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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Acids and Esters Page 1

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Carboxylic Acids

 

The organic compounds containing a carboxyl group

are called carboxylic acids.

For example, acetic acid, CH3-COOH.
The general formula of monocarboxylic acids is CnH2nO2 / CnH2n+1COOH.

They are represented as

where R is an alkyl group (except formic acid where

R is a hydrogen atom).

 

 

Nomenclature of Acids

 

(i) Common Name: The lower members are known by the names derived from the source (origin).
For example, H
-COOH formic acid (obtained from red ant known as 'formica' in Latin)
CH3
- COOH, acetic acid (obtained from vinegar known as 'acetum' in Latin)

(ii) IUPAC Name: The suffix
-e of the corresponding alkane is replaced by -oic acid.
For example, H
- COOH, Methanoic acid; CH3 - COOH, Ethanoic acid.

 

 

Structure of Carbon Atom

 

In the carboxylic acid group (-COOH), the carbon atom is sp2 hybridised.

 

 

Formic Acid (HCOOH)

 

Formic acid

contains both aldehyde group and acid group. Hence, it shows the

reducing properties of aldehydes, i.e. it can reduce Fehling's solution, Tollen's reagent, etc.

 

 

Preparation of Monocarboxylic Acids

 

a. By Oxidation:

  1. From Primary Alcohols:

R-CH2OH + [O]

  R-CHO + H2O

R-CHO + [O]

R-COOH

  1. From Aldehydes:

R-CHO + [O]

R-COOH

b. By Hydrolysis of Alkyl Cyanides (Nitriles):

  1. Using Acids:

R-C N + 2H2O

R-COOH + NH3

  1. (NH3 + HCl NH4Cl)
  2. Using Alkalies:

R- C N + H2O

R-COONa + NH3

  1. R- COONa + HCl R-COOH + NaCl

 c. By Grignard Reagent and Solid Carbon Dioxide:
      It is a nucleophilic addition reaction.

+ R-MgX

 

 

Properties

 

Carboxylic acids are weak acids. The acidic nature of carboxylic acid decreases as the molecular weight increases. This acidic nature is due to the resonance stabilisation of carboxylate ion.

 R-

-OH + H2O H3O+ +

Among all the acids, formic acid is the strongest acid.

 

 

Reactions:

 

a. Acidic Properties:
They undergo nucleophilic substitution reactions.

  1. Reaction with Reactive Metals (M) like Na, K:
    2R
    - COOH + 2M 2R - COOM + H2
  2. Reaction with Alkalies:

R - COOH + NaOH

  R- COONa + H2O

 (or KOH)

 

  (or R- COOK)

  1.  
  2. Reaction with Carbonates and Bicarbonates:
    2R-COOH + Na2CO3 2 R - COONa + H2O + CO2
    R
    -COOH + NaHCO3 R-COONa + H2O + CO2
  3. Reaction with Ammonia:
    R
    -COOH + NH3 R-COONH4

b. Anhydride Formation:

     

    P2O5 + H2O 2HPO3
                               metaphosphoric acid

c. Esterification:

R-COOH + R - OH

R -

COO

- R` +H2O

 

ester

 

The reverse of esterification is known as hydrolysis.

D. Amide Formation:
R
-COOH + NH3 R-COONH4

 R-COONH4

R -

CONH2

+H2O

 

amide

 

 

 

 

 

Esters:

 

The derivatives of carboxylic acids, in which the 'H' atom of the carboxylic group is replaced by an alkyl group, are called esters.
They are the condensation products of acids and hydroxy compounds. They have general formula, R
- COOR where R is an alkyl group. For example: Ethyl acetate, CH3-COOC2H5.

 

 

Nomenclature of Esters

 

  1. Common Name: Esters are named by replacing the suffix -ic acid of the carboxylic acid by -ate and adding the name of alkyl group of the alcohol as a separate word before it.
  2. IUPAC System: Esters are named as alkyl alkanoate. For example: H-COOCH3, methyl methanoate.

 

 

Preparation of Esters

 

a. From Carboxylic Acids and Alcohols (Esterification):

R-COOH + R -OH

  R- COOR+H2O

b . From Acid Anhydrides and Alcohols:

c. From Acid Chlorides and Alcohols:

R- CO 

Cl + H

 O - R' 

 R- COOR' + HCl

 


d. From Silver Salt of an Acid and Alkyl Halide:

R- COO  

Ag + X

 - R'

 R- COOR' + AgX

 

 

 

 

 

Reactions of Ethyl Acetate

 

  1. Hydrolysis:
    1. Acid Hydrolysis:

CH3COOC2H5 + H - OH  

 CH3COOH + C2H5OH

acid (H+)

 

    1.  
    2. Alkaline Hydrolysis:

CH3COOC2H5 + NaOH  

 CH3COONa + C2H5OH

    1.  
  1. Action of Grignard Reagent:



                                         

                                

    With excess of Grignard reagent:



                                        

 

 

 

 

Important Uses of Esters:

 

  1. Esters are used as solvents for inks, paints, oils, gums, resins, etc.
  2. As esters have a pleasant smell, they are used in the manufacture of perfumes, essences, and flavouring agents. For example, isoamyl acetate is used as banana flavour, methyl butyrate as pineapple flavour, octyleacetate as orange flavour.
  3. Esters are used as plasticizers in the manufacture of plastics.
  4. Esters are used as perfumes in soaps, cosmetics, etc.

 

 

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