Making Alcohols with Grignard Reagents (NEET Chemistry)
Chemistry · Alcohols, Phenols And Ethers · NEET
A Grignard reagent is written RMgX (like CH3MgBr). Its R group jumps onto the carbon of a C=O bond, and after adding water you get an alcohol. The rule for the class of alcohol is easy: formaldehyde (HCHO) gives a 1° alcohol, any other aldehyde gives a 2° alcohol, and a ketone gives a 3° alcohol. Memory hook: "0-1-2 rings on the carbon = 1°, 2°, 3° alcohol" — count how many carbon groups the carbonyl already has.
Count the carbon groups on the carbonyl carbon: formaldehyde (0) gives a 1° alcohol, any other aldehyde (1) gives a 2° alcohol, and a ketone (2) gives a 3° alcohol. The Grignard always adds one more carbon group plus, after water, an -OH.
Your doubts, answered
How do I know if the product is a 1°, 2° or 3° alcohol?
Count the carbon groups already attached to the carbonyl carbon (the C in C=O), then add one more from R of the Grignard. Formaldehyde HCHO has 0 carbon groups, so H-CHO + RMgX gives a 1° alcohol (RCH2OH). Any other aldehyde RCHO has 1 carbon group, so you get a 2° alcohol. A ketone R-CO-R has 2 carbon groups, so you get a 3° alcohol. This is the single most tested idea for NEET.
Acetone + CH3MgCl gives what? I keep getting it wrong.
Acetone is (CH3)2C=O, a ketone. The methyl from CH3MgCl adds to the carbonyl carbon, so that carbon now holds three CH3 groups and one OH: (CH3)3C-OH. That is tert-butyl alcohol (2-methylpropan-2-ol), a 3° alcohol. Ketone + Grignard is always a 3° alcohol. This was the exact answer in NEET 2020.
Why must the reaction be done in dry ether with no water?
The C-Mg bond acts like a carbanion (a carbon that carries a negative charge). It grabs any H+ it can find. Water is a source of H+. If water is present, RMgX just becomes R-H (an alkane) and is wasted, instead of adding to the carbonyl. So NEET expects you to know: even traces of moisture must be avoided, and the solvent is dry (anhydrous) ether.
What does the '(ii) H2O / H+' step actually do?
When R adds to C=O, you first get an alkoxide salt bonded to magnesium: R-C-O-MgX. This is not the alcohol yet. Adding water (acidic work-up) simply swaps the -OMgX for -OH, giving the neutral alcohol. So the reaction is always two steps: (i) Grignard in dry ether, then (ii) H2O/H+ to release the alcohol.
What if I react the Grignard with CO2 instead of a carbonyl?
Then you do NOT get an alcohol. CO2 (dry ice) plus RMgX, followed by acid, gives a carboxylic acid R-COOH. This adds one carbon as -COOH. Watch the reagent: C=O of an aldehyde/ketone gives an alcohol, but CO2 gives a carboxylic acid. NEET mixes these to trap students.
⚠️ The NEET trap ✗ Acetone + C2H5MgBr gives pentan-3-ol or 2-methylpropan-2-ol. ✓ Acetone + C2H5MgBr gives 2-methylbutan-2-ol. The ethyl group adds to the carbonyl carbon of (CH3)2C=O, giving (CH3)2C(OH)-CH2CH3 — a 3° alcohol with a 4-carbon chain and a methyl branch at C-2. 🧠 After adding, name the longest chain through the new bond, THEN mark branches. Ethyl (2C) + one CH3 of acetone in the chain = 4 carbons = butan-2-ol, with the other CH3 as a 2-methyl branch.
Real NEET questions
NEET 2020
Reaction between acetone and methylmagnesium chloride followed by hydrolysis will give:
A · tert-Butyl alcohol ✓
B · Isobutyl alcohol
C · Isopropyl alcohol
D · sec-Butyl alcohol
Solution: Acetone (CH3-CO-CH3) is a ketone, so a Grignard reagent gives a 3° alcohol. The CH3 from CH3MgCl adds to the carbonyl carbon: CH3COCH3 + CH3MgCl gives (CH3)3C-OMgCl, and hydrolysis gives (CH3)3C-OH. That carbon now carries three methyl groups plus OH, which is 2-methylpropan-2-ol = tert-butyl alcohol. Answer: (A).
NEET 2021
What is the IUPAC name of the product? CH3COCH3 -> (i) C2H5MgBr, dry ether (ii) H2O, H+ -> Product
A · pentan-3-ol
B · 2-methylbutan-2-ol ✓
C · 2-methylpropan-2-ol
D · pentan-2-ol
Solution: Acetone is a ketone, so the ethyl group of C2H5MgBr adds to the carbonyl carbon and dry-ether then acidic work-up gives a 3° alcohol: (CH3)2C(OH)-CH2CH3. Take the longest chain through the new bond: CH3-CH2-C(OH)(CH3)- gives a 4-carbon butan-2-ol chain, with a methyl branch at C-2. IUPAC name = 2-methylbutan-2-ol. Answer: (B).
Solved Alcohols, Phenols And Ethers NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is an alkyl or aryl magnesium halide, RMgX (for example CH3MgBr), where the C-Mg bond behaves like a carbanion and can add its R group to a carbonyl carbon.
Which carbonyl gives a primary alcohol with a Grignard reagent?
Only formaldehyde (HCHO, methanal). HCHO + RMgX, then H2O, gives RCH2OH, which is a primary (1°) alcohol. Every other aldehyde gives a secondary alcohol.
Does a ketone always give a tertiary alcohol?
Yes. A ketone already has two carbon groups on the carbonyl carbon; the Grignard adds a third. So the product carbon has three carbon groups and one OH, which is a tertiary (3°) alcohol.
Why is dry ether used as the solvent?
Ether is unreactive and dissolves the Grignard reagent, and it is kept dry because water (or any H+ source) destroys RMgX by turning it into an alkane R-H before it can react with the carbonyl.
Grignard with CO2 gives an alcohol or an acid?
An acid. RMgX + CO2, then H+, gives a carboxylic acid RCOOH, not an alcohol. Only aldehydes and ketones give alcohols.
