2.6 Halogenoalkanes

Question 1

What are halogenoalkanes?

Answer 1

Halogenoalkanes are organic compounds where at least one hydrogen atom on an alkane has been replaced by a halogen.

Question 2

What is a nucleophile?

Answer 2

A nucleophile is a species that donates a lone pair of electrons in a reaction.

Question 3

What do the displayed, structural and skeletal formulae of the halogenoalkane with the molecular formula C2H5X look like?

Answer 3

Question 4

What is the difference between primary, secondary and tertiary halogenoalkanes?

Answer 4

Question 5

What is produced when a halogenoalkane reacts with an aqueous alkali, like KOH?

Answer 5

An alcohol is produced.
The OH- ion acts as a nucleophile and a nucleophilic substitution reaction takes place.

Question 6

What is formed when a halogenoalkane reacts with ethanolic potassium hydroxide? What type of reaction takes place?

Answer 6

An alkene is produced.
The reaction is an elimination reaction.
(One of the reactions)

Question 7

What is formed when a halogenoalkane reacts with warm ethanolic ammonia?

Answer 7

An amine is produced.
Ammonia acts as the nucleophile in a nucleophilic substitution reaction.

Question 8

What is formed when a halogenoalkane reacts with alcoholic potassium cyanide?

Answer 8

A nitrile is produced.
The cyanide ion acts as the nucleophile in a nucleophilic substitution reaction.

Question 9

Write the chemical equation for the reaction between bromoethane and ethanolic potassium hydroxide

Answer 9

Question 10

What reaction can be carried out to increase the number of carbons in the halogenoalkane chain?

Answer 10

Nucleophilic substitution of the halogenoalkane with a cyanide ion in alcoholic conditions.
The cyanide ion, CN-, contains a carbon atom so the carbon chain increases by one unit.

Question 11

How can halogenoalkanes be qualitatively identified?

Answer 11

Add ethanolic NaOH or KOH, followed by nitric acid then aqueous silver nitrate solution to the halogenoalkane. If a halogenoalkane is present then a silver halide precipitate will form in the solution.

Question 12

What colour precipitate will form when chloroethane is added to silver nitrate?

Answer 12

AgCl - White precipitate

Question 13

What colour precipitate will form when bromoethane is added to silver nitrate?

Answer 13

AgBr - Cream precipitate

Question 14

What colour precipitate will form when iodoethane is added to silver nitrate?

Answer 14

AgI - Yellow precipitate

Question 15

What is the name of the mechanism for the reaction between a halogenoalkane and ammonia?

Answer 15

Nucleophilic substitution

Question 16

What is the condition required for a halogenoalkane to produce an alcohol rather than an alkene when it reacts with KOH?

Answer 16

The KOH needs to be aqueous.
If the KOH is ethanolic, an alkene will be produced.

Question 17

What is the trend in reactivity of primary, secondary and tertiary halogenoalkanes?

Answer 17

Tertiary > Secondary > Primary
The tertiary halogenoalkane is the most reactive and the primary halogenoalkane is the least reactive.

Question 18

What is the trend in reactivity of chloro-, bromo- and iodo- halogenoalkanes? Explain in terms of bond enthalpy

Answer 18

iodo- > bromo- > chloro-
Iodo- halogenoalkanes are the most reactive. This is because the C-I bond is the weakest (low bond enthalpy) and so it is broken easily.
Chloro- halogenoalkanes are the least reactive because the C-Cl bond has a high bond enthalpy and therefore requires a lot of energy to break

Question 19

What are some common uses of halogenoalkanes?

Answer 19

• Solvents
• Refrigerants
• Anaesthetics

Question 20

Why are there tight regulations surrounding the use of halogenoalkanes?

Answer 20

Halogenoalkanes are toxic and have a negative impact on the environment, so their use is regulated.

Question 21

What are CFCs?

Answer 21

Chlorofluorocarbons.
These are organic compounds containing only carbon, chlorine and fluorine atoms.

Question 22

What is the environmental problem associated with the use of CFCs (chlorofluorocarbons)?

Answer 22

CFCs damage the ozone layer. The ozone is in the upper atmosphere and it absorbs a lot of UV radiation.
CFCs are broken down by UV light, releasing a chlorine radical. This chlorine radical reacts with ozone and breaks it down to oxygen. This means there is less protection from UV radiation.

Question 23

Give the equations for the reactions that take place between a chlorine free radical and ozone

Answer 23

Question 24

Explain the impact of the bonds C-H, C-F and C-Cl on the atmosphere in reference to the strength of the bond

Answer 24

The C-Cl bond is the weakest of the three bonds so it breaks first. This means the C-Cl bond has the greatest impact on the environment as the free radical is produced most easily. The C-H and C-F bonds are much stronger, with C-F having the highest bond strength.

Question 25

What is a reflux setup?

Answer 25

Reflux is the boiling setup with a vertical condenser that allows the vapours to return to the same mixture once they condense.