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A short class with Sarah and her students. The other ways are under More.
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Look at CH3COCH3 on the board. What is bonded to the carbonyl carbon?
Two methyl groups and the oxygen. So there is a carbon on each side of it.
Exactly, and that is what makes it a ketone rather than an aldehyde. Why can't a ketone have fewer than three carbon atoms?
Because the carbonyl carbon needs one carbon on each side, so the smallest ketone has three.
Good. The carbonyl carbon is sp2 hybridised. What shape does that give?
Flat, with bond angles of about 120 degrees.
Right. And since oxygen pulls the shared electrons towards itself, the carbon is left slightly positive. Hold on to that idea, because it explains most of the chemistry of ketones.
Let's name CH3CH2COCH2CH3. What is the longest chain containing the carbonyl carbon?
Five carbons, so pentane. The carbonyl is on carbon 3 from either end, so pentan-3-one.
Perfect. And its common name?
It has two ethyl groups, so diethyl ketone.
Now CH3COCH2CH3. Why don't we bother writing butan-2-one?
Because in a four-carbon ketone the carbonyl can only be on carbon 2. On carbon 1 it would be an aldehyde, so the name is just butanone.
Exactly. We add a number only when more than one position is possible.
Which compounds with a carbonyl group have the formula C3H6O?
Propanal and propanone.
What kind of isomers are they?
Functional isomers, because one is an aldehyde and the other is a ketone.
Good. How many ketones can you draw for C5H10O?
Pentan-2-one, pentan-3-one and 3-methylbutan-2-one. That makes three.
Correct. The first two are position isomers, and the branched one is a chain isomer of both.
the carbonyl carbon is bonded to two carbon atoms (R–CO–R′), so the smallest ketone has three carbons.
an sp2 carbon makes the group planar, with bond angles of about 120°.
the C=O bond is polar, with a δ+ carbon and a δ− oxygen.
longest chain containing the carbonyl carbon, suffix -one, lowest number for the carbonyl carbon.
ketones are functional isomers of aldehydes and show position and chain isomerism among themselves.
CH3COCH2CH2CH3 is pentan-2-one: the chain has five carbons, and numbering from the left gives the carbonyl carbon the number 2 rather than 4.
(CH3)2CHCOCH(CH3)2 is 2,4-dimethylpentan-3-one: a five-carbon chain with the carbonyl carbon at position 3 and methyl groups on carbons 2 and 4.
C6H5COCH3 is acetophenone: its IUPAC name is 1-phenylethan-1-one and its common name is methyl phenyl ketone.
Carbon on the left and carbon on the right, the ketone's C=O sits in the middle tight.
Aldo the aldehyde always stood at the end of the queue, holding a hydrogen. His cousin Keto refused to stand at the end, so he always squeezed into the middle with a carbon friend on each side.
K for Ketone, K for Kept inside: the carbonyl group is kept inside the chain.
L.O.N. - Longest chain, One as the ending, Number lowest for the carbonyl carbon.
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General formula of a ketone
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Hybridisation and bond angle at the carbonyl carbon
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IUPAC name of CH3CH2COCH2CH3
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Why is there no butan-1-one?
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Functional isomer of propanone
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A compound in which the carbonyl group is bonded to two carbon atoms; general formula R–CO–R′.
A carbon atom joined to an oxygen atom by a double bond (>C=O).
The mixing of one s and two p orbitals to give three orbitals in one plane at 120°, as on the carbonyl carbon.
A ketone with two identical groups on the carbonyl carbon, such as propanone.
A ketone with two different groups on the carbonyl carbon, such as butanone.
Compounds with the same molecular formula but different functional groups, such as propanal and propanone.
Isomers that differ in the alkyl groups on either side of the same functional group, such as pentan-2-one and pentan-3-one.
Ketones are carbonyl compounds in which the C=O group is joined to two carbon atoms. This lesson explains how a ketone is built, how to name it by the IUPAC and common systems, and the isomerism ketones show.
A ketone has the general formula R–CO–R′, with the carbonyl carbon inside the carbon chain rather than at its end. The carbonyl carbon is sp2 hybridised, so the group is flat with bond angles of about 120°, and it is polar because oxygen pulls the shared electrons towards itself. IUPAC names end in -one, and the chain is numbered to give the carbonyl carbon the lowest possible number; common names list the two groups on the carbonyl carbon followed by the word ketone. Ketones are functional isomers of aldehydes and also show position and chain isomerism among themselves.
A ketone is an organic compound in which the carbonyl group (>C=O) is bonded to two carbon atoms. Its general formula is R–CO–R′, where R and R′ are alkyl or aryl groups. When R and R′ are the same, the ketone is a simple (symmetrical) ketone, such as propanone, CH3COCH3. When they are different, it is a mixed (unsymmetrical) ketone, such as butanone, CH3COCH2CH3. Because the carbonyl carbon needs a carbon atom on each side, the smallest ketone has three carbon atoms. Open-chain saturated ketones have the general formula CnH2nO (n ≥ 3), the same as aldehydes with the same number of carbon atoms.
Common names list the two groups attached to the carbonyl carbon, in alphabetical order, followed by the word ketone:
In common names, the carbon atom next to the carbonyl group is called the α-carbon, the next one the β-carbon, and so on.
How a ketone is built and named is the base for the rest of this chapter: its preparation, its reactions and its uses all depend on a polar, planar carbonyl group held between two carbon atoms.