In a nutshell

Grignard reagents have the general formula $RMgX$ . They are made by refluxing a haloalkane with magnesium in dry ether. They can also be used to make carboxylic acids and alcohols.

Equations

These are general chemical equations. You will need to be able to use these when you write equations for specific substances.

$RX \xrightarrow[dry \ ether]{Mg} RMgX \\ \ \\RMgBr \xrightarrow[2. \ dil. \ HCl]{1. CO_2, \ \ dry \ ether} RCOOH \ + \ MgBrCl \\ \ \\R_1MgBr \xrightarrow[2. \ dil. \ HCl]{1. R_2COR_3, \ \ dry \ ether} R_1C(OH)R_2R_3\ + \ MgBrCl$

Making Grignard reagents

Grignard reagents have the general formula $RMgX$ . They are made under reflux by reacting a haloalkane with magnesium in a non-aqueous solvent, such as dry ether.

$RX \xrightarrow[dry \ ether]{Mg} RMgX$

Example

$bromomethane \xrightarrow[dry \ ether]{Mg} Grignard \ reagent \\ \ \\CH_3Br \xrightarrow[dry \ ether]{Mg} CH_3MgBr$

Making carboxylic acids

Grignard reagents react with carbon dioxide to form carboxylic acids via two steps. Firstly, carbon dioxide gas is bubbled through the Grignard reagent in dry ether. Then a dilute acid is added.

$RMgBr \xrightarrow[2. \ dil. \ HCl]{1. CO_2, \ \ dry \ ether} RCOOH \ + \ MgBrCl$

Example

$Grignard \ reagent \xrightarrow[2. \ dil. \ HCl]{1. CO_2, \ \ dry \ ether} ethanoic \ acid \ + \ magnesium \ bromide \ chloride \\ \ \\CH_3MgBr \xrightarrow[2. \ dil. \ HCl]{1. CO_2, \ \ dry \ ether} CH_3COOH \ + \ MgBrCl$

A new C-C bond is formed during the reaction. This occurs between the carbon of $C-Mg$ from the Grignard reagent and the carbon of carbon dioxide. One of the $C=O$ bonds in carbon dioxide is broken so it can go on to become the - $COO^-$ group, which is then protonated in the second step to give - $COOH$ .

Making alcohols

Grignard reagents react with aldehydes and ketones to form alcohols. A new C-C bond is formed during the reaction. This occurs between the carbon of $C-Mg$ from the Grignard reagent and the carbon of carbonyl group. The $C=O$ bond in the carbonyl breaks and upon protonation an alcohol group is made.

$R_1MgBr \xrightarrow[2. \ dil. \ HCl]{1. R_2COR_3, \ \ dry \ ether} R_1C(OH)R_2R_3\ + \ MgBrCl$

Example

In the following example the carbonyl is propanone which goes on to form propan- $2$ -ol.

$Grignard \ reagent$	$\xrightarrow[2. \ dil. \ HCl]{1. CH_3COCH_3, \ dry \ ether}$	$2-methylpropan-2-ol \newline + \ magnesium \ bromide \ chloride$
$CH_3MgBr$	$\xrightarrow[2. \ dil. \ HCl]{1. CH_3COCH_3, \ dry \ ether}$	$CH_3C(CH_3)OH CH_3\ + \ MgBrCl$