chapter 15 haloalkanes Flashcards
why are carbon halogen bonds polar?
halogens are more electronegative than carbon so the electron pair in the bond is closer to the halogen atom than the carbon atom causing slight positive and negative charges
what is a nucleophile?
an atom or group of atoms that is attracted to an electron deficient carbon atom, where it donates a pair of electrons to form a new covalent bond
what is an electrophile?
an atom or group of atoms that is attracted to an electron-rich centre, where it accepts a pair of electrons
why can haloalkanes attract nucleophiles?
the carbon atom bonded to the halogen is slightly positive
what are three common examples of nucleophiles?
:OH-
H2O:
:NH3
what is nucleophilic substitution?
a reaction in which a nucleophile is attracted to an electron-deficient carbon atom and replaces an atom or group of atoms on it.
describe the hydrolysis of a haloalkane.
•the nucleophile OH- approaches the carbon atom on the opposite side to the halogen to minimise repulsion
•lone pair on OH- is attracted to C
•new bond is formed and carbon halogen breaks by heterolytic fission
•an alcohol and a halide is formed
how can haloalkanes be converted into alcohols?
aqueous hydroxides e.g NaOH
this is very slow at room temperature so it is heated under reflux to get a good yield
what is the order of bond strength of carbon halogen bonds?
•the bond enthalpy decreases down the halogen group (C-F strongest C-I weakest)
•this means less energy is required to break C-I bond
what can we predict from the bond ethalpies of the carbon halogen bonds?
•iodoalkanes react faster than bromoalkanes
•bromoalkanes react fast than chloroalkanes
•fluoroalkanes are relatively unreactive because a large amount of energy is needed to break the C-F bond
how do we measure the rate of reaction for haloalkanes?
•use 1-haloalkanes and react them with AgNO3(aq).
•as the reaction takes place halide ions are formed which react with Ag+ to form different coloured precipitates at different rates
•the nucleophile is the water present in the aqueous solution
why is the hydrolysis of a tertiary haloalkane faster than the hydrolysis of the same primary haloalkane?
•primary haloalkane has a one step mechanism
•tertiary haloalkane has a two step mechanism
•tertiary are more stable
what are the uses of organohalogen compounds?
many pesticides, solvents, making polymers, flame retardants, refrigerants
where is the ozone layer found?
outer edge of stratosphere from around 10km-40km above the surface
what does the ozone layer do?
absorbs most of the biologically damaging UV radiation (UV-B) from suns rays.
what does UV-B do?
radiation most commonly linked to sunburn and genetic damage
how is ozone formed in the stratosphere?
•very high energy breaks down O2 into 2O.
•then a steady reversible reaction is set up where ozone is formed and broken O2 + O <~> O3
what are CFCs and what were they used in?
chlorofluorocarbons and they were used in many refrigerants, AC units and as aerosol propellants.
why are CFCs stable at surface level?
due to the strength of the carbon halogen bond
why are CFCs not stable as they rise?
in the stratosphere the strong carbon halogen bonds break due to UV radiation causing chlorine radicals to form
why are chlorine radicals specifically formed in the stratosphere?
C-F bond has higher bond enthalpy so C-Cl bond breaks first
what is the process of radiation initiating the breakdown of CFCs called?
photodissociation
Name and write out the three mechanism steps and overall equation for CFCs breaking down ozone.
photodissociation: CF2Cl2 ~> CF2Cl• + Cl•
propagation 1: Cl• + O3 ~> ClO• + O2
propagation 2: ClO• + O ~> Cl• + O2
overall: O3 + O ~> 2O2
why is it bad that propagation step 2 regenerates a Cl•?
it allows the cycle to repeat meaning one CFC molecule can promote the breakdown of 100,000 O3 molecules
what is another radical other than Cl• that can break down O3? how is it formed?
NO• it is formed naturally from lightning strikes and also is a result of aircraft travel
