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NEET NEET Questions — Free MCQ Practice

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Reaction intermediate of E1 reaction is:

A.

Carbocation

B.

carbanion

C.

Carbene

D.

free radical

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Explanation

In an E1 reaction, the rate-determining step involves the formation of a carbocation intermediate by the heterolytic cleavage of a carbon-hydrogen or carbon-carbon bond. This carbocation then undergoes elimination of a proton to form an alkene product.

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Reaction intermediate of E1cb reaction is:

A.

Carbocation

B.

Carbanion

C.

benzyne

D.

free radical

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In the given reaction:

${CH}_{3} - {CH}_{2} - \underset{{CH}_{3}}{\underset{|}{CH}} - {CH}_{2} - Br \overset{alc . KOH / ∆}{\to} [X]$

[X] will be:

A.

$C H_{3} - C H_{2} - C = C H_{2} I C H_{3}$

B.

$C H_{3} - C H = C - C H_{3} l C H_{3}$

C.

$C H_{2} = C H - C H - C H_{3} l C H_{3}$

D.

$C H_{3} - C H = C H - C H_{2} - C H_{3}$

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Explanation

${CH}_{3} - {CH}_{2} - CH - {CH}_{2} - R \overset{E 2}{\to} {CH}_{3} - {CH}_{2} - C = {CH}_{2} | | {CH}_{3} {CH}_{3}$

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Which one of the following pairs is correctly matched?

A.

Saytzeff rule, least substituted alkene

B.

Hofmann rule, most substituted alkene

C.

E1cb reaction, Hofmann elimination

D.

E1 reaction, Hofmann elimination

If both the assertion and the reason are true and the reason is a correct explanation of the assertion
If both the assertion and reason are true but the reason is not a correct explanation of the assertion
If the assertion is true but the reason is false
If both the assertion and reason are false

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Explanation

In E1cb reaction reaction intermediate is carbanion. Hence, product formation takes place according to Hofmann rule.

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Arrange the reactivity of alcohols in decreasing order for E1 elimination reaction:

(i) $O H l C H_{3} - C H - C H_{3}$

(ii) $O H l C H_{3} - C - C H_{3} l C H_{3}$

(iii) $O H l C_{6} H_{5} - C - C H_{3} l C H_{3}$

(iv) $C H_{3} - C H_{2} - C H_{2} O H$

Select the correct answer from the codes given below:

Codes:

A.

(iii), (ii), (iv), (i)

B.

(ii), (iii), (iv), (i)

C.

(ii), (iii), (i), (iv)

D.

(iii), (ii), (i), (iv)

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Explanation

Reactivity $α$ stability of carbocation

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In the given reaction,

$\overset{Br}{\overset{|}{{CH}_{2}}} - C H = C H_{2} \overset{N a N H_{2} / ∆}{\to} [X]$

[X] will be:

A.

$C H_{3} - C \equiv C H$

B.

$H_{2} C = C \equiv C H_{2}$

C.

$C H_{3} - C \equiv \overset{⊝}{C} \overset{\oplus}{N} a$

D.

$H_{2} C = C \equiv \overset{⊝}{C} H \overset{\oplus}{N} a$

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Explanation

$B r - C H_{2} - C H = C H_{2} \overset{\overset{⊝}{N} H_{2}}{\to} [C H_{2} = C = C H_{2}] \overset{R e a r r a n g e m e n t}{\to} [C H_{3} - C \equiv C H] \overset{N a N H_{2}}{\to} C H_{3} - C \equiv \overset{⊝}{C} \overset{\oplus}{N} a$

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Consider the following statements :

(i) Pyrolytic elimination is always syn elimination.

(ii) In pyrolytic elimination, product formation takes place by most stable eclipsed conformation.

(iii) In pyrolytic elimination, product formation takes place by Hofmann rule.

(iv) In pyrrolytic elimination, product formation takes place by Saytzeff rule.

Of these statements :

A.

(i), (ii) and (iv) are correct

B.

(i), (ii) and (iii) are correct

C.

(i) and (ii) are correct

D.

(ii) and (iii) are correct

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Explanation

The term pyrolytic elimination literally means an elimination reaction occurring in the organic substrate due to the application of heat (Greek word pyr meaning fire). Pyrolytic eliminations do not require any additional reagent or solvent to occur, and mostly carried out in the gaseous phase though can be performed in inert solvents. Thus, these type of eliminations are different from other types of eliminations (E1,E2 or E1cB) as all other types of elimination reactions require an added base (external base or solvent) to proceed. These thermal elimination reactions of molecules can also be termed as pyrolytic Ei (Elimination internal) reactions. Carboxylic esters, xanthates, amine oxides, sulfoxides and selenoxides are some substrates which commonly undergo pyrolytic eliminations.

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Dichloromethane on treatment with alc. KOH/ $∆$ gives:

A.

Carbene

B.

Diclorocarbene

C.

Singlet chlorocarbene

D.

Triplet chlorocarbene

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A compound A of formula C₃H₆Cl₂ on reaction with alkali can give B of formula C₃H₆O or C of formula C₃H₄. B on oxidation gave a compound of the formula C₃H₆O. C with dilute H₂SO₄ containing Hg²⁺ ion gave D of formula C₃H₆O, which with bromine and NaOH gave the sodium salt of C₂H₄O₂. Then A is :

A.

CH₃CH₂CHCl₂

B.

CH₃CCl₂CH₃

C.

CH₂ClCH₂CH₂Cl

D.

CH₃CHClCH₂Cl

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Identify the product (A) in following reaction series, ${CH}_{3} CN \overset{Na / C_{2} H_{5} OH}{\to}$ (X) $\overset{{HNO}_{2}}{\to}$ (Y) $\overset{[O]}{\to}$ (Z) $\overset{Tollens reagent}{\to}$ (A) :

A.

CH₃CHO

B.

CH₃CONH₂

C.

CH₃COOH

D.

CH₃- CH₂-NHOH

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Explanation

(c) ${CH}_{3} CN \overset{Na / C_{2} H_{5} OH}{\to}$ ${CH}_{3} {CH}_{2} {NH}_{2}$ $\overset{{HNO}_{2}}{\to}$ ${CH}_{3} {CH}_{2} OH$ $\overset{[O]}{\to}$ ${CH}_{3} CHO$ $\overset{Tollens reagent}{\to}$ ${CH}_{3} COOH$

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