OZ - Organic reactions *03 Flashcards
haloalkanes
an alkane with at least one halogen atom in place of a hydrogen atom
haloalkanes - naming
- longest part of carbon chain = last part of compounds name
- name and position of halogen atom at start
- chloro
- bromo
- iodo - if more then one of the same halogen atoms
- di
- tri
- tetra
haloalkanes - boiling points down group , why?
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
haloalkanes - carbon-halogen bond
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
what is a nucleophile?
an electron pair donor
it donates an electron pair to somewhere without enough electrons
nucleophilic substitution - what?
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
nucleophilic substitution - haloalkanes + hydroxide ions
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
nucleophilic substitution - haloalkane + water
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
nucleophilic substitution - haloalkanes + ammonia
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
reactivity of haloalkanes
carbon-halogen bonds strength decides reactivity
C-F strongest —> has the highest bond enthalpy
reactivity increase down the group
stronger bond = slower reaction