Haloalkanes

Question 1

What is the general formula of haloalkanes?

Answer 1

CnH2n+1X

Question 1

What are haloalkanes?

Answer 1

alkanes that contain a halogen

Question 2

What are the three ways haloalkanes can be classified?

Answer 2

Primary, secondary, tertiary

Question 3

What is a primary haloalkane?

Answer 3

A haloalkane where the halogen is attached to a C, which is only attached to one other C

Question 4

What is a secondary haloalkane?

Answer 4

A haloalkane where the halogen is attached to a C, which is only attached to two other C’s

Question 5

What is a tertiary haloalkane?

Answer 5

A haloalkane where the halogen is attached to a C, which is attached to three other C’s

Question 6

How are haloalkanes formed?

Answer 6

When you combine an alkane with a halogen in the presence of bright sunlight or a UV lamp

Question 7

Why is light essential in the formation of haloalkanes?

Answer 7

it starts the reaction

Question 8

What is the reaction to form a haloalkane classed as?

Answer 8

a substitution reaction

Question 9

What is the reaction to form a haloalkane?

Answer 9

it is a chain reaction called a free radical substitution mechanism - it has three stages

Question 10

What are the three stages of the free radical substitution mechanism?

Answer 10

Initiation, propagation and termination

Question 11

What is the definition of the initiation stage?

Answer 11

defined as a stage which results in the formation of a free radical

Question 12

What is a free radical?

Answer 12

a species which has a single unpaired electron on its outer shell

Question 13

What is a key property of a free radical?

Answer 13

they are extremely reactive as they no longer have a noble gas configuration

Question 14

What is involved in the initiation stage?

Answer 14

This stage involves the breaking of the covalent bond between the diatomic halogen molecule to form two halogen atoms. UV light provides the energy needed to break this bond. Both halogen atoms have donated one pair of electrons into the covalent bond, and when the bond breaks they each take back their electron. This is called HOMOLYTIC FISSION. As soon as the halide radicals have been formed this triggers the next stage in the mechanism.

Question 15

What is first stage in a free radical substitution mechanism?

Answer 15

initiation

Question 16

What is the second stage in a free radical substitution mechanism?

Answer 16

propagation

Question 17

What is the third stage in a free radical substitution mechanism?

Answer 17

termination

Question 18

What is the definition of the propagation stage?

Answer 18

defined as a stage which prolongs the life of the free radical

Question 19

How many steps are involved in the propagation stage?

Answer 19

two

Question 20

What is involved in the first step of the propagation stage?

Answer 20

The halide radical collides with the alkane and removes a hydrogen atom. This produces HX. The alkane however is now missing one hydrogen atom and so this is now a radical as one carbon will have a single unpaired electron.

Question 21

What is involved in the second step of the propagation stage?

Answer 21

During the initiation stage not all of the halogen will react and some will remain as the diatomic molecule. In the second propagation step the alkane radical will collide with one of these diatomic molecules and split it apart. This produces another halide radical and haloalkane (the desired product).

Question 22

How do the propagation steps show how the free radical substitution mechanism is as a chain reaction?

Answer 22

The second propagation reaction forms the desired haloalkane product, but also regenerates another chloride radical. This can then react again and again.

Question 23

What is the definition of the termination stage?

Answer 23

defined as a stage in which free radicals are removed

Question 24

When can termination occur?

Answer 24

when any two free radicals collide and form a covalent bond between them - this can happen in one of 3 combinations

Question 25

What are the three combinations of two free radicals colliding?

Answer 25

• two halide radicals
• two alkane radicals
• one halide radical and one alkane radical
  (In an exam if you’re only given enough space to draw out one termination reaction you should always select the reaction which produces the haloalkane product)