Structure and names of aldehydes and ketones

Three smells, one group

Crush a bitter almond, and you smell benzaldehyde. Open a bottle of nail varnish remover, and you smell propanone, also called acetone. A specimen jar in a biology laboratory holds methanal in water, called formalin.

These compounds look different. Yet each has a carbon atom double bonded to an oxygen atom: $\text{C}{=}\text{O}$. This is the carbonyl group. It is found in aldehydes, ketones, carboxylic acids, esters and amides. In this chapter we meet the first three.

• In an aldehyde, the carbonyl carbon is bonded to one hydrogen atom and to one carbon group, $\text{RCHO}$. The simplest aldehyde, $\text{HCHO}$, has two hydrogen atoms.
• In a ketone, the carbonyl carbon is bonded to two carbon groups, $\text{RCOR}'$.
• In a carboxylic acid, the carbonyl carbon is bonded to an $\text{OH}$ group as well, $\text{RCOOH}$.

The letter R stands for any alkyl or aryl group. An aldehyde group always sits at the end of a chain. A ketone group is always inside the chain.

The shape and the polarity of the carbonyl group

The carbonyl carbon uses three $sp^2$ orbitals to form three sigma bonds. These bonds lie in one plane, about $120^\circ$ apart. The fourth electron of the carbon is in a $p$ orbital. It overlaps sideways with a $p$ orbital of oxygen, and this makes the pi bond. The pi cloud lies above and below the plane.

Oxygen pulls electrons more strongly than carbon. So the pi electrons sit closer to oxygen. The oxygen carries a small negative charge, $\delta^-$, and the carbon a small positive charge, $\delta^+$. We can say the same thing with two resonance forms. The first is $\text{C}{=}\text{O}$ and the second is $\text{C}^{+} \text{-}\text{O}^{-}$. The first form contributes much more, but the second explains the polarity.

Three results follow, and we use them for the whole chapter:

1. The carbon is electrophilic. It attracts a nucleophile, a species with a lone pair to give.
2. The oxygen is weakly basic. It can take a proton from an acid.
3. A hydrogen on the carbon next to the carbonyl, the alpha hydrogen, is acidic. You will use this in the aldol reaction.

A measure of the polarity is the dipole moment. Propanone has $2.88\ \text{D}$, and propene, which has the same shape of skeleton but no oxygen, has only about $0.4\ \text{D}$.

IUPAC names

We build the name from the longest chain that contains the carbonyl carbon.

1. For an aldehyde, replace the final -e of the alkane with -al. The carbonyl carbon is always carbon 1, so no number is needed: propanal, not propan-1-al.
2. For a ketone, replace the final -e with -one. Number the chain so that the carbonyl carbon gets the smaller number. Write the number before the ending: pentan-2-one. Propanone and butanone need no number, because there is only one place.
3. In a ring, the ketone is a cycloalkanone: cyclohexanone. An aldehyde group on a ring is named with -carbaldehyde: cyclohexanecarbaldehyde.
4. Other groups are named as prefixes with their numbers, as in alkanes: 3-methylbutanal.
5. If a group of higher priority is also present, the carbonyl is named as a prefix: oxo for ${=}\text{O}$ inside a chain, formyl for $\text{-}\text{CHO}$. The order of priority is acid, then aldehyde, then ketone, then alcohol. So $\text{CH}_{3}\text{COCH}_{2}\text{COOH}$ is 3-oxobutanoic acid.

Many compounds keep a common name, and you must know them:

• $\text{HCHO}$: formaldehyde, IUPAC methanal.
• $\text{CH}_{3}\text{CHO}$: acetaldehyde, IUPAC ethanal.
• $\text{CH}_{3}\text{CH}_{2}\text{CHO}$: propionaldehyde, IUPAC propanal.
• $\text{C}_{6}\text{H}_{5}\text{CHO}$: benzaldehyde, also benzenecarbaldehyde.
• $\text{CH}_{3}\text{COCH}_{3}$: acetone, IUPAC propanone.
• $\text{CH}_{3}\text{COCH}_{2}\text{CH}_{3}$: ethyl methyl ketone, IUPAC butanone.
• $\text{C}_{6}\text{H}_{5}\text{COCH}_{3}$: acetophenone, IUPAC 1-phenylethanone.
• $\text{C}_{6}\text{H}_{5}\text{COC}_{6}\text{H}_{5}$: benzophenone, IUPAC diphenylmethanone.

In common names, the carbon atoms next to the carbonyl are labelled by Greek letters: alpha, beta, gamma. The hydrogens on the alpha carbon are the alpha hydrogens.

Start at 5 carbons and place 2. The carbonyl is the second carbon, so it is pentan-2-one. Now place 4. You still read pentan-2-one, because you count from the other end. Place 1, and it becomes an aldehyde.

Isomers of carbonyl compounds

Carbonyl compounds show chain isomerism (butanal and 2-methylpropanal), position isomerism (pentan-2-one and pentan-3-one) and functional isomerism. An aldehyde and a ketone of the same formula are functional isomers: propanal and propanone are both $\text{C}_{3}\text{H}_{6}\text{O}$. Both have the general formula $\text{CnH}_{2}\text{nO}$ when saturated.

To count them, remember that an aldehyde is $\text{HCO}$ joined to one alkyl group, and a ketone is $\text{CO}$ joined to two alkyl groups. The number of alkyl groups with 1, 2, 3, 4 and 5 carbons is 1, 1, 2, 4 and 8.

Practise with new numbers

Try it in Python

The program counts the alkyl groups by building them from smaller ones. Then it adds the aldehydes and ketones of each formula.

Why they boil where they do

The carbonyl group has a dipole. So the molecules of an aldehyde or a ketone attract one another by dipole-dipole forces. Their boiling points are higher than those of hydrocarbons and ethers of similar molar mass.

They are lower than those of alcohols. An alcohol has an $\text{O}\text{-}\text{H}$ group, so its molecules form hydrogen bonds with one another. A carbonyl compound has no $\text{O}\text{-}\text{H}$ or $\text{N}\text{-}\text{H}$ group, so it cannot form hydrogen bonds with its own kind.

The picture shows five compounds of about $58$ to $60\ \text{g/mol}$. Butane has only weak forces. Propan-1-ol has hydrogen bonds. The aldehyde and the ketone sit between them.

Some more facts:

• Methanal is a gas at room temperature, with a boiling point of $-19\ ^\circ\text{C}$. Ethanal boils at $20\ ^\circ\text{C}$. The higher aldehydes and ketones are liquids, and the very large ones are solids.
• The oxygen of the carbonyl group can accept a hydrogen bond from water. So methanal, ethanal and propanone mix with water in every ratio. The solubility falls as the carbon chain grows, because the hydrocarbon part does not suit water. Benzaldehyde is only slightly soluble.
• The lower aldehydes have a sharp smell. The higher aldehydes and ketones have pleasant smells and are used in perfumes and flavours.

Common mistakes

• Giving the aldehyde a number. The aldehyde carbon is always carbon 1. Write propanal, not propan-1-al.
• Numbering from the wrong end. The carbonyl carbon of a ketone must get the smaller number.
• Saying that aldehydes and ketones form hydrogen bonds with themselves. They do not. They only accept hydrogen bonds from water or alcohols.
• Forgetting the aldehyde isomer of a ketone. Propanone has the isomer propanal.
• Putting the C=O of a ketone at the end of a chain. A carbonyl at the end of a chain has a hydrogen on it, so the compound is an aldehyde.

Remember these

• The carbonyl carbon is $sp^2$, flat, with angles near $120^\circ$. It is $\delta^+$, and the oxygen is $\delta^-$.
• Aldehyde: -al, carbon 1. Ketone: -one, the lowest number. Oxo and formyl are the prefixes.
• Boiling points: hydrocarbon and ether, then aldehyde and ketone, then alcohol.
• The first three members mix with water, and the solubility falls with chain length.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name $(\text{CH}_{3})_{2}\text{CHCH}_{2}\text{CHO}$.

2. Which has the higher boiling point, butanal or butan-1-ol? Why?

3. How many aldehydes have the formula $\text{C}_{4}\text{H}_{8}\text{O}$?

4. Why is the carbon of a carbonyl group attacked by nucleophiles?

References

• OpenStax, Chemistry 2e, Chapter 20, section 20.3 Aldehydes, Ketones, Carboxylic Acids, and Esters.
• NCERT, Chemistry Part II for Class XII, Chapter 8, the sections on nomenclature, the structure of the carbonyl group and physical properties.