halogenoalkanes and mechanisms Flashcards
why do we aim to get an alkane into a halogenoalkane?
alkanes are unreactive due to;
non-polar bonds (very similar electronegativities)
so overall non-polar
what is the mechanism for turning an alkane into a halogenoalkane?
free radical substitution
what gets substituted in free radical substitution?
a hydrogen atom for a halogen (Cl/Br)
what are the reagents and conditions for free radical substitution?
reagent; chlorine or bromine mostly- don’t use flourine
conditions; UV light (doesn’t possess enough energy to break flourine bond so we don’t use flourine)
what kind of fission occurs in free radical substitution?
homolytic - each atom receives 1 electron each
THIS IS THE ONLY HOMOLYTIC MECHANISM IN AS CHEMISTRY
what are the four steps of free radical substitution?
- Initiation
- Propagation 1
- Propagation 2
- Termination
outline the aims of each step of free radical substitution?
- Initiation- formation of the radical
- Propagation 1 - formation of the HX and radical
- Propagation 2- formation of the haloalkane
- Termination- removal of radical
what are CFCs and what do they do?
chloroflourocarbons lead to the depletion of the ozone layer
what did scientists discover about waste CFCs?
they didn’t break down easily and would rise up to the upper atmosphere and break down to form chlorine radicals
these chlorine radicals catalyse the breakdown of ozone, and have led to a hole in the ozone layer
how does ozone form?
O2 + O* ———> O3
(* represents radical)
why is the ozone layer beneficial?
it absorbs UV radiation that can cause skin cancer
how do chlorine radicals catalyse the breakdown of ozone ?
Cl* + O3 ———> O2 + ClO*
ClO* + O3 ———> 2O2 + Cl*
Cl* reacts and is regenerated (never used up ) i.e. its a catalyst
what is the overall equation for depletion of the ozone?
2O3 ———–> 3O2
what are alternatives to CFCs?
HFC’s (hydroflourocarbons) that don’t contain chlorine
or HCFC’s (hydrochloroflourocarbons)- less stable than CFCs so break down easily (less likely to form radicals) but are banned in USA and UK
what can’t we use in the UK and why?
CFC’s and HCFC’s because they contain C-Cl bonds and a chlorine radical can form
what are nucleophiles?
electron pair donors
what are the three nucleophiles we need to know?
:OH- (forms alcohol)
:NH3 (forms amine)
:CN- (forms nitrile)
what basically happens in nucleophilic substitution?
replacing halogen for nucleophile
explain why haloalkanes are reactive?
what is the exception?
they are polar
polarity is due to the difference in electronegativity between carbon and halogen
non-polar C-I bond because electronegativities are similar due to increased shielding of iodine
what kind of fission is present in nucleophilic substitution?
heterolytic - electrons distributed unevenly when the covalent bond breaks
what is the process called for nucleophilic substitution for alcohols?
hydrolysis- adding water to break a bond
halogenoalkanes react with aqueous hydroxides to form alcohols- known as hydrolysis
what are the reagents and conditions for nucleophilic substitution of alchohols?
reagent; sodium hydroxide solution
conditions; warm, aqueous
what are the reagents and conditions for nucleophilic substitution of nitriles?
reagent; KCN (potassium cyanide)
conditions; ethanol, warm
what is important to remember about reacting a halogenoalkanes with cyanide?
increases the length of the carbon chain
what are the reagents and conditions for nucleophilic substitution of amines?
reagent; excess ammonia
conditions; warm in a sealed tube, ethanol solvent
what reaction can we do to measure the effect of halogens on the rate of nucleophilic substitution?
reagents; AgNO3 (aq)
conditions; warm ethanol at 60 degrees
see time taken for each different colour precipitate to form for each of the haloalkanes
what are the 3 haloalkanes we test the rate of nucleophilic substitution on and what colour precipitate do we look for?
1-chlorobutane ; white
1-bromobutane; cream
1-iodobutane; yellow
order the 3 halogenoalkanes in order of how fast it takes the precipitate to appear and explain?
chloro slowest (bond
enthalpy
strongest
since attraction
stronger)
bromo
iodo fastest
(bond enthalpy
weakest)
how do we get from a halogenoalkane to an alkene?
elimination
what is elimination?
a reaction in which an atom or group of atoms is removed from a reactant
what is removed in elimination?
a hydrogen atom and a halogen atom
what are the reagents and conditions for elimination reactions?
reagent; sodium (or potassium) hydroxide
conditions; reflux in ethanol (hot)
what is the benefit of using a reflux in elimination reactions?
allows you to use really high temperatures without any solvents evaporating
how do we get from an alkene to a halogenoalkane?
electrophilic addition
what is an electrophile?
electron pair acceptor
what acts as an electrophile in electrophilic addition?
X2 or HX
how are dipoles able to be created in Cl-Cl bonds for electrophilic addition?
double bond in the alkene is a region of high electron density which induces polarity in Cl-Cl
what is a symmetrical alkene?
the groups/ atoms attached to either end f the carbon- carbon double bond are the same
a symmetrical alkene will only ever give you one product in electrophilic addition
what is an asymmetrical alkene?
the groups/ atoms attached to either end of the carbon-carbon double bond are different
asymmetrical alkenes electrophilic addition results in 2 different products
explain how major/minor products work in electrophilic addition?
the major (product you get most of) comes from the most stable carbocation
explain why a tertiary carbocation is more stable than a secondary carbocation which is more stable than a primary carbocation?
greatest positive inductive effect (due to most alkyl groups)
how do we make alkyl hydrogensulfates?
electrophilic addition
reagent; concentrated H2SO4
condition; room temp
