Propose a mechanism involving a hydride shift or an alkyl shift for each solvolysis reaction.Explain how each rearrangement forms a more stable intermediate.
Solution 26P :
Here, we have to Propose a mechanism that involves a hydride shift or an alkyl shift for each solvolysis reaction and also explain how each rearrangement forms a more stable intermediate.
First, let’s see what is a hydride shift or an alkyl shift and solvolysis reaction and then we will proceed to propose mechanism for it.
A rearrangement pathway where an unstable carbocation can be transformed into stable carbocation is called as hydride shift or it is also called as alkyl shift.
Solvolysis reactions are usually substitution reactions in which an atom or a group of atoms in a molecule is replaced by another atom or group of atoms.
For the rearrangement of the compound, we should first analyze the structure of carbocations and see if the migration of any groups of the carbons in the compound will lead to more stable carbocation only then the rearrangement for compound can be done to form a more stable intermediate.
For the above solvolysis reaction, the mechanism is as follows :
Here, atom I(Iodine) is the halogen group, the leaving group that dissociates from the compound and carbocation is generated. The carbocation generated here is secondary carbocation as the carbon atom bearing positive charge is associated or bonded with 2 carbon atoms. This carbocation is a strong electrophile that accepts electrons. In the next step of mechanism, this carbocation undergoes attack from the nucleophile. Here the nucleophile used is methanol. The nucleophilic attack on the carbocation is on the unrearranged carbocation , this is shown below:
In the above reaction we see that the methanol which is nucleophile attacks the carbocation from the top and it produces a product. This product has positive charge and is unstable and hence it further reacts with methanol undergoing acid base reaction and produces a product that is stable and doesn’t bear any charge by losing the proton with its bonding electrons.
In the above reaction, the product is un rearranged product. For the rearrangement of the product, we should first analyze the structure of carbocations and see if the migration of any groups of the carbons in the compound will lead to more stable carbocation. The carbocation is rearranged and the nucleophilic attack that happens is shown below :
Here, in the above compound, Cl(Chlorine) is the halogen group and it dissociates from the compound and forms secondary carbocation. This secondary carbocation is unrearranged. Then , the nucleophile ethanol is added for further reaction. The nucleophilic attack on the unrearranged carbocation is shown below:
Here, the nucleophile attacks the carbocation from bottom and the nucleophile is substituted and product is formed. This product has positive charge on it and it further reacts with ethanol, where the proton is lost with its bonding electrons and stable product is formed.