Halogenoalkanes

Question 1

how do we react alkanes with halogens and what do they form

Answer 1

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

Question 2

what is a free radical

Answer 2

a free radical is a reactive species due to the presence of an unpaired electron

Question 3

what is it called when a halogen atom replaces a hydrogen atom on the alkane

Answer 3

a substitution reaction and because this reaction includes free radicals it’s called free radical substitution

Question 4

when free radical substitution of alkanes reaction has began what may happen afterwards

Answer 4

the process may continue until all hydrogen atoms on the alkane have been substituted with a halogen

Question 5

what can we see when we look at propagation step 1 and 2 together?

Answer 5

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.

Question 6

what are chlorofluorocarbons?

Answer 6

they are halogenoalkanes containing both chlorine and fluorine but no hydrogen

Question 7

what are shortchain chlorofluorocarbons used for

Answer 7

they are gases and were used in refrigerators

Question 8

what are long chain chlorofluorocarbons used for

Answer 8

used for dry cleaning and degreasing solvents

Question 9

are cfcs reactive or no

Answer 9

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

Question 10

how are cfcs decomposed

Answer 10

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

Question 11

what is ozone

Answer 11

O3

Question 12

why is the ozone layer important

Answer 12

the ozone layer is important as it absorbs harmful UV radiation from the sun that causes skin cancer by damaging DNA in skin cells

Question 13

which chemicals cause ozone molecules to decompose

Answer 13

chlorofluorocarbons

Question 14

why do we call the chlorine free radical in ozone breakdown a catalyst

Answer 14

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

Question 15

what have chemists developed as an alternative to chlorofluorocarbons?

Answer 15

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

Question 16

the halogenoalkanes are a homologous series with the functional group…

Answer 16

CnH2n+1X

where X is a halogen

Question 17

how can halogenoalkanes be classified

Answer 17

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

Question 18

what is a nucleophile

Answer 18

a species that has a lone pair of electrons

Question 19

what is nucleophilic substitution

Answer 19

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.

Question 20

how do we form alcohols from halogenoalkanes using nucleophillic substitution

Answer 20

reagents = NaOH / KOH (source of :OH-)

conditions = halogenoalkanes are WARMED with AQUEOUS NaOH / KOH to form alcohol

Question 21

how do we form nitriles using nucleophilic substitution with halogenoalkanes

Answer 21

reagent = potassium cyanide KCN
:CN-

conditions = halogenoalkanes are WARMED in AQUEOS ETHANOL solution

this adds an extra carbon to the chain

Question 22

how do we form primary amines with halogenoalkanes

Answer 22

when halogenoalkanes are warmed with excess of ammonia in a sealed container