Aldehydes and ketones are organic compounds built around the carbonyl group, C=O. This group appears in sugars, fragrances, hormones, solvents, and many important reaction pathways in living systems and industry. Learning how to identify and name aldehydes and ketones helps connect molecular structure to physical properties and chemical behavior.
Their reactions are a central part of organic chemistry because the carbonyl carbon is strongly influenced by the electronegative oxygen atom.
In a carbonyl group, oxygen pulls electron density away from carbon, making oxygen partially negative and carbon partially positive. This polarity makes the carbonyl carbon a good target for nucleophiles, which are electron-rich species. Aldehydes have at least one hydrogen attached to the carbonyl carbon, while ketones have two carbon groups attached.
This structural difference affects naming, oxidation, and how easily the molecule reacts.
Key Facts
- Carbonyl group: C=O, with O as δ- and the carbonyl carbon as δ+.
- Aldehyde general formula: RCHO, where the carbonyl carbon is at the end of a carbon chain.
- Ketone general formula: RCOR', where the carbonyl carbon is within a carbon chain.
- Aldehyde names usually end in -al, such as ethanal, while ketone names usually end in -one, such as propanone.
- Nucleophilic addition occurs because Nu:- attacks the δ+ carbonyl carbon, then the oxygen is protonated.
- Aldehydes are generally more reactive than ketones because they have less steric hindrance and fewer electron-donating alkyl groups.
Vocabulary
- Carbonyl group
- A functional group made of a carbon atom double-bonded to an oxygen atom, written C=O.
- Aldehyde
- An organic compound with a carbonyl group at the end of a carbon chain and at least one hydrogen attached to the carbonyl carbon.
- Ketone
- An organic compound with a carbonyl group bonded to two carbon groups.
- Nucleophile
- An electron-rich atom, ion, or molecule that donates an electron pair to form a new bond.
- Oxidation
- A reaction that increases bonding to oxygen, decreases bonding to hydrogen, or removes electrons from a molecule.
Common Mistakes to Avoid
- Calling every C=O compound an aldehyde is wrong because ketones, carboxylic acids, esters, and amides also contain carbonyl groups.
- Numbering a ketone chain from the wrong end is wrong because the carbonyl carbon should receive the lowest possible position number.
- Forgetting the aldehyde hydrogen is wrong because an aldehyde must have at least one H attached directly to the carbonyl carbon.
- Assuming aldehydes and ketones oxidize the same way is wrong because aldehydes are readily oxidized to carboxylic acids, while ketones usually resist mild oxidation.
Practice Questions
- 1 Name the compound CH3CH2CHO and identify whether it is an aldehyde or a ketone.
- 2 A ketone has the formula CH3COCH2CH3. Give its IUPAC name and state the position number of the carbonyl carbon.
- 3 Explain why the carbonyl carbon in aldehydes and ketones is attacked by nucleophiles, using bond polarity and partial charges in your answer.