15.1 - Haloalkanes Flashcards
what are haloalkanes?
haloalkanes are compounds that contain C, H and at least one halogen.
how are haloalkanes named when two or more halogens are in a structure?
they are listed in alphabetical order.
when is a haloalkane tertiary?
when the halogen is attached to a carbon with three alkyl groups attached.
why is the C-halogen bond polar?
halogen atoms are more electronegative than carbon atoms, so halogen pulls electron pair closer than carbon atom.
so C-halogen bond is polar.
what is attracted to the slight positive charge of the C atom in a haloalkane?
nucleophiles - electron pair donors
they are attracted to the electron-deficient C atom
what type of reaction is it when a haloalkane reacts with a nucleophile?
a nucleophilic substitution reaction
what happens during the hydrolysis of a haloalkane?
the halogen atom is replaced by an OH- group.
happens on primary haloalkane
reaction mechanism for hydrolysis of haloalkane by OH-
1) OH- is nucleophile. it approaches C attached to haloalkane on opposite side of molecule to halogen.
2) this minimises repulsion between delta negative halogen and negative OH- ion.
3) lone pair of electrons on hydroxide ion attracted, then donated to the delta positive carbon atom.
4) carbon-halogen bond breaks by heterolytic fission. (both go to halogen)
5) new product is alcohol.
halide ion also formed.
how can haloalkanes be converted to alcohols?
using aqueous NaOH. (has OH- ions)
reaction very slow so has to be heated under reflux for good yield.
what does the rate of hydrolysis of a haloalkane depend on?
the strength of the C-halogen bond in haloalkane
which is the weakest carbon-halogen and why?
C-I
because it has a lower bond enthalpy so less energy required to break it.
which haloalkanes react fastest?
iodoalkanes because C-I bond is weakest
how can the rate of reaction of hydrolysis of haloalkanes be measured?
carry out reaction in presence of aqueous silver nitrate
as reaction takes place, Ag+ ions react with forming Cl- ions.
forms silver halide (we know what colour precipitates they form)
also, water comes from ‘aqueous’ silver nitrate
(should take place in ethanol, because allows single solution to be made rather than two layers)
what would be seen when aqueous silver nitrate is added to 1-iodobutane?
a yellow precipitate forms rapidly
what would be seen when aqueous silver nitrate is added to 1-bromobutane?
a cream precipitate
forms slower than 1-iodobutane but faster than 1-chlorobutane
