Question

How will you prepare 2-methyl-propan−2−ol from methyl magnesium bromide?

Answer 1

Preparation of 2-Methyl-propan-2-ol from Methyl magnesium bromide:

Step 1 Adduct formation:

• Methyl magnesium bromide is known as the Grignard reagent.
• Here, $\mathrm{Mg}$ metal is bonded with an alkyl or aryl group by a highly polar covalent bond and $\mathrm{Mg}$ is bonded with halogen by an ionic bond.
• Grignard reagent plays a major role in the synthesis of aldehydes from alcohol.
• Propanone is reacted with Methyl magnesium bromide to form an adduct which is then hydrolyzed to form 2-Methyl-propan−2−ol.
• Here the double bond between the Carbon and Oxygen atoms will break, then Oxygen gets a negative charge and Carbon gets a positive charge.
• The bond between the alkyl group and the magnesium will also break and, the methyl group gets a negative charge, and then the Magnesium bromide ion gets a positive charge.
• Therefore, a bond formed between the negatively charged oxygen and positively charged Magnesium bromide, then another bond formed between the negatively charged Methyl group and positively charged Carbon, thus an adduct is formed.
• The adduct is a direct product of a reaction that contains all the atoms of the reactants.

$\underset{\mathrm{Methyl}\mathrm{magnesium}\mathrm{bromide}}{{\mathrm{CH}}_3-\mathrm{Mg}-\mathrm{Br}}+\underset{\mathrm{Propanone}}{{\mathrm{CH}}_3-\mathrm{CO}-{\mathrm{CH}}_3}\to \underset{\mathrm{Adduct}}{\mathrm{MgBr}-\mathrm{O}-\mathrm{C}{\left({\mathrm{CH}}_3\right)}_3}$

Step 2 Hydrolysis:

• During hydrolysis of water, the bond between $\mathrm{MgBr}$ and Oxygen breaks and $\mathrm{MgBr}$ gets a positive charge and Oxygen gets a negative charge.
• The positive charged Oxygen gets bonded with positively charged Hydrogen and forms 2-Methyl-propan-2-ol
• Then negatively charged Hydroxyl group is attached to negatively charged $\mathrm{MgBr}$. and form Hydroxy magnesium bromide.

$\underset{\mathrm{Adduct}}{\mathrm{MgBr}-\mathrm{O}-\mathrm{C}{\left({\mathrm{CH}}_3\right)}_3}\stackrel{{\mathrm{H}}_2\mathrm{O}}{\to }\underset{2-\mathrm{Methyl}\mathrm{propan}-2-\mathrm{ol}}{\mathrm{HO}-\mathrm{C}{\left({\mathrm{CH}}_3\right)}_3}+\underset{\mathrm{Hydroxy}\mathrm{magnesium}\mathrm{bromide}}{\mathrm{Mg}\left(\mathrm{OH}\right)\mathrm{Br}}$