[ksr985]

Can a hydride or methide shift occur more than once to give more stable cationic intermediates?

for example, if i try dehydrate with H2SO4, the following compound:

PhCH2CH(CH3)CH2CH2OH.

Will the alkene formed be

PhCH=C(CH3)CH2CH3

or

PhCH2C(CH3)=CHCH3

( i think the first is correct )

and will the final intermediate carbocation have the +ve charge on the 3rd or 4th carbon, staring from the right of the chain, ie, from the carbon initially attached to -OH.

[Mitch]

Can a hydride or methide shift occur more than once to give more stable cationic intermediates?

Yes, although calling it "stable" is a misnomer

[ksr985]

but what will be the final intermediate?

[Donaldson Tan]

can someone explain the concept of methyl shift?

[movies]

can someone explain the concept of methyl shift?

It's like a hydride shift, but with a methyl group.

 

Basically, it's a rearrangement that usually forms a more stable carbocation.  Say you had 3,3-dimethyl-2-butanol and treated it with strong acid to form the secondary carbocation (like the first step of an S_N1 reaction).  One of the methyl groups can migrate to quench the secondary carbocation and form a new tertiary carbocation.  Subsequent treatment with a nucleophile like methanol could then capture the carbocation and form a product like 2-methoxy-2,3-dimethylbutane.



This can happen with all kinds of alkyl groups.

[Donaldson Tan]

if i have cyclopentane ring, is it possible for a methyl shift from a ring substituent to enter the ring to form cyclohexane?

[ksr985]

i still dont know what the final intermediate in my question is...

[movies]

if i have cyclopentane ring, is it possible for a methyl shift from a ring substituent to enter the ring to form cyclohexane?

If you had just methylcyclopentane with a endo-cyclic secondary carbocation, no that wouldn't happen.  You would see a hydride shift to make the tertiary carbocation.

If you had methylcyclopentane and an exo-cyclic primary carbocation then you might see a ring expansion product (forming a secondary carbocation), but more likely you would again see a hydride shift to make the tertiary carbocation.

i still dont know what the final intermediate in my question is...

This will depend on whether a tertiary carbocation is more stable than a benzylic carbocation!