ketone
/ KEE-tone /
A ketone is a carbonyl group (C=O) sandwiched in the middle of a carbon chain, with carbon atoms on both sides. The general shape is R-CO-R', where both R and R' are carbon-containing groups. Acetone, the nail-polish remover smell, is the simplest and most famous ketone.
Naming again starts from the parent alkane, but you drop the -e and add -one, and you give the carbonyl carbon the lowest possible number along the chain. Propanone (acetone) is CH3-CO-CH3; butanone is CH3-CO-CH2-CH3; in longer chains a locant is needed, as in pentan-2-one. Because a ketone carbon sits between two other carbons, it can never be the very first carbon, which distinguishes it on sight from an aldehyde.
Ketones are slightly less reactive than aldehydes toward nucleophiles, for two reasons: the two flanking carbon groups crowd the carbonyl carbon (steric hindrance) and they also push a little electron density toward it (an electron-donating inductive and hyperconjugative effect), making the carbon less electron-poor. They cannot be oxidized further under mild conditions because there is no H on the carbonyl carbon to remove. They are abundant in nature, from sugars to steroid hormones, and are workhorse intermediates in synthesis.
Acetone (propanone), CH3-CO-CH3, is the carbonyl carbon flanked by two methyl groups. Your body makes it in small amounts during fasting (ketosis), which is why breath can smell faintly fruity.
A ketone has the carbonyl in the middle, with carbons on both sides.
Aldehydes being more reactive than ketones is a trend, not an absolute law; both steric crowding and electron donation by the alkyl groups contribute, and substituents can shift the balance.