Carbonyl chemistry
The idea
The carbonyl group, C=O, is the most important functional group in organic chemistry, and its reactivity flows from one fact: oxygen is far more electronegative than carbon, so the bond is strongly polarized, leaving the carbonyl carbon electrophilic (δ+) and the oxygen nucleophilic (δ−). Aldehydes and ketones therefore undergo nucleophilic addition: a nucleophile attacks the carbon, the π bond opens, and the oxygen becomes an alkoxide that is then protonated to an alcohol. Carboxylic acid derivatives (esters, amides, acyl halides) add a second act — nucleophilic acyl substitution — because they carry a leaving group that can depart after the nucleophile adds.
Reactivity ranks predictably. Aldehydes are more electrophilic than ketones, because a ketone's two alkyl groups both donate electron density and add steric bulk around the carbon. Among acid derivatives, the better the leaving group, the more reactive: acyl chlorides exceed esters exceed amides. Powerful carbon nucleophiles — Grignard and organolithium reagents — add irreversibly to build new carbon–carbon bonds, the workhorse reaction for assembling larger molecules. Counting carbons before and after such an addition is the first habit to build.
A frequent error is forgetting that organometallic reagents are destroyed by acidic protons. A Grignard reagent is a strong base as well as a nucleophile, so it cannot survive in the presence of water, alcohols, or acids — those proton sources quench it before it reaches the carbonyl. The addition is run in a dry, aprotic solvent, and the acidic aqueous workup comes only at the end.
Worked example
Propanal (CH₃CH₂CHO) is treated with methylmagnesium bromide (CH₃MgBr) in dry ether, followed by aqueous acid workup. Give the structure and IUPAC name of the alcohol product, and classify it.
- Identify the electrophilic site: the carbonyl carbon of propanal carries the δ+ charge, so the carbon nucleophile will attack there.
- Add the nucleophile: the methyl group of CH₃MgBr delivers a methyl carbanion equivalent to the carbonyl carbon, opening the C=O π bond and leaving the oxygen as a magnesium alkoxide.
- Protonate on workup: aqueous acid converts the alkoxide to an OH group; the carbon that was the carbonyl now bears the new methyl group, the original H, the original ethyl chain, and the OH.
- Assemble the structure: the product is CH₃CH₂CH(OH)CH₃ — a four-carbon chain (propanal's three carbons plus the added methyl) with OH on the second carbon.
- Name and classify, then count to check: the chain is butane, OH on C2 gives butan-2-ol, and the OH carbon bears two other carbons, so it is a secondary alcohol — and carbon count goes from 3 (propanal) to 4 (product), confirming one new C–C bond was formed.
Answer. The product is butan-2-ol (CH₃CH₂CH(OH)CH₃), a secondary alcohol formed by Grignard addition.
Check your understanding
- Why is an aldehyde more reactive toward nucleophilic addition than a ketone with a similar carbon skeleton?
- Why must a Grignard reaction be kept rigorously free of water and alcohols, and what happens to the reagent if it is not?
- How does nucleophilic acyl substitution differ from the simple addition seen with aldehydes, and what structural feature makes the difference?
- How would the product change if you used formaldehyde or a ketone instead of propanal, and what class of alcohol would each give?
Build the foundations first
Carbonyl chemistry builds on these concepts. If any feel shaky, start there.