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Hydrogen Bonding

The correct d...

Question

The correct decreasing order of enthalpies of reaction for producing carbocation is :

Δ H_{1}^{o} > Δ H_{2}^{o} > Δ H_{3}^{o} > Δ H_{4}^{o}

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Δ H_{4}^{o} > Δ H_{1}^{o} > Δ H_{2}^{o} > Δ H_{3}^{o}

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Δ H_{3}^{o} > Δ H_{2}^{o} > Δ H_{1}^{o} > Δ H_{4}^{o}

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Δ H_{2}^{o} > Δ H_{1}^{o} > Δ H_{4}^{o} > Δ H_{3}^{o}

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Solution

The correct option is B $Δ H_{4}^{o} > Δ H_{1}^{o} > Δ H_{2}^{o} > Δ H_{3}^{o}$
For the given reactions, the correct decreasing order of enthalpies of reaction for producing carbocation is :
$Δ H_{4}^{o} > Δ H_{1}^{o} > Δ H_{2}^{o} > Δ H_{3}^{o}$ .
The carbocation (III) is most stable carbocation as it is stabilized by resonance with benzene ring. Hence, it has lowest value of enthalpy change.
The carbocation (IV) is least stable as the positive charge is present on more electronegative $s p^{2}$ carbon atom. Moreover, there is no resonance of this charge with benzene ring. Hence, it has maximum value of enthalpy change.

Carbocation (II) is more stable than carbocation (I). There is resonance between positive charge and $C = C$ double bond. There is no such resonance stabilisation in case of (I). Hence, the enthalpy change for (II) is lower than the enthalpy change for (I).

Hence, the correct option is $B$

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