Which of the following alcohols is dehydrated most easily with conc. $\ce{H_2SO_4}$?

Q: Which of the following alcohols is dehydrated most easily with conc. $\ce{H_2SO_4}$?

3. $\ce{p−CH_3OC_6 H_4CH(OH)CH_{3 }}$ The dehydration reaction is as: Alcohol $+ H_2SO_{4 }$ $\rightarrow$ alkene is an elimination reaction that goes via carbocation formation. Higher is the stability of carbocation more easily it can be dehydrated. The corresponding carbocations formed by given molecules are shown in the figure. Due to the $--$Inductive effect $(-I)$and $--$Mesomeric effect $(-M)$ of $NO_2 $ group, it will decrease the electron density on benzene and will, therefore, destabilise the carbocation the most and makes it difficult to dehydrate. In case of $\ce{p−ClC_6H_4CH(OH)CH_3 }$, the Cl has stronger $-I$ effect $($due to high electronegativity$)$ than its +M effect (due to a lone pair of electrons) thus will destabilise the carbocation and unfavour dehydration. In case of $\ce{p−CH_3OC_{6}H_4CH(OH)CH_3}$ , the $CH_3O$ has the stronger $+M$ effect due to a lone pair of electrons on $O$ than its $-I$ effect and will stabilise the carbocation by increasing the electron density in the benzene ring and will favour dehydration. $\ce{C_3H_5CH(OH)CH_3}$ has $+I$ effect that stabilises the carbocation but its impact is lower than the $+-$mesomeric effect of the methoxy group. The alcohol that is dehydrated most easily with conc. $\ce{H_{2}SO_4}$ is $\ce{p−CH_3OC_{6}H_4 CH(OH)CH_3}$

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Solution

$\ce{p−CH_3OC_6H_4CH(OH)CH_{3}}$

The dehydration reaction is as: Alcohol $+ H_2SO_{4 }$
$\rightarrow$ alkene is an elimination reaction that goes via carbocation formation.
Higher is the stability of carbocation more easily it can be dehydrated.
The corresponding carbocations formed by given molecules are shown in the figure.
Due to the $--$Inductive effect $(-I)$and $--$Mesomeric effect $(-M)$ of $NO_2$ group, it will decrease the electron density on benzene and will, therefore, destabilise the carbocation the most and makes it difficult to dehydrate.
In case of $\ce{p−ClC_6H_4CH(OH)CH_3}$, the Cl has stronger $-I$ effect $($due to high electronegativity$)$ than its +M effect (due to a lone pair of electrons) thus will destabilise the carbocation and unfavour dehydration.
In case of $\ce{p−CH_3OC_{6}H_4CH(OH)CH_3}$, the $CH_3O$ has the stronger $+M$ effect due to a lone pair of electrons on $O$ than its $-I$ effect and will stabilise the carbocation by increasing the electron density in the benzene ring and will favour dehydration.
$\ce{C_3H_5CH(OH)CH_3}$ has $+I$ effect that stabilises the carbocation but its impact is lower than the $+-$mesomeric effect of the methoxy group.
The alcohol that is dehydrated most easily with conc. $\ce{H_{2}SO_4}$ is $\ce{p−CH_3OC_{6}H_4CH(OH)CH_3}$

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