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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.

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149.

Define Markownikoff's rule giving an example.

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150. Give the mechanism of Markownikoffs rule as applied to unsaturated hydrocarbons.

Propene (unsymmetrical alkene) reacts with hydrogen iodide (unsymmetrical reagent) to form 2-iodopropane. Addition takes place according to Markownikoffs rule.

Mechanism: The reaction involves electrophilic addition. It is initiated by the attack of an electrophile (H⁺) on propene molecule. Two types of intermediate carbocations are formed.

The carbocation which is mainly formed depends on upon its relative stability. Since secondary carbocation (I) is more stable than the primary carbocation (II), therefore, it would be formed faster and than in preference to carbocation (II).
The carbocation (I) then takes up the iodide ion

of the HI to form isopropyl iodide as the main product.

On the basis of above explanation. Markownikoffs rule may also be stated as The electrophilic addition to an unsymmetrical alkene always occurs through the formation of a more stable carbocation intermediate.

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