Carbonyl addition reactions are central to organic chemistry because aldehydes and ketones contain a strongly polarized C=O bond. The oxygen pulls electron density away from carbon, making the carbonyl carbon partially positive and attractive to nucleophiles. These reactions build new carbon bonds, form alcohols, and connect directly to carbohydrate chemistry.
Learning the pattern helps students predict many reactions from one core mechanism.
In nucleophilic addition, a nucleophile attacks the carbonyl carbon while the pi bond electrons move onto oxygen, forming a tetrahedral alkoxide intermediate. A proton source then protonates the alkoxide to give an alcohol or related product. Aldehydes usually react faster than ketones because they are less sterically crowded and have a more electrophilic carbonyl carbon.
Under acid catalysis, the carbonyl oxygen is protonated first, which makes the carbonyl carbon even more electrophilic.
Key Facts
- Carbonyl polarity: R2C=O has Cδ+ and Oδ− because oxygen is more electronegative than carbon.
- General addition pattern: R2C=O + Nu− + H+ gives R2C(OH)Nu.
- Basic mechanism step 1: Nu− attacks the carbonyl carbon and the C=O pi electrons move to oxygen.
- Basic mechanism step 2: the alkoxide intermediate R2C(O−)Nu is protonated to form R2C(OH)Nu.
- Aldehydes are generally more reactive than ketones because they have less steric hindrance and less electron donation from alkyl groups.
- Hemiacetal formation: aldehyde or ketone + alcohol gives a hemiacetal with both OH and OR on the former carbonyl carbon.
Vocabulary
- Carbonyl group
- A functional group containing a carbon atom double bonded to an oxygen atom, written C=O.
- Nucleophile
- An electron-rich species that donates an electron pair to form a new covalent bond.
- Electrophile
- An electron-poor species that accepts an electron pair during a reaction.
- Alkoxide intermediate
- A negatively charged oxygen species formed after a nucleophile adds to a carbonyl carbon.
- Hemiacetal
- A compound in which one carbon is bonded to both an OH group and an OR group, often formed by addition of an alcohol to a carbonyl.
Common Mistakes to Avoid
- Attacking the carbonyl oxygen with the nucleophile is wrong because the carbonyl carbon is the electrophilic site in aldehydes and ketones.
- Forgetting to move the C=O pi electrons onto oxygen is wrong because carbon cannot exceed four bonds in the tetrahedral intermediate.
- Drawing the final product as an alkoxide without protonation is incomplete when the reaction conditions include water, alcohol, or acid.
- Assuming ketones are always more reactive than aldehydes is wrong because ketones are usually more hindered and less electrophilic due to two carbon groups.
Practice Questions
- 1 Acetaldehyde, CH3CHO, reacts with HCN followed by protonation. Draw the product and identify the new bond formed.
- 2 A ketone, (CH3)2C=O, reacts with CH3MgBr followed by H3O+. Write the structure of the alcohol product and count the number of carbon atoms in it.
- 3 Explain why protonating the carbonyl oxygen under acidic conditions makes nucleophilic addition faster, using partial charges and electron density in your answer.