4.6 - Amines Flashcards
What are the physical properties of amines
Solubility
- small amines are very soluble in water, primary, secondary and tertiary amines can form hydrogen bonds with water due to the lone pair of electrons on the nitrogen. solubility decreases as chain length increases as the significance of the lone pair decreases
Boiling point
Primary amines: lower boiling point than H2O as less lone pairs of electrons, as chain length increases boiling point increase
Secondary amines: lower boiling point than primary amines as the lone pair is between the alkyl groups
Tertiary amines: lowest boiling point as there is no lone pair of electrons to form hydrogen bonds
How are primary amines formed from halogenoalkanes
- nucleophilic substitution
Heat with concentrated ammonia in ethanol in a sealed tube
CH3CH2Br > CH3CH2NH3] > CH3CH2NH2 + NaBr + H2O
- to get a primary amine add NaOH and heat
CH3CH2NH3+Br- + NaOH > CH3CH2NH2 + NaBr + H2O
- excess NH3 forms secondary and tertiary amines
How are amines formed from nitriles
- reduce nitriles to form amines
LiAlH4 in dry ether
eg. CH3CH2CN + 4[H] > CH3CH2CH2NH2 (treat product with dilute acid)
Or react with H2 and Nickel catalyst
CH3CN + 2H2 > CH3CH2NH2
How are aromatic amines formed
- nitrobenzene is reduced
Heat using Tin and HCl - derivatives of nitrobenzene are reduced in a similar way
What is role of chromosphore in amines
Chromophore: a structural unit of a molecule primarily responsible for absorption of radiation of a certain wavelength
If absorption is in the visible region, the colour that is not absorbed is seen, the wavelength of light absorbed is dependent on other groups in the molecule. Intensity of the colour depends upon concentration of the compound
Yellow - 384nm
Orange - 476nm
Red - 520nm
What is the basicity of amines
Bronsted theory a base is a H+ acceptor
Lewis theory a base is a lone pair donator
Amines behave similarly to NH3
NH3 + H+ > NH4
Lone pair accepts a proton by a coordinate bond
What is the reaction of amines with water
Amines are weak bases, the position of equilibrium lies to the left, but they are stronger than NH3. The alkyl group in amines ‘push’ electrons to N making it slightly more positive
Phenylamine is less basic than amines and NH3 because the lone pair on N atom is drawn into the delocalised ring system
Amines react with acids to form salts
eg. CH3CH2NH2 + HCl > CH3CH2NH3+Cl- (ethylammonium chloride)
What is ethanoylation of amines
- addition elimination reaction with acid chloride
eg. CH3NH2 + CH3COCl > CH3NHC=OCH3 + HCl
- CH3NHC=OCH3 is N-methylethanamide
What are the reactions of amine with HNO2
To form HNO2
NaNO2 + HCl > HNO2 + NaCl
a. Primary amine
ethylamine + HNO2 > ethanol + N2 + water
CH3CH2NH2 + HNO2 > CH3CH2OH + N2 + H2O
- effervescence from N2 (g)
Very poor yield of alcohol so not used in synthesis
b. Aromatic amine
Below 10 degrees - no reaction as too slow
0-10 degrees
- phenylamine > (NaNO2 + HCl) benezenediazonium chloride + water
Above 10 degrees
- forms benezenediazonium chloride salt but is unstable so decomposes to phenol
What are the properties of benzenediazonium salts
Benzenediazonium salts are very reactive, they react with phenols and aromatic amines to form azogroup compounds
Benzendiazonium ions are weak electrophiles and undergo electrophilic substitution with NH2 or OH, they form azo dyes either red, orange or yellow
These are coupling reactions and are carried out in alkaline solutions, coupling usually occurs at position 4 but can occur at position 2
What are the reactions of benzenediazonium ions
- With phenol
benezenediazonium chloride + phenol > 4-phenylazophenol + HCl - With amine
benezenediazonium chloride + aromatic amine > 4-phenylazodimethylphenylamime + H2O
