ketones
Class_12_Aldehydes_&_Ketones_Wolf_Kishner_Reaction_1
  • By action of red phosphorous on HI we get, hydrocarbons [ temperature 423 – 473k] as shown below:
Class_12_Aldehydes_&_Ketones_Wolf_Kishner_Reaction_2
  • Reduction to Pinacol – For this ,we take 2 moles of acetone and reduce them as shown:

.Class_12_Aldehydes_&_Ketones_Reduction_To_Pinacol

Methods for preparing only  aldehydes

From acid chlorides – Rosenmund reaction

Class_12_Aldehydes_&_Ketones_Preparation_Of_Aldehydes_From_Acid_Chlorides

Drawback – Formaldehyde can’t be prepard by this method : formyl chloride H – COCl  is unstable .Aldehydes can be further reduced to alcohol. So, in order to stop it at aldehyde stage we have to poison it.

2.From reduction of alkyl cyanides – known as Stephen’s reduction (use of SnCl2)

Class_12_Aldehydes_&_Ketones_Preparation_Of_Aldehydes_From_Alkyl_Cyanides

This can also be occur in presence of other reducing agent like  – DIBAL. (Disobutyl aluminiun hydride ).It is used  for long chain nitrils.

Class_12_Aldehydes_&_Ketones_Preparation_Of_Aldehydes_From_Alkyl_Cyanides_1

2.Oxidation reaction – from hydrocarbons

  •  Oxidation of methyl benzene
Class_12_Aldehydes_&_Ketones_Preparation_Of_Aldehydes_From_Hydrocarbons
  •      Use of chromyl chloride – Etard reaction
Class_12_Aldehydes_&_Ketones_Preparation_Of_Aldehydes_Etard_Reaction
  •  Side chain chlorination followed by hydrolysis
  • Gatterman kosh reaction
Class_12_Aldehydes_&_Ketones_Preparation_Of_Aldehydes_Gatterman_Kosh
  • Miscellaneous reactions: Aldehyde / ketone
Class_12_Aldehydes_&_Ketones_Cross_Aldol_Condensation_3
  • Aldol condensation: If we take two molecules of aldehyde and they are condensed .The product obtained is a compound with aldehyde and alcoholic group. These reactions are given by compounds that have – alpha C atom.

            Example:                                                      

  • Self-aldol condensation : In these 2 molecules of same aldehyde are taken:
Class_12_Aldehydes_&_Ketones_Self_Aldol_Condensation

Mechanism of this reaction

  1. First step is slow and is rate determining step. In this OH act on aldehyde.
Class_12_Aldehydes_&_Ketones_Self_Aldol_Condensation_First_Step
  1. Second step – The enolic form react with another molecule.
Class_12_Aldehydes_&_Ketones_Self_Aldol_Condensation_Second_Step
  1. Addition of hydrogen ion
Class_12_Aldehydes_&_Ketones_Cross_Aldol_Condensation_2
  • Cross- aldol condensation : In this we take molecules that are different:
Class_12_Aldehydes_&_Ketones_Cross_Aldol_Condensation_1

       We can take other ways also in which We take alpha Hydrogen of CH3HO(ethanal).

Class_12_Aldehydes_&_Ketones_Cross_Aldol_Condensation
  • Cannizzaro Reaction– It is shown by those that do not have alpha Hydrogen. In these two products are formed – alcohol and acid.
Class_12_Aldehydes_&_Ketones_Cannizaro_Reaction

Intermolecular Cannizzaro reaction– The reaction occurs within  same molecule, which has more than 2 or more carbonyl groups present.

Class_12_Aldehydes_&_Ketones_InterMolecular_Cannizaro_Reaction

One “O” gets connected to OH and other to acid.

Halogenation Reaction –

 In this alpha hydrogen is substituted by a halogen and we get product accordingly.

Class_12_Aldehydes_&_Ketones_Halogenations_Reaction

Ring substitution reaction: It is meta directing

Class_12_Aldehydes_&_Ketones_Ring_Substitution_Reaction

The electrophile can’t attack at ortho and para position. As they are   meta directing compounds. So, all substitution occurs on Meta position.

Halogenations

Class_12_Aldehydes_&_Ketones_Halogenations

With Ketone

Class_12_Aldehydes_&_Ketones_With_Ketone

Nitration

Class_12_Aldehydes_&_Ketones_Nitration

 Sulphonation 

Preparation of only ketones

1.From acyl chloride :  In this we make use of dialkyl cadmium ,then we react with RCOCL. For preparation we take grignard reagent :

Class_12_Aldehydes_&_Ketones_Preparation_Of_Ketones_From_Acyl_Chloride

Lets prepare acetophenone by this method :

Class_12_Aldehydes_&_Ketones_Preparation_Of_Ketones_From_Acteophone

 2.From nitriles : In this we make use of grignard reagent and nitriles as shown in reaction :

Class_12_Aldehydes_&_Ketones_Preparation_Of_Ketones_From_Nitriles

In a similar manner, we can prepare acetophenone by taking grignard reagent as phenyl magnesium bromide and methyl cyanide.

Test for aldehydes and ketones

  1. For aldehydes
  • Tollen’s reagent – It is a solution of Ammonical silver nitrate and it has formula [ Ag (NH3)2 ]+ OH ] .Now ,Let’s take any aldehyde like :Formaldehyde and react with it as shown below .
Class_12_Aldehydes_&_Ketones_Tollens_Reagent

Therefore, called as “Silver mirror test”. This test of oxidation that is Silver mirror test is not given by ketones.

  • Fehling solution test
  • It is prepared by missing equal amount of Fehling solution A and Fehling solution B.(Fehling A has Aq. CuSO4 and Fehling B has Rochelle salt) .
  • We take test tube and add both solutions A & B equally and resulting solution is blue due to CuSO4).
  • Now, when we add aldehyde this blue solution turns red. The medium is

            alkaline due to Fehling solution B.

  • The following reaction occurs.
Class_12_Aldehydes_&_Ketones_Benedict's_Reaction_1

Identification: The Colour changes from blue to red .This test is not given by ketones.

  •  Benedict’s solution test– It is alkaline solution of cupric ion complied with citrate ions. When we react with aldehyde the following reaction occur:
Class_12_Aldehydes_&_Ketones_Benedict's_Reaction_1

Identification: The Colour change from blue to red .This test is not given by ketones.

Test for Ketones

  • Iodoform test
  • By performing this test, we get yellow precipitate due to Formation of Iodoform.
  • In this, we react aldehyde with Iodine in presence of NaOH. In this 3H are substitute by Iodine as shown below:
Class_12_Aldehydes_&_Ketones_IodoForm_Test_1

Uses of Aldehydes & Ketones:-

Class_12_Aldehydes_&_Ketones_Uses_Of_Aldehydes_&_Ketones
Class_12_Aldehydes_&_Ketones_IodoForm_Test_2
  • Limitation of this test: This is given only by methyl ketones i.e. one of the R in ketone should be CH3 ,and also by ethanol as it also contains methyl group.
  •   See we have few examples Which will give and which will not give this test –
Class_12_Aldehydes_&_Ketones_Limitation_Of_IodoForm_Test

Physical properties of aldehydes and ketones :

  • Lower members have unpleasent smell and up to 11 members they are liquids.
  • Solubility : Aldehydes are more polar in nature.As we know, in ketones two alkyl groups are present. So, if electron density on carbon increases, the polarity of bond decreases, due to +I effect .

Therefore, solubililty of aldehyde is more as they have more tendency to form hydrogen bonds .As number of carbon atoms increase, the solubility decrease because of non polar nature .

  • Boiling points : Boiling point is always more for polar compounds .
  • ketones have higher boiling points than aldehyde because of more polarity .
  • Their boiling point is always lower than alcohols and carboxylic acids because they are less polar then them .
  • In aldehydes and ketones, dipole dipole interactions are comparitively less .

CARBOXYLIC ACID

Class_12_Chemistry_Carboxylic_Acid_Names_Of_Carboxylic_Acids
Class_12_Chemistry_Carboxylic_Acid_Names_Of_Carboxylic_Acids_1

Method of preparation         

 It is prepared basically from oxidation and hydrolysis method.

  1.  From alcohols by oxidation: For this alkaline kMnO4 , CrO3 in H2 SO4,acidified K2 Cr2 Olike oxidising agents are used.The reaction occurs as shown below :
	/u-img/00/01/06/00010638.jpg
  1. Oxidation of Aldehyde  and Ketones: In this aldehyde and ketones can be oxidised in presence of oxidising agents like alkaline potassium permagnante as shown below .
Class_12_Chemistry_Carboxylic_Acid_Methos_Of_Preparation_By_Oxidation

3.Hydrolysis of nitriles: In this cyanides are hydrolysed to form amides which further on hydrolysis yields carboxylic acid .

Class_12_Chemistry_Carboxylic_Acid_Methos_Of_Preparation_By_Hydrolysis

4.From Grignard reagent : In this grignard reagent is made to react with carbondioxide .

Class_12_Chemistry_Carboxylic_Acid_Methos_Of_Preparation_From_Grignard_Reagent

5.From acyl halide: In this hydrolysis of acyl halide is hydrolysed to form respective carboxylic acid .

Class_12_Chemistry_Carboxylic_Acid_Methos_Of_Preparation_By_Aycl_Halide

6.From alkyl benzene :In this alkyl benzene is oxidised in presence of oxidising agent like alkaline potassium permagnate to form respectice carboxylic acid .

Class_12_Chemistry_Carboxylic_Acid_Methos_Of_Preparation_From_Alkyl_Benzene

Physical properties

  1.  Existence: They are Colourless liquids with pungent smell. The higher members are oily liquids and still higher members are waxy solids.
  2. Solubility : First four members are soluble in H2O that is-  HCOOH , CH3 , COOH , C2H5 COOH , C3H7 But as number of Carbon atoms increase , solubility decrease because  size of non-polar group increases. Solubility of smaller  members is due to formation of Hydrogen bonds.
  3. Boiling points:Boiling points are higher due to formation of hydrogen bonds . As number of Carbon atoms increases , boiling point also increases.
Class_12_Chemistry_Carboxylic_Acid_Acetic_Acid_As_Dimer
  1. Melting point : Show un-usual behaviour.
  • Those having odd number of carbon atoms possess low Melting point –
  • CH3 CH2 COOH  =  251k
  • High Melting point of members with even number of Carbon atoms –
  • CH3 CH2 CH2 COOH   =   367k
  • It is due to symmetrical structure.
Class_12_Chemistry_Carboxylic_Acid_Types_Of_CarboxylicAcids

Chemical properties of carboxylic acid

 Main four  types of reactions shown by them:

  1. Reaction due to Hydrogen atom of carboxylic acid: This property actually shows acidic character of carboxylic acid acids that is :
Class_12_Chemistry_Carboxylic_Acid_Reaction_Of_Carboxylic_Acids_WithHydrogen

 If we talk about Strength of acidic character of CH3 COOH,then we have : 

   CH3 COOH + H2O –> CH3 COO + H3O+

According to Arhenius acid and Bronstedconcept: Carboxylic acid is regarded as an acid.We can write its expression for dissociation constant as written below.

Class_12_Chemistry_Carboxylic_Acid_Reaction_Of_Carboxylic_Acids_ArcheniusAcid

It is seen that :

  • Higher the value of ka, stronger is the acid.
  • Pka = -log ka
  • Higher the value of Pka, weaker is the acid.

Effect of substituents on strength of acid

We can have subsituents of two types :

  • Electron withdrawing
  • Electron Releasing groups

 Electron withdrawing subsituents: They will neutralize the intensity of  -ve charge and hence, RCOO ion gets stablized. Therefore , the acid srength increases. Eg:Halogens

Electron releasing subsituents : They will increase the intensity of negative charge  and hence ,RCOO ion gets less stablized. Therefore, acidic strength due to this decreases.

The acidic strength also depends upon other factors such as :

  1. Type of halogen attached :

        F>Cl>Br

Fluroine is much more electronegative

Acidic Strength CH3COOF more stronger acid > CH3COCl

BrCH3COOH > CH3COOI

  1. Acidic strength depending upon number of halogen atoms attached :
Class_12_Chemistry_Carboxylic_Acid_Acidic_Strength
  1.  Depending upon the position, where X is attached
Class_12_Chemistry_Carboxylic_Acid_Acidic_Strength_1
  1. Depending upon hybridisation:
Class_12_Chemistry_Carboxylic_Acid_Acidic_Strength_2

Comparison of acidic strength of alcohol , phenols and carboxylic acid

  • If we compare Alcohol and carboxylic acid, we see that :
Class_12_Chemistry_Carboxylic_Acid_Comparison_Of_Alcohol_Phenols_Acids

It’s seen that more the ion is stablized , more the reaction  will be favoured in found direction.Below you can see carboxylic acid is resonance stabilized. So, it is more stable and on the other hand the R group attached to O in alcohol intensifies its charge. Hence, the stability is lowered in mparison to carboxylic acid.

Class_12_Chemistry_Carboxylic_Acid_Acidic_Strength_4

Phenols and carboxylic acid : Acidic character

Similarly, if we look for phenoxide ion and carboxylate ion that is formed by phenol and carboxylic acid after loosing hydrogen ion, we can easily make out that carboxylate ion is more stabilized. In it negative charge resides on electronegative ion (resonating structure ) whereas in case of phenoxide ion the negative charge is on carbon.let’s see. the structures below :

Class_12_Chemistry_Carboxylic_Acid_Comparison_Of_Alcohol_Phenols_Acids_1

That is the reason ,carboxylic acid is stronger than phenols .

Reactions to prove acidic character of carboxylic acid

  •  Reaction with metal
Class_12_Chemistry_Carboxylic_Acid_Acidic_Character_Of_Acids

Reaction involving OH group: In this we react carboxylic acid with compounds like With PCl5 , PCl3 , SOCl2.

Class_12_Chemistry_Carboxylic_Acid_Reaction_With_OH_Group
  • Reaction with alcohol : Esterification
Class_12_Chemistry_Carboxylic_Acid_Esterfication
  • Reaction with NH3
Class_12_Chemistry_Carboxylic_Acid_Reaction_With_Ammonia
  • Formation of acid anhydride
Class_12_Chemistry_Carboxylic_Acid_Formation_Of_Acid_Anhydride

3.Reaction involving COOH group

Class_12_Chemistry_Carboxylic_Acid_Decarboxyation_And_ElectrolyticDecarboxylation
  • Reaction of carboxylic salt of Calcium

            Class_12_Chemistry_Carboxylic_Acid_Reaction_With_Calcium 4.Reduction reaction

  • Partial Reduction

The acids on reduction in presence of reducing agents like are  Li AlH4 etc forms alcohols that is :

Class_12_Chemistry_Carboxylic_Acid_Partial_Reduction
  • Complete reduction : When complete reduction is carried out, it forms alkane in presence of red Phosphorous that is :
Class_12_Chemistry_Carboxylic_Acid_Complete_Reduction

Special name reactions                     

Class_12_Chemistry_Carboxylic_Acid_Some_Name_Reactions

Ring subsitution reactions :

  • Bromination
Class_12_Chemistry_Carboxylic_Acid_Bromination
  • Sulphonation
Class_12_Chemistry_Carboxylic_Acid_Sulphonation
  •    Nitration
Class_12_Chemistry_Carboxylic_Acid_Nitration

Uses of carboxylic acid

  1. Methanoic acid is used as coagulating agent in rubber industry.
  2. Benzoic acid is used as antiseptic and also in perfumery.
  3. Ethanoic acid is used as solvent and also in cooking as vinegar and much more.
Chapter-12-Aldehydes-Ketones-and-Carboxylic-Acids

CBSE Class 12 Chemistry Important Questions Chapter 12 – Aldehydes Ketones and Carboxylic Acids

1 Mark Questions

1. Give one use of Formalin.

Ans. Formalin is used as a disinfectant, preservative for biological specimens and in leather industry.

2. What is the chemical name of Tollen’s reagent and Fehling’s solution.

Ans. Tollen’s reagent  = Ammoniacal Silver Nitrate

Fehlings solution =  Sodium Potassium Tartarate.

3. Write the structure of alkenes that on ozonolysis will give ketone only.

Ans.

4.  What is the function of in rosenmund reaction?

Ans. acts as a catalytic poison which prevents further reduction of aldehyde to alcohol.

5.  Name the isomers with molecular formula. Which one will have high boiling point?

Ans. The two isomers are and. Acetone boils at higher temperature due to presence of two electron donating alkyl groups.

6. Write a chemical test to distinguish between aldehyde and ketone.

Ans. Aldehydes and ketones can be distinguished by Tollen’s test. Aldehydes give a silver mirror on reacting with Tollen’s reagent whereas ketones will not react.

7. What happens when acetaldehyde is kept with a trace of sulphuric acid? Write the structure of product.

Ans. A trimer of acetaldehyde, called paraldchyde is formed.

8.  What is the Hofmann bromamide reaction? Illustrate with one example.

Ans. Hoffman bromamide reaction is a reaction in which amides are converted to amines of one carbon less than the starting amide. It is a very important step – down reaction.

9. Give IUPAC names of following

(i)

(ii)

(iii)

(iv)

(v) 

(vi)

(vii)

(viii)

(ix) HOOC – CH = CH – COOH

(x)

 Ans. (i).  5-Chloro -3- ethylpentan -2-one.

(ii). 2 –(2-bromophenyl) ethanal

(iii). 2- Phenylpropanal

(iv). 5- Chloro -3- methyl pentan -2-one

(v). 4- Hydroxypentan -2- one

(vi).  3- Methylbutan -2-one

(vii).  2- Ethyl -2- methylbutanoic acid

(viii).  2- (3- Bromophenyl) ethanal

(ix).  But-2- en -1,4 –dioic acid

(x).  4- Methoxybenzaldehyde

10. Draw the structure of the following –

(i) 4- Methoxybenzaldehyde

 (ii) 5- Bromo -3- Chloro -2- iodobenzoic acid

(iii) 3,3 – Dimethyl -1- Chlorobutane

(iv) 2,3- Dihydroxy -4-methylpentanal

(v) 3- Hydroxy-2-methyl -propanal

(vi) 2,4 –Dimethyl -3- pentanone

(vii) 1,2 –Ethaneodioc acid

 (viii) 3- Pentene -2-one

(ix) 1,3 – Propane –dioic acid

Ans. (i).

(ii).

(iii).

(iv).

(v).

(vi).

(vii).

(viii).

(ix).

2 Marks Questions

1. Ethanoic acid has molar mass of 120 in vapour state.

Ans. Carboxylic acid on dissociation form carboxylate ion which is stabilized by two equivalent resonance structure in which negative charge is at the more electronegative oxygen atom, whereas the conjugate base of phenol, phenoxide ion, has non – equivalent resonance structures in which negative charge is at the less electronegative carbon atom. Therefore resonance is not as important as it is in carboxylate ion. Moreover the negative charge is delocalized over two more electronegative oxygen atoms in carboxylate ion whereas it is less effectively delocalized over one oxygen atom and one carbon atom in phenoxide ion. Therefore the carboxylate ion is more stabilized than phenoxide ion and carboxylic acids are stronger acids than phenol.

2. Carboxylic acids do not give characteristic reactions of Carboxylic acid is stronger acid than phenol.

Ans. Ethanol can form intermolecular Hydrogen bonding with water molecules, ethyl chloride can not. Therefore ethanol is soluble in water and ethyl chloride is not.

3. Ethanol is more soluble in water than ethyl chloride

Ans. Aldehydes are more reactive than Ketones due to steric and electronic reasons. In Ketones due to presence of two relatively large alkyl groups, the approach of nucleophile is more hindered than in aldehydes having only one such substitute. More over the +I effect of alkyl groups reduces the electophilicity of carbonyl group more in Ketone than in aldehydes.

4. Aldehydes are more reactive than Ketones towards nucleophilic additions.

Ans. Carboxylic acids have more extensive association of molecules through intermolecular hydrogen bonding than alcohols. Moreover their boiling points are higher than alcohols of same carbon atoms.

5. Carboxylic acids has higher boiling points than alcohols of same no. of carbon atoms.

Ans. Ethanoic acid exists as dimer in vapour state in which two molecules remain together by hydrogen bonding. This increases the effective molecular mass to 120.

6. carbonyl group.

Ans. In carboxylic acids due to presence of resonance, the C=O group is not a pure carbonyl group & therefore they do not show characteristic reactions of carbonyl group.

7. Formaldehyde does not undergo aldol condensation.

Ans. Formaldehyde does not have any – hydrogen and therefore it can not show aldol condensation.

8. Floro acetic acid is a stronger acid than acetic acid.

Ans. In fluoroacetic acid, Fluorine being electron withdrawing group stabilizes the conjugate base through delocalization of the negative charge


Therefore fluoroacetic acid is a stronger acid than acetic acid.

3 Marks Questions

1.  Toluene to benzaldehyde  

Ans.

2.  Acetaldehyde to Acetamide

Ans.

3.  Methanol to acetic acid

Ans.

4.  Methanol to Ethanol  

Ans.

5.  Acetic acid to Propionic acid

Ans.

6.  Ethyl alcohol to acetone

Ans.

7.  Acetone to tert butyl alcohol

Ans.

8.  Toluene to m- nitrobenzoic acid

Ans.

9.  Phenol to acetophenone

Ans.

10.  Acetaldehyde to Acetone

Ans.

11. Give the IUPAC names of the following compounds:

(i) 

(ii) 

(iii)

(iv)

Ans. (i) 3-Phenylpropanoic acid

(ii) 3-Methylbut-2-enoic acid

(iii) 2-Methylcyclopentanecarboxylic acid

(iv)2,4,6-Trinitrobenzoic acid

NCERT Solutions for Class 12 Chemistry Chapter 12 Aldehydes Ketones and Carboxylic Acids

NTEXT Questions

Question 1. ( Aldehydes Ketones and Carboxylic Acids )
Write the structures of the following compounds.

  1. α-Methoxypropionaldehyde
  2. 3-Hydroxybutanal
  3. 2-Hydroxycyclopentanecarbaldehyde
  4. 4-Oxopentanal
  5. Di-sec.butyl ketone
  6. 4-Fluoroacetophenone

Solution:

Question 2. ( Aldehydes Ketones and Carboxylic Acids )
Write the structures of the products of the following reactions :

Solution:

Question 3. ( Aldehydes Ketones and Carboxylic Acids )
Arrange the following compounds in increasing order of their boiling points.
CH3CHO, CH3CH2OH, CH3OCH3, CH3CH2CH3
Solution:
CH3CH2CH3 < CH3OCH3 < CH3CHO < CH3CH2OH

Question 4. ( Aldehydes Ketones and Carboxylic Acids )
Arrange the following compounds in increasing order of their reactivity in nucleophilic addition reactions.

  1. Ethanal, Propanal, Propanone, Butanone.
  2. Benzaldehyde, p-Tolualdehyde,

p – Nitrobenzaldehyde, Acetophenone. Hint: Consider steric effect and electronic effect.
Solution:

  1. Butanone < Propanone < Propanal < Ethanal
  2. Acetophenone < p-Tolualdehyde, Benzaldehyde < p-Nitrobenzaldehyde

Question 5. ( Aldehydes Ketones and Carboxylic Acids )
Predict the products of the following reactions :

Solution:

Question 6. ( Aldehydes Ketones and Carboxylic Acids )
Give the IUPAC names of the following compounds :

Solution:

  1. 3-Phenylpropanoic acid
  2. 3-Methylbut-2-enoic acid
  3. 2-Methylcyclopentanecarboxylic acid
  4. 2,4,6-Trinitrobenzoic acid

Question 7. ( Aldehydes Ketones and Carboxylic Acids )
Show how each of the following compounds can be converted to benzoic acid.

  1. Ethylbenzene
  2. Acetophenone
  3. Bromobenzene
  4. Phenylethene (Styrene)

Solution:

Question 8. ( Aldehydes Ketones and Carboxylic Acids )
Which acid of each pair shown here would you expect to be stronger ?
(i) CH3CO2H or CH2FCO2H
(ii) CH2FCO2H or CH2ClCO2H
(iii) CH2FCH2CH2CO2H or CH3CHFCH2CO2H

Solution:
(i) H2CFCOOH will be stronger of the two. The presence of electronegative F atom at the α-C causes electron withdrawal from the COOH and facilitates the release of H+.
(ii) CH2FCO2H is a stronger acid for the same reason as stated above. F is more electronegative than Cl, so it withdraws electrons from the carboxyl group to a greater extent.
(iii) CH3CHFCH2COOH is stronger. Although both the givenacidshaveFatomin them, it is the proximity of F in CH3CHFCH2COOH to the COOH group which makes it more acidic.

NCERT Exercises

Question 1. ( Aldehydes Ketones and Carboxylic Acids )
What is meant by the following terms? Give an example of the reaction in each case.

  1. Cyanohydrin
  2. Acetal
  3. Semicarbazone
  4. Aldol
  5. Hemiacetal
  6. Oxime
  7. Ketal
  8. Imine
  9. 2,4-DNP derivative
  10. Schiff’s base

Solution:

Question 2. ( Aldehydes Ketones and Carboxylic Acids )
Name the following compounds according to IUPAC system of nomenclature :

  1. CH3CH(CH3)CH2CH2CHO
  2. CH3CH2COCH(C2H5)CH2CH2Cl
  3. CH3CH=CHCHO
  4. CH3COCH2COCH3
  5. CH3CH(CH3)CH2C(CH3)2COCH3
  6. (CH3)3CCH2COOH
  7. OHCC6H4CHO-p

Solution:

  1. 4-Methylpentanal
  2. 6-Chloro-4-ethylhexan-3-one
  3. But-2-en-1-al
  4. Pentane-2, 4-dione
  5. 3, 3, 5-Trimethylhexan-2-one
  6. 3, 3-Dimethylbutanoic acid
  7. Benzene-1, 4-dicarbaldehyde

Question 3. ( Aldehydes Ketones and Carboxylic Acids )
Draw the structures of the following compounds.

  1. 3-Methylbutanal
  2. p-Nitropropiophenone
  3. p-Methylbenzaldehyde
  4. 4-Methylpent-3-en-2-one
  5. 4-Chloropentan-2-one
  6. 3-Bromo-4-phenylpentanoic acid
  7. p, p’-Dihydroxybenzophenone
  8. Hex-2-en-4-ynoic acid

Solution:

Question 4. ( Aldehydes Ketones and Carboxylic Acids )
Write the IUPAC names of the following ketones and aldehydes. Wherever possible, give also common names.

Solution:

Question 5. ( Aldehydes Ketones and Carboxylic Acids )
Draw structures of the following derivatives.

  1. The 2,4-dinitrophenylhydrazone of benz- aldehyde
  2. Cyclopropanone oxime
  3. Acetaldehydedimethylacetal
  4. The semicarbazone of cyclobutanone
  5. The ethylene ketal of hexan-3-one
  6. The methyl hemiacetal of formaldehyde

Solution:

Question 6. ( Aldehydes Ketones and Carboxylic Acids )
Predict the products formed when cyclohexanecarbaldehyde reacts with following reagents.

  1. PhMgBr and then H3O+
  2. Tollens’reagent
  3. Semicarbazide and weak acid
  4. Excess ethanol and acid
  5. Zinc amalgam and dilute hydrochloric acid

Solution:

Question 7. ( Aldehydes Ketones and Carboxylic Acids )
Which of the following compounds would undergo aldol condensation, which the Cannizzaro reaction and which neither? Write the structures of the expected products of aldol condensation and Cannizzaro reaction.

  1. Methanal
  2. 2-Methylpentanal
  3. Benzaldehyde
  4. Benzophenone
  5. Cyclohexanone
  6. 1-Phenylpropanone
  7. Phenylacetaldehyde
  8. Butan-1-ol
  9. 2,2-Dimethylbutanal

Solution:
Aldol condensation is shown by those aldehydes or ketones which have at least one α-H atom while Cannizzaro reaction is undergone by aldehydes that have no α-H atom.

Question 8. ( Aldehydes Ketones and Carboxylic Acids )
How will you convert ethanal into the following compounds?

  1. Butane-1,3-diol
  2. But-2-enal
  3. But-2-enoic acid

Solution:

Question 9. ( Aldehydes Ketones and Carboxylic Acids )
Write structural formulas and names of four possible aldol condensation products from propanal and butanal. In each case, indicate which aldehyde acts as nucleophile and which as an electrophile
Solution:
The possible products of aldol condensation from propanal and butanal are

Question 10. ( Aldehydes Ketones and Carboxylic Acids )
An organic compound with the molecular formula C9H10O forms 2,4-DNP derivative, reduces Tollens’ reagent and undergoes Cannizzaro reaction. On vigorous oxidation, it gives 1, 2-benzenedicarboxylic acid. Identify the compound.
Solution:
(i) It is given that the compound reduces Tollens’ reagent. This proves that the compound is an aldehyde. Further, the fact that it undergoes Cannizzaro reaction shows that it lacks an α-H atom.

(ii) On oxidation it yields 1,2-benzenedicarboxylic acid. This shows that it is an o-substituted benzaldehyde. The only possible structure for the compound is :

Question 11. ( Aldehydes Ketones and Carboxylic Acids )
An organic compound (A) (molecular formula C8H16O2) was hydrolysed with dilute sulphuric acid to give a carboxylic acid (B) and an alcohol (C). Oxidation of (C) with chromic acid produced (8). (C) on dehydration gives but-1-ene. Write equations for the reactions involved.
Solution:

Question 12. ( Aldehydes Ketones and Carboxylic Acids )
Arrange thefollowing compounds in increasing order of their property as indicated :

  1. Acetaldehyde, Acetone, Di-tert-butyl ketone, Methyl fert-butyl ketone (reactivity towards HCN)
  2. CH3CH2CH(Br)COOH,CH3CH(Br)CH2COOH, (CH3)2CHCOOH, CH3CH2CH2COOH (acid strength)
  3. Benzoic acid, 4-Nitrobenzoic acid, 3,4-Dinitrobenzoic acid, 4-Methoxybenzoic acid (acid strength)

Solution:

Question 13. ( Aldehydes Ketones and Carboxylic Acids )
Give simple chemical tests to distinguish between the following pairs of compounds.

  1. Propanal and Propanone
  2. Acetophenone and Benzophenone
  3. Phenol and Benzoic acid
  4. Benzoic acid and Ethyl benzoate
  5. Pentan-2-one and Pentan-3-one
  6. Benzaldehyde and Acetophenone
  7. Ethanal and Propanal

Solution:
The given set of compounds may be distinguish by the following reaction.

Question 14. ( Aldehydes Ketones and Carboxylic Acids )
How will you prepare the following compounds from benzene? You may use any inorganic reagent and any organic reagent having not more than one carbon atom

  1. Methyl benzoate
  2. m-Nitrobenzoic acid
  3. p-Nitrobenzoic acid
  4. Phenylacetic acid
  5. p-Nitrobenzaldehyde

Solution:

Question 15. ( Aldehydes Ketones and Carboxylic Acids )
How will you bring about the following conversions in not more than two steps?

  1. Propanone to Propene
  2. Benzoic acid to Benzaldehyde
  3. Ethanol to 3-Hydroxybutanal
  4. Benzene to m-Nitroacetophenone
  5. Benzaldehyde to Benzophenone
  6. Bromobenzene to 1 -Phenylethanol
  7. Benzaldehyde to 3-Phenyipropan-1 -ol
  8. Benzaldehyde to a-Hydroxyphenylacetic acid
  9. Benzoic acid to m-Nitrobenzyl alcohol

Solution:

Question 16. ( Aldehydes Ketones and Carboxylic Acids )
Describe the following :

  1. Acetylation
  2. Cannizzaro reaction
  3. Cross-aldol condensation
  4. Decarboxylation

Solution:

Question 17. ( Aldehydes Ketones and Carboxylic Acids )
Complete each synthesis by giving missing starting material, reagents or products

Solution:

Question 18. ( Aldehydes Ketones and Carboxylic Acids )
Give plausible explanation for each of the following :

  1. Cyclohexanone forms cyanohydrin in good yield but 2, 2,6 trimethylcyclohexa- none does not.
  2. There are two – NH2 groups in semicarbazide. However, only one is involved in the formation of semicarbazones.
  3. During the preparation of esters from a carboxylic acid and an alcohol in the presence of an acid catalyst, the water or the ester should be removed as soon as it is formed.

Solution:

Question 19. ( Aldehydes Ketones and Carboxylic Acids )
An organic compound contains 69.77% carbon, 11.63% hydrogen and rest, oxygen. The molecular mass of the compound is 86. It does not reduce Tollens’ reagent but forms an addition compound with sodium hydrogensulphite and give positive iodoform test. On vigorous oxidation it gives ethanoic and propanoic acid. Write the possible structure of the compound.
Solution:
(a) The given compound does not reduce Tollens’ reagent which implies that it is not an aldehyde.

Question 20. ( Aldehydes Ketones and Carboxylic Acids )
Although phenoxide ion has more number of resonating structures than carboxylate ion, carboxylic acid is a stronger acid than phenol. Why?
Solution:
(i) Phenoxide ion has the following resonating structures :

(iii) The negative charge that rests on the electronegative O atom in carboxylate ion. We know that the presence of negative charge on an electronegative atom makes the ion more stable. For the same reason RCOO is more stable than the phenoxide ion where the oxygen has no negative charge on it. For the above two reasons carboxylate ion is more stable and has higher acidity than phenol.

Related Links

Class 9th

Class 10th

Class 11th

Class 12th


Leave a Reply

Your email address will not be published. Required fields are marked *