Carbocation Rearrangement

[balambfishie]

For reactions where rearrangement is possible does the reaction only form the rearranged products or does it also form nonrearranged products as minor products?

I'm thinking both types of products are made, but I can't find the answer conclusively anywhere in my ochem textbook.

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[mcloaf]

You should expect a mixture of products.

 

[chiddler]

For reactions where rearrangement is possible does the reaction only form the rearranged products or does it also form nonrearranged products as minor products?

I'm thinking both types of products are made, but I can't find the answer conclusively anywhere in my ochem textbook.

it depends on conditions.

a good example to illustrate this is diene reactions (1,4 addition and 1,2 addition). Look up the mechanism; you'll see that a 1,2 addition occurs at low temperatures more frequently because rearrangement is unfavorable in such conditions. Whereas at higher temperatures, you get the more stable product 1,4 addition.

If you recall, you can divide these two products into the kinetic product (produced faster with lower activation energy, less thermodynamically stable) and the thermodynamic product (slower due to higher activation energy, more thermodynamically stable)

 

[milski]

Both products are formed. Carey & Giuliano, 8th ed. has an example of butanol being dehydrated to butene (12%) and 2-butene (88%).

 

[milski]

it depends on conditions.

a good example to illustrate this is diene reactions (1,4 addition and 1,2 addition). Look up the mechanism; you'll see that a 1,2 addition occurs at low temperatures more frequently because rearrangement is unfavorable in such conditions. Whereas at higher temperatures, you get the more stable product 1,4 addition.

If you recall, you can divide these two products into the kinetic product (produced faster with lower activation energy, less thermodynamically stable) and the thermodynamic product (slower due to higher activation energy, more thermodynamically stable)

That's not the same as carbocation rearrangement - you have two resonance structures of the same carbocation in the 1,2/1,4 additions. With the carbocation rearrangement you have two distinct carbocations.

 

[chiddler]

That's not the same as carbocation rearrangement - you have two resonance structures of the same carbocation in the 1,2/1,4 additions. With the carbocation rearrangement you have two distinct carbocations.

can you please give an example to help illustrate.

 

[MedPR]

can you please give an example to help illustrate.

Carbocation rearrangement involves actual movement of a group (hydride or alkyl), whereas in 1,2 and 1,4 addition there is resonance stabilization, in which certain resonance structures will react more readily than other resonance structures. In resonance, however, substituents do not move and all resonance structures are the same overall structure. Carbocation rearrangement actually results in a different carbocation.

For instance, if you have 2-bromo-3-methylbutane. When the bromine leaves, you now have a secondary carbocation (at C-2). There will be a hydride shift, however, to create a tertiary carbocation (now at the original C-3).

For 1,2 and 1,4 addition, I'll link to wikipremed.

 

[milski]

Carbocation rearrangement involves actual movement of a group (hydride or alkyl), whereas in 1,2 and 1,4 addition there is resonance stabilization, in which certain resonance structures will react more readily than other resonance structures. In resonance, however, substituents do not move and all resonance structures are the same overall structure. Carbocation rearrangement actually results in a different carbocation.

For instance, if you have 2-bromo-3-methylbutane. When the bromine leaves, you now have a secondary carbocation (at C-2). There will be a hydride shift, however, to create a tertiary carbocation (now at the original C-3).

MedPR already gave a good example for the rearrangement, thanks!

In contrast to what you have in his example, in the allylic cation case you have a single cation. It has 3 carbon atoms sharing pi orbitals. If the group is asymmetrical for some reason (tertiary C being at one end, etc), it will have asymmetric charge distribution which causes the different products.

On the second to last line of MedPR's post which has [... or ...]: these are not two separate entities, you can never isolate either of them. They are just a non-ideal representation of what the resonance structure is.

Carbocations in the first example are distinct cations which can be observed.

 

[chiddler]

oh i see. I think the mechanism makes it easy to forget that resonance structures are not discrete.

 

[milski]

oh i see. I think the mechanism makes it easy to forget that resonance structures are not discrete.

Oh, you already knew what it was all about.

 

[MedPR]

oh i see. I think the mechanism makes it easy to forget that resonance structures are not discrete.

Yea, it's something that might get slid into an MCAT question just to see how closely you're paying attention.

 

[chiddler]

forgot to say thank you

thank you

 

[ljc]

This is a bit tangential - but I was under the impression that adding Br2 across a double bond did not have a carbocation intermediate, and thus could not undergo rearrangement?

I thought the transition state was the formation of cyclic Br across the double bond?

 

[chiddler]

This is a bit tangential - but I was under the impression that adding Br2 across a double bond did not have a carbocation intermediate, and thus could not undergo rearrangement?

I thought the transition state was the formation of cyclic Br across the double bond?

i think for straight Br2 + alkene, then that is the reaction you're thinking of. The reaction that was discussed was with diene which is slightly different.

 

[MedPR]

This is a bit tangential - but I was under the impression that adding Br2 across a double bond did not have a carbocation intermediate, and thus could not undergo rearrangement?

I thought the transition state was the formation of cyclic Br across the double bond?

i think for straight Br2 + alkene, then that is the reaction you're thinking of. The reaction that was discussed was with diene which is slightly different.

Yea, if dienes followed the same mechanism as isolated or alkenes, there would never be 1,4 additions of Br2