Does attachment of phenyl to an alkene increases its reactivity towards electrophilic addition reaction?

Question

According to me phenyl should decrease the reactivity as its electron withdrawing nature destabilises the carbocation.

Answer 1

Not always, and it can depend upon the groups attached to a phenyl ring, like $\ce{-OMe, -COOH, -NH2, -NO2, -X}$, etc.

The attachment of a phenyl ring to a carbon-containing double bond can affect both the stability of carbocation and the possibility of attacking with the help of the compound's $\pi$-electron cloud.

If you have only the phenyl ring with no substitution, first of all the $\pi$-electron cloud of the double bond will be in high conjugation with the phenyl ring, and thus the $\pi$-electron density will be reduced on the $\ce{C=C}$ double bond, which can reduce the possibility of attack of that double bond to a electrophile. Though the carbocation formed can be stabilised through resonance with the phenyl ring (as it is a benzyl carbocation), the step of carbocation formation can have a very high activation energy ($E_\mathrm a$) if you don't have enough stabilisation of the carbocation. But, if you have the carbocation which can stabilise through resonance with multiple benzene rings, the actual potential energy of the carbocation can be reduced to a great extent. In that case, the rate of reaction will increase.

So, actually this situation can be thought as a competition between the stability of carbocation and the decreased electron density. Generally, the stability of carbocation is the dominating effect if it is stabilised to a large extent.

The rate will be decreased even more if an electron withdrawing group is attached to the phenyl ring due to even more delocalisation of $\pi$-electron cloud, and less stabilisation of the carbocation.

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If you have a strong electron donating group attached to a phenyl ring and the double bonded carbon is ortho/para w.r.t that group, that can itself exhibit the $+R$ effect, and increase electron density near the double bond through delocalisation of its own electron density through the $\pi$-bonds. In that case, the attacking of $\ce{C=C}$ double bond to a electrophile will be much easier, and the carbocation formed will be even more stable. Then, the rate of reactivity of the double bond in the electrophilic addition can increase for attachment of a substituted phenyl ring.