NITROGEN COMPOUNDS (Amines) Physical properties of amines Formation of amines Basicity of amines Reactions of phenylamines

Amines : Physical properties Amines are derivatives of ammonia ( 1 hydrogen atoms replaced by carbon chains). Amines are classified as primary, secondary or tertiary. Quaternary ammonium salt are organic equivalent of ammonium compounds. E.g: NH4+Cl- ammonium chloride (CH3)4N+Cl- tetramethylammonium chloride Amines have higher boiling points than hydrocarbon with similar Mr intermolecular hydrogen bonding.

But amines have lower boiling point than alcohol because intermolecular hydrogen bonding in amines are weaker than alcohols. Amines are soluble in water form hydrogen bonds with water molecules Solubility in water decreases with increasing num. of C atoms. Has a fishy smell.

Naming amines Use the suffix –ylamine after a stem indicating the number of carbon atoms. E.g : CH3NH2 methylamine 1 (CH3)2NH dimethylamine 2 Exercise : Name the following compounds. (CH3CH2)2NH (CH3)3N Write the structural formulas of the following compounds: Propylamine 2-nitrophenylamine

Formation of amines Nitrile reduction Reagent and condition : LiAlH4 in dry ether or mixture of Na and ethanol or H2, Ni, 140C. Product : alkyl amine Eg. : R–CN RCH2NH2 LiAlH4 Dry ether

2) Reduction of nitrobenzene Reagent & condition : reflux with tin and excess conc. HCl or Zn or Fe and excess of HCl or H2(g), Ni, 300C. Product : Phenylamine The metal and acid gives off H2(g) which performs the reduction. The phenylamine is separated through steam distillation.

Basicity of amines Amines are weak base. Due to ability of lone pair in the nitrogen atom to accept a proton (form a dative bond). Are weak alkalis in water. Reacts with acid, forming a salt. Strength of base depends on: Availability of lone pair on N to accept proton. Stability of its conjugate acid ion. Order of base strength : ethylamine > ammonia > phenylamine pKb : 3.3 4.8 9.4 * The lower the pKb value, the stronger the base. Base strength decrease

Ethylamine is stronger base than ammonia. The EDG ethyl group in ethylamine increases the electron density on the N atom more available to accept proton than ammonia. The ethyl group pushes electron onto the –NH3+ group on the conjugate acid stabilises the ion.

Phenylamine is a much weaker base than ammonia. This is due to delocalisation of lone pairs of electrons on the nitrogen atom over the benzene ring hence less available to accept a proton.

Effect of substituent on basicity of amines. Electron-donating group (EDG) increases basicity of amines. E.g : propylamine is more basic than ammonia. The R-group increases electron density on N atom, making it more available to accept a proton. Electron withdrawing group (EWG) decreases basicity of amines. E.g 3-chlorophenylamine is weaker then phenylamine. Electron-withdrawing Cl atom makes the lone pair in N atom less available to accept proton.

Reactions of Phenylamine Reaction with HNO2 Reagent : NaNO2 and conc. HCl Condition : <5°C Product : diazonium salt The aromatic diazonium ion formed is more stable due to delocalisation of electrons to the benzene ring.

The benzenediazonium ion reacts with nucleophiles : The diazonium ion is not stable above 10°C will decompose to form N2(g) and phenol. Aliphatic amines (e.g ethylamine) will not give diazonium salt in this reaction. This is because the diazonium group, -N+ N, is unstable. It decompose quickly to give N2(g), alcohol and alkene. The benzenediazonium ion reacts with nucleophiles :

Coupling of benzenediazonium chloride and phenol – formation of dyes. The –N+2 group of the benzenediazonium ion is a strong electrophile. It can attack the benzene ring, especially those that have EDG such as –OH group attached to it. Reagent : diazonium salt and phenol Condition : Cold alkaline solutions Product : bright orange precipitate of (4-hydroxyphenyl) azobenzene. E.g

The compound formed is stable due to the extensive delocalisation of electrons via the N=N bond. Because it is very stable, the dyes do not fade. This coupling reaction is used in formation of dyestuff. Other aromatic amines and arenes can be used to make dyes.

Reactions of phenylamine. Reaction with Aqueous Bromine (electrophilic substitution) Aliphatic amines have no reaction with aqueous bromine. Condition : room temperature. Observation : reddish-brown bromine solution decolourised, white ppt of 2,4,6-tribromophenylamine formed and steamy white fumes of HBr liberated. Phenylamine is more reactive than benzene due to delocalisation of lone pair electrons from N atom into the benzene ring.