3.3 Halogenoalkanes Flashcards
General formula of halogenoalkanes
CₙH₂ₙX
(X = F,Cl, Br, I)
Functional group of halogeoalkanes
C-X
Why are halogenoalkanes more reactive than alkanes?
They have a polar bond
Uses of halogenoalkanes
- Refrigerants
- Solvents
- Pharmaceuticals
What order are halogens named in a halogenoalkane
Alphabetical order
Nucleophile
An electron pair donator
Why do halogenoalkanes have nucleophilic substitution reactions?
They have a polar carbon-halogen bond
What happens in a nucleophilic substitution reaction?
- Nucleophile donates electron pair to the δ+C atom
- This forms a new colvalent bond
- The C-X bond breaks
- Halide ion is formed
Product of the reaction between halogenoalkanes and aqueous hydroxides
Alcohols
What type of reaction is halogenoalkane + aqueous hydroxide?
Hydrolysis (the mechanism is nucleophilic substitution)
Conditions for hydrolysis of halogenoalkanes with aqueous hydroxides
- Warm
- Aqueous
Reagent for hydrolysis of halogenoalkanes with aqueous hydroxides
NaOH / KOH
General equation for hydrolysis of halogenoalkanes
RX + NaOH → ROH + NaX
R - hydrocarbon chain
X - halogen
Ethanolic
Solution in ethanol
Product of the reaction between halogenoalkanes and potassium cyanide
Nitriles
Conditions for the reaction between halogenoalkanes and potassium cyanide
- Warm
- Ethanolic
Nitrile group
C≡N
General equation for the reaction between halogenoalkanes and potassium cyanide
RX + KCN → RCN + KX
R - hydrocarbon chain
X - halogen
What effect does reacting halogenoalkanes with cyanide have on the carbon chain?
Increases the chain length
Mechanism for halogenoalkane + potassium cyanide
Nucleophilic substitution
Mechanism for halogenoalkane + ammonia
Nucleophilic substitution
Halogenoalkanes are reacted with ______ ammonia to form ______
Excess, amines
Reagent for forming amines from halogenoalkanes
Excess ammonia
Conditions for nucleophilic substitution of halogenoalkanes + ammonia
- Warm
- Sealed tube
- Ethanolic
General equation for nucleophilic substitution of halogenoalkanes with ammonia
RX + 2NH3 –> RNH2 + NH4 + X
R - Carbon chain
X - Halogen
What are the two roles of the ammonia molecules in nucleophilic substitution of halogenoalkanes
As a nucleophile and as a base
What determines the rate of substitution reactions in halogenoalkanes?
The carbon-halogen bond enthalpy
Which halogeneoalkanes have the fastest rate of hydrolysis?
Iodoalkanes
C-I bonds have the lowest enthalpy so are the weakest and easiest to break
How can the rates of hydrolysis of halogenoalkanes be compaired?
- React chloro- bromo- and iodo-alkanes with aqueous silver nitrate
- Ethanol as a cosolvent
Which variables need to be controlled when comparing the rates of hydrolysis in different halogenoalkanes?
- Equal number of moles of halogenoalkanes
- Same chain length
- Water bath to control temperature
Elimination
A reaction in which an atom or group of atoms is removed from a reactant.
Product of an elimination reaction with a halogenoalkane
An alkene
Reagent for elimination reactions with halogenoalkanes
NaOH / KOH
Conditions for elimination reactions with halogenoalkanes
- Ethanol as solvent
- Reflux
Reflux
- A solution is heated to its boiling point
- Gas produced enters a condenser and returns to the solution
Which atoms are removed in elimination of halogenoalkanes?
The halogen + a H from an adjacent C
Role of OH⁻ in elimination of halogenoalkanes
As a base
Ozone
O₃
How is ozone formed?
O₂ + O → O₃
In the upper atmosphere
Why is ozone beneficial?
It absorbs harmful UV radiation
CFCs
ChloroFluoroCarbons
How do CFCs cause the breakdown of ozone?
- They rise to the upper atmosphere
- Break down to form chlorine radicals
- Chlorine radicals catalyse the breakdown of ozone
Equations for the breakdown of ozone
Cl· + O₃ → O₂ + ClO·
ClO· + O₃ → 2O₂ + Cl·
Overall: 2O₃ → 3O₂
Alternative to CFCs
HFCs (Hydrofluorocarbons)
Uses of CFCs
Solvents and refrigerants
Properties of CFCs
- Non-toxic
- Low reactivity
Why is an excess of ammonia used in nucleophilic substitution reactions?
To prevent further substitution reactions as the intermediate is a nucleophile