Grignard reagents

​​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.

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

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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.

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