Aromatic chemistry and amines definitions

Benzene: its structure and the evidence for delocalisation

Kekulé structure

A model of benzene as a ring of six carbon atoms joined by alternating single and double bonds, cyclohexa-1,3,5-triene.

Delocalised electrons

Electrons that are shared between more than two atoms and are not fixed in one bond.

Delocalised model of benzene

A planar ring of six carbon atoms joined by σ bonds, in which a p orbital on each carbon overlaps with those on both neighbours, giving a ring of six delocalised electrons above and below the plane.

Arene

A hydrocarbon containing one or more benzene rings; compounds containing a benzene ring are called aromatic.

Enthalpy of hydrogenation

The enthalpy change when one mole of an unsaturated compound reacts completely with hydrogen to give a saturated compound.

Delocalisation energy

The difference in energy between benzene and the Kekulé structure, 152 kJ mol⁻¹, by which the delocalised ring is more stable; also called resonance energy.

Phenyl group

The C₆H₅– group, a benzene ring with one hydrogen atom removed, attached to the rest of a molecule.

Phenol

A compound with an OH group bonded directly to a benzene ring; also the name of the simplest, C₆H₅OH.

Phenoxide ion

The ion C₆H₅O⁻ formed when phenol loses a proton, its negative charge delocalised into the ring.

Halogenoarene

A compound in which a halogen atom is bonded directly to a carbon atom of a benzene ring, such as chlorobenzene.

Electrophilic substitution: nitration and Friedel–Crafts acylation

Electrophile

An electron pair acceptor: a species that accepts a pair of electrons to form a new covalent bond.

Electrophilic substitution

A reaction in which an electrophile replaces an atom, usually hydrogen, on an electron-rich compound such as benzene.

Nitronium ion

The electrophile in nitration, NO₂⁺, formed when concentrated sulfuric acid protonates nitric acid, which then loses water.

Acylium ion

The electrophile in Friedel–Crafts acylation, RCO⁺, with the positive charge on the carbonyl carbon.

Halogen carrier

A compound such as AlCl₃ that accepts a lone pair from a halogen atom and so generates a positive electrophile.

Friedel–Crafts acylation

The electrophilic substitution of an acyl group, RCO, onto a benzene ring, using an acyl chloride and an aluminium chloride catalyst, giving a ketone.

Friedel–Crafts alkylation

The electrophilic substitution of an alkyl group onto a benzene ring, using a halogenoalkane and an aluminium chloride catalyst.

Activating group

A group that releases electron density into a benzene ring, so that the ring reacts with electrophiles faster than benzene; OH, NH₂ and alkyl groups direct to positions 2, 4 and 6.

Deactivating group

A group that withdraws electron density from a benzene ring, so that the ring reacts with electrophiles more slowly than benzene; NO₂, COOH and COR direct to position 3.

Amines: preparation, basicity and nucleophilic reactions

Primary amine

An amine in which the nitrogen atom is bonded to one carbon group, RNH₂.

Secondary amine

An amine in which the nitrogen atom is bonded to two carbon groups, R₂NH.

Tertiary amine

An amine in which the nitrogen atom is bonded to three carbon groups, R₃N.

Quaternary ammonium salt

An ionic compound containing a nitrogen atom bonded to four carbon groups and carrying a positive charge, R₄N⁺, with a negative ion.

Cationic surfactant

A quaternary ammonium salt with a long hydrocarbon chain, whose positive head is attracted to negatively charged surfaces such as fabric and hair.

Positive inductive effect

The release of electron density by an alkyl group towards the atom it is bonded to.

Nucleophile

An electron pair donor: a species with a lone pair of electrons that it donates to an electron-deficient atom to form a covalent bond.

Diazonium ion

An ion containing the –N₂⁺ group bonded to a benzene ring, such as C₆H₅N₂⁺, made from an aromatic amine and nitrous acid below 10 °C.

Azo compound

A compound in which two benzene rings are joined by an –N=N– group, made by coupling a diazonium ion with a phenol or an aromatic amine.

Amino acids, proteins, enzymes and DNA

Amino acid

A compound with an amine group and a carboxylic acid group; in the α-amino acids that make proteins both are on the same carbon atom.

Zwitterion

An ion with both a positive and a negative charge and no overall charge, as in ⁺H₃NCH(R)COO⁻.

Isoelectric point

The pH at which an amino acid exists as the zwitterion and has no overall charge.

Peptide link

The –CONH– group that joins two amino acids, formed by a condensation reaction that eliminates water.

Primary structure

The sequence of amino acids in a polypeptide chain.

Secondary structure

The coiling of a polypeptide chain into an α-helix or its folding into a β-pleated sheet, held by hydrogen bonds between peptide links.

Tertiary structure

The folding of the whole polypeptide chain into a three-dimensional shape, held by hydrogen bonds, ionic bonds and disulfide bridges between side chains.

Active site

The region of an enzyme, formed by its tertiary structure, to which the substrate binds and where the reaction is catalysed.

Stereospecific active site

An active site that can bond to only one enantiomeric form of a substrate or drug, because the site itself is chiral.

Nucleotide

The monomer of DNA, made of a phosphate group, the sugar 2-deoxyribose and one of four bases.

Electrophoresis

The separation of charged species, such as amino acids in a buffer of fixed pH, by their movement towards the oppositely charged electrode when a potential difference is applied.