CH6 : Halogenoalkanes

Question 1

What is a primary,secondary and tertiary halogenoalkanes?

Answer 1

PRIMARY : carbon attached to halogen, is itself attached to one other alkyl group
SECONDARY : halogen is attached to a carbon that itself is attached to two other alkyl groups
TERTIARY: halogen is attached to a carbon that itself is attached to three other alkyl groups

Question 2

What is a nucleophile?

Answer 2

An electron rich species that can donate a pair of electrons

Question 3

How do halogenoalkanes form alcohols?

Answer 3

Halogenoalkanes are hydrolysed in a nucelophilic substitution reaction
- can be done by adding water (water is a weak nucleophile - slower)

Halogenoalkane + water —> alcohol + H+ + halide ion

• or by adding OH- ions from potassium/sodium hydroxide in ethanol (better nucleophile as carries full negative charge)
  Carry out UNDER REFULX
• OH nucleophile replaces halide

Question 4

How to compare reactivity of halogenoalkanes using hydrolysis ?

Answer 4

1. In 50 degree water bath, set up 3 different halogenoalkanes, ethanol (solvent) and acidifed silver nitrate (contains water)
2. time how long takes for precipitate to form
   - silver ions react with halide ions as soon as they form to form - silver halide precipitate
   - faster it forms, the faster the rate of hydrolysis
   (Iodoalkanes are fastest bc largest and fluoroalkanes are slowest- smallest)

Experiment can be used to compare rates of hydrolysis between primary, secondary and tertiary haloalkanes
- tertiary is fastest, primary is slowest

Question 5

What determines the rate at which halogenoalkanes are hydrolysed

Answer 5

• the carbon-halogen bond enthalpy
  Weaker C-halogen bonds break easier - react faster

Bond enthalpy depends on size of halogen - size increases down group 7
- larger the halogen , the longer the C-X bond , lower the bond enthalpy

Question 6

How can we form nitriles from halogenoalkanes?

Answer 6

Reflux halogenoalkane + potassium cyanide in ethanol
- cyanide ion react by nucleophilic substitution reaction - FORM NITRILE

Mechanism : CN- ion is nucelophile and replaces halogen
- This substitution extends carbon chain by adding extra carbon atom

Question 7

How do amines form from halogenoalkanes ?

Answer 7

Warm the halogenoalkane with excess ethanolic ammonia (NH3 is nucleophile)
- ammonia replaces halogen to from primary amine - nucleophilic substitution reaction

Mechanism :
- NH3 acts as nucleophile and replaces halogen
- Then second ammonia molecule removes a hydrogen from NH3 group to leave amine

Question 8

Why is excess ammonia used in reaction to form nitriles?

Answer 8

Because product is more reactive than ammonia so further substitution could occur

Question 9

How do alkenes form from halogenoalkanes?

Answer 9

ELIMINATION REACTION - hydroxide ions act as a base to remove H+ ion from halogenoalkane

Halogenoalkanes heated with ethanolic sodium hydroxide (alkali)
- Heated under reflux

HALOALKANE + NaOH(ethanol) —> ALKENE + WATER + NaX