6.8 nitrogen compounds

Question 1

state and explain the relative basicities of amide, primary amine, secondary amine and phenylamine

Answer 1

• amide < phenylamine < primary amine < secondary amine
• amide is neutral because the lone pair of electrons delocalises into the adjacent C=O bond and is unavailable to form dative bond with H+ ions
• phenylamine is weaker than aliphatic amines because the lone pair of electrons on the nitrogen atom is delocalised into the benzene ring, hence lone pair on N atom is less available to form dative bond with H+ ions
• secondary amine is more basic than primary amine because there is one more electron-donating alkyl group bonded to the N atom, which increases the electron density on the N atom. hence, the lone pair on the N atom is more available for dative bond with H+ ions

Question 2

what are the methods to prepare amines (5)?

Answer 2

1. reduction of nitriles (LiAlH4 in dry ether OR H2 with Ni catalyst, heat at high pressure)
2. nucleophilic substitution of halogenoalkanes (excess NH3 in ethanol, heat in a sealed tube)
3. basic hydrolysis of amides (KOH/NaOH, heat under reflux)
4. reduction of amides (LiAlH4 in dry ether)
5. reduction of nitrobenzene to phenylamine (Sn in excess conc HCl, heat under reflux, followed by addition of NaOH)

Question 3

what are the reactions of amines (4)?

Answer 3

1. nucleophilic substitution to halogenoalkane and NHR (limited R-X in ethanol, heat in a sealed tube)
2. neutralisation with acids
3. condensation with anhydrous acyl chlorides to form amides
4. electrophilic substitution for nitrobenzene (Br2(aq))

Question 4

which amine cannot undergo condensation with acyl chlorides and why?

Answer 4

tertiary amine because they do not have a H atom to lose during condensation

Question 5

why can amines not undergo condensation with carboxylic acids?

Answer 5

carboxylic acids will neutralise the amine instead

Question 6

what are the reactions of amides (3)?

Answer 6

1. reduction to CH2 from C=O (LiAlH4 in dry ether)
2. acidic hydrolysis to carboxylic acid and ammonium salt (HCl/H2SO4, heat under reflux)
3. basic hydrolysis to carboxylate salt and amine (NaOH/KOH, heat under reflux)

Question 7

which types of amino acid rotate plane-polarised light and why?

Answer 7

naturally occuring amino acids, they are found only in one mirror image form.
lab-synthesised amino acids are found in both enantiomers, giving a racemic mixture

Question 8

define zwitterions

Answer 8

a zwitterion is a compound with no overall electrical charge but containing separate parts which are positively and negatively charged

Question 9

define isoelectric point

Answer 9

the point of specific pH where only the zwitterion form exists

Question 10

what points can amino acids act as buffers?

Answer 10

near its pKa values, because equal proportions of the acid and conjugate base are present

Question 11

how do you determine the direction and distance of travelling in electrophoresis?

Answer 11

overall charge determines direction, whether it travels towards the cathode or anode.
mass determines distance, higher Mr shorter distance