Test: Grignard Reagent - Chemistry MCQ

In the first step, the Grignard forms the carbon-carbon bond. This results in an alkoxide (the conjugate base of an alcohol). To form the alcohol, it’s necessary to add acid at the end of the reaction and this reaction usually occurs in Et₂O or THF followed by H₃O+ work-ups.

Oxirane (ethylene oxide or epoxide) when treated with phenylmagnesiuim bromide (C₆​H₅​−Mg−Br) followed by acid workup, will yield 2-phenylethanol.

A Grignard’s reagent can be formed by reacting magnesium with Ethyl iodide, as shown below;
C₂H₅I + Mg + Dryether → C₂H₅ − Mg − I (Ethylmagnesium iodide)

It is supposed that the Mg-C bond is strongly polar covalent, not ionic. Grignard’s reagents are less reactive than organosodium, -potassium and -lithium compounds, that is the reason why it is more convenient to work with them.

Grignard reagents are made from halogenoalkanes (haloalkanes or alkyl halides) and introduces some of their reactions. A Grignard reagent has a formula RMgX where X is a halogen, and R is an alkyl or aryl (based on a benzene ring) group.

Acetaldehyde would not give tert-butyl alcohol when treated with excess methylmagnesium bromide, as the Grignard Reaction is the addition of an organomagnesium halide (Grignard reagent) to a ketone or aldehyde, to form a tertiary or secondary alcohol, respectively.

Ethylene oxide reacts with Grignard reagents to give, after protonation, primary alcohols with two additional carbon atoms.

Pentanal gives a secondary alcohol upon reaction with methylmagnesium bromide, as the Grignard Reaction is the addition of an organomagnesium halide (Grignard reagent) to a ketone or aldehyde, to form a tertiary or secondary alcohol, respectively.

3-Methylpentanal does not give a tertiary alcohol upon reaction with methylmagnesium bromide (a Grignard reagent).

• Grignard reagents react with carbonyl compounds to form alcohols.
• Aldehydes (like 3-methylpentanal) react with Grignard reagents to form secondary alcohols.
• Ketones react to form tertiary alcohols.
• Esters react to form tertiary alcohols after two equivalents of Grignard reagent.

Since 3-methylpentanal is an aldehyde, it forms a secondary alcohol, not a tertiary one.

Alkyl halides can be converted into Grignard reagents by warming them with magnesium powder in dry ether.
RX + Mg + Dry ether → R−Mg−X