Grignard and Wurtz Reactions | JEE Notes

Metals turn the carbon of a halide from a target into an attacker. This is how a short chain becomes a longer one.

Grignard reagents

C H_{3} C H_{2} B r + M g \frac{d r y e t h e r}{} \to C H_{3} C H_{2} M g B r

The C-Mg bond is polar with carbon at the negative end. So a carbon that was attacked by nucleophiles now acts as one. The reagent must be kept dry.

Watch the animation: Carbon changes sign: the Grignard reagent

Keep water out.

R M g X + H_{2} O \to R H + M g (O H) X

Any compound with an acidic hydrogen gives the alkane. This includes water, alcohols, amines and terminal alkynes.

Counting the methane given by $C H_{3} M g B r$ with a molecule is a way to count its acidic hydrogens.

RMgX with	After $H_{3} O^{+}$
HCHO	Primary alcohol
Other aldehydes	Secondary alcohol
Ketones	Tertiary alcohol
$C O_{2}$	Carboxylic acid, RCOOH

Grignard reagent with ethanal

The carbon of the C-Mg bond is negative, so it attacks the slightly positive carbonyl carbon. The new C-C bond forms here.

Dilute acid in a second, separate step gives propan-2-ol. Add the acid first and the reagent would die as methane.

Reactivity of the halide with magnesium follows the bond strength: $R I > R B r > R C l$ .

Sodium couplings in dry ether

Name	Reaction
Wurtz	$2 R X + 2 N a \to R - R + 2 N a X$
Fittig	$2 A r X + 2 N a \to A r - A r + 2 N a X$
Wurtz-Fittig	$A r X + R X + 2 N a \to A r - R + 2 N a X$

Two different alkyl halides in a Wurtz reaction give three alkanes, so Wurtz is good only for symmetrical alkanes. Methane cannot be made this way, because the product always has at least two carbons.

The JEE Advanced syllabus names Grignard reactions under alkyl halides, and Fittig and Wurtz-Fittig under haloarenes.

Worked example: plan a Grignard route

Give two ways to make 2-methylbutan-2-ol, $(C H_{3})_{2} C (O H) C H_{2} C H_{3}$ , from a Grignard reagent and a ketone.

Show the solution

Cut one bond at the alcohol carbon. The piece you cut becomes the Grignard reagent, and the rest becomes the ketone.

Cut an ethyl group: $C H_{3} C H_{2} M g B r$ with propanone.
Cut a methyl group: $C H_{3} M g B r$ with butan-2-one.

Add dilute acid in a second step each time.

Worked example: counting active hydrogens

0.01 mol of a compound gives 448 mL of methane at STP with excess $C H_{3} M g B r$ . Take the molar volume as 22.4 L. How many active hydrogens does one molecule have?

Show the solution

Methane is $0.448 / 22.4 = 0.02$ mol. That is 2 mol of methane per mol of compound, so each molecule has 2 active hydrogens.

Grignard reagents and the Wurtz family

In 60 seconds

Grignard reagents

Grignard reagent with ethanal

Sodium couplings in dry ether

Worked example: plan a Grignard route

Worked example: counting active hydrogens