OZ - Organic reactions *03

Question 1

haloalkanes

Answer 1

an alkane with at least one halogen atom in place of a hydrogen atom

Question 2

haloalkanes - naming

Answer 2

1. longest part of carbon chain = last part of compounds name
2. name and position of halogen atom at start
   - chloro
   - bromo
   - iodo
3. if more then one of the same halogen atoms
   - di
   - tri
   - tetra

Question 3

haloalkanes - boiling points down group , why?

Answer 3

increase down group

• boiling points depends on strength of intermolecular bonds
• down group 7 atomic radius and no. of electron shells increase
• so there are stronger ID-ID forces between molecules
• more energy is needed to overcome them

Question 4

haloalkanes - carbon-halogen bond

Answer 4

polar

• fluorine, chlorine and bromine are much more electronegative than carbon –> so the bond is polar
• the electronegtaive atom pulls electron density away from the carbon, so the carbon is e- deficient
• —> this means it can be attacked by a nucleophile

Question 5

what is a nucleophile?

Answer 5

an electron pair donor

it donates an electron pair to somewhere without enough electrons

Question 6

nucleophilic substitution - what?

Answer 6

substitution = one functional group is replaced by another functional group

nucleophilic substitution = a nucleophile attacks a slightly +ve carbon and replaces the slightly -ve group

Question 7

nucleophilic substitution - haloalkanes + hydroxide ions

Answer 7

conditions:

• warm aq sodium hydroxide
• reflux

mechanism:

• the C-Br bond is polar
• the slightly +ve carbon attracts the lone pair of electrons on OH-
• the C-Br bond breaks heterolytically, a new bond forms between he C and OH- ion

general equation:
R-X + NaOH —-> ROH + NaX

Question 8

nucleophilic substitution - haloalkane + water

Answer 8

conditions:
- warming a haloalkane with water

mechanism:

• slightly +ve carbon attracts a lone pair on H2O
• the C-Br bond breaks
• an intermediate forms with an oxygen that has 3 bonds —> this is unstable so one O-H bond breaks
• an alcohol is formed

general equation:
R-X + H2O —-> R-OH + HBr

Question 9

nucleophilic substitution - haloalkanes + ammonia

Answer 9

conditions:
warm with excess ethanolic ammonia (ammonia dissolves in ethanol)

mechanism:

• NH3 nucleophile attacks the slightly +ve carbon atom
• forms an unstable intermediate —> R-N+H3
• another ammonia molecule than removes a hydrogen group to leave an amine —> R-NH2 and forms ammonium (N+H4)

general equation:
R-Br + ammonia —> R-N+H3 + NH3 < —-> R-NH2 + N+H4

Question 10

reactivity of haloalkanes

Answer 10

carbon-halogen bonds strength decides reactivity

C-F strongest —> has the highest bond enthalpy

reactivity increase down the group

stronger bond = slower reaction