Aldehydes and Ketones: The Carbonyl Group and Oxidizing Alcohols

Smells of the kitchen

Open a jar of cinnamon, a bottle of vanilla or a bottle of almond extract. The main flavour molecules are cinnamaldehyde, vanillin and benzaldehyde, and all three are aldehydes. Nail polish remover is mostly acetone, a ketone. Both families contain the carbonyl group, C=O.

• In an aldehyde, the carbonyl carbon is at the end of a chain, bonded to at least one H. It is written –CHO (not –COH).
• In a ketone, the carbonyl carbon is inside the chain, bonded to two carbons: R–CO–R′.

An aldehyde and a ketone with the same number of carbons are functional-group isomers: propanal and propanone are both C₃H₆O.

Naming aldehydes and ketones

Aldehydes: replace the -e of the alkane with -al: methanal (HCHO), ethanal (CH₃CHO), propanal, butanal. The aldehyde carbon is always C1, so it needs no number. Branches are numbered from the CHO end: (CH₃)₂CHCH₂CHO is 3-methylbutanal.

Ketones: replace -e with -one and give the carbonyl carbon the lowest possible number: CH₃COCH₂CH₂CH₃ is pentan-2-one (2-pentanone) and CH₃CH₂COCH₂CH₃ is pentan-3-one. Propanone and butanone need no number: the C=O can only be on C2. A ketone needs at least three carbons.

Common names you will meet: formaldehyde (methanal), acetaldehyde (ethanal), acetone (propanone) and methyl ethyl ketone, or MEK (butanone), a solvent in some glues and paints.

Properties

• The C=O bond is very polar, so aldehydes and ketones have dipole-dipole forces. Their boiling points are higher than those of alkanes of similar mass.
• They have no O–H, so they cannot hydrogen-bond to each other. Their boiling points are lower than those of alcohols of similar mass.

Compare molecules of almost the same mass: butane (58 g/mol) −1 °C; propanal (58 g/mol) 48 °C; propanone (58 g/mol) 56 °C; propan-1-ol (60 g/mol) 97 °C.

• The carbonyl O can accept hydrogen bonds from water, so small aldehydes and ketones dissolve well: methanal, ethanal and propanone mix with water in all proportions; butanone dissolves to about 27 g per 100 mL. Solubility falls as the chain grows.
• Many have strong odours. Small aldehydes smell sharp; larger ones often smell fruity or flowery and are used in perfumes and flavours.
• Methanal (formaldehyde) is a gas sold dissolved in water as formalin, used to preserve biological specimens and to make resins in plywood and particleboard. It is a known human carcinogen, which is why building products in Canada have limits on how much formaldehyde they can release.

Making aldehydes and ketones: oxidizing alcohols

In organic chemistry, oxidation means a carbon compound gains O or loses H. Common oxidizing agents are acidified potassium dichromate (K₂Cr₂O₇ with H₂SO₄) and potassium permanganate (KMnO₄). In equations they are often written as [O].

To be oxidized, the carbon that holds the –OH must have at least one H to lose. So:

• Primary alcohol → aldehyde → carboxylic acid. The aldehyde forms first. If it stays in the flask with more oxidizing agent, it is oxidized again to a carboxylic acid (Lesson 10). To stop at the aldehyde, chemists distil it out as soon as it forms (aldehydes boil lower than their alcohols).
• Secondary alcohol → ketone. Ketones resist further oxidation.
• Tertiary alcohol → no reaction. The C–OH carbon has no H.

CH₃CH₂OH + [O] → CH₃CHO + H₂O

CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O

The reverse, adding H₂ to a C=O with a metal catalyst, is a reduction: aldehydes are reduced to primary alcohols and ketones to secondary alcohols.

Your body uses the same chemistry: liver enzymes oxidize ethanol to ethanal and then to ethanoic acid. Ethanal is toxic, which is one reason drinking alcohol harms the body.

Investigation: telling 1°, 2° and 3° alcohols apart (lab only)

Question: Can oxidation with acidified dichromate distinguish primary, secondary and tertiary alcohols?

Hypothesis: Primary and secondary alcohols will be oxidized (orange → green); the tertiary alcohol will not.

Method (in a school lab):

1. Put 2 mL of acidified potassium dichromate solution in each of three labelled test tubes.
2. Add 5 drops of propan-1-ol, propan-2-ol or 2-methylpropan-2-ol.
3. Warm the tubes in a hot-water bath (about 60 °C) for 5 minutes. No open flames: the alcohols are flammable.
4. Record colour changes. Carefully waft to compare odours (the aldehyde from propan-1-ol has a sharp smell).

Variables: independent: type of alcohol; dependent: colour change; controlled: volumes, temperature, time.

Safety (A1.4): potassium dichromate contains chromium(VI): it is toxic, a carcinogen and a strong oxidizer, and the solution contains sulfuric acid. Wear goggles, gloves and a lab coat; work in a fume hood; collect all chromium waste in the labelled heavy-metal container. Many schools use acidified potassium permanganate instead (purple → colourless with 1° and 2° alcohols).