Amines and Amino Acids
Properties
Amines have the functional group N. We have primary, secondary and tertiary amines; their structures are outlined in the diagram below.
Amines are basic because of a lone pair of electrons on the nitrogen atom, this means it can accept protons (hydrogen ions) and is therefore a Brønsted-Lowry base.
The strength of the base is related to the availability of the lone pair and hence the electron density. Ammonia (an amine with H as R) is a weaker base than primary amines because the alkyl group pushes electrons towards the lone pair.
However, aromatic amines, are less basic than ammonia, because the delocalized electrons in the benzene ring reduce the electron density.
Making Amines
Amines can be made by the nucleophillic substitution of ammonia and amines with haloalkanes, the reaction meachanism is as follows.
However, as discussed above, primary amines have the nitrogen lone pair even more available than ammonia, this means that further substitution will occur to produce secondary, tertiary amines and quaternary ammonium salts. These are used as cationic surfactants which have a variety of applications, including in fabric softeners, paints and laxatives.
Primary amines can also be made by reacting a haloalkane with cyanide. This substitutes the Br to produce a nitrile: which has the functional group -C≡N, and this is then reduced to an amine. Note how the carbon chain increases by one in this method.
RBr + CN- ® RC≡N + Br-
RC≡N + 2H2 ® RCH2NH2
Aromatic amines can be produced by the reduction of C6H6NO2, by using a Nickel catalyst.
C6H6NO2 + 3H2 ® C6H6NH2 + 2H2O
Amino Acids
Below is the structure of an amino acid, where the R represents a side chain. Notice how the amino acid has two functional groups: the amine and carboxylic acid group.
The way that amino acids are named, is that the carbon in the carboxylic group is carbon 1. Alpha (α) amino acids have the amine on the second carbon; β (beta) amino acids have the amine on the third carbon and so on. However, all naturally occuring amino acids (those that are used in the body in proteins) are α-amino acids.
The amino acid is charged differently according to the pH conditions it's in. If it is in acidic conditions there is an abundance of H+ so the nitrogen lone pair accepts a proton. However if it is in alkaline conditions the H+ in the COOH is donated.
At a certain pH (specific to each amino acid), the isoelectric point is reached, and a zwitterion is formed. This has both a positive charge on the nitrogen and a negative charge on the oxygen.
Proteins
A protein is a naturally occuring polymer of amino acids. They are formed by a condensation reaction which forms a peptide bond between two amino acids, and water as a byproduct.
The reverse of the condensation reaction is hydrolysis, which requires water and will break a protein in to its constituent amino acids.
The 3D structure of proteins is what gives them their biological properties, this is determined by hydrogen bonding. So the different sequences of amino acids produces atoms in different places and hence, a unique shape.