Q43 E - Organic Chemistry (Mcmurry)

Question

Question: Addition of HBr to a double bond with an ether  substituentoccurs regiospecifically to give a product in which the  andare bonded to the same carbon. Draw the two possible carbocation intermediates in this electrophilic addition reaction, and explain usingresonance why the observed product is formed.

Step-by-Step Solution

Answer

Grignard reagent can’t be synthesized because the alcoholic group present on this alkyl halide is slightly acidic. Another molecule with no Grignard reagent can be $BrC{H}_{2}C{H}_{2}C{H}_{2}COOH$.

Step-by-Step Solution Step 1: Resonance

Resonance shows various structures of a molecule, possibly due to electrons delocalization. More, the resonating structure will be stable.

Step 2: Alkene reaction with HBr

When HBr reacts with an alkene, it forms alkyl halide, H and Br of HBr can attach on either side of the alkene, giving two products.

Step 3: Two possible carbocation intermediates

The two possible carbocations formed are named A and B here.

Carbocation A and B

In A, the lone pair of electron of oxygen is able to stabilize carbocation, and it has three electron-donating alkyl groups. In contrast, in B, the carbocation has only two alkyl groups to donate electrons. 

Step 4: Reason for production of A molecule most

A molecule is formed here as it is favored due to resonance stabilization. Resonance provides molecules with extra stability.