Carbonyl group


Properties of Carbonyls

Carboyls have a C bonded to an O. There is a large difference in electronegativity between these two atoms and so the C is partially positive and the O is partially negative.
There is a dipole-dipole force between the two atoms and therefore they have higher boiling points than alkanes of a similar molecular mass (who only have van der Waals forces). Carbonyl compounds have lower boiling points than alcohols (who have hydrogen bonding).

Carbonyl compounds are soluble in water because they are polar. As chain length increases, solubility decreases.

The double bond is strong, but it is still very reactive as the carbon is partially positive, so is attacked by nucleophiles, and the double bond means that the compound is not saturated. Nucleophilic addition reactions occur.

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Carbonyl reactions

The nucleophile attacks the partially positive carbon, and the high electron-density double bond gives an electron to the partially negative oxygen. The negative ion atom gives an electron to an H+ ion (given from the solution) to form an alcohol with a nucleophile attached.

Using hydrogen cyanide as the nucleophile allows the carbon chain length to increase by one. this produces a hydroxynitrile.

Aldehydes can be oxidised to carboxylic acids, but ketones cannot because a C--C bond must be broken. A common oxidising agent is potassium dichromate (vi).

Fehlings test --> made from Cu2+ ion, an alkali and a complexing agent. The aldehyde creates a brick red precipitate of copper (i) oxide whereas no reaction is seen with a ketone.

Silver mirror test --> Tollen's reagent is used, which is made of [Ag(NH3)2]+. The complex ion is reduced to Ag + NH3 and the aldehyde is oxidised to a carboxylic acid.

NaBH4 is a reducing agent, used in the reduction of aldehydes and ketones to alcohols.The hydride ion (H-) is the nucleophile.

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Carboxylic acids and Esters

Carboxylic can form hydrogen bonds as there is a H attached to an O atom. This can happen in the solid state and therefore has a very high melting point. It also forms hydrogen bonds in water and so is very soluble.

An ester is made by reacting an alcohol with a carboxylic acid. It is named using the alcohol then the acid: ethanoic acid + methanol --> methyl ethanoate.

Carboxylic acids are polarised. The H from OH can be lost to form a carboxylate ion and H+. The negative charge on the carboxylate ion is shared over the carbonyl group so it is stable.

Carboxylic acids can react with sodium hydrogencarbonate to form an ionic salt, carbon dioxide and water.

Carboxylic acid with...

alkali --> ionic salt, water

metal carbonates --> ionic salt, water, carbon dioxide

Esters are made with a strong acid catalyst. It is a reversable reaction.

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Esters can be hydrolysed as the water can act as a nucleophile and attacks the partially positive carbon atom. This produces a carboxylic acid and an alcohol. This is kept in equilibrium.

This hydrolysis occurs at room temperature when a strong acid catalyst is used.

Bases can also catalyse this, but the salt of the acid is produced instead of the acid itself. An equilibrium is not established and so the reaction goes to completion.

Methyl ethanoate + water <---> ethanoic acid + methanol.

ethanoic acid + methanol + NaOH --> sodium ethanoate + water.

Fats and oils are made of esters. These consist of 3 long-chain molecules of carboxylic acid called fatty acids.

Fats and oils are hydrolysed by boiling with NaOH. Glycerol and sodium salts are made. The salts are soaps. They are ionic and dissociate to form Na+ and RCOO-.

RCOO- has two ends: a hydrocarbon chain and a COO- group. The hydrocarbon chain mixes with grease whilst the COO- mixes with water. It is therefore a cleaning agent.

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