3.3.3 Halogenoalkanes Flashcards
How do you name halogenoalkanes?
Based on original alkane, with a prefix indicating halogen atom:
Fluoro for F; Chloro for Cl; Bromo for Br; lodo for I.
Substituents are listed alphabetically
What is a primary, secondary and tertiary halogenoalkane?
Primary halogenoalkane
One carbon attached to the carbon atom adjoining the halogen
Secondary halogenoalkane
Two carbons attached to the carbon atom adjoining the halogen
Tertiary halogenoalkane
Three carbons attached to the carbon atom adjoining the halogen
What types of reactions can halogenoalkanes undergo?
Halogenoalkanes undergo either substitution or elimination reactions.
What are nucleophilic substitution reactions?
• In a substitution reaction, the halogen atom is replaced by another atom/group.
• Halogenoalkanes are susceptible to attack by nucleophiles (lone pair donors) such as OH^-, CN^- and NH3
• This is because the C-halogen bond is polar with the C atom being partially+
• The C-halogen bond is polar because the halogen atom is more electronegative than the carbon atom.
• The rate of the reaction is affected by the strength of the C-halogen bond. The weaker the bond, the faster the reaction.
(Fastest) iodoalkanes>bromoalkanes>chloroalkanes>fluoroalkanes (Slowest)
Weakest bond = C-I Strongest bond = C-F
What are the conditions needed for nucleophilic substitution with NaOH?
Aqueous, warm.
What are the conditions needed for nucleophilic substitution with KCN?
Aqueous ethanol, warm.
What are the conditions needed for nucleophilic substitution with NH3?
Excess concentrated ammonia dissolved in ethanol at pressure in a sealed container.
What are elimination reactions?
• When halogenoalkanes react with OH^- ions, an elimination reaction can compete with the nucleophilic substitution reaction.
• Elimination is favoured if hot, ethanolic KOH is used instead of warm, aqueous NaOH.
• The change from NaOH to KOH is simply about solubility, with KOH being more soluble than NaOH in ethanol. The key changes are the solvent (from water to ethanol) and the higher temperature.
• In elimination, an H and X are removed from adjacent C atoms giving an alkene.
• This reaction can give a mixture of alkenes, either because the H can come from different adjacent C atoms, or because the alkenes formed have E/Z stereoisomerism.
• If there is no H on a C atom adjacent to the C-X then no elimination is possible.
• In elimination, the OH^- ion acts as a base. In substitution, the OH ion acts as a nucleophile.
What are the conditions needed for elimination with KOH?
Ethanolic, hot.
What are the uses of halogenoalkanes?
Chloroalkanes and chlorofluoroalkanes can be used as solvents.
Halogenoalkanes have also been used as refrigerants, pesticides and aerosol propellants.
What is ozone?
The naturally occurring ozone (O3) layer in the upper atmosphere is beneficial as it filters out much of the sun’s harmful UV radiation.
Ozone in the lower atmosphere is a pollutant and contributes towards the formation of smog.
What do CFCs do to the ozone layer and how?
Man-made chlorofluorocarbons (CFC’s) caused a hole to form in the ozone layer.
Chlorine radicals are formed in the upper atmosphere when energy from ultra-violet radiation causes C-Cl bonds in chlorofluorocarbons (CFCs) to break.
CF2Cl2 →(uv) •CF2Cl + Cl•
The chlorine free radical atoms react with ozone - they catalyse the decomposition of ozone, due to these reactions, because they are regenerated. (They provide an alternative route with a lower activation energy)
These reactions contributed to the formation of a hole in the ozone layer.
Cl• + O3 → ClO• + O2
ClO• + O3 → 2O2 + Cl•
Overall equation
2O3 → 3O2
The regenerated Cl radical means that one Cl radical could destroy many thousands of ozone molecules (chain reaction).
How was the issue of CFCs depleting the ozone layer resolved?
HFCs (Hydro fluoro carbons) e.g. CH2FCF3 are now used for refrigerators and air-conditioners. These are safer as they do not contain the C-Cl bond.
The C-F bond is stronger than the C-Cl bond and is not affected by UV.
