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 CnH2n+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 pKa value. Higher the value of Ka or lower value of pKa, 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 Ka and lower value of pKa .
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.