[lip]

1.)Please explain why in Hydrogen Bromide, Phenyl Acetylene electrophilic addition mechanism ,cabocation form is one with positive carbon  adjacent to phenyl group.As per my knowledge phenyl is electron withdrawing and positive carbon is Sp hybridized so how come this is more stable?


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[discodermolide]

Try drawing the resonance contributions to the stabilisation.
See if that gets you anywhere.

[lip]

positive carbon is SP hybridize so empty P orbital is perpendicular to conjugated pI electron system . As per my knowledge cannot draw resonance(two double bonds next to each other and P orbital orthogonal to pi cloud) .

[Mitch]

Besides resonance, you can think about it through Markovnikov's rule. The carbocation will form on the most substituted carbon.

[lip]

yap, I agree that this is Markovnikov addition but , substituent on carbon is a phenyl group(electron withdrawing? ) not a alkyl group and carbon is SP ,so I am unable to understand rationale for stability

[Mitch]

That p-orbital is not necessarily perpendicular. It can rotate freely through that single bond, and thus resonance stabilize.

[lip]

that sound reasonable, but to rotation to occur single bond has to overcome the conjugation (there is a extended pi system with double bond), so is it possible to rotate without destroying conjugated system ?
and if resonance is drawn,  structure with two double bond adjacent to each other (Allene C=C=C) resulting. as per my knowledge this is not stable.  only possible explanation come to my mind after all these is that inductive effect of phenyl group. If phenyl is electron donating, mechanism is reasonable .but as per my knowledge   phenyl is electron withdrawing group.

[discodermolide]

What's wrong with this as one of the resonance structures?


Just move the double bonds and hence the positive charge around the ring

[lip]

as per my knowledge molecular orbital diagram for this is as follows, so it is contradict the resonance proposed by you

[discodermolide]

Well, if you don't like my suggestion, where is your idea?

[willug]

Your molecular orbital diagram looks wrong to me. How many carbons do you have? Check that first maybe.

[lip]

IT has seven carbon yap I think I missed an one carbon atom  . I will put a correct one

[opsomath]

Phenyl isn't electron withdrawing. Does that help?

[lip]

yap if it electron donating, stability can be explained by inductive effect.