Haloalkanes Flashcards
What is the functional group for a haloalkane?
a halogen atom
What is the state of haloalkanes at RMT?
gas
why do the BP increase as the no. of C atoms increase?
theres an increases in surface contact area between molecules which means an increase in london forces so more energy is needed to break them
Why are haloalkanes insoluble in water?
cannot form H bonds
What type of reaction is a reaction of an alkane and a halogen?
radical substitution
What is the overall equation of radical substitution?
CH3CH3 + Br2 —-> CH3CH2Br + HBr
Why is radical substitution not a good way of making a haloalkane?
more than one H atom can be substituted and in different positions
so a mixture of products is formed with a low % yeild of the desired product
What 3 ways can a haloalkane be made?
radical substitution
electrophilic addition
alcohol and hydrogen halide
Why is electrophillic addition more efficient than radical substitution?
100% atome economy
only 1 or 2 possible products formed
What are the conditions of reacting an alcohol with a hydrogen halide?
warm the alcohol with a mixture of the NaX
concentrated H2SO4 catalyst
What are the conditions of nucleophilic substitution?
heating the mixture with aq sodium hydroxide
What is the overall equation for nucleophilic substitution?
CH3CH2Br + OH- —-> CH3CH2OH + Br-
What is a nucleophile?
an electron pair donor
What are the key points to include when drawing the nucleophilic sub mechanism?
curly arrow from lone pair on :OH- to C attached to halogen
dipoles on C-Br bond
curly arrow from C-Br bond to Br
:Br- product
What type of bond breaking is present in nucleophilic subsitution?
heterolytic fission of the carbon-halogen bond
What is the trend of the rate of hydrolysis of the haloalkanes?
ROR increases down the group
Why does the ROR of hydrolysis increase down the group?
bonds get weaker going down the group so theres a lower activation energy and a faster reaction
What is the exprt. used to investigate the ROR of hydrolysis?
add ethanol and aq silver nitrate to each haloalkane
warm the tubes in the same hot water bath - same temp
observe the rate the precipitate forms
Why is ethanol used in the hydrolysis expt.?
to act as a solvent - allows water and haloalkanes to mix and produce a single solution rather than two layers
What are the 2 successive reactions that occur in the hydrolysis expt.?
each organic compound reacts with water:
C2H5X +H2O —> C2H5OH + H+ + X-
the halide ions released react with the silver ions:
Ag+ (aq) + X- (aq) —-> AgX (s)
What are the common uses of chlorofluoroalkanes?
solvents
aerosol propellants
How are CFCs harmful to the envrionment?
they cause damage to the ozone layer
Why is ozone important for the atmosphere?
it absorbs UV radiation from the sun, which could cause mutations if it reached the surface
What is the reaction with CF2Cl2 forming Cl radicals?
CF2Cl2 –UV—> CF2Cl. + Cl.
How do the Cl radicals break down the ozone molecules?
they act as a catalyst, breaking them down in a chain reaction
the radicals are used up then regenerated
What are the chain reactions of cl radicals breaking down ozone?
Cl. + O3 —-> ClO. + O2
ClO. + O —> Cl. + O2
What is the overall reaction of ozone being broken down?
O3 + o —> 2 O2
What are the chain reactions of nitrogen oxide breaking down ozone?
.NO + O3 —> .NO2 + O2
.NO2 + O—-> .NO + O2
How do nitrogen oxides form?
form naturally as a result of lightning acting on nitrogen and oxygen in the air
How is the amount of nitrogen oxide produced increased in higher altitude?
by the effect of jet aeroplane engine - the intense heat
What are 3 examples of nucleophiles?
hydroxide ions
water molecules
ammonia molecules
What happens during hydrolysis of a haloalkane (nucleophilic substitution)?
- OH nucleophile approaches carbon atom attached to halogen
- the lone pair on the OH ion is attracted and donated to the delta+ carbon atom
- a new bond is formed between the oxygen atom and the carbon atom
- the carbon-halogen bond breaks by heterolytic fission
- alchohol and halide ion formed
Why does the hydroxide nucleophile in hydrolysis, approach from the opposite side of the halogen?
minimises the repulsion between nucleophile and the delta- halogen atom
What base is used to make alchohols from haloalkanes?
sodium hydroxide
what is the trend of C-X bond enthalpies down group 2?
bond enthalpy decreases down group 2
What is the link between bond enthalpies and ROR?
the compound with the slowest ROR is the one with the strongest carbon-halogen bond
What are organohalogen compounds?
molecules that contain at least one halogen atom joined to a carbon chain
