Unit 12: Organometallics and Grignard Reagents
Turning a carbon-halogen bond into a carbon-metal bond flips carbon from electrophile to nucleophile, and hands a chemist a new carbon-carbon bond.
4 topics · 15 questions
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Making a Grignard Reagent
- How it worksHow does magnesium metal insert itself directly into the carbon-bromine bond of an alkyl halide?
- Worked exampleFind the Grignard reagent formed when bromobenzene reacts with magnesium metal in dry ether.
- Common mistakeWhy must a Grignard reagent be prepared and used in perfectly dry, anhydrous solvent the whole time?
Your turn1 of 2
Bromobenzene reacts with magnesium metal in dry ether. What Grignard reagent forms?Grignard Addition to Carbonyl Compounds
- How it worksHow does a Grignard's carbanion attack a carbonyl carbon and push its electrons onto the oxygen as a new alkoxide?
- How it worksHow does an aqueous acid workup protonate that alkoxide afterward to deliver the final neutral alcohol?
- Worked exampleFind the alcohol product when phenylmagnesium bromide reacts with formaldehyde, followed by aqueous acid workup.
- Worked exampleFind the alcohol product when methylmagnesium bromide reacts with cyclohexanone, followed by aqueous acid workup.
- Going furtherWhat happens to the number of new carbon-carbon bonds formed when a Grignard attacks an ester instead of a ketone?
Your turn1 of 2
Phenylmagnesium bromide reacts with formaldehyde, followed by an aqueous acid workup. What alcohol forms?Organolithium Reagents
- How it worksHow does lithium's smaller size and lower electronegativity make the carbon-lithium bond even more ionic than carbon-magnesium?
- Worked exampleFind the alcohol product when n-butyllithium reacts with acetone, followed by an aqueous workup.
- Going furtherWhat happens when an organolithium reagent is used on a substrate that also carries an acidic proton nearby?
Your turn1 of 2
N-butyllithium reacts with acetone, followed by an aqueous workup. What alcohol forms?Limitations of Organometallic Reagents with Protic Groups
- How it worksHow does a Grignard reagent act as a base and grab a proton from any O-H or N-H group before it can add to a carbonyl?
- Worked exampleFind what happens when phenylmagnesium bromide is added to a substrate carrying both a ketone and a free hydroxyl group.
- Common mistakeWhy can't a Grignard reagent be used directly on a molecule that already has an unprotected alcohol or amine on it?
- Ties togetherHow does this incompatibility with acidic protons explain why protecting groups become essential in a real synthesis?
Your turn1 of 2
Phenylmagnesium bromide is added to a substrate carrying both a ketone and a free, unprotected hydroxyl group. What happens?