(A) The molecule in option $A$ is cyclobutadiene. It is a $4n \pi$ electron system ($n=1$,total $4 \pi$ electrons),which makes it anti-aromatic accord

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(A) The molecule in option $A$ is cyclobutadiene. It is a $4n \pi$ electron system ($n=1$,total $4 \pi$ electrons),which makes it anti-aromatic according to $H$ückel's rule. Anti-aromatic compounds are highly unstable and reactive,and thus cannot exist as stable molecules at room temperature. The other options represent stable or relatively stable species.

Class 11 Chemistry 8-4.Organic Chemistry : Reaction mechanism Reactive Intermediates

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Which of the following cannot exist at room temperature?

A
Cyclobutadiene
B
$CH_2=CH-CH=CH_2$
C
Cyclobutene
D
Cyclopentadienyl cation

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8-4.Organic Chemistry : Reaction mechanism

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Reactive Intermediates

The stability of carbanions follows the order:
$I: CH_3CH_2CH_2\bar{C}H_2$
$II: CH_3\bar{C}HCH_2CH_3$
$III: (CH_3)_3\bar{C}$
$IV: CH_3\bar{C}(Ph)CH_2CH_3$

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Which of the following is the most stable among the given ions?

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Choose the most reasonable reaction intermediate for the following reaction.

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Rank the following radicals in order of increasing stability (least $ < < < $ most).

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For the following carbocations,the correct order of stability is:
$I: ^+CH_2-COCH_3$
$II: ^+CH_2-OCH_3$
$III: ^+CH_2-CH_3$

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Which of the following cannot exist at room temperature?

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The stability of carbanions follows the order:$I: CH_3CH_2CH_2\bar{C}H_2$$II: CH_3\bar{C}HCH_2CH_3$$III: (CH_3)_3\bar{C}$$IV: CH_3\bar{C}(Ph)CH_2CH_3$

Which of the following is the most stable among the given ions?

Choose the most reasonable reaction intermediate for the following reaction.

Rank the following radicals in order of increasing stability (least $ < < < $ most).

For the following carbocations,the correct order of stability is:$I: ^+CH_2-COCH_3$$II: ^+CH_2-OCH_3$$III: ^+CH_2-CH_3$

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The stability of carbanions follows the order:
$I: CH_3CH_2CH_2\bar{C}H_2$
$II: CH_3\bar{C}HCH_2CH_3$
$III: (CH_3)_3\bar{C}$
$IV: CH_3\bar{C}(Ph)CH_2CH_3$

For the following carbocations,the correct order of stability is:
$I: ^+CH_2-COCH_3$
$II: ^+CH_2-OCH_3$
$III: ^+CH_2-CH_3$