Organic Chemistry I and II · Carbonyl reactions
Grignard reaction practice problems: predict the product, then plan the synthesis
A Grignard reagent, R–MgX, is the first carbon nucleophile most students meet. It makes a new C–C bond to a carbonyl carbon, and after an acid workup you have an alcohol. Exams ask it two ways: forward (what forms?) and backward (which Grignard and which carbonyl would make this alcohol?). This page gives you practice with both.
Jump to: what a Grignard does · product map · the two traps · worked examples · build the alcohol · 9 practice problems · FAQ
What a Grignard reagent does
You make it by stirring an alkyl or aryl halide with magnesium metal in dry ether or THF: R–Br + Mg → R–MgBr. The magnesium sits between the carbon and the halogen, and because carbon is much more electronegative than magnesium, the C–Mg bond is strongly polarized toward carbon. Treat that carbon as a carbanion, R⁻: a strong nucleophile and an extremely strong base.
Every Grignard addition to an aldehyde or ketone follows the same two steps:
- Addition. R⁻ attacks the carbonyl carbon, and the C=O pi electrons move onto oxygen. You get a magnesium alkoxide, with a new C–R bond.
- Workup. Dilute acid (H₃O⁺) is added afterwards to protonate the alkoxide, giving the alcohol.
The carbon that used to be the C=O carbon is the one that ends up holding the OH. Call it the carbinol carbon. It always gains the Grignard's R group, which is the key to working backwards.
Product map: what each partner gives
| Partner | Product | What to notice |
|---|---|---|
| Formaldehyde (H₂C=O) | Primary alcohol, R–CH₂OH | Adds one carbon |
| Any other aldehyde (R′CHO) | Secondary alcohol, R–CH(OH)–R′ | New stereocenter (racemic), unless R and R′ are the same |
| Ketone (R′COR″) | Tertiary alcohol | Three carbon groups on the carbinol carbon |
| Ester (R′CO₂R″) or acid chloride | Tertiary alcohol with two identical R groups from the Grignard | Adds twice; the OR″ or Cl is lost |
| CO₂, then H₃O⁺ | Carboxylic acid, R–COOH | Adds one carbon, as COOH |
| Ethylene oxide, then H₃O⁺ | Primary alcohol, R–CH₂CH₂OH | Adds two carbons; attacks the less hindered epoxide carbon |
| Nitrile (R′C≡N), then H₃O⁺ | Ketone, R–CO–R′ | Stops at the ketone; the imine is hydrolyzed in workup |
| Water, alcohol, acid, D₂O | Alkane, R–H (or R–D with D₂O) | The Grignard is destroyed as a base |
The two traps that cost the most points
Trap 1: any O–H or N–H kills it
A Grignard reacts as a base faster than it adds as a nucleophile. Alkanes have pKa values of 50 or more, so R⁻ will rip a proton off water (15.7), an alcohol (about 16), a carboxylic acid (about 5), an amine N–H (about 35), or a terminal alkyne (about 25) before it does anything else. That rules out wet solvents, and it rules out a molecule that has its own O–H, N–H or C≡C–H alongside the halide. The fix is to protect the group first (for example, an alcohol as a silyl ether).
Trap 2: esters add twice
The first equivalent adds, then the OR group leaves and gives a ketone. That ketone is more reactive than the ester you started with, so a second equivalent adds right away. You can't stop at the ketone. The product is a tertiary alcohol in which two of the three groups came from the Grignard.
Worked examples
Example 1: Forward, ketone
Phenylmagnesium bromide (C₆H₅MgBr) and acetone, then H₃O⁺.
- The phenyl carbanion attacks the carbonyl carbon of acetone, (CH₃)₂C=O.
- That carbon now holds two methyls, a phenyl and O⁻ (as the magnesium salt).
- Acid workup protonates the oxygen.
Product: 2-phenylpropan-2-ol, (CH₃)₂C(OH)C₆H₅, a tertiary alcohol.
Example 2: Forward, ester (same product, different route)
Methyl benzoate (C₆H₅CO₂CH₃) with two equivalents of CH₃MgBr, then H₃O⁺.
- CH₃⁻ adds to the ester carbonyl. The tetrahedral intermediate kicks out CH₃O⁻, leaving acetophenone, C₆H₅COCH₃.
- The second CH₃MgBr adds to acetophenone, which is more reactive than the ester.
- Workup gives the alcohol. The carbinol carbon holds phenyl (from the ester) and two methyls (both from the Grignard).
Product: 2-phenylpropan-2-ol again. Two identical groups on the carbinol carbon are the sign an ester route is possible.
Example 3: Backward, three ways to one alcohol
Target: 3-methylhexan-3-ol, CH₃CH₂–C(OH)(CH₃)–CH₂CH₂CH₃.
- Find the carbinol carbon (the one with the OH). Its three carbon groups are ethyl, methyl and propyl.
- Break one bond to that carbon at a time. The group you break off becomes the Grignard. The other two stay on the carbonyl carbon.
- Break ethyl: CH₃CH₂MgBr + pentan-2-one. Break methyl: CH₃MgBr + hexan-3-one. Break propyl: CH₃CH₂CH₂MgBr + butan-2-one.
All three routes work and give the same (racemic) alcohol. No ester route here, because no two groups are the same.
Example 4: The one that fails
A student plans to make the Grignard reagent from 4-bromobutan-1-ol, HO–CH₂CH₂CH₂CH₂–Br, and add it to acetone.
- As soon as any C–Mg forms, that carbanion meets the O–H of the same kind of molecule.
- Acid–base wins over addition: the carbanion takes the proton, and the reagent is destroyed.
- Fix: protect the OH first (for example as a trimethylsilyl ether), do the Grignard reaction, then remove the protecting group.
Before making any Grignard, scan the whole molecule for O–H, N–H, C≡C–H and C=O groups.
Want more problems like these? OrgoDrill covers alcohols and carbonyl chemistry with practice for every reaction, and review brings back what you missed until it's automatic.
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Build the alcohol
For each target, choose a Grignard reagent and a carbonyl partner, then check. Most targets have more than one right answer, and the checker accepts any pair that works. After the acid workup, of course.
Grignard practice problems
Pick an answer and press Check answer. Assume an acid workup (H₃O⁺) after each Grignard step.
Frequently asked questions
What does a Grignard reagent react with?
Mainly carbonyl compounds. It adds to formaldehyde to give primary alcohols, other aldehydes to give secondary alcohols, ketones to give tertiary alcohols, and esters twice to give tertiary alcohols. It also adds to CO₂ to give carboxylic acids, to ethylene oxide to give primary alcohols two carbons longer, and to nitriles to give ketones.
Why must Grignard reactions be kept dry?
A Grignard's carbon behaves like a carbanion, the conjugate base of an alkane with a pKa of 50 or more. Water is a vastly stronger acid, so even a trace of moisture protonates the reagent and turns it into an alkane before it can react with the carbonyl.
Why does a Grignard add twice to an ester?
After the first addition, the alkoxy group leaves and a ketone forms. Ketones are more reactive toward Grignard reagents than esters, so the ketone reacts with a second equivalent as soon as it forms. The product is a tertiary alcohol with two identical groups from the Grignard.
How do I work out which Grignard and carbonyl make an alcohol?
Find the carbon holding the OH and list its three other groups. Break the bond to any one carbon group: that group becomes the Grignard (R–MgBr), and the carbon with the OH becomes a C=O carrying the other two groups. If two of the groups are identical, an ester plus two equivalents of Grignard also works.
Sources
- OpenStax Organic Chemistry, 10.6 Reactions of Alkyl Halides: Grignard Reagents
- OpenStax Organic Chemistry, 17.5 Alcohols from Carbonyl Compounds: Grignard Reaction and 17.8 Protection of Alcohols
- OpenStax Organic Chemistry, 18.5 Reactions of Epoxides: Ring-Opening
- OpenStax Organic Chemistry, 20.5 Preparing Carboxylic Acids and 20.7 Chemistry of Nitriles
- OpenStax Organic Chemistry, 21.6 Chemistry of Esters
Grignard questions show up in Orgo I and all through Orgo II. Practice them with the rest of the carbonyl reactions.
Start practicing freeChapters 1 and 11 are free, plus a sample of every other chapter. No login, no card. Nothing expires.