Chemistry Class 12 NCERT Solutions: Chapter 12 Aldehydes Ketones and Carboxylic Acids Part 1 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q: 1. What is meant by the following terms? Give an example of the reaction in each case.

(i) Cyanohydrin

(ii) Acetal

(iii) Semicarbazone

(iv) Aldol

(v) Hemiacetal

(iv) Oxime

(vii)

(viii) Imine

(ix) 2,4, -DNP-derivative

(x) Schiff՚s base

Answer:

(i) Cyanohydrin:

Cyanohydrins are organic compounds having the formula , where R and R′ can be alkyl or aryl groups.

Cyanohydrins Are Organic Compounds

Aldehydes and ketones react with hydrogen cyanide (HCN) in the presence of excess sodium cyanide as a catalyst to field cyanohydrin. These reactions are known as cyanohydrin reactions.

Cyanohydrins are useful synthetic intermediates.

(ii) Acetal:

Acetals are gem − di-alkoxy alkanes in which two alkoxy groups are present on the terminal carbon atom. One bond is connected to an alkyl group while the other is connected to a hydrogen atom.

General Structure of an Acetal

When aldehydes are treated with two equivalents of a monohydric alcohol in the presence of dry gas, hemiacetals are produced that further react with one more molecule of alcohol to yield acetal.

Aldehydes Hemiacetal Acetal

(iii) Semicarbarbazone:

Semicarbazones are derivatives of aldehydes and ketones produced by the condensation reaction between a ketone or aldehyde and semicarbazide.

The Semicarbazone

Semicarbazones are useful for identification and characterization of aldehydes and ketones.

(iv) Aldol:

A -hydroxy aldehyde or ketone is known as an aldol. It is produced by the condensation reaction of two molecules of the same or one molecule each of two different aldehydes or ketones in the presence of a base.

Aldehydes or Ketones in the Presence of a Base

(v) Hemiacetal

Hemiacetals are alkoxyalcohols

Hemiacetals Are Α-Alkoxyalcohols

General structure of a hemiacetal

Aldehyde reacts with one molecule of a monohydric alcohol in the presence of dry gas.

General Structure of a Hemiacetal

(vi) Oxime:

Oximes are a class of organic compounds having the general formula , where R is an organic side chain and R′ is either hydrogen or an organic side chain. If R′ is H, then it is known as aldoxime and if R′ is an organic side chain, it is known as ketoxime.

Oximes Are a Class of Organic Compounds

On treatment with hydroxylamine in a weakly acidic medium, aldehydes or ketones form oximes.

On Treatment with Hydroxylamine

(vii) Ketal:

Ketals are gem − dialkoxyalkanes in which two alkoxy groups are present on the same carbon atom within the chain. The other two bonds of the carbon atom are connected to two alkyl groups.

Structure of Ketal

Ketones react with ethylene glycol in the presence of dry gas to give a cyclic product known as ethylene glycol ketals.

Ketones React with Ethylene Glycol

(viii) Imine:

Imines are chemical compounds containing a carbon nitrogen double bond

Structure of an Imine

Imines are produced when aldehydes and ketones react with ammonia and its derivatives.

Aldehydes and Ketones React with Ammonia

(ix) 2,4, -DNP-derivative:

2,4 − dinitrophenylhydragones are 2,4 − DNP − derivatives, which are produced when aldehydes or ketones react with 2,4 − dinitrophenylhydrazine in a weakly acidic medium.

2,4 − Dinitrophenylhydrazone

To identify and characterize aldehydes and ketones, 2,4 − DNP derivatives are used.

(x) Schiff՚s base:

Schiff՚s base (or azomethine) is a chemical compound containing a carbon-nitrogen double bond with the nitrogen atom connected to an aryl or alkyl group-but not hydrogen. They have the general formula . Hence, it is an imine. It is named after a scientist, Hugo Schiff.

Schiff՚s Base

Aldehydes and ketones on treatment with primary aliphatic or aromatic amines in the presence of trace of an acid yields a Schiff՚s base.

Schiff՚s Base Formation

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