paper 4 organic

Question 1

By ref to the formation of sigma and pi bonds, describe and explain the shape of a benzene molecule

Answer 1

• Benzene has a shape of trigonal planar bond angle of
  120°, C-C bonds have the same length.
• It also contains delocalised e- in the pi system
• All carbon atoms undergoes sp2 hybridisation.
• The sp2 C-H and C-C bonds overlaps to form sigma bond
• The unhybridised p-orbital in carbon atoms overlap sideways with each other above and below to form pi bond

Question 2

Describe the relative basicities of NH3, ethylamine, phenylamine, (least to most basic) and explain

Answer 2

• phenylamine < ammonia < ethyamine
• In phenylamine, the LP on N can be delocalized into benzene ring, making it less available to accept PROTON
• In ethylamine, the e- donating alkyl grp w +ve inductive effect increases the e- density on N, making N more available to accept PROTON

Question 3

CH3COOH, CCl3COOH, CH3CONH2

rank in order of acidity (least –> most) and explain

Answer 3

• CH3CONH2 is neutral
• In CCl3COOH, chlorine is an e- withdrawing grp with -ve
  inductive effect, and weakens the strength of O-H bond.
• The structure of -ve ion is more stableised due to the
  delocalization of -ve charge by the C=O, hence more H+
  produced.

Question 4

State and explain the relative acidities of Cl3CCOOH CH2ClCOOH, CH3COOH

Answer 4

• Relative acidities increases in the order CH3COOH,
  CH2ClCOOH, Cl3CCOOH
• Cl is an e- withdrawing grp with -ve inductive effect and
  weakens the strength of O-H bond.
• The more -ve ion, the more stable the -ve charge
  becomes , more H+ ion is produced.
• Therefore Cl3CCOOH is the strongest acid
• In CH3COOH, the e- donating alkyl grp with +ve
  inductive effect will ↑ the strength of O-H bond and less
  likely to ionise

Question 5

Compare and Explain the relative acidities of hexanoic acid, hexan-1-ol and phenol

Answer 5

• Relative acidities increases in order hexan-1-ol, phenol, hexanoic acid
• In hexanoic acid, the C=O is an e- withdrawing group
  with -ve inductive effect and weakens the strength of O-H bond. More H+ produced
• Besides, the -ve ion stabilizes the structure of the -ve structure due to delocalization of -ve charge
• In phenoxide ion, the lone pair e- in oxygen ion can delocalize into the benzene ring and stabilise the structure of -ve ion due to negative inductive effect of ring. O-H bond weakened, more H+ produced
• In hexan-1-ol, the e- donating alkyl group increases the e- density and strengthens the O-H bond
• Hence, it is less likely to ionise, less H+ produced

Question 6

Describe 2 observations when HOOCCOOH is warmed with acidified manganate (VII) ions

Answer 6

• the purple colour of KMnO4 decolorises

- effervescence takes place

Question 7

Polyalkenes biodegrade very slowly.

Explain why by referring to the structures of polymers,

Answer 7

• C–C bonds are non-polar.

- No dipoles so cannot be hydrolysed

Question 8

Some polymers will degrade in the environment.

Describe 2 processes by which this can happen.

Answer 8

1. Hydrolysis using acid/base

2. Action of UV light

Question 9

Chlorobenzene and Phenol both show lack of reactivity towards reactants that cause the breaking of the C–X bond.
Explain why

Answer 9

• Both C–Cl and C–O are stringer in Chlorobenzene and
  Phenol
• the p-orbital in Cl or lone pair of e- in O or Cl can be
  delocalised into the benzene ring

Question 10

eg of non-biodegradable polymers

Answer 10

• PVC

- polyethene

Question 11

eg of biodegradable polymers

Answer 11

• polyesters

- polyamides

Question 12

state and explain the relative rates of hydrolysis of the following compounds : CH3CH2CL , CH3COCL , C6H5CL

Answer 12

• relative rates of hydrolysis ↑ in the order: C6H5CL,
  CH3CH2CL, CH3COCL
• In C6H5CL, the p-orbital on CL overlaps with pi system,
  no hydrolysis takes place
• In CH3COCL, the carbon in C-CL is more e- deficient
  since it is attached to an O atom, C-CL bond is the
  weakest
• In CH3CH2CL, the C-CL bond is strengthened by the
  +ve inductive effect alkyl grp

Question 13

Write an eqn to show how compound H, C10H13NH2 behaves as a base

Answer 13

C10H13NH2 + H30+ ⇌ C10H13NH3+ + H20

Question 14

Phe-tyr, Phe and Tyr

Suggest why the other 2 species give spots R and S that are so close together

Answer 14

mass/charge ratios are about the same for phe-tyr and phe (charge density)

Question 15

Construct an eqn for the formation of NO2+ in nitration of benzene

Answer 15

HNO3 + H2SO4 –> HSO4- + NO2+ + H2O

Question 16

Suggest why bromination of phenol occurs more readily than bromination of benzene

Answer 16

• LP of e- on O can be delocalized into benzene ring, e- density of benzene ring is higher
• Phenol can induce a dipole on the Br2

Question 17

Write the expression for the kw of D2O

Answer 17

Kw = [D+] [DO-}

Question 18

Many polymers are degradeble

State 2 diff processes by which some polymers can be degraded

Answer 18

• hydrolysis

- action of UV

Question 19

Suggest how the type of amino acids in a protein determines its 3D structure

Answer 19

• Ionic attractions btw ⎼COO- and ⎼NH3+
• VDW btw alkyl grps
• HB btw ⎼OH, ⎼NH2, ⎼COOH, ⎼NH

Question 20

Suggest why poly(ethene) is non-biodegradable

Answer 20

• non-polar

- doesn’t undergo hydrolysis