A-level Chemistry/WJEC/Module 4/Aldehydes and ketones

Aldehydes () and ketones () are both carbonyl compounds. Aldehydes are organic compounds in which the carbonyl carbon is connected to an H atom, while a ketone has both its vacancies satisfied by carbon.

Naming Aldehydes and Ketones

Ketones are named by replacing the -ane in the alkane name with -one. The carbon chain is numbered so that the ketone carbon, called the carbonyl group, gets the lowest number.

Alternatively, functional class nomenclature of ketones is also recognised by IUPAC, which is done by naming the substituents attached to the carbonyl group in alphabetical order, ending with the word ketone. Butanone can also be named ethyl methyl ketone ("EMK") using this method.

If two ketone groups are on the same structure, the ending -dione would be added to the alkane name, such as heptane-2,5-dione.

Aldehydes replace the-ane ending of an alkane with -anal for an aldehyde. Since an aldehyde is always at the carbon that is numbered one, a number designation is not needed. For example, the aldehyde of pentane would simply be pentanal.

Boiling Points and Bond Angles

Aldehyde and ketone polarity is characterised by the high dipole moments of their carbonyl group, which makes them rather polar molecules. They are more polar than alkenes, though because they lack hydrogen, they cannot participate in hydrogen bonding like alcohols, thus making their relative boiling points higher than alkenes, yet lower than alcohols.

Typical bond angles between the carbonyl group and its substituents show minor deviations from the trigonal planar angles of 120 degrees, with a slightly higher bond angle between the O=C-R bond than the R-C-R bond on the carbonyl carbon (with R being any substituent).

Preparing Aldehydes and Ketones

Partial oxidation of primary alcohols to aldehydes

A primary alcohol can be oxidised into an aldehyde using acidified potassium dichromate(VI) and heating with distillation. The aldehyde has a lower boiling point than the alcohol and so it can be distilled off as it forms. If reflux apparatus is used, the aldehyde cannot escape the oxidising mix and is oxidised fully to a carboxylic acid.

The orange dichromate(VI) ion, Cr₂O₇^2-, is reduced to the green Cr³⁺(aq) ion.

Aldehyde synthesis by oxidation. Oxidation can be shown as loss of electrons, a loss of hydrogen atoms [H], or a gain of oxygen atoms [O].

Oxidation of secondary alcohols to ketones

A secondary alcohol can be oxidised into a ketone using acidified potassium dichromate(VI) and heating under reflux.

The orange dichromate(VI) ion, Cr₂O₇^2-, is reduced to the green Cr³⁺(aq) ion.

Oxidation of a ketone. R¹ and R² are alkyl and/or aryl groups e.g. -CH₃, -CH₂CH₃ or -C₆H₅.

Reactions of Aldehydes and Ketones

Oxidation

Aldehydes but not ketones may be oxidised into carboxylic acids using strong oxidising agents. Typical reagents include

• Tollens' Reagent (Ag₂O in aqueous ammonia forms the [Ag(NH₃)₂]⁺ ion)
• acidified dichromate
• Benedict's/Fehling's reagent (essentially alkaline Cu²⁺).

This is one way to test for the presence of an aldehyde in a sample compound; An aldehyde will become a carboxylic acid when reacted with a suitable oxidising agent, but a ketone will not react.

Using Benedict's Reagent to test for aldehydes: The blue copper(II) citrate ([CuC₆H₅O₇]^–, left) is converted to a red precipitate of copper(I) oxide (Cu₂O, right). The intermediate colours are mixtures of copper(II) and copper(I) oxide. Fehling's Reagent is based on copper(II) tartrate ([Cu(C₄H₄O₆)₂]^2–) but gives exactly the same results.

Using Tollen's Reagent to test for aldehydes: The [Ag(NH₃)₂]⁺ ions of the original colourless solution (right) have been reduced to Ag by the aldehyde. The silver coats the inside of the test tube (left).

Nucleophilic addition

Since aldehydes and ketones contain a polar carbonyl group, the partially positive carbon atom can act as an electrophile. Nucleophiles are able to attack this carbonyl carbon, resulting in a net addition to the molecule.

With cyanide, nucleophilic addition occurs to give a hydroxynitrile:

R¹R²C=O + :CN^– + H⁺ → R¹R²C(OH)CN

e.g. propanal → 2-HydroxyButaneNitrile

Mechanism of nucleophilic addition to aldehyde or ketone. You need to learn this mechanism. The nucleophile is usually cyanide (:CN^–) which adds a carbon to the chain.

The condensation reaction with 2,4-DNPH

2,4-DNPH ("Brady's Reagent") will react with aldehydes and ketones to form distinctive orange/red precipitates. This is a clear test for aldehydes and ketones. The reactions with Tollen's/Benedict's/Fehling's reagents (see above) will indicate if the compound is an aldehyde or a ketone.

Reaction of 2,4-DNPH with cyclohexanone.

The 2,4-DNPH derivatives are easy to purify. The melting points of the derivatives were commonly used to identify the compound which has produced them.

Aldehydes, ketones and the melting points of their 2,4-DNPH derivatives:

• ethanal 168 °C
• propanal 150 °C
• butanal 123 °C
• pentanal 98 °C
• propanone 127 °C
• butanone 117 °C
• pentan-2-one 144 °C
• pentan-3-one 156 °C

Reduction

Both NaBH₄(aq) and LiAlH₄(ether) will reduce aldehydes and ketones to primary and secondary alcohols respectively.

e.g. CH₃CH₂CHO + 2[H] → CH₃CH₂CH₂OH

e.g. CH₃COCH₃ + 2[H] → CH₃CHOHCH₃

Iodoform test

Alkaline iodine solution reacts with specific organic structures to form CHI₃, "iodoform". Iodoform is a pale yellow precipitate with a distinctive "antiseptic" smell. Alkaline iodine can be made directly with iodine and sodium hydroxide solutions, or made in situ by reacting solutions of iodide and the oxidising agent sodium chlorate(I).

Iodoform in a test tube. In a typical test, iodoform appears as a precipitate in a solution.

Negative (left) and positive (right) iodoform test results.

If iodoform is produced, the organic structure is either a methyl ketone (CH₃CO-) or a methyl alcohol (CH₃CHOH-). Examples of compounds which produce iodoform include propanone and propan-2-ol, ethanal and ethanol. Propanal and propan-1-ol will not make iodoform. Pentan-2-one and pentan-2-ol will make iodoform, but pentan-3-one and pentan-3-ol will not.