Naming and Making Aldehydes and Ketones: Quick Revision

A brief note on Naming and Making Aldehydes and Ketones. Read it before a test. Try each question at the end before you open its answer.

The carbonyl group

• $\text{C}{=}\text{O}$ is flat, with the carbon $sp^2$ and angles near $120^\circ$. The carbon is $\delta^+$ and the oxygen $\delta^-$.
• An aldehyde is $\text{RCHO}$. A ketone is $\text{RCOR}'$. The carbonyl carbon of an aldehyde is carbon 1.
• Alpha hydrogens (on the carbon next to C=O) are acidic.

Names

• Aldehyde: alkane name, -e becomes -al. Ketone: -e becomes -one, with the smaller number for C=O.
• Common names: formaldehyde (methanal), acetaldehyde (ethanal), acetone (propanone), acetophenone (1-phenylethanone), benzophenone (diphenylmethanone).
• Prefixes when a higher group is present: oxo and formyl.
• Isomers of the carbonyl compounds of formula $\text{CnH}_{2}\text{nO}$: chain, position, and aldehyde against ketone.

Physical properties

• Boiling point: ether and hydrocarbon, then aldehyde and ketone, then alcohol. There are no hydrogen bonds between the molecules of one carbonyl compound.
• Methanal, ethanal and propanone mix with water in every ratio. Solubility falls as the chain grows.

Preparation

• Secondary alcohol, oxidant: ketone. Primary alcohol: PCC for the aldehyde, dichromate for the acid.
• Cu at $573\ \text{K}$: dehydrogenation. Alkyne, Hg$^{2+}$ and dilute acid: ethanal from ethyne, a methyl ketone from other terminal alkynes.
• Rosenmund: acyl chloride, H$_2$, Pd-BaSO$_4$. Stephen: nitrile, SnCl$_2$ and HCl, then water. Ester with DIBAL-H: aldehyde.
• Nitrile with a Grignard reagent, then water: ketone.
• Etard: toluene, CrO$_2$Cl$_2$ in CS$_2$, then H$_3$O$^+$. Gattermann-Koch: benzene, CO, HCl, anhydrous AlCl$_3$ and CuCl.
• Friedel-Crafts acylation: benzene, acyl chloride, anhydrous AlCl$_3$ (a little over one mole).

A worked example

Name the product of benzene with ethanoyl chloride and anhydrous aluminium chloride, and find the mass from $7.85\ \text{g}$ of the acid chloride ($78.50\ \text{g/mol}$) in a $75\ \%$ yield. The ketone has $120.15\ \text{g/mol}$.

1. Product. Acetophenone, 1-phenylethanone.
2. Moles. $\dfrac{7.85}{78.50} = 0.100\ \text{mol}$.
3. Mass. $0.100 \times 120.15 \times 0.75 = 9.01\ \text{g}$.

Practice with new numbers

A program to try

The program prints the molar mass of the saturated carbonyl compounds with 3 to 6 carbons.

Mistakes to avoid

• A number for the aldehyde group. It is always carbon 1.
• Hydrogen bonds between carbonyl molecules. There are none.
• Dichromate for an aldehyde. It carries on to the acid. Use PCC.
• Acyl chloride in the wrong reaction. Rosenmund uses H$_2$ and Pd-BaSO$_4$. Friedel-Crafts uses AlCl$_3$.

Check yourself

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

2. Which reaction makes benzaldehyde from toluene and chromyl chloride?

3. Why does butan-1-ol boil higher than butanal?

4. What does hydration of propyne give?

Sources

• OpenStax, Chemistry 2e, Chapter 20, section 20.3 Aldehydes, Ketones, Carboxylic Acids, and Esters.
• NCERT, Chemistry Part II for Class XII, Chapter 8.