Grignard Reagents & Reactions with Metals

CHEReactions of Haloalkanes

Treating haloalkanes with metals gives organometallics — above all the Grignard reagent $R\text{-}MgX$ , a versatile carbanion source for building C–C bonds — along with the Wurtz and Frankland reactions.

Grignard formation

R\text{-}X + Mg \xrightarrow{\text{dry ether}} R\text{-}MgX

reactivity

R\text{-}I > R\text{-}Br > R\text{-}Cl

; medium must be anhydrous

Grignard with carbonyls

R\text{-}MgX + HCHO \rightarrow 1^\circ\,\text{alcohol} \quad ; \quad R\text{-}MgX + CO_2 \rightarrow R\text{-}COOH

aldehydes (other than HCHO) give

2^\circ

and ketones give

3^\circ

alcohols after hydrolysis

Wurtz reaction

2\,R\text{-}X + 2Na \xrightarrow{\text{dry ether}} R\text{-}R + 2NaX

couples two halides to a symmetrical alkane with an even number of carbons

Formation: $R\text{-}X + Mg \xrightarrow{\text{dry ether}} R\text{-}MgX$ (alkyl magnesium halide); reactivity $R\text{-}I > R\text{-}Br > R\text{-}Cl$ .
The medium must be anhydrous: Grignard reagents react with any active hydrogen ( $H_2O$ , alcohols, $NH_3$ , acids) to give the alkane $R\text{-}H$ — this is why "dry ether" is essential.
With carbonyls: + HCHO → $1^\circ$ alcohol; + other aldehydes → $2^\circ$ alcohol; + ketones → $3^\circ$ alcohol; + $CO_2$ → carboxylic acid.
Wurtz reaction: $2\,R\text{-}X + 2Na \xrightarrow{\text{dry ether}} R\text{-}R + 2NaX$ — a symmetrical alkane with an even carbon count.
Variants: Wurtz–Fittig (aryl + alkyl halide + Na) → alkylarene; Fittig (two aryl halides + Na) → biaryl.
Frankland reaction: $R\text{-}X + Zn \rightarrow R_2Zn$ (dialkylzinc).
The C–Mg bond is highly polar ( $C^{\delta-}\text{-}Mg^{\delta+}$ ), making the carbon strongly nucleophilic — it behaves like a carbanion and attacks the electrophilic carbonyl carbon.
A Grignard reagent attacks $C{=}O$ to give an alkoxide salt, which on acidic work-up ( $H_3O^+$ ) gives the alcohol; this is the standard C–C bond-forming step.
Because they react instantly with active-H compounds, Grignard reagents also give alkanes from water, so they can be used to introduce a $-H$ (e.g. $CH_3MgI + H_2O \rightarrow CH_4$ ).
Wurtz cannot cleanly make an odd-carbon alkane: coupling two different halides ( $R\text{-}X + R'\text{-}X$ ) gives a mixture of $R\text{-}R$ , $R'\text{-}R'$ and $R\text{-}R'$ , so it is reliable only for symmetrical alkanes.
Wurtz–Fittig couples an aryl halide with an alkyl halide (Na, dry ether) to attach an alkyl chain to a ring (e.g. chlorobenzene + methyl chloride → toluene).
Grignard and Wurtz both require strictly dry conditions and reactive halides; iodides react fastest but bromides are commonly used as a practical compromise.

\mathrm{R\text{–}X} + \mathrm{Mg} \xrightarrow{\text{dry ether}} \mathrm{R\text{–}Mg\text{–}X}

ConditionsDry ether — the solvent must be scrupulously anhydrous

Any compound with an active hydrogen — water, alcohol, an acid, even an amine — destroys the reagent by protonating the carbanion to give the alkane. 'Why dry ether?' is a one-mark question with exactly this answer.

Grignard reacts with	Product after hydrolysis
Methanal (formaldehyde)	Primary alcohol
Any other aldehyde	Secondary alcohol
A ketone	Tertiary alcohol
Carbon dioxide, $\mathrm{CO_2}$	Carboxylic acid — one carbon longer

The class of alcohol is decided entirely by the carbonyl compound, so this table answers every 'predict the product' question in the subtopic.

\mathrm{C_6H_5Br} + 2\,\mathrm{Na} + \mathrm{CH_3Br} \xrightarrow{\text{dry ether}} \mathrm{C_6H_5CH_3} + 2\,\mathrm{NaBr}

ConditionsWurtz–Fittig reaction

Wurtz couples two alkyl halides; Fittig couples two aryl halides; Wurtz–Fittig couples one of each to give an alkylarene. The name tells you which halides went in.

Where the marks go

Forgetting that water, alcohols, or any active hydrogen destroys the Grignard reagent — "dry ether" is compulsory.
Assigning the wrong alcohol class: HCHO gives a primary alcohol, other aldehydes secondary, ketones tertiary; a common slip is calling the HCHO product secondary.
Using Wurtz for an odd-carbon or unsymmetrical alkane and ignoring the mixture of products it gives.
Confusing Wurtz, Fittig and Wurtz–Fittig — Fittig joins two aryl halides (biaryl), Wurtz–Fittig joins an aryl with an alkyl halide.
Omitting the acidic work-up step ( $H_3O^+$ ) and writing the alcohol directly from the carbonyl addition.

How the board asks it

Conversiongrignard with carbonyls and $CO_2$
How will you obtain (i) propan- $2$ -ol and (ii) ethanoic acid starting from a suitable Grignard reagent? Give the equations involved.
Give reasonsgrignard destroyed by active hydrogen
Account for the fact that a Grignard reagent must be prepared and used in perfectly dry ether and never in the presence of water or alcohol.
Predict the productalcohol class from carbonyl type
Write the product (with its alcohol class) formed when $CH_3MgI$ , followed by acidic hydrolysis, reacts with (i) $HCHO$ and (ii) propanone.
Distinguishwurtz vs wurtz–fittig
Distinguish between the Wurtz reaction and the Wurtz–Fittig reaction, giving one equation for each.
Conversionwurtz coupling of haloalkanes
How will you convert methyl bromide into ethane by the Wurtz reaction? Give a reason why the Wurtz reaction is unsuitable for preparing propane.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.