Question

Write the reaction equations of-
i.2-Bromo-2-methylpropane + magnesium in the dry ether
ii.Product of (i) + water

Answer 1

Hint: The first reaction is between a halo-alkane and Magnesium in the presence of dry ether so it is clear that this is the synthesis reaction of Grignard’s reagent. The second equation is the hydrolysis of Grignard’s reagent as it is a reaction with water.
Complete step by step answer:
(i)We’ve understood that the first reaction is the formation of Grignard’s reagent. Let’s see the general reaction of the synthesis of a Grignard reagent. Here R-X is the halo-alkane. So, R-X will react with Mg to give R-MgX as the product. It can be shown in the reaction form as follows:
$R-X\text{ + Mg }\xrightarrow{Dry\text{ e}ther}\text{ R-Mg-X }\left( Grignard\text{ Reagent} \right)$
Now, let’s substitute R-X with 2-Bromo-2-methylpropane $\left[ {{\left( C{{H}_{3}} \right)}_{3}}C-Br \right]$.
When this 2-Bromo-2-methylpropane $\left[ {{\left( C{{H}_{3}} \right)}_{3}}C-Br \right]$ reacts with Mg in the dry ether it will form $\left[ {{\left( C{{H}_{3}} \right)}_{3}}C-MgBr \right]$ as the final product. The reaction equation is as follows,
${{\left( C{{H}_{3}} \right)}_{3}}C-Br\text{ + Mg }\xrightarrow{Dry\text{ e}ther}\text{ }{{\left( C{{H}_{3}} \right)}_{3}}C-MgBr$
(ii) The reaction between product (i) and water denotes the hydrolysis of the Grignard reagent. When the Grignard reagent is hydrolyzed it forms an alkane as the major product.
The general reaction equation of hydrolysis of Grignard reagent is as follows:
$R-MgX\text{ + }{{\text{H}}_{2}}O\text{ }\to \text{ R-H + MgX}\left( OH \right)$
The mechanism of this reaction shows that ${{R}^{-}}$ (alkyl group) separates the Grignard compound from the alkane with an ${{H}^{+}}$ion. The ${{R}^{-}}$ acts as a strong base so it removes the proton ${{H}^{+}}$ to form the respective alkane.
Also, $Mg{{X}^{+}}$pairs with \[O{{H}^{-}}\]to form the $\text{MgX}\left( OH \right)$.
The reaction between $\left[ {{\left( C{{H}_{3}} \right)}_{3}}C-MgBr \right]$and water is given below,
$\left[ {{\left( C{{H}_{3}} \right)}_{3}}C-MgBr \right]\text{ + }{{\text{H}}_{2}}O\text{ }\to \text{ }{{\left( C{{H}_{3}} \right)}_{3}}C-H\text{ + }MgBr(OH)$.
The major product of hydrolysis is 2-methylpropane.
The reaction equations are:
i.${{\left( C{{H}_{3}} \right)}_{3}}C-Br\text{ + Mg }\xrightarrow{Dry\text{ e}ther}\text{ }{{\left( C{{H}_{3}} \right)}_{3}}C-MgBr$
ii.$\left[ {{\left( C{{H}_{3}} \right)}_{3}}C-MgBr \right]\text{ + }{{\text{H}}_{2}}O\text{ }\to \text{ }{{\left( C{{H}_{3}} \right)}_{3}}C-H\text{ + }MgBr(OH)$

Note:
Do not confuse the reaction of a halo-alkane and magnesium i.e. Grignard synthesis with Wurtz synthesis reaction. In the latter, Na is used along with dry ether. Also, during the hydrolysis of the Grignard reagent, MgBr(OH) formed is a gelatinous substance. So, the hydrolysis is carried out in the presence of a dilute acid that can dissolve the basic salt produced.