Unit 3.2.8 - Haloalkanes

Question 1

What mechanism makes chloroalkanes from alkanes?

Answer 1

Free radical substitution

Question 2

Draw the mechanism for the free radical substitution reaction between methane and chlorine.

Answer 2

See Flash Card

Question 3

What are the five steps for a free radical substitution mechanism?

Answer 3

initiation step, first propagation step, secondary propagation step, termination step and minor termination steps.

Question 4

What is an essential condition for free radical substitution?

Answer 4

UV light

Question 5

Why is UV light essential during free radical substitution?

Answer 5

It is required to break the covalent bonds and form free radicals.

Question 6

What is a free radical?

Answer 6

A species which contains an unpaired electron.

Question 7

What are free radicals caused by?

Answer 7

Homolytic fission of a covalent bond.

Question 8

What is Homolytic fission?

Answer 8

Breaking of a covalent bond in such a way that one electron goes to each atom.

Question 9

What happens if excess chlorine is used during free radical substitution between methane and chlorine?

Answer 9

Further substitution happens until all the hydrogen bonds have been substituted out.

Question 10

How do you ensure chloromethane is the major product during free radical substitution between methane and chlorine?

Answer 10

Use excess methane.

Question 11

What can chloroalkanes and chlorofluroalkanes be used for?

Answer 11

Solvents

Question 12

What is the main use of CFCs?

Answer 12

Aerosols and old fridges.

Question 13

What happens to CFCs when the are realised into the atmosphere and how does this lead to the breakdown of the ozone layer?

Answer 13

CFCs undergo homolytic fission to produce cholorine free radicals and these break down the ozone layer.

Question 14

Draw the mechanism for how CFCs break down the ozone layer.

Answer 14

See flash card

Question 15

What about haloalkanes means they attract nucleophiles?

Answer 15

The CX bond is polar which means the carbon is positively charged.

Question 16

What is a nucleophile?

Answer 16

A species with a lone pair of electrons which can be used to bond with a +ve carbon atom.

Question 17

Name three common nucleophiles and state their formula.

Answer 17

Hydroxide ions (OH-)
Cyanide ions (CN-)
Ammonia (NH3)

Question 18

What reaction happens between haloalkanes and nucleophiles?

Answer 18

Nucleophilic substitution.

Question 19

Draw the mechanism for the nucleophilic substitution between a haloalkane and a hydroxide ion.

Answer 19

See flash card

Question 20

What is produced when cyanide ions react with haloalkanes?

Answer 20

Nitriles

Question 21

Draw the mechanism for the nucleophilic substitution between a haloalkane and a cyanide ion.

Answer 21

See flash card

Question 22

When haloalkanes are heated with ethanolic ammonia what do they produce?

Answer 22

A primary amine

Question 23

Draw the mechanism for the nucleophilic substitution between a haloalkane and ammonia.

Answer 23

See flash card

Question 24

Name the two mechanisms that an OH- ion can have with a haloalkane.

Answer 24

Nucleophilic substitution and elimination.

Question 25

What does elimination of a haloalkane produce?

Answer 25

Alkene

Question 26

Draw the mechanism for the elimination reaction between a haloalkane and a hydroxide ion.

Answer 26

See flash card

Question 27

During elimination where is the hydrogen atom lost from?

Answer 27

From the carbon atom next to the carbon attached to the halogen.

Question 28

Why can different products be formed during elimination?

Answer 28

It depends which carbon atom the hydrogen is lost from.

Question 29

What decides whether nucleophilic substitution or elimination happens?

Answer 29

The conditions.

Question 30

What saying can be used to decide whether a reaction is a nucleophilic substitution or elimination?

Answer 30

Cheet pawn

Question 31

What does Cheet Pawn stand for?

Answer 31

``` Concentrated NaOH Hot Ethanol solvent Elimination Tertiary/secondary ``` Primary Aqueous solvent Warm Nucleophilic

Question 32

During elimination what does the hydroxide ion act as?

Answer 32

A base.

Question 33

What is a base?

Answer 33

A proton acceptor

Question 34

During nucleophilic substitution what does the hydroxide ion act as?

Answer 34

Nucleophile

Question 35

How does the rate of reaction change between halogen alkanes?

Answer 35

Iodo alkanes > Bromo alkanes > Chloroalkanes > Fluoralkanes

Question 36

Why are Iodo alkanes the most reactive?

Answer 36

The C-I bonds are long and easier to break making them more reactive.

Question 37

During elimination which product will be the major one?

Answer 37

The one where the carbon atoms in the C=C bond are attached to the most carbons

Question 38

Why is the major product of elimination the one where the carbon atoms in the C=C are attached to the most carbon atoms?

Answer 38

It is the most stable