4.6 Amines

Structure of an amine

• Contain a nitrogen atom bonded directly to a carbon atom of a fully alkyl or aryl group

• if the nitrogen atom is bonded to one carbon the compound is a primary amine

• secondary aminr have the nitrogen atom bonded to two carbon etc

Making primary aliphatic amines from halogenoalkanes

• Reacted with ammonia using water/ethanol solvent to produce an amine

• CH3CH2CH2Br + NH3 = CH3CH2CH2NH2 + HBr

If mixture is warmed

Ammonia gas is lost from the mixture reducing the yield

an excess of ammonia is used to react with acidic gas hydrogen bromide to give ammonium bromide

if in excess of the halogenoalkane is used so the substitution can occur giving a secondary amiine

• CH3CH2CH2NH2 + CH3CH2CH2Br — (CH3CH2CH2)2NH + HBr

further reaction occurs because the a min reacts as a base and attack speed positive carbon atom in the carbon- bromine Bond

nucleophilic substitution

Nucleophilic substitution in the formation of primary aliphatic amines

Propyl ammonium bromide

CH3CH2CH2NH2 + HBr — CH3CH2CH2+NH3 Br-

propyl ammonium bromide is a substituted ammonium bromide with one of the hydrogen atoms replaced with a propyl group

if an ammonium salt is heated with the base, ammonia is produced.

Propyl ammonium bromide heated with aqueous sodium hydroxide

Propyl amine is obtained

CH3CH2CH2+NH3 Br- + NaOH — CH3CH2CH2NH2 +NaBr + H2O

Making primary amines from nitriles

Nitriles can be reduced with a reducing agent, typically lithium tetra hydridoaluminate

dissolved in ethoxyethane as a solvent

Formation of aromatic a

amines from nitrobenzines

Benzene is not easily attacked by nucleophiles like ammonia

this is because the ring electrons repel nucleophiles but are attracted to electrophiles

eg. Phenylamine

Traditional reducing agent of this reaction

• Tin metal and hydrochloric acid

• after the initial fast reaction is over, the mixture is heated to 100 degrees for 30 minutes

• product at this stage is not phenyle aiming but contains a complex tin (IV) salt of formula: (C6H5NH3)2 SnCl6

After cooling the mixture

Aqueous sodium hydroxide is added to decompose the salt in phenyl aimine, sodium chloride and tin (lV) oxide

mixture is steam distilled to produce a distillate containing phenylamine and water

the immiscible phenylamine is separated and dried to remove traces of water

redistilled to reduce pure phenylamine

Basicity of amines

Like ammonia

have lone pair of electrons on the nitrogen atom these can be used to accept a proton by means of a coordinate bond

Amines are weak bases

Equilibrium position is well to the left

fishy strong smell indicates free methylamine in present in the solution

Methylamines and other alkylamines are stronger bases than ammonia

electrons

Because the alkyl groups ‘push’ electrons slightly towards the nitrogen atom making it more delta negative when compared with ammonia

Phenylamine is a much weaker base than ammonia

As the nitrogen lone pair of electrons become part of the delocalized pi system

this makes the nitrogen relatively less delta negative

mean is more susceptible to electrophilic ring substitution than benzene because of the nitrogen lone pair effect

although the NH2 group of amines can hydrogen bond with water, phenylamine is only slightly soluble in water because of the hydrophobic effect of the benzene ring

will react with acids to form salts

Ethanolation of primary amines

The nitrogen lone pair enables amines to react as nucleophiles

they attack the Delta positive carbon atom in the carbonyl group

in an acid chloride

Formation of Nmethyl ethanamide

N= methyl group is bonded to the nitrogen atom this compound is an n substituted derivative of ethanamide (CH3CONH2)

Phenyl reacts similar to produce N Phenylethanamide

The NHCO is a peptide link/ peptide bond

eg. paracetemol

Reaction of cold nitric acid with primary amines

Nitric acid, HNO3, is an unstable compound and is made when required by the reaction of dilute acid (eg HCL) on sodium nitrate (lll) NaNO2

NaNO2 + HCL — HNO3+ H2O

Primary aliphatic amine react with nitric acid

R-NH2 + HNO2 — R-OH + N2 + H2O

yeild of alcohol is poor

1 propylcarbocation

The products of 1 propylcarbocation suggest that is formed as an intermediate which can undergo isomerism to 2 propylcarbocation

or else lose protons to give the alkene

reaction may initially form an alkyl diazonium ion ( R—N+) which loses nitrogen to give a primary carbocation

Phenylamine Diazonium

Primary aromatic amine

most stable than alkyl diazonium Ion and if the temperature is between 0 to 10 degrees: a solution containing benzene diazonium ion is produced

Aq Sulfuric acid instead of hydrochloric acid

The intermediate is benzene diazonium hydrogen sulfate rather than benzene daiazonium chloride

above 10° decomposition of the benzene diazonium compound occurs giving phenol

if the reaction is performed under 0 degrees the production of benzene diazonium compound is too slow

Coupling reactions of benzene diazonium salts

Benzene diazonium compounds are very reactive and can be used as intermediate in the formation of other compounds

in the formation of phenol from pension diosonium chloride the nitrogen atoms are lost as nitrogen gas, as the unstable ion decomposers

Below 10° benzene diazonium compounds

React with phenols and aromatic a means to produce compounds with theN=N p- azo group is retained

Benzenediazonium Ion is a weak electrophile

Will bond, by electrophilic substitution, with aromatic compounds where the ring has been activated by the presence of hydroxide or amine groups.

The resulting compound is highly colored and is called an azo dye

Coupling occurs at the 4 position and the 2 position relative to the hydroxide or amine

Coupling occurs with other aromatic systems

eg. Naphthalene-2-ol

Azo dye forms as a red solid and is used under the name Sudan1

Chromophere

A structural unit in a molecule that is primarily responsible for the absorption of radiation of a certain wavelength

generally in the visible or a ultraviolet region

If the absorption is in visible region

The colour that is not absorbed is the color observed

the actual wavelength of light absorbs depends on other groups present in the molecule

Intensity of the colour that is observed in solution

Depends on the concentration of the compound present, and is basis of colorimetry

Red- blue green

Violet- Green

blue- yellow