nitrogen compounds

Question 1

comment on the bp of aliphatic amines

Answer 1

• higher than hydrocarbons of comparable mass because pri and sec amines are able to form intermolecular H bonds in addition to id-id interactions between non-polar alkyl grps hence more E needed to seperate amine molecules
• lower than alc because N-H bonds are less polar than O-H bonds since N is less electronegative than O so the H bonds formed are weaker.

Question 2

comment on the bp of aliphatic amines

Answer 2

• soluble in water as they can form H bonds with water moelcules and the E released can overcome interactions between amine molecules and between water molecules (state what type of interactions)

Question 3

why are phenylamines less soluble than aliphatic amines

Answer 3

due to presence of hydrophobic benzene ring

Question 4

how to compare the strength of amines

Answer 4

1. availability of lp of electrons on N. A lewis base is an electron pair donor
2. stability of the conjugate acid. A bronsted-lowry base is a proton acceptor
   RNH2 + H20 —> RNH3+ + OH-

Question 5

what are the 3 factors that affect availability of lp for donation

Answer 5

1. inductive effect of substituents
2. delocalisation of e- pair
3. steric hindrance

Question 6

explain how delocalisation of e- pair affects availability of lp for donation and give exp

Answer 6

p orbital of N overlaps with the pi electron cloud of the benzene ring leading to delocalisation of lp of e-, making them less available for donation
C6H5NH2 < NH3

2. Makes phenylamine a weaker nucleophile
3. enchances e- density in the benzene ring so it is more susceptible to electrophilic attack

Question 7

explain how steric hindrance affects availability of lp for donation

Answer 7

bulky groups on N can hinder donation of lp of e- to the approaching proton

Question 8

explain how inductive effect of substituents affects availability of lp for donation and give exp

Answer 8

alkyl groups on N exert an electron donating inductive effect which increases the availability of lp for donation. Hence tertiary > secondary > primary

Question 9

how to form amines

Answer 9

1. nucleophilic sub of halogenoalkanes
   - alc NH3 heat under pressure in a sealed tube
2. reduction of nitriles
   - LiAlH4 in dry ether at rtp
3. reduction of amides
   - LiAlH4 in dry ether at rtp or H2 gas with ni…

Question 10

what reactions do amines undergo

Answer 10

1. neutralisation of dilute acids to form salt
2. nucleophilic sub of halogenoalkanes
   - alc RX heat under pressure in sealed tube link back to halogenoalkanes and alc NH3
3. condensation with Acyl chloride to form amides
   - RCOCl at rtp

Question 11

how to form phenylamines

Answer 11

Reduction of nitrobenzene
- Sn and concentrated HCl with heating. followed by addition of alcohol like NaOH

Question 12

what reaction does phenylamine undergo that amines do not

Answer 12

electrophilic sub to form tri subbed product - delocalisation of lp of N into the benzene ring increases e- density in the ring making it more susceptible to electrophilic attack due to overlap of p orbitals of N and pi electron cloud of benzene
- Br2(aq)

Question 13

why are amides neutral while amines are basic

Answer 13

the lp of e- on N is unavailable for donation as it is delocalised due to interaction with pi electrons of the CO double bond
( show CO double bond with arrow going into the O and lp on N arrow to the C-N bond. reversible arrow. show lp on O- arrow into CO bond and CN bond arrow to positive N)

Question 14

what reactions do amides undergo

Answer 14

1. acid/base hydrolysis to form carboxylic acid or carboxylic salt
   - acid — ammonium salt
   - base — NH3/amine
2. reduction to form amines
   - LiAlH4 in dry ether, rtp

Question 15

what are amino acids

Answer 15

organic compounds that contain an acidic carboxyl grp and a basic amine group

Question 16

explain how amino acids exists as zwitterions

Answer 16

• amino acids undergo intramolecular acid-base reaction where basic amine group accepts a proton to become NH3+ while CO2H loses a proton to become COO-
• this makes amino acids have similar phy properties to ionic compounds

Question 17

comment on the bp of amino acids

Answer 17

• high due to their giant ionic structure containing dipolar ions held by strong electrostatic forces of attraction between opp charged grps.

Question 18

comment on the solubility of amino acids

Answer 18

• insoluble in non-polar solvents as E released from ion-induced dipole interactions between zwitterions and non-polar solvent are insufficient to overcome ionic bonds between zwitterions

Question 19

what is the structure of an amino acid in different ph

Answer 19

when pH < pKa, then protonated form will be predominant (CO2H and NH4+) and the converse applies

Question 20

rank a-CO2H, alkyl-CO2H, a-NH3+ and alkyl-NH3+ in terms of increasing pKa and explain why

Answer 20

same order
1. CO2H is more acidic than NH3+ due to resonance stabilization of the carboxylate ions produced
2. a-CO2H more acidic as it is closer to the a-NH3+ group which exerts e- withdrawing inductive effect leading to the dispersal of negative charge on carboxylate ion, stabilizing it
3. a-NH3+ more acidic than alkyl-NH3+ as it is closer to partially positive C in a-CO2- which exerts an e-withdrawing inductive effect, leading to intensification of positive charge, destabilizing it making it more reactive

Question 21

what reaction do amino acids undergo

Answer 21

condensation - two amino acids join together forming a peptide linkage between CO2H group and NH2 group, eliminating a water molecule between them

Question 22

what reaction do proteins undergo

Answer 22

complete hydrolysis with dilute acid or dilute base both with heat
- acid – carboxylic acid and protonated NH3+
- base – carboxylic salt and NH2