2.6 Halogenoalkanes Flashcards
what are halogenoalkanes?
Halogenoalkanes are organic
compounds where at least one hydrogen
atom on an alkane has been replaced by
a halogen.
what is a nucleophile ?
A nucleophile is a species that donates a
lone pair of electrons in a reaction
What is the difference between primary, secondary and tertiary haloalkanes?
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
What is produced when a halogenoalkane reacts with an aqueous alkali, like KOH?
An alcohol is produced.
The OH- ion acts as a nucleophile and a
nucleophilic substitution reaction takes place.
What is formed when a halogenoalkane
reacts with ethanolic potassium
hydroxide? What type of reaction takes
place?
An alkene is produced.
The reaction is an elimination reaction.
What is formed when a halogenoalkane
reacts with warm ethanolic ammonia?
An amine is produced.
Ammonia acts as the nucleophile in a
nucleophilic substitution reaction.
Write the chemical equation for the
reaction between bromoethane and
ethanolic potassium hydroxide
CH3CH2Br + KOH →CH2CH2+ H2O + KBr
What reaction can be carried out to
increase the number of carbons in the
halogenoalkane chain?
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.
How can halogenoalkanes be
qualitatively identified?
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
What colour precipitate will form when
chloroethane is added to silver nitrate?
AgCl - White precipitate
What colour precipitate will form when
bromoethane is added to silver nitrate?
AgBr - Cream precipitate
What colour precipitate will form when
iodoethane is added to silver nitrate?
AgI - Yellow precipitate
What is the name of the mechanism for
the reaction between a halogenoalkane
and ammonia?
Nucleophilic substitution
What is the condition required for a
halogenoalkane to produce an alcohol
rather than an alkene when it reacts with
KOH?
The KOH needs to be aqueous.
If the KOH is ethanolic, an alkene will be
produced.
What is the trend in reactivity of primary,
secondary and tertiary
halogenoalkanes?
Tertiary > Secondary > Primary
The tertiary halogenoalkane is the most
reactive and the primary halogenoalkane is
the least reactive.
What is the trend in reactivity of chloro-,
bromo- and iodo- halogenoalkanes?
Explain in terms of bond enthalpy
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
What are some common uses of halogenoalkanes?
- Solvents
- Refrigerants
- Anaesthetics
Why are there tight regulations
surrounding the use of
halogenoalkanes?
Halogenoalkanes are toxic and have a
negative impact on the environment, so
their use is regulated.
What are CFCs?
Chlorofluorocarbons.
These are organic compounds containing
only carbon, chlorine and fluorine atoms.
What is the environmental problem
associated with the use of CFCs
(chlorofluorocarbons)?
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.
Give the equations for the reactions that
take place between a chlorine free
radical and ozone
Cl* + O3 → O2 + ClO*
ClO* + O3 → 2O2 + Cl*
Overall: 2O3 → 3O2
Explain the impact of the bonds C-H, C-F
and C-Cl on the atmosphere in reference
to the strength of the bond
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.
What is a reflux setup?
Reflux is the boiling setup with a vertical
condenser that allows the vapours to
return to the same mixture once they
condense.
