Halogenoalkanes

Question 1

how do alkanes react with halogens

Answer 1

in the presence of UV light

Question 2

what is formed when alkanes and halogens react together in the presence of UV light

Answer 2

a halogenoalkane and hydrogen halide

Question 3

what is the reaction for the formation of halogenoalkanes and why

Answer 3

a substitution reaction

a hydrogen in the alkane is replaced by the halogen

The hydrogen lost from the alkane will join with the second half of the diatomic
halogen molecule to form HX

Question 4

what’s the mechanism for the formation of halogenoalkanes

Answer 4

free radical substitution

Question 5

what’s a free radical

Answer 5

a reactive species due the presence of an unpaired electron

Question 6

what are the 3 stages in free radical substitution

Answer 6

1) initiation

2) propagation 1 and 2

3) termination

Question 7

explain the initiation step in free radical
substitution for formation of halogenoalkanes

Answer 7

ultraviolet light causes the covalent bond between halogen atoms to break forming 2 halogen free radicals

Question 8

what happens in 1st propagation for formation of halogenoalkanes

Answer 8

the halogen radical reacts with the alkane

the halogen radical removes a hydrogen from alkane

creating a hydrogen halide and alkyl radical

Question 9

what happens in 2nd propagation for formation of halogenoalkanes

Answer 9

the alkyl radical reacts with halogen molecule and

creates halogenoalkane and halogen radical

Question 10

what does the halogen radical created in the end of 2nd propagation do after

Answer 10

the halogen radical created at the end of propagation 2 goes on to react with another alkane molecule and 1st propagation is repeated

there will be a chain reaction until no more halogen molecules or alkane molecules

Question 11

what happens in termination step for formation of halogenoalkanes

Answer 11

2 radicals react to form neutral molecules

Question 12

what are chlorofluorocarbons (CFC’s)

Answer 12

they are halogenoalkanes containing both chlorine and fluorine but no hydrogen

Question 13

describe short chain CFCS and what they were used for

Answer 13

they are gasses

they were used in refrigerators

Question 14

what were long chain CFCS used for

Answer 14

they are used for degreasing solvents
and dry cleaning

Question 15

how are CFC gases like under normal conditions

Answer 15

very unreactive

Question 16

what happens when CFC gasses reach the atmosphere

Answer 16

they decompose to produce chlorine free radicals

Question 17

why is the ozone layer important

Answer 17

it absorbs harmful UV radiation from sun that causes skin cancer by damaging DNA in skin cells

Question 18

what layer is ozone layer in

Answer 18

stratosphere

Question 19

what do chlorine radicals do to ozone

Answer 19

chlorine radicals decompose ozone in the stratosphere creating a hole in the earths ozone layer

Question 20

name all the steps in the decomposition of ozone

Answer 20

1) the c-cl bond is broken in the presence of uv radiation to produce chlorine radical

R-Cl —> R* + Cl*

2) the chlorine radical decomposes ozone in the stratosphere

Cl* + O3 —> ClO* +O2

3) the resulting ClO3 radicals attack more ozone molecules and regenerate Cl*

ClO* + O3 —> 2O2 * + Cl*

Question 21

what can we call the Cl* in regards to the breakdown of O3

Answer 21

as the Cl* is not destroyed and is regenerated we can say it acts as a catalyst in the breakdown of ozone to oxygen

Question 22

what’s the overall equation for decomposition of ozone

Answer 22

2O3 —> 3O2

Question 23

what are the safer alternatives to CFCS

Answer 23

chemists have developed safer chlorine free compounds which is primarily used in air conditioning and refrigerators

Question 24

why are the C-X bonds polar

Answer 24

because the halogen atoms are more electronegative than carbon

the electrons in the C-X bond are more attracted to the halogen leaving it delta negative and the carbon delta positive

Question 25

what is the delta positive carbon atom attacked by

Answer 25

nucleophile

Question 26

what’s a nucleophile

Answer 26

electron pair donor

Question 27

name the 3 types of nucleophillic substitution with halogenoalkanes

Answer 27

1) NaOH/KOH (:OH ions) to form alcohols

2) KCN ( :CN- ions) to form nitriles

3) with excess ammonia (:NH3) to form primary amines

Question 28

how do we form alcohols from halogenoalkanes

Answer 28

Reagent: NaOH / KOH

Conditions : Warm and aqueous

When Halogenoalkanes are warmed with aqueous sodium hydroxide (NaOH) or potassium hydroxide (KOH), alcohols are formed. NaOH and KOH are ionic compounds containing the nucleophile: OH.

Question 29

how do we form nitriles from halogenoalkanes

Answer 29

Reagent: KCN (:CN- ions)

Conditions: warmed in aqueous ethanol

When Halogenoalkanes are warmed with an aqueous ethanol solution of potassium cyanide (KCN), nitriles are formed. This adds an extra carbon to the chain. The nucleophile in KCN is :CN-

Question 30

how are primary amines formed from halogenoalkanes

Answer 30

reagent: NH3

conditions: warmed, excess ammonia, sealed container

When Halogenoalkanes are warmed with an excess of ammonia in a sealed container, primary amines are formed (2 hydrogen atoms attached to the nitrogen). The nucleophile is ammonia, in which nitrogen atom has the lone pair of electrons:

Question 31

what mechanism is alkene formed from halogenoalkanes

Answer 31

elimination mechanism

Question 32

describe reagents and conditions elimination mechanism

describe the different roles of hydroxide ions

Answer 32

reagents: NaOH

conditions: hot in ethanol

In the reaction between aqueous sodium hydroxide and a Halogenalkane, the hydroxide ions act as a nucleophile (electron pair donor) and an alcohol is formed by nucleophilic substitution.

• However, the hydroxide ions can also act as a base (accept H*), so an alternative elimination reaction also takes place in which one of the hydrogen atom and the halogen atom are eliminated from the Halogenalkane to give an alkene.

Question 33

describe reagents and conditions elimination mechanism

describe the different roles of hydroxide ions

Answer 33

reagents: NaOH

conditions: hot in ethanol

In the reaction between aqueous sodium hydroxide and a Halogenalkane, the hydroxide ions act as a nucleophile (electron pair donor) and an alcohol is formed by nucleophilic substitution.

• However, the hydroxide ions can also act as a base (accept H*), so an alternative elimination reaction also takes place in which one of the hydrogen atom and the halogen atom are eliminated from the Halogenalkane to give an alkene.

Question 34

what are the steps in elimination reaction

Answer 34

Stage 1: The Hydroxide ion removes a hydrogen atom from an adjacent carbon to the halogen atom.

Stage 2: The electrons in the carbon-hydrogen bond move onto the carbon-carbon bond to create a double bond.

Stage 3: the electrons in the carbon-halogen bond move onto the halogen atoms, breaking the bond and creating a halide ion.