Amines These end in –amine. There is, however, rather confusingly two ways of using this suffix. The exam board tend to use the common version where the name stem ends in -yl propylamine. The IUPAC version of the same chemical is propan-1-amine. (This is used in the same way as naming alcohols) If there is another priority functional group as well as the amine group then propylamine the prefix amino is used. Or propan-1-amine 2-aminopropanoic acid. H3C CH C O O H H2N If the amine is secondary and has two alkyl groups attached to the nitrogen, then each chain is named and the smaller alkyl group is preceded by an –N which plays the same role as a number in positioning a side alkyl chain CH3CH2CH2NHCH3 N-methylpropylamine (common name) N-methylpropan-1-amine (IUPAC name) Diethylamine (common name- does not use N if chains are same length) N-ethylethanamine (IUPAC name does still use N). In the common naming version if the chain lengths are the same an –N is not used CH3CH2CH2 N CH3 CH3 If a tertiary amine similar rules apply, and each alkyl side group is given an N N,N-dimethylpropylamine (common name) N,N-dimethylpropan-1-amine (IUPAC name) If there are two amine groups then it is easiest to use amino prefix N H (CH2 )6 N H H H It could also be named 1,6-diaminohexane
Nomenclature (not explicitly stated but necessary for subsequent topics)
Primary aliphatic amines act as Bronsted-Lowry Bases because the lone pair of electrons on the nitrogen is readily available for forming a dative covalent bond with a H+ and so accepting a proton. Primary aliphatic amines are stronger bases than ammonia as the alkyl groups are electron releasing and push electrons towards the nitrogen atom and so make it a stronger base. Base strength of aromatic amines Primary aromatic amines such as Phenylamine do not form basic solutions because the lone pair of electrons on the nitrogen delocalise with the ring of electrons in the benzene ring. This means the N is less able to accept protons. Base Properties Amines as bases react with acids to form ammonium salts. CH3NH2 (aq) +HCl (aq) CH3NH3+Cl- (aq) Methylamine methylammonium chloride Addition of NaOH to an ammonium salt will convert it back to the amine The ionic salts formed in this reaction means that the compounds are soluble in the acid. e.g. Phenylamine is not very soluble in water but phenylammonium chloride is soluble These ionic salts will be solid crystals, if the water is evaporated, because of the strong ionic interactions. Secondary amines are stronger bases than primary amines because they have more alkyl groups that are substituted onto the N atom in place of H atoms. Therefore more electron density is pushed onto the N atom (as the inductive effect of alkyl groups is greater than that of H atoms). One might expect using the same trend that tertiary amine would be the strongest amine base but the trend does not hold. The tertiary amines and corresponding ammonium salts are less soluble in water and this makes them less strong bases than the secondary amines. (This point will not be examined) NH2 phenylamine Reactions with acids Making a basic buffer from an amine Basic buffers can be made from combining a weak base with a salt of that weak base e.g. Ammonia and ammonium chloride Methylamine and methylammonium chloride Ethylamine and ethylammonium chloride. Overall order of base strength Aromatic amines < ammonia< primary amines< tertiary amines< secondary amines Weaker bases Stronger bases
18.104.22.168 Base properties (A-level only)
Amines are weak bases.
The difference in base strength between ammonia, primary aliphatic and primary aromatic amines.
Students should be able to explain the difference in base strength in terms of the availability of the lone pair of electrons on the N atom.
22.214.171.124 Preparation (A-level only)
Primary aliphatic amines can be prepared by the reaction of ammonia with halogenoalkanes and by the reduction of nitriles.
Aromatic amines, prepared by the reduction of nitro compounds, are used in the manufacture of dyes.
126.96.36.199 Nucleophilic properties (A-level only)
Amines are nucleophiles.
The nucleophilic substitution reactions of ammonia and amines with halogenoalkanes to form primary, secondary, tertiary amines and quaternary ammonium salts.
The use of quaternary ammonium salts as cationic surfactants.
The nucleophilic addition–elimination reactions of ammonia and primary amines with acyl chlorides and acid anhydrides.
Students should be able to outline the mechanisms of:
• these nucleophilic substitution reactions
• the nucleophilic addition–elimination reactions of ammonia and primary amines with acyl chlorides.