4.2.2 Haloalkanes Flashcards
Classifying haloalkanes
Primary
Secondary
Tertiary
Primary haloalkanes
One carbon attached to carbon atom adjoining the halogen
Secondary haloalkane
Two carbons attached to the carbon atom adjoining the halogen
Tertiary haloalkane
Three carbons attached to the carbon atom adjoining the halogen
Nucleophile
Electron pair donator
Substitution
Swapping a halogen atom for another atom or group of atoms
What do nucleophiles always have
Lone pair
So they can donate the electrons
Mechanism of nucelophilic substitution
Nucleophile attacks carbocation (has slight positive change as electro negativity difference between carbon and halogen)
Curly arrow from lone pair of electrons to carbocation then centre of the bond
What do the rate of substitution reactions depend on
Strength of carbon-halogen bond
Weaker bond the easier to break the faster the reaction
Speed of nucleophilic substitution
C-I
C-Br
C-Cl
C-F
Iodo is fastest and fluoro is slowest as strength of C-F is such that fluoro is unreactive
Nucleophilic substitution with aqueous hydroxide ions
Haloalkane->alcohol Reagent: potassium or sodium hydroxide Conditions: in aqueous solution, heat under reflux Mechanism: nucleophilic substitution Type of reagent: nucleophile, OH-
Why is it important that ions will be aqueous
If the solvent is changed to ethanol an elimination reaction occurs
What is hydrolysis
Splitting of a molecule by a reaction with water
Water as a nucleophile
Poor as it reacts slowly with haloalkanes in a substitution reaction
Use reflux or heat for more than 20 minutes
adding aqueous silver nitrate
halide group leaving combines with a silver ion to form silver halide precipitate
this can be used to determine the reactivity of the halides
faster ppt formed, faster substitution reaction, more reactive the halide
reactivity fastest to slowest halogens
iodine
bromine
chlorine
colour ppt formed by halides
iodine- yellow
bromine-cream
chlorine- white
equation showing reaction between haloalkane and silver nitrate
CH3CH2I + H2O -> CH3CH2OH + I- + H+
Ag+ (aq) + I- (aq) -> AgI (s) yellow ppt
what is a CFC
chlorofluorocarbon
original reason for development of CFC’s
aerosols
refrigerants
air-conditioning
due to their low reactivity, volatility and non-toxicity
why have lots of CFC’s been banned
due to toxicity of haloalkanes and detrimental effect on ozone layer in atmosphere
what are now used instead of CFC’s and why
HFC’s (hydro fluoro carbons)
much safer as they don’t contain the C-Cl bond
why is the natural ozone layer beneficial
O3
filters out most of the sun’s harmful UV radiation
ozone in the lower atmosphere
pollutant
contributes to formation of smog
how is ozone continually being formed and broken down
in the stratosphere
by action of ultraviolet radiation
reaction for formation of ozone
O + O2 (reversible) O3
ozone depletion and reaction
reverse of the formation reaction
energy is supplied by ultraviolet light
O3+ uv light -> O2 + O
ozone formation
uv light causes an oxygen molecule to split into free radicals
O2 + UV light -> O. + O.
then free radical hits another O2 molecule ozone forms
O. + O2. -> O3
what does rate of ozone formation equal
rate of ozone depletion
what may catalyse the breakdown of ozone
radicals from CFC’s
nitrous oxides from thunderstorms or aircrafts
how are chlorine radicals formed in the upper atmosphere
when energy from ultra-violet radiation causes C-Cl bonds in CFC’s to break
chlorine free radicals and ozone explanation
free radicals catalyse the decomposition of ozone because they are regenerated
provide an alternative route with lower activation energy
contribute to the formation of a hole in the ozone layer
equation of ozone decomposition with chlorine
Cl. + O3 -> ClO. + O2
ClO. + O. -> O2 +Cl.
overall: O3 + O. -> 2 O2
the chlorine free radical is regenerated so can destroy thousands of ozone molecules
ozone decomposition including nitrous oxides
NO + O3 -> NO2 + O2
NO2 + O. -> O2 + NO
overall: O3 + O. -> 2 O2
remaining concern of CFC’s
still entering the atmosphere from disused items and still used for some purposes in certain countries
long lifetime in the atmosphere and takes a long time for CFC’s to reach the upper atmosphere
