Question

Identify A and B in the following reaction

Answer 1

Step 1 :

• This is a grignard reaction
• THF is required for the production of the Grignard reagent. In the Grignard reagent, lone pair electrons of molecules create a complex with the magnesium. This combination aids in the stabilization of the organometallic and boosts its reactivity.
• As we know, $\mathrm{Mg}$ exists as ${\mathrm{Mg}}^{2+}$and will attack chlorine due to the higher electron density of chlorine. As a result, this will be a displacement reaction.

$$\begin{gathered} {\mathrm{C}}_6{\mathrm{H}}_4\mathrm{BrCl}\stackrel{\mathrm{Mg}/\mathrm{THF}}{\to }{\mathrm{BrC}}_6{\mathrm{H}}_4{\mathrm{Mg}}^{+}{\mathrm{Cl}}^{-} \\ \left(\mathrm{a}\right) \end{gathered}$$

Step 2 :

• The Grignard Reaction occurs when an organomagnesium halide (Grignard reagent) is added to a ketone or aldehyde to generate tertiary or secondary alcohol.
• Magnesium chloride is substituted by aldehyde, resulting in the formation of alcohol.
• The handing reagent that quenches the magnesium alkoxide product of the Grignard addition is ammonium chloride.

$$\begin{gathered} {\mathrm{BrC}}_6{\mathrm{H}}_4{\mathrm{Mg}}^{+}{\mathrm{Cl}}^{-}\underset{\left(\mathrm{II}\right){\mathrm{NH}}_3\mathrm{Cl}}{\overset{\left(\mathrm{i}\right){\mathrm{CH}}_3\mathrm{CHO}}{\to }}{\mathrm{BrC}}_6{\mathrm{H}}_4\mathrm{C}\left(\mathrm{OH}\right){\mathrm{CH}}_3 \\ \left(\mathrm{a}\right)\left(\mathrm{b}\right) \end{gathered}$$

• Hence overall reaction is: