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Hydrocarbons

Short Answer Type

141.

What happens when:

(i) Ethyl alcohol is heated in the presence of Al₂O₃ at 493 K?

(ii) Ethylene dibromide is heated with zinc dust?

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Long Answer Type

142.

Explain the mechanism of the following reaction:

$straight C subscript 2 straight H subscript 5 OH space rightwards arrow from 440 space straight K to Conc. space straight H subscript 2 SO subscript 4 of space CH subscript 2 space equals space CH subscript 2 space plus space straight H subscript 2 straight O$

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143.

How are alkenes prepared from Alkynes ?

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Short Answer Type

144.

The $straight pi$ -bond is weaker than carbon-carbon σ bond. Explain.

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145.

Why are alkenes more reactive than alkanes ?

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Long Answer Type

146.

Why do alkenes undergo electrophilic addition reactions?

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147.

Discuss the mechanism of addition of bromine to ethylene.

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148.
Discuss the mechanism of addition hydrogen acids to symmetrical alkenes. Justify the order of reactivity of halogen acids HI > HBr > HCl.

Alkenes react with halogen acids to form mono-haloalkanes called alkyl halides.

The order of reactivity of the halogen acids is HI > HBr > HCl.

The above order is justified on the basis of bond dissociation energies of the halogen acids.
HI (300 kJ mo^-1) < HBr (365 kJ mol^-1) < HCl (430 kJ mor^-1) For example,

Mechanism: It is electrophilic addition reaction and consists of the following steps:

(i) Electromeric effect and electrophilic attack:

(ii) Attack of nucleophile: The nucleophile released in the slow step combines with carbocation to give chloroethane.