When haloalkanes and aryl and vinyl halides react with magnesium metal they yield which reagent?
- (a)Tollens’ reagent
- (b)Fehling reagent
- (c)Grignard reagent
- (d)Hinsberg reagent
Answer
Why
Correct — C. A halide treated with magnesium metal in dry ether gives an alkyl or aryl magnesium halide, RMgX, which is the Grignard reagent.
The magnesium inserts itself between the carbon and the halogen. That leaves carbon bonded to a metal and strongly nucleophilic, so it attacks carbonyl carbons and builds new carbon-carbon bonds.
The other three names in the list are detection reagents from qualitative analysis, not products of a reaction with magnesium — option (c).
Why the others are wrong
- (a)Tollens’ reagent — Tollens' reagent is ammoniacal silver nitrate. It oxidises aldehydes and deposits the silver mirror, and no magnesium is involved in making or using it.
- (b)Fehling reagent — Fehling's solution is an alkaline copper(II) tartrate complex used to detect aliphatic aldehydes, which throw a red-brown precipitate of copper(I) oxide.
- (d)Hinsberg reagent — Hinsberg's reagent is benzenesulphonyl chloride, used to tell primary, secondary and tertiary amines apart by how each reacts with it.
Concept
A Grignard reagent has the formula RMgX, where R is an alkyl, aryl or vinyl group and X a halogen. It is prepared by stirring the halide with magnesium turnings in dry ether.
Its use is synthesis, not testing. Added to an aldehyde, a ketone or an ester and then hydrolysed, it gives alcohols, which is the textbook route to a longer carbon chain.
Water destroys the reagent, so the solvent and the glassware must be dry.
The stem follows the textbook line that haloalkanes and haloarenes react with magnesium in dry ether. In the laboratory the less reactive aryl and vinyl halides usually need the more strongly coordinating solvent THF, a refinement the exam does not test.
Key facts
- A Grignard reagent is an alkyl or aryl magnesium halide, RMgX.
- It is prepared from an organic halide and magnesium in dry ether.
- Grignard reagents react with carbonyl compounds and, after hydrolysis, give alcohols.
- Victor Grignard shared the Nobel Prize in Chemistry in 1912 for this work.
Study next
Common traps
- Picking a reagent you recognise from qualitative tests, when the stem describes a preparation.
- Forgetting that moisture destroys a Grignard reagent, which is why dry ether is specified.
The same reagent is asked from the formula side on 13 Sep 2024, 16:00, GA Q.23, which shows four structures and asks which one represents the Grignard reagent.
Know the name-to-preparation and the name-to-formula direction both ways.
Related PYQs
No directly related past PYQ was found.