Chemistry Class-11: Chapter –13. Hydrocarbons Part – 9

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VI. Long Answer Type

Question: 48

An alkyl halide reacts with ethanolic to give an alkene, which reacts with to give a compound , which on dehydrobromination gives an alkyne . On treatment with sodium metal in liquid ammonia one mole of ‘D’ gives one mole of the sodium salt of ‘D’ and half a mole of hydrogen gas. Complete hydrogenation of ‘D’ yields a straight chain alkane. Identify A,B, C and D. Give the reactions involved

Answer:

The reactions suggest that (D) is a terminal alkyne. This means triple bond is at the end of the chain. It could be either (I) or (II).

Since alkyne ‘D’ on hydrogenation yields straight chain alkane, therefore structure I is the structure of alkyne (D).

Hence, the structures of A, B and C are as follows:

(A)

(B)

(C)

Question: 49

vapour of a hydrocarbon ‘A’ having carbon and hydrogen weighs at STP. Hydrogenation of ‘A’ gives methylpentane. Also ‘A’ on hydration in the presence of and gives a ketone ‘B’ having molecular formula . The ketone ‘B’ gives a positive iodoform test. Find the structure of ‘A’ and give the reactions involved.

Answer:

Step vapour of weighs

vapour of weighs

Step II

Empirical formula of ‘A’

Empirical formula mass

Molecular mass is double of the empirical formula mass.

Molecular Formula is

Step III

Structure of 2-methylpentane is

Hence, the molecule has a five carbon chain with a methyl group at the second carbon atom. ‘A’ adds a molecule of in the presence of and , it should be an alkyne. Two possible structures for ‘A’ are:

Since the ketone (B) gives a positive iodoform test, it should contain group. Hence the structure of ketone is as follows:

Therefore structure of alkyne is II.

Question: 50

An unsaturated hydrocarbon ‘A’ adds two molecules of and on reductive ozonolysis gives butanedial, ethanal and propanone. Give the structure of ‘A’, write its IUPAC name and explain the reactions involved.

Answer:

Two molecules of hydrogen add on ‘A’ this shows that ‘A’ is either an alkadiene or an alkyne. On reductive ozonolysis ‘A’ gives three fragments, one of which is dialdehyde. Hence, the molecule has broken down at two sites. Therefore, ‘A’ has two double bonds. It gives the following three fragments: and

Hence, its structure as deduced from the three fragments must be

Reactions

(A)

Question: 51

In the presence of peroxide addition of to propene takes place according to anti Markovnikov’s rule but peroxide effect is not seen in the case of and . Explain.

Answer:

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