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Consider the compounds, (I) - 1,2-diphenylethene and (II) - 1-phenylpropene. Compare their rate of hydrohalogenation (HX addition)

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I feel that the answer should be (II) > (I), but to my surprise it's given to be (I) > (II).

My thoughts:

  1. In (I), phenyl group consists of sp2 carbons, operates -I effect and destabilises the carbocation intermediate.

  2. In (II), protonation should occur at the carbon directly attached to the phenyl group. The methyl group (instead of phenyl, as in (I)) operates +I effect and stabilises the carbocation.

  3. In both cases, the carbocation formed enjoys resonance stabilisation from a phenyl group - so it's not a factor we need to compare.

So, what is the real order of rates of $\ce{HX}$ addition? Is my reasoning correct, or have I missed something?

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Assumung fruitfull collissions ,for structure (II) , intermediates 1 and 2 are formed. Intermediate 2 is relatively more stable then intermediate 1 .

The intermediate I has 5 resonance structures ( scheme 1) .Further stablised by inductive effects of ethyl group.

For structure I, intermediates 3 and 4 are formed (Scheme 2).And 3 & 4 are similar. Intermediate 3 has 5 resonance structures. Similarly Intermediate 4 also has 5 resonance structures.

Totally 10 resonance structures are possible for structure I.(Since delocalization is more, energy of such intermediate should be less)

Stable carbocations for both I & II are formed but , they are dissimilar.

Assuming fruitfull collisions lead to intermediates 1,2,3 & 4 ,statistically since number of resonance structures are more for intermediate formed from structure I , it should lead to formation of major product.

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Hence hydrohaogenation for (I) - 1,2-diphenylethene ais greater then (II) 1-phenylpropene

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  • $\begingroup$ Why does the nucleophilicity of alkene not in question here? That in consideration, yields the reverse order. $\endgroup$ – arya_stark Jun 5 at 10:51
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I would suggest that there is a potential kinetic effect here. While the most stable carbocation formed in either case is similar, the number of collisions that result in the formation of the more stable carbocation is higher in I than in II.

If we take a very simplified view of the possible collisions in II, we can see that in a reaction with $\ce{HCl}$ only "one" of the orientations of the collisions will result in the formation of the desired carbocation. enter image description here

In the case of I, "both" of the possible collisions will lead to the formation of the same stable carbocation. enter image description here

If the number of possible effective collisions is higher, the reaction will be faster.

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