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atom on an alkane has been replaced by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 is the difference between primary, secondary and tertiary haloalkanes?

Answer 3

Primary haloalkanes have only one alkyl group attached to the carbon to which the halogen is bonded, secondary
haloalkanes have two and tertiary have three

Question 4

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

Answer 4

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

Question 5

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

Answer 5

An alkene is produced.
The reaction is an elimination reaction.

Question 6

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

Answer 6

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

Question 7

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

Answer 7

CH3CH2Br + KOH →CH2CH2+ H2O + KBr

Question 8

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

Answer 8

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 9

How can halogenoalkanes be
qualitatively identified?

Answer 9

Add ethanol, followed by aqueous silver
nitrate solution to the halogenoalkane. If a
halogenoalkane is present then a silver halide
precipitate will form in the solution

Question 10

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

Answer 10

AgCl - White precipitate

Question 11

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

Answer 11

AgBr - Cream precipitate

Question 12

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

Answer 12

AgI - Yellow precipitate

Question 13

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

Answer 13

Nucleophilic substitution

Question 14

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

Answer 14

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

Question 15

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

Answer 15

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

Question 16

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

Answer 16

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 17

What are some common uses of halogenoalkanes?

Answer 17

• Solvents
• Refrigerants
• Anaesthetics

Question 18

Why are there tight regulations
surrounding the use of
halogenoalkanes?

Answer 18

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

Question 19

What are CFCs?

Answer 19

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

Question 20

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

Answer 20

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 21

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

Answer 21

Cl* + O3 → O2 + ClO*
ClO* + O3 → 2O2 + Cl*
Overall: 2O3 → 3O2

Question 22

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 22

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 23

What is a reflux setup?

Answer 23

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