Functional Groups (Cambridge (CIE) A Level Chemistry): Revision Note

Functional Groups of Organic Compounds (AL)

• Many organic compounds contain one or more functional groups

• A functional group is an atom or group of atoms in an organic molecule, that determines its characteristic chemical and physical properties

Arenes

• Arenes are aromatic compounds that contain a benzene ring

The arene functional group

Chemical properties

• Due to the delocalised electron ring (π system of electrons), these compounds are electron-rich and therefore can undergo electrophilic attack under the right conditions

• However, because the delocalised electron ring system makes benzene so stable, it is resistant to addition reactions

• This is very different to alkenes, which are very reactive and readily undergo addition reactions

Physical properties

• Benzene has van der Waals dispersion forces of attraction between the molecules and has a boiling point of 80 C

• The presence of the non-polar hydrocarbon part in the arene functional group means that these compounds are often insoluble in water

• Benzene would have to break many hydrogen bonds between the water molecules to be soluble in water, which does not happen as it is not energetically favourable

Halogenoarenes

• These are aromatic compounds that contain a halogen bonded to a benzene ring

The halogenoarene functional group

• They are also known as aryl halides

Chemical properties

• These compounds are also prone to electrophilic attack because of the π system of delocalised electrons

• The halogens can also take part in substitution reactions

Physical properties

• Chlorobenzene, bromobenzene and iodobenzene are all liquid at room temperature with an oily texture

• As you might expect, the boiling points increase as the size of the halogen attached increases, because the number of electrons within the molecule increases

• Like other arenes, halogenoarenes are insoluble in water because of the non-polar hydrocarbon part of the ring

• These molecules are large relative to the size of water molecules, and as with the arenes it is not energetically favourable for the halogenoarene molecules to break the hydrogen bonds between the water molecules, so it does not happen

Phenols

• Phenols are another type of aromatic compounds containing a hydroxide bonded to a benzene ring

The phenol functional group

Chemical properties

• The -OH group in phenols is more acidic than in alcohols as the oxygen donates one of its lone pairs of electrons into the ring system

• This causes an increased electron density of the ring, causing it to become much more reactive than benzene itself

• It also makes it easier for the hydrogen of the -OH group to be donated

• Phenols can also react with reactive metals such as sodium to form alkoxide ions

Physical properties

• Phenol is a white, crystalline solid, and it has a disinfectant-like smell

• Due to the -OH group in phenols, they can form hydrogen bonds with water molecules, and therefore to a degree phenol is soluble in water

Acyl chlorides

• Acyl chlorides are (carboxylic) acid derivatives containing:

  • A chlorine atom attached to a C=O group (replacing what would have been the -OH group of a carboxylic acid)

  • An acyl (hydrocarbon) group attached to a C=O group

The acyl chloride functional group

• Acyl chlorides are also known as 'acid' chlorides

Chemical properties

• They are fuming liquids and are colourless, with a strong smell

• Acyl chlorides are extremely reactive and readily take part in substitution reactions in which the chlorine atom is substituted by other species

• This reactivity is why they are fuming liquids and why they have such a strong smell - they react with any water vapour in the air

Physical properties

• Acyl chlorides react violently with water, so we cannot say whether or not they would be soluble in water

Amines

• Amines are compounds with the -NH₂ (primary amine), -NH (secondary amine) or -N (tertiary amine) group

 The amine functional group

• Classification of amines

  • In primary amines, the N of the amine group is bonded to one R group (and two hydrogen atoms)

  • In secondary amines, the N of the amine group is bonded to two R groups (and one hydrogen atom)

  • In tertiary amines, the N of the amine group is bonded to three R groups

Chemical properties

• Due to the lone pair of electrons on the nitrogen, amines are basic compounds

Physical properties

• The lone pair on the N of the amine group means that they can form hydrogen bonds

• They are often soluble in water because they form hydrogen bonds with water molecules

• The smaller amines are very soluble in water, but their solubility decreases as the non-polar hydrocarbon chain gets longer

• They often have a fishy smell, especially as the size of the amines increases

Amides

• Amides are compounds containing:

  • An amine (-NH₂) group

  • A carbonyl group (C=O)

  • The amide group is -CONH₂

The amide functional group

Chemical properties

• Amides are less basic than amines, as the lone pair of electrons on the nitrogen is delocalised

Physical properties

• Amides are often soluble in water as they can form hydrogen bonds with water molecules

• The smaller amides are very soluble in water, but their solubility decreases as the non-polar hydrocarbon chain gets longer

Amino Acids

• Amino acids are the building blocks of proteins and consists of:

  • An amine (-NH₂) group

  • A carboxyl (-COOH) group

The α-amino acid functional group

Chemical properties

• Amino acids react with bases to form salts

• They also react with alcohols to form esters

• The reaction of amino acids with amines gives amides

Physical properties

• Most of the amino acids are soluble in water but insoluble in organic solvents

• Amino acids have chiral centres and exhibit optical isomers (except for glycine)