Chapter 15 - Haloalkanes Flashcards
like alcohols, can be ?
classified as primary, secondary or tertiary
nucleophiles include?
- Hydroxide ions
- H2O
- Ammonia (lone pair on the N)
what happens when a haloalkane reacts w a nucleophile?
the nucleophile replaces the halogen in a substitution reaction.
haloalkanes can b converted to alcohols using?
aqueous sodium hydroxide. The reaction is very slow at room temp so the mixture is heated under reflux to obtain a good yield of product
relative reactivities?
- iodoalkanes > bromoalkanes > chloroalkanes > fluoroalkanes (most to least)
- depends on bond enthalpy value
hydrolysis of haloalkanes procedure?
1) set up 3 test tubes, each containing 1cm3 of ethanol and 2 drops of each haloalkane
2) stand test tubes in a water bath @ 50 degrees
3) place a test tube containing 0.1moldm-3 silver nitrate in the water bath and allow all tubes to reach a constant temp
4) add 1cm3 of the silver nitrate solution quickly to each test tube. Immediately start a stop clock
5) record time taken for precipitate to form, calculate rate using 1/ time
as radiation inititates the breakdown in CFCs this process is called?
photodissociation
are CFCs responsible for all ozone depleting reactions?
- no. other radicals also catalyse the breakdown of ozone
- NO radicals are formed naturally during lightning strikes and as a result of aircraft travel in the stratosphere
nitrogen radical overall equation?
is the same as w the chlorine radicals , showing that the radicals act as a catalyst for the process
O3 + O -> 202
haloalkanes are?
sat.
can be classed as ?
primary, secondary, tertiary
what is a primary haloalkane?
only has 1 C (alkyl gr) attached to the C that is bonded to the halogen
why does the polarity of the C-X bond ⬇ down G7?
The electroneg of the halogen decreases down the gr
how do we assess reactivity?
- compare relative rates of hydrolysis of C-X bond
- add AgNO3 solution
- the speed of silver halide precipitate formation indicated the speed of hydrolysis
which haloalkane is hydrolysed fastest and why?
iodoalkane - smallest bond enthalpy
nucleophiles are?
Nucleo = + philes = liking
are attracted to e- deficient areas - usually delta + areas
rate of hydrolysis is faster when?
heated w an (aq) alkaline solution, rather than H2O
CFCs = ?
Chlorofluorocarbons e.g. dichlorodifluromethane
why were CFCs used sm?
they are inert and non toxic
CFCs?
- Are gases at room temp
- after use, they are able to diffuse thru layers of the atmosphere
- in the stratosphere, CFCs become exposed to UV radiation
- UV light causes C-Cl bond in the CFCs to undergo homolytic fission, forming Cl free radicals
N radicals?
- NO * + O3 -> NO2 * + O2
- NO2 + O -> NO* + O2
- overall : O3 + O -> 2O2
- This occurs naturally when lighting strikes- high temp around the lightning bolt causes the reaction
what makes haloalkanes reactive?
they are polar molecules. They r often used to synthesise other chemicals
GF?
CnH2n+1X
X =F, Cl, Br, I
Uses - Dichlorodifluoromethane
used as refrigerants and aerosol propellants (CCl2F2)
Uses - which polymers are haloalkanes?
PVC & Teflon
Uses - Bromotrifluoromethane ?
flame retardants (BrCF3)
Uses - Carbontetrachloride?
dry cleaning solvent - CCl4
polarity of the C-X bond?
- decreases down the G
- C-F is the most polar bond, but these are the least reactive of the haloalkanes
reactivity of the C-X bond?
- the strength of the C-X bond is more important
- the C-I bond requires least energy to break so iodoalkanes are the most reactive
- Reactivity of haloalkanes ⬆ down G7
Hydrolysis definition?
the breaking of a bond by the reaction of H2O or OH-
Reagent and Condition for hydrolysis?
R: NaOH (aq), or K(OH) & AgNO3(aq) when looking at RATE
C: hot/ reflux and in ethanol
what is a nucleophile?
a particle w a lone pair of e-s that can be donated to form a covalent bond (an electron pair donor) e.g. H2O, OH-, NH3
a substitution reaction is?
a reaction where one atom or group of atoms is swapped for a diff atom or group of atoms
nucleophilic substitution - what happens?
the halogen is swapped for the OH
e.g. of nucleophilic substitution - bromoethane and sodium hydroxide?
CH3CH2Br +NaOH -> CH3CH2OH + NaBr
describe what happens in nucleophilic substitution?
- the lone pair of e-s on the O of the nucleophile is donated to the e- definicient C, delta +, forming a Covalent bond
- the C-Br bond breaks heterolytically and both e-s are passed to the Br forming Br-. (w a lone pair)
which haloalkane would hydrolyse the fastest?
- iodoalkanes
- the C-I bond has the lowest bond enthalpy so will break w least energy
- 2nd fastest is bromoalkanes
- slowest: choloroalkanes
what are the essential reagents and conditions for the hydrolysis of haloalkanes?
- heat in water bath @ 50 degrees C to heat flammable ethanol and keep constant temp
- haloalkanes need to be dissolved in ethanol - allows mixing w AgNO3(aq)
- AgNO3(aq) to supply the H2O (aq) for hydrolysis and to detect the halide ion by formation of AgX(s)
what would u see when carrying out an experiment to find out rate of hydrolysis?
- precipitate for yellow AgI(s) forms instantly, followed by cream precipitate of AgBr (s) and the slow formation of a white precipitate of AgCl (s) - C-I bond is the weakest so iodoalkanes hydrolyse fastest
what is the formula for atomic Oxygen?
O or O radical
Ozone molecule formula?
O3
O, O2, O3 all exist in?
the stratosphere
what does Ozone do?
forms a layer of ozone gas in the stratosphere where it filters out short wavelength UV radiation that is harmful to humans and can cause skin cancer
how is the amount of ozone in the stratosphere kept constant?
- Ozone in the stratosphere is continually being formed and broken down by the action of uv radiation
- V high energy uv radiation sspits O2 into O radicals: O2 -> O-
- An equil is then set up and ozone is formed and broken down by the same rate: O3 -> (REVERSIBLE) O- + O2
If no interference by mankind,?
the amounts of O2, O-, O3 would remain unchanged, hwvr for many yrs we used CFCS
Why is ozone destroyed in the stratopshere?
as a consequence of the use of CFCs
what r CFCs?
Chlorofluorocarbons, they break down in the presence of uv light to form chlorine radicals
when CFCs reach the stratosphere?
uv light breaks the C-Cl bonds forming Cl free radicals
what kind of bond breaking occurs when the C-Cl bond breaks?
homolytically - each atom takes 1 e- from the bond forming a radical w one unpaired e-
the equation for ozone decomposing in the stratosphere is?
O3 + O- -> 2O2 - this reaction is catalysed by the presence of gaseous Cl radicals
the O in the 2nd step form continuously in the stratosphere when?
uv light reacts w O3 and O2 molecules
the role of Cl and ClO radicals was proven conclusively by who?
an American-led team of scientists who flew into the hole in the ozone layer recording concs of O3 and ClO radicals
Which other radical destroys ozone?
NO - nitrogen monoxide radical
2 types of catalyst ?
homogeneous or heterogeneous
homogeneous catalysts?
catalysts and reactants in the SAME state - think HOMOsexual
heterogeneous catalyst?
catalyst n reactant are in diff states e.g. NO and CO in exhaust fumes are gases but the Pt in a catalytic converter is a solid catalyst
what does hydrolysis of a haloalkane produce?
alcohol and halide ions
?breakdown of ozone 3 steps?
- CCl2F2 -> Cl- + CClF2-
- O3 + Cl- -> ClO- + O2
- ClO- + O- -> O2 + Cl-
alternatives to CFCs- HCFCs?
- Hydrofluoroalkanes
- these r non flammable and non toxic but still damage the ozone layer but only 1/10th as much as CFCs
alternatives to CFCs- ozone friendly products?
hydrocarbon refrigerants & propellants, like butane do not damage the ozone layer, are flammable.
