13. The R group of a particular Grignard’s reagent is an un-branched alkyl chain with
n carbons. How many chiral carbons do you expect the product to have if this reagent
reacts with formaldehyde?
a. 0
b. 1
c. n
d. n + 1
The correct answer is a. 0. A Grignard reagent (R-MgX) with an unbranched alkyl R group containing n carbons reacts with formaldehyde (HCHO) to form R-CH₂-OH, a primary alcohol after hydrolysis. This product has no chiral carbons because the CH₂OH carbon bears two identical hydrogens, and the straight-chain R group (CH₃-(CH₂)ₙ₋₁-) contains only carbons with two identical hydrogens or three.
Reaction Mechanism
Grignard reagents act as nucleophiles, with R⁻ attacking formaldehyde’s carbonyl carbon to form R-CH₂-OMgX, which hydrolyzes to R-CH₂-OH. Formaldehyde uniquely yields primary alcohols since its carbonyl carbon has two hydrogens. Unbranched R ensures a linear chain without branching that could create asymmetry.
Option Analysis
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a. 0: Correct. The product R-CH₂-OH has CH₂(OH) with substituents R, OH, H, H (two H’s prevent chirality). Internal chain carbons are -CH₂- (two H’s each); terminal is CH₃ (three H’s).
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b. 1: Incorrect. No new stereocenter forms; CH₂OH lacks four different groups, unlike secondary/tertiary alcohols from other carbonyls.
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c. n: Incorrect. Unbranched chains have zero chiral carbons inherently; reaction adds none.
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d. n + 1: Incorrect. Original chain contributes zero; new carbon also achiral.
Grignard reagent formaldehyde reactions are essential in organic chemistry for CSIR NET Life Sciences preparation, producing primary alcohols with 0 chiral carbons when using unbranched alkyl chains. The key phrase “Grignard reagent formaldehyde chiral carbons” highlights why option a. 0 is correct—no stereocenters form in straight-chain products.
Product Structure Breakdown
The reaction R-MgX + HCHO → R-CH₂-OH creates a linear primary alcohol. Chiral carbons require four different substituents, absent here:
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New carbon: R-CH₂-OH (H, H, OH, R)
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Chain carbons: All -CH₂- or CH₃-
Exam Relevance for CSIR NET
This tests stereochemistry understanding in organometallic reactions. Unbranched specification eliminates branching-induced chirality, unlike cyclic/branched R groups.
Common Mistakes to Avoid
Students often pick b. 1, confusing with aldehyde reactions forming secondary alcohols (one chiral center). Formaldehyde’s H₂C=O uniquely prevents this.
