Carbonyl addition reactions are central reactions of aldehydes and ketones because the polarized bond creates an electrophilic carbon. This cheat sheet helps students connect mechanism, reagent choice, and product structure in one reference. It is especially useful for comparing aldehydes and ketones, predicting addition products, and recognizing acid-catalyzed versus base-promoted pathways.
The core idea is that a nucleophile attacks the carbonyl carbon and the oxygen becomes an alkoxide or alcohol after proton transfer. Aldehydes are usually more reactive than ketones because they are less hindered and less electron donating. Important reaction families include hydration, alcohol addition to form hemiacetals and acetals, amine addition to form imines and enamines, cyanide addition to form cyanohydrins, and hydride addition to form alcohols.
Key Facts
- A carbonyl group is polarized as , so nucleophiles attack the carbonyl carbon and electrophiles or protons interact with oxygen.
- The general nucleophilic addition pattern is .
- Aldehydes react faster than ketones in most additions because has less steric hindrance and less alkyl electron donation than .
- Hydration gives a geminal diol by , and electron-withdrawing groups shift equilibrium toward the hydrate.
- Alcohol addition first forms a hemiacetal by , then acid-catalyzed excess alcohol can form an acetal .
- Primary amines form imines by , usually under mildly acidic conditions.
- Cyanide addition forms cyanohydrins by , creating a new carbon-carbon bond.
- Hydride reagents reduce aldehydes to primary alcohols and ketones to secondary alcohols, such as and .
Vocabulary
- Carbonyl group
- A functional group containing a carbon-oxygen double bond, written as , with an electrophilic carbon atom.
- Nucleophilic addition
- A reaction in which a nucleophile attacks a multiple bond, such as , and forms a new sigma bond to carbon.
- Aldehyde
- A carbonyl compound with at least one hydrogen attached to the carbonyl carbon, written as .
- Ketone
- A carbonyl compound with two carbon groups attached to the carbonyl carbon, written as .
- Hemiacetal
- A compound containing both and on the same carbon, with general structure .
- Imine
- A nitrogen analog of a carbonyl compound with a carbon-nitrogen double bond, written as .
Common Mistakes to Avoid
- Attacking the carbonyl oxygen with the nucleophile instead of the carbonyl carbon is wrong because the carbonyl carbon is electrophilic in .
- Forgetting the proton-transfer step gives an alkoxide as the final product when neutral workup should produce an alcohol such as .
- Using strong acid with cyanide or amines without considering reagent compatibility is wrong because and amines can be protonated and lose nucleophilicity.
- Predicting ketones to be more reactive than aldehydes is wrong because ketones usually have greater steric hindrance and stronger alkyl electron donation.
- Calling every alcohol-addition product an acetal is wrong because one equivalent of alcohol usually gives a hemiacetal before full acetal formation.
Practice Questions
- 1 Predict the major product when benzaldehyde, , is treated with followed by aqueous workup.
- 2 Draw the product of acetone, , reacting with and name the new functional group formed.
- 3 For the reaction , identify whether the product is a hydrate, hemiacetal, acetal, imine, or cyanohydrin.
- 4 Explain why aldehydes generally undergo nucleophilic addition faster than ketones, using both steric and electronic reasoning.