Aldehydes and ketones

Aldehydes and ketones are organic compounds containing the carbonyl functional group, C=O. The key difference lies in the position of this group. In an aldehyde, the carbonyl carbon is bonded to at least one hydrogen atom, meaning the C=O group is always at the end of a carbon chain. In a ketone, the carbonyl carbon is bonded to two other carbon atoms, so the C=O group is within a carbon chain. The C=O bond is polar (Cδ+ and Oδ−), which is the source of their reactivity. To name aldehydes, replace the '-e' from the parent alkane with '-al' (e.g., propane becomes propanal). For ketones, replace '-e' with '-one' and use a number to indicate the position of the C=O group if the chain is longer than four carbons (e.g., pentan-2-one).

Aldehydes and ketones are typically formed by the oxidation of alcohols. The type of alcohol determines the product. Primary alcohols are oxidised to form aldehydes. To prevent further oxidation to a carboxylic acid, the aldehyde must be distilled off as it forms. This works because the aldehyde has a lower boiling point than the alcohol (due to lack of hydrogen bonding). Secondary alcohols are oxidised to form ketones. Since ketones are resistant to further oxidation, the reaction can be carried out under reflux to ensure a high yield. The standard oxidising agent for these reactions is acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄), which turns from orange (Cr₂O₇²⁻) to green (Cr³⁺) as the reaction proceeds.

Reduction of carbonyl compounds is the reverse of the oxidation of alcohols. Aldehydes are reduced to form primary alcohols, and ketones are reduced to form secondary alcohols. This is a form of addition reaction where two hydrogen atoms are added across the C=O double bond. The reaction requires a reducing agent. Two common reducing agents used in the lab are sodium borohydride (NaBH₄) and lithium aluminium hydride (LiAlH₄). NaBH₄ is safer and can be used in aqueous or alcoholic solution. LiAlH₄ is a much more powerful reducing agent and reacts violently with water, so it must be used in a dry, non-reactive solvent like dry ether.

The polar Cδ+=Oδ− bond in carbonyls allows them to undergo nucleophilic addition. The carbon atom is electron-deficient and is attacked by nucleophiles. A good example is the reaction with hydrogen cyanide (HCN). The cyanide ion, :CN⁻, acts as the nucleophile, attacking the δ+ carbon atom and breaking the C=O π-bond. This forms a negatively charged intermediate, which is then protonated by H⁺ to form a 2-hydroxynitrile. Because liquid HCN is extremely toxic and volatile, the reaction is usually carried out in situ by mixing sodium or potassium cyanide (NaCN or KCN) with a dilute acid (e.g., H₂SO₄). This reaction is important in synthesis as it increases the length of the carbon chain by one carbon.

A simple chemical test can confirm the presence of a carbonyl group in a compound. The reagent used is 2,4-dinitrophenylhydrazine, often abbreviated as 2,4-DNPH or called Brady's reagent. When a few drops of 2,4-DNPH solution are added to a sample containing an aldehyde or a ketone, a bright orange or yellow precipitate is formed. This precipitate is a 2,4-dinitrophenylhydrazone derivative. This is a positive test for the carbonyl group. Alcohols, carboxylic acids, and esters do not give this result. The test is very reliable. Furthermore, the solid precipitate can be filtered, purified by recrystallisation, and its melting point determined. By comparing this melting point to a database of known values, the specific identity of the original aldehyde or ketone can be determined.

Once you've confirmed a carbonyl group is present using 2,4-DNPH, you can distinguish between an aldehyde and a ketone using mild oxidising agents. Aldehydes are easily oxidised to carboxylic acids, whereas ketones are resistant to oxidation.

1. Tollens' Reagent: This is a solution of ammoniacal silver nitrate, containing the [Ag(NH₃)₂]⁺ complex ion. When warmed with an aldehyde, the aldehyde is oxidised, and the Ag⁺ ions are reduced to metallic silver, forming a beautiful 'silver mirror' on the inside of the test tube. Ketones do not react, so the solution remains colourless.

1. Fehling's Solution: This is an alkaline solution containing copper(II) ions complexed with tartrate ions, which is deep blue. When warmed with an aldehyde, the aldehyde is oxidised, and the blue Cu²⁺ ions are reduced to form a brick-red precipitate of copper(I) oxide (Cu₂O). Ketones do not react, so the solution remains blue.

The tri-iodomethane (or iodoform) test is a specific chemical test used to identify the presence of certain structural features in a molecule. It gives a positive result for any compound containing a methyl ketone group (CH₃-C=O) or a structure that can be oxidised to a methyl ketone under the reaction conditions. This includes ethanol (CH₃CH₂OH) and secondary alcohols with the structure CH₃CH(OH)-R. The reagents are alkaline aqueous iodine (iodine dissolved in sodium hydroxide solution). When a compound gives a positive test, a pale yellow precipitate of tri-iodomethane (CHI₃) is formed. This solid has a characteristic 'antiseptic' or 'hospital' smell.

Infrared (IR) spectroscopy is an analytical technique used to identify functional groups in a molecule. When a molecule is exposed to IR radiation, its bonds vibrate by stretching and bending at specific frequencies. The frequencies that are absorbed are recorded on a spectrum. The carbonyl group (C=O) has a very strong, sharp absorption band in a characteristic region of the IR spectrum, between 1680–1750 cm⁻¹. This makes it one of the easiest functional groups to identify. While the C=O peak confirms a carbonyl, other peaks can help distinguish between aldehydes and ketones. Aldehydes show two weak but distinct C-H stretching absorptions around 2720–2820 cm⁻¹ and 2820–2920 cm⁻¹, which are absent in ketones. A very broad absorption from 2500-3300 cm⁻¹ would indicate an O-H group in a carboxylic acid, which also contains a C=O group.