NEET Chemistry Notes Aldehydes, Ketones and Carboxylic Acids – Uses of Aldehydes and Ketones

NEET Chemistry Notes Aldehydes, Ketones and Carboxylic Acids – Uses of Aldehydes and Ketones

Uses of Aldehydes and Ketones

Uses of Aldehydes and Ketones

• In chemical industry, aldehydes and ketones are used as solvents, starting materials and reagents for the synthesis of other products.
•  Formaldehyde, under the name formalin (40% solution) is used to preserve biological specimens.
•  Formaldehyde is also used to prepare bakelite, urea-formaldehyde glues and other polymeric products.

Carboxylic Acids

Carbon compounds containing a functional group, —COOH are called carboxylic acids. Some higher members of aliphatic carboxylic acids (C₁₂-C₁₈ ) known as fatty acids, occur in natural fats as esters of glycerol.

Nomenclature

According to IUPAC nomenclature system, name of a carboxylic acid is obtained by replacing the terminal ‘e’ of the corresponding alkane by the suffix ‘oic acid’. Their general formula is C_nH_2n+1  COOH.

Nature of the Carboxylic Acids

• Carboxylic acids are acidic in nature but they are weaker acids than mineral acids but stronger acids than phenols (because the carboxylate ion is more stabilised as compared to phenoxide ion).
• The carboxylic acids form salts with alkalies.
•  Strength of acid is indicated by pK_a value. Higher the value of K_a or lower value of pK_a, stronger is the acid.
• Aromatic acids are more acidic than aliphatic acids.

Preparation

(i) From Acid Derivatives

All acid derivatives like amides (RCONH₂), acid halides (COCl), esters (RCOOR’), acid anhydrides (RCO —O—COR) on hydrolysis give carboxylic acids.
All acid derivatives break form RC⁺Q.

(ii) From Alkyl Benzene

Alkyl benzene when treated with strong oxidising agent like H₂Cr0₄ (chromic acid), acidic or alkaline KMn0₄  gives benzoic acid.

Chemical Properties

Reactions Showing Acidic Behaviour
Reactions with Metals and Alkalies

Carboxylic acids dissociate in water to give resonance stabilised carboxylate anions and hydronium ion.

Effect of Substituents on the Acidity of Carboxylic Acids

Electron withdrawing group (EWG) stabilises the carboxylate anion find strengthens the acid.

Electron donating group (EDG) destabilises the carboxylate anion and weakens the acid. Strong acids have higher value of K_a and lower value of pK_a .

Following acids are arranged in decreasing order of acidity

In case of aromatic carboxylic acids, more the –R effect, more is the acidic nature.

Reactions Involving Cleavage of C —OH Bond

Reduction of—COOH Group

Carboxylic acids are reduced to primary alcohols by LiAlH₄ or better with B₂H₆.B₂H₆ does not easily reduce functional groups such as esters, nitro, halo, etc. NaBH₄ does not reduce the carboxyl group.

Methanoic acid and its sodium salt acts as reducing agent due to — CHO group. It reduces Tollen’s reagent, Fehling’s reagent, KMn0₄, etc.

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