Halogenoalkanes Flashcards
how do we react alkanes with halogens and what do they form
alkanes react with halogens in the presence of UV light to form halogenoalkanes and a hydrogen halide
a hydrogen from the alkane is replaced by a halogen
the hydrogen lost from the alkane will join with the second half of the diatomic halogen molecule to form HX
what is a free radical
a free radical is a reactive species due to the presence of an unpaired electron
what is it called when a halogen atom replaces a hydrogen atom on the alkane
a substitution reaction and because this reaction includes free radicals it’s called free radical substitution
when free radical substitution of alkanes reaction has began what may happen afterwards
the process may continue until all hydrogen atoms on the alkane have been substituted with a halogen
what can we see when we look at propagation step 1 and 2 together?
propagation step 1 and 2 is a chain reaction
1) the halogen radical made in propagation step 2 can now go back and react with alkane molecules in propagation step 1.
what are chlorofluorocarbons?
they are halogenoalkanes containing both chlorine and fluorine but no hydrogen
what are shortchain chlorofluorocarbons used for
they are gases and were used in refrigerators
what are long chain chlorofluorocarbons used for
used for dry cleaning and degreasing solvents
are cfcs reactive or no
cfc gasses are very unreactive in normal conditions however they eventually end up in the atmosphere where they are decomposed to give CHLORINE FREE RADICALS
how are cfcs decomposed
cfcs make there way up to the stratosphere and once in the stratosphere ultra violet radiation causes the carbon-chlorine bond to break to form a chlorine free radical and a cfc free radical
what is ozone
O3
why is the ozone layer important
the ozone layer is important as it absorbs harmful UV radiation from the sun that causes skin cancer by damaging DNA in skin cells
which chemicals cause ozone molecules to decompose
chlorofluorocarbons
why do we call the chlorine free radical in ozone breakdown a catalyst
because as the chlorine free radical is not destroyed and is regenerated we can say it acts as a catalyst in the break down of ozone into oxygen
what have chemists developed as an alternative to chlorofluorocarbons?
chemists have developed safer alternative chlorine free compounds which are used in air conditioning and refrigerators
^ such as hydrofluorocarbons which only contain C-H and C-F bonds which do not produce chlorine free radicals and these bonds are stronger
the halogenoalkanes are a homologous series with the functional group…
CnH2n+1X
where X is a halogen
how can halogenoalkanes be classified
halogenoalkanes can be classified into as:
primary
secondary
tertiary
(primary = the halogen is boned to a carbon where that is bonded to another carbon)
secondary = the halogen is bonded to a carbon where that carbon is bonded to 2 other carbons
what is a nucleophile
a species that has a lone pair of electrons
what is nucleophilic substitution
The halogen atoms in Halogenoalkanes are more electronegative than C, so the carbon-halogen bonds are polar. The electrons in the C-X bond are attracted towards the halogen atom which gains a slight negative charge, leaving the carbon atom electron deficient or with a slight positive charge
• The nucleophile will donate its lone pair of electrons to carbon, forming a bond. The carbon-halogen bond then breaks and a halide ion is released.
• This type of reaction is known as nucleophilic substitution because the halide ion is substituted for a nucleophile.
how do we form alcohols from halogenoalkanes using nucleophillic substitution
reagents = NaOH / KOH (source of :OH-)
conditions = halogenoalkanes are WARMED with AQUEOUS NaOH / KOH to form alcohol
how do we form nitriles using nucleophilic substitution with halogenoalkanes
reagent = potassium cyanide KCN
:CN-
conditions = halogenoalkanes are WARMED in AQUEOS ETHANOL solution
this adds an extra carbon to the chain
how do we form primary amines with halogenoalkanes
when halogenoalkanes are warmed with excess of ammonia in a sealed container
