HALOGENOALKANES

Question 1

State and explain the trend in polarity of C-X (halogen) bonds going down the group

Answer 1

from C-F to C-I bond polarity decreases as you go down the group. This is because Electronegativity decreases as you go down the group, so the difference in electronegativity between carbon and the halogen decreases, therefore the polarity of the bond is less.

Question 2

Name the two forces that interact between Halogenoalkane molecules

Answer 2

Van der Waals forces
Permanent dipole-dipole interactions

Question 3

What is the trend in boiling point as the length of the haloalkane chain increases?

Answer 3

VdWs increase as the relative molecular mass/ number of electrons increases. Therefore as chain length increases, boiling point increases.

Question 4

What is the pattern of boiling points in different halogenoalkanes

Answer 4

VdWs increase due to the greater molecular mass of halogens further down the group, the boiling point increases. Even though the permanent dipole-dipole interactions are less, because the C-X bond becomes less polar.

Question 5

Are halogenoalkanes soluble in water?

Answer 5

Not soluble or only slightly soluble.

Question 6

What is a nucleophile?

Answer 6

A species with a lone pair of electrons

Question 7

Which 3 nucleophiles do halogenoalkanes react with?

Answer 7

Cyanide ions
Hydroxide ions
Ammonia

Question 8

How does nucleophilic substitution work?

Answer 8

The C-X bond is polar leaving the carbon slightly positive. The lone pair on the species is very negative so it is attracted to the positive carbon which forms a new bond, kicking out the halogen

Question 9

What does nucleophilic substitution of a halogenoalkane by OH produce

Answer 9

an Alcohol
(+ an ionic substance halide e.g Sodium Bromide from Sodium Hydroxide)

Question 10

Iodoethane + Potassium Hydroxide&raquo_space;>

Answer 10

Ethanol + Potassium iodide

Question 11

What does nucleophilic substitution of a halogenoalkane by CN produce

Answer 11

a Nitrile
(+ ionic halide compound e.g Sodium Bromide from Sodium Hydroxide)

Question 12

Bromoethane + Potassium Cyanide&raquo_space;> (hint add 1 C)

Answer 12

Propanenitrile + Potassium Bromide

Question 13

What does nucleophilic substitution of a halogenoalkane by NH3 produce

Answer 13

an Amine
(+ ammonium halide)

Question 14

Bromoethane + Ammonia&raquo_space;>

Answer 14

Ethylamine + Ammonium Bromide

Question 15

When drawing a mechanism, how many arrows are there in total for OH and CN

Answer 15

2

Question 16

When drawing a mechanism, how many arrows are there in total for NH3

Answer 16

3

Question 17

Where do the 2 arrows go in OH mechanism

Answer 17

Lone pair on the O to the slightly positive Carbon

C-X bond to the halogen

Question 18

Where do the 2 arrows go in CN mechanism

Answer 18

From lone pair on the C of the CN to the slightly positive C in the chain

From the C-X bond to the halogen

Question 19

Where do the arrows go in the NH3 mechanism

Answer 19

From lone pair on the N to the slighty postive C

From the C-X bond to the halogen

From one of the N-H bonds to the H

Question 20

What are the conditions of an elimination reaction?

Answer 20

Hot ethanolic

HYDROXIDE ION ONLY

Question 21

Does the OH act as a base or nucleophile in elimination

Answer 21

Base

Question 22

What does elimination of a halogenoalkane form?

Answer 22

Alkene
Halide ion
Water

Question 23

How does elimination work?

Answer 23

• OH attacks a hydrogen that is attached to the carbon that is attached to the halogen
  X-C-C-H
• OH (accepts a proton) takes H to form water
• The electrons from the C-X bond go to form a C=C double bond

-The halogen C now has 5 bonds so kicks out the halogen

Question 24

What is the chemical formula of Ozone?

Answer 24

O3

Question 25

Is ozone very reactive?

Answer 25

Yes

Question 26

What are CFCs

Answer 26

Chlorofluorocarbons

Question 27

Are CFCs reactive?

Answer 27

Not particularly

Question 28

What is the Ozone layer and what does it do?

Answer 28

1) The ozone layer is found in the stratosphere and is important for preventing harmful UV rays from entering our atmosphere

Question 29

What were CFCs used for?

Answer 29

Refrigerants and aerosols

Question 30

How do CFCs damage the ozone layer?

Answer 30

• CFCs were broken down by UV light to form UV radicals
• The radicals react with ozone to produce o2, this damages and depletes the ozone layer
• The process is through free radical substitution
• One chlorine radical can cause many more ozone molecules to be broken down. This is because at the end of the propagation steps, more chlorine radicals are produced.

Question 31

To what extent has the ozone layer recovered?

Answer 31

The ozone layer has still not fully recovered

Even though there are no more CFCs in use, chlorine radicals are still being produced which break down ozone.

The ozone layer has recovered to an extent since CFCs were banned worldwide in 1987

Question 32

What has replaced CFCs and why are they better?

Answer 32

Hydrogenfluorocarbons HFCs
HFCs break down more easily in the atmosphere (lower) instead of becoming a radical