Question

3. Mechanism of carbocation Rearrangements (hydride shift and methyl shift).

Can you please explain

what you need to understand to do the mechanism

what possible reagents could be used

An example (how do you know that this is the mechanism you will use?)

Any tips/ tricks/videos to understanding this mechanism

Answer 1

FEW THINGS YOU NEED TO KNOW:

The rearrangement is generally taken as the evidence that a reaction is by S_N1 (unimolecular nuceophilic substituion)

In S_N1 reactions the substrate should form a carbocation and the the order of stability of carbocations is

tertiary (3⁰) > secondary (2⁰) > primary (¹⁰) l

Thus iso propyl carbocation (2⁰) is more stable than n-propyl carbocation (1⁰)

Order of stability of butyl carbocations is

t-butyl > sec butyl > iso butyl > n- butyl.

A n--ropyl substrate for example yields the n- propyl cation ; this will rearrange to the isopropyl cation which will combine with the nucleophile to give the isopropyl product.

Similarly isobuyl cation rearrnges to the tert-butyl ation

3-methyl -2-butyl cation(2⁰) rearrnges to the 2-methyl-2-butyl cation(3⁰)

The neopentyl cation(1⁰) rearranges to tert-pentyl cation.(3⁰)

3,3- dimethyl-2-butyl cation (2⁰) rearranges to 2,3- dimethyl-2-butyl cation (3⁰)

In each case the rerrangement takes place in such a way that a less stable carbocation is converted into a more stable one.

a primary into a a seconary, a primary ino a tertiary or a secondary into a tertiary.

You can anticipate rearrangement in all reactions which involve a carbocation intermediate Example : S_N1 , E₁ etc reactions

FORMATION OF CARBOCATIONS

Carbocations can be formed in a number of ways.but one of the most common methods when a rearrangement is desired is the acid treatment of an alcohol.The alcohol undergoes protonation followed by the removal of H₂O molecule to form a cation

Other ways of generating carbocations:

1.A direct ionization in which a goup attached to C arbon atom leaves with its pair of electrons .

R-X = R ⁺ + X^-

Example : sec butyl bromide ionizing to form sec buty carbocation which will rearrange to tert-butyl carbocation

2.A proton or other positive species adds to one atom of an unsaturated system leaving the adjacent carbo atom with a positive charge.

TIPS

:Whenever you are dealing with a reaction involving carbocations you

i) fix the nature of the carbocation,

ii) If it is primary or secondary ,look for the possibility of the carbocation rearranging to a more stable tertiary carbocation.by hydride(H^-) or alkyl (R^-)shift

iii) In S_N1 reactions observe whether the substitution is at the original C atom or at the adjacent C- atom .If it is at the adjacent C-atom then rearrangent is definitely involved.

iv) In E1 reactions the double bond will be found at unexpected positions .For example you may expect the double bond between 1 and 2 C-atoms but the bond wil be found between 2 and 3 C atoms.