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Hydrocarbons

Short Answer Type

171.

Give reduction of phenol ?

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Long Answer Type

172.

Discuss in brief the polymerisation reactions of alkenes ?

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Short Answer Type

173.

Enlist three uses of ethylene.

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Long Answer Type

174.

What are alkynes?

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175.

For the following compounds, write structural formulas and IUPAC names for all possible isomers having the number of double or triple bond as indicated:

$left parenthesis straight i right parenthesis space space straight C subscript 4 straight H subscript 8 left parenthesis one space double space bond right parenthesis left parenthesis ii right parenthesis space straight C subscript 5 straight H subscript 8 space left parenthesis one space triple space bond right parenthesis$

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176. Write structures of different isomers corresponding to the 5th member of alkyne series. Also, write IUPAC names of all the isomers ?

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Short Answer Type

177.

How is acetylene obtained from:
(i) Ethylene dibromide?
(ii) Tetrabromoethane ?

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Long Answer Type

178.

Discuss the preparation of higher alkynes.

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179.

Why do alkynes undergo electrophilic addition reactions ?

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Short Answer Type

180. Alkynes are less reactive towards electrophilic addition reactions than alkenes. Explain.

In alkynes, the carbon atom is sp hybridised whereas in alkenes, it is sp² hybridised. The sp hybrid orbitals have greater s-character (50%) and lesser effective size than sp²hybrid orbitals (33.3%). Carbon-carbon triple bond length (– C ≡ C –) is 120 pm while carbon-carbon double bond length (C = C) is 134 pm. Consequently,

-electrons are more tightly held by the nuclei of carbon atoms in alkyne than in alkene. As a result, the

electrons are less easily available for combination with the electrophiles. Hence alkynes are less reactive than alkenes.

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