Amines

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

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

Nucleophilic properties Primary amines can be formed by the nucleophilic substitution reaction between halogenoalkanes and ammonia in a one step reaction. However, as the lone pair of electrons is still available on the N in the amine formed, the primary amine can react in the same nucleophilic way in a successive series of reactions forming secondary, tertiary amines and quaternary ammonium salts. This is therefore not a good method for making a primary amine because of the further reactions. It would mean the desired product would have to be separated from the other products. CH3CH2Br + 2NH3  CH3CH2NH2 + NH4Br Forming a primary amine in a one step reaction of halogenoalkanes with ammonia Ammonia dissolved in ethanol is the initial nucleophile H3C C H H Br 3HN: H3C C H H NH3+ Br – In the first step of the mechanism the nucleophile attacks the halogenoalkane to form an intermediate H3C C H H NH2 + H :NH3 In the second step of the mechanism a second ammonia removes a proton from the intermediate (acts as base) to form the amine H3C C H H NH2 + NH4Br H3C CH2 N + CH2 CH3 CH2 CH3 H Reaction forming a tertiary amine The same reaction mechanism occurs with the secondary amine reacting to form a tertiary amine H3C CH2 NH +CH2 CH3 H Using an excess of Ammonia can limit the further subsequent reactions and will maximise the amount of primary amine formed Reaction forming secondary amine The primary amine formed in the reaction above has a lone pair of electrons on the nitrogen and will react further with the halogenoalkane. H3C C H H Br CH3CH2NH2 : H3C CH2 NH2+ CH2 CH3 Br – :NH3 H3C CH2 NH CH2 CH3 + NH4Br In this second step of the mechanism either ammonia or the amine can remove a proton from the intermediate (acts as base) to form the amine. Diethylamine H3C C H H Br : H3C CH2 NH CH2 CH3 H3C CH2 N CH2 CH3 CH2 CH3 H3C CH2 NH +CH2 CH3 CH2 CH3 :NH3 triethylamine Further reactions 3 Reaction forming a quaternary ammonium salt Only the first step of the mechanism occurs when forming the quaternary salt Using an excess of the halogenoalkane will promote the formation of the quaternary salt H3C(CH2)11 N (CH2)11CH3 (CH2)11 CH3 (CH2)11 CH3 quaternary ammonium salt + ClQuaternary Salts can be used as cationic surfactants The positive nitrogen is attracted toward negatively charged surfaces such as glass, hair, fibres and plastics. This helps in their uses as fabric softeners, hair conditioners and sewage flocculants Surfactants reduce the surface tension of liquids H3C C H H Br : H3C CH2 N CH2 CH3 CH2 CH3 H3C CH2 N + CH2CH3 CH2 CH3 CH2 CH3 Tetraethylammonium ion Quaternary ammonium salts are not amines Overall scheme of reactions Where RX is the haloalkane Using a large excess of Ammonia will maximise the amount of primary amine formed Using an excess of the halogenoalkane will promote the formation of the quaternary salt N H H H : N R H H : N R R H : N R R R : N + R R R R RX RX RX RX N Goalby chemrevise.org Some questions will involve substituting an amine onto a halogenoalkane which has a different length of carbon chain from the amine CH3CH2CH2NH2 CH3Br CH3CH2CH2 NH CH3 CH3Br CH3CH2CH2 N CH3 CH3 CH3Br CH3CH2CH2 N +CH3 CH3 CH3 Br – propylamine N-methylpropylamine N,N-dimethylpropylamine Trimethylpropyl- ammonium bromide Using excess bromomethane would promote the final quaternary salt 4 Preparing Amines from Nitriles in a 2 step reaction Using the method above of reacting halogenoalkanes and ammonia is not an efficient method for preparing a high yield of the primary amine because of the further substitution reactions that occur. A better method is to use the following 2 step reaction scheme Step 1. convert halogenoalkane to nitrile by using KCN in ethanol (heat under reflux) CH3CH2Br + CN-  CH3CH2CN + Br – Step 2. reduce nitrile to amine by using LiAlH4 in ether or by reducing with H2 using a Ni catalyst CH3CH2CN + 4[H] CH3CH2CH2NH2 Reducing nitroarenes to aromatic amines The nitro group on an arene can be reduced an amine group as follows Reagent: Sn and HCl or Fe and HCl Conditions: Heating Mechanism:reduction NO2 NH2 + 6[H] + 2H2O As the reaction is carried out in HCl the salt C6H5NH3+Cl- will be formed. Reacting this salt with NaOH will give phenylamine. phenylamine nitrobenzene See the last topic for how to form nitrobenzene from benzene. This reduction reaction can also be done with catalytic hydrogenation ( H2 using a Ni catalyst). A disadvantage of this method is that it is a two step reaction that may therefore have a low yield. Also KCN is toxic. Other reactions of amines Aliphatic amines and phenylamine can react with acyl chlorides and acid anhydrides to form amides in a nucleophilic addition-elimination reaction- see previous chapter on reactions of C=O bond for more details. N Goalby chemrevise.org Change in functional group: acyl chloride  secondary amide Reagent: primary amine Conditions: room temp. Change in functional group: acid anhydride  secondary amide Reagent: primary amine Conditions: room temp. RCOCl +2CH3NH2  RCONHCH3 + CH3NH3+Cl- (RCO)2O +2CH3NH2  RCONHCH3 + RCO2 -CH3NH3+ CH3 C O Cl+ 2CH3NH2  + CH3NH3+Cl- CH3 C O O CH3 C O +2CH3NH2  + CH3CO2 -CH3NH3+ N-methylethanamide N-methylethanamide