3.3.3 - halogenoalkanes Flashcards
what does CFCs stand for?
chlorofluorocarbons
what are CFCs?
halogenoalkane molecules where all hydrogen atoms have been replaced by chlorine and fluorine atoms
what is the ozone layer made from?
(O)3
what does the ozone layer do?
absorbs lots of UV radiation from the sun
dangers of UV radiation
- sunburn
- skin cancer
how is the ozone layer formed?
oxygen molecule is broken down into 2 free radicals by UV radiation
first step of ozone layer formation
O2 (+UV) = O. +O.
second step of ozone layer formation
O2 + O. = O3
CFCs and the ozone layer
CFCs contribute to damaging the ozone layer
what is the first step of how CFCs damage the ozone layer?
chlorine radicals are formed in the upper atmosphere when
C-Cl bonds in CFCs are broken down by UV radiation
e.g.
CCl3F (+UV) =.CCl2F +Cl.
what is the second step of how CFCs damage the ozone layer
Cl. + O = O2 + ClO.
ClO. + O3 = 2(O2) + Cl.
what safer alternatives to CFCs have been made?
- HCFCs (hydrochlorofluorocarbons)
- HFCs (hydrofluorocarbons)
what did CFCs mainly used to be used in?
- propellers in aerosols
- fire extinguishers
- coolant gas in fridges
- added to foam plastics to make insulation/packaging materials
properties of CFCs
- pretty unreactive
- non flammable
- non toxic
what do most aerosols use now instead of CFCs as a propellant
-pump spray system
- nitrogen as a propellant
what is now used as a coolant gas in fridges instead of CFCs
ammonia or hydrocarbons
what is now used to make foamed polymers instead of CFCs?
carbon dioxide is used to make foamed polymers
what are halogenoalkanes
an alkane with at least one halogen atom in place of a hydrogen atom
polarity of carbon-halogen bonds
most carbon-halogen bonds are polar
what is the reasoning for the polarity of carbon-halogen bonds
halogens are usually much more electronegative than carbon
what charges would the carbon and halogen have in a carbon-halogen bond
carbon has a slight positive charge and the halogen has a slight negative charge
what is the reason for the charges in a carbon-halogen bond
halogen atom is more electronegative than carbon atom so withdraws electron density from the carbon atom
what is a nucleophile
nucleophile is an electron pair donor, donates an electron pair to somewhere without enough electrons
what are some examples of nucleophiles that will react with halogenoalkanes
-cyanide ion
-ammonia
-hydroxide ion
how would you illustrate a cyanide ion?
-
: CN
how would you illustrate an ammonia ion?
: NH3
how would you illustrate a hydroxide ion?
-
: OH
what happens in a nucleophilic substitution reaction?
nucleophile attacks a polar molecule, kicks out a functional group and takes its place
what is the general formula for the nucleophilic substitution of a halogenoalkane
CH3,CH2,X + Nu-
= CH3,CH2,Nu + X-
where X represents a halogen
and Nu represents a nucleophile
what is produced in the nucleophilic substitution reaction of halogenoalkanes with hydroxides
halogenoalkanes react with hydroxides to produce alcohols
what are the conditions used to make an alcohol from a halogenoalkane by using nucleophilic substitution
warm aqueous sodium or potassium hydroxide
(water=solvent)
what group do nitriles have
nitriles have CN groups, C atom and N atom are held together with a triple bond
how do you form nitriles
if you warm a halogenoalkane with ethanolic potassium cyanide, you get a nitrile
(happens under reflux, ethanol used as solvent)
what are amines
-an amine has the structure R3N
-R groups can be hydrogen or another group
- nitrogen always has a lone pair, shown as a pair of dots
how are amines formed in nucleophilic substitution reactions
if you warm a halogenoalkane with excess ethanolic ammonia (ethanol as solvent) in a sealed tube the ammonia swaps place with the halogen to form an amine
bond enthalpy from highest to lowest of carbon-halogen bonds
highest bond enthalpy
C-F
C-Cl
C-Br
C-I
lowest bond enthalpy
what carbon-halogen bonds are weakest
C-I as it has the lowest bond enthalpy
if you have a molecule with more than one halogen, how do you know which halogen gets replaced first?
the halogen with the lowest bond enthalpy will get replaced first
what is an elimination reaction?
in an elimination reaction, a small group of atoms breaks away from a larger molecule, this small group isn’t replaced by anything else
what are the conditions for an elimination reaction to occur with a halogenoalkane and hydroxide ions
- hydroxide ions are dissolved in ethanol (ethanol used as solvent)
- reaction under reflux
what is formed in elimination reactions of halogenoalkanes with hydroxide
alkenes
in a nucleophilic substitution reaction with a halogenoalkane and hydroxide, what is the role of the OH-
under aqueous conditions, OH- acts as a nucleophile, donates electron pair to slightly positive carbon atom
in an elimination reaction with a halogenoalkane and hydroxide, what is the role of the OH-
OH- acts as a base - removes a hydrogen atom from the halogenoalkane
