Halogenoalkanes Flashcards
Naming halogenoalkanes
Based on original alkane, with a prefix indicating halogen atom:
Fluoro for F; Chloro for Cl; Bromo for Br; Iodo for I.
Substituents are listed alphabetically
Classifying halogenoalkanes
Halogenoalkanes can be classified as primary, secondary or tertiary depending on the number of carbon atoms attached to the C-X functional group.
What is a primary halogenoalkane
One carbon attached to the
carbon atom adjoining the
halogen
Secondary halogenoalkanes
Two carbons attached to the carbon atom adjoining the halogen
Tertiary halogenoalkanes
Three carbons attached to the carbon atom adjoining the halogen
What are the two reactions of halogenoalkanes
Halogenoalkanes undergo either
substitution or elimination reactions
Nucleophilic substitution meaning
Substitution: swapping a halogen atom for another atom or groups of atoms
Nucleophile: electron pair donator e.g. :OH-, :NH3, CN-
:Nu represents any nucleophile – they
always have a lone pair and act as
electron pair donators
Drawing nucleophilic substitution
The nucleophiles attack the positive carbon atom
carbon has a small positive charge because of the electronegativity difference between the carbon and the halogen
curly arrow will always start from a lone pair of electrons or
the centre of a bond
The rate of substitution reactions depending on the strength of the C-X bond
The weaker the bond, the easier it is to break and the faster the reaction.
The iodoalkanes are the fastest to substitute and the fluoroalkanes are the slowest. The strength of the C-F bond is such that fluoroalkanes are very unreactive
Hydrolysis of halogenoalkanes
Hydrolysis is defined as the splitting of a molecule ( in this case a halogenoalkane) by a reaction with water
Water is a poor nucleophile but it can
react slowly with halogenoalkanes in a
substitution reactio
Silver nitrate being added to halogenoalkanes
Aqueous silver nitrate is added to a halogenoalkane. The halide leaving group combines with a silver ion to form a silver halide precipitate.
The precipitate only forms when the halide ion has left the halogenoalkane and so the rate of formation of the precipitate can be used to compare the reactivity of the different
halogenoalkanes.
CH3CH2X + H2O CH3CH2OH + X- + H+
Hydrolysis of iodoalkane
CH3CH2I + H2O —> CH3CH2OH + I- + H+
Ag+ (aq) + I- (aq) —> AgI (s) - yellow precipitate
The iodoalkane forms a precipitate with
the silver nitrate first as the C-I bond is
weakest and so it hydrolyses the quickest
Comparing the rates if hydrolysis reactions
The quicker the precipitate is formed, the faster the substitution reaction and the more reactive the halogenoalkane.
The rate of these substitution reactions depends on the strength of the C-X bond. The weaker the bond, the easier it is to breal
and the faster the reaction.
What happens with nucleophilic substitution with aqueous hydroxide ions
Change in functional group: halogenoalkane —> alcohol
Reagent: potassium (or sodium) hydroxide
Conditions: In aqueous solution; warm
Mechanism: Nucleophilic substitution
Type of reagent: Nucleophile, OH-
The aqueous conditions needed is an important point. If the solvent is changed to ethanol
an elimination reaction occurs.
Alternative mechanism for tertiary halogenoalkanes
Tertiary halogenoalkanes undergo this mechanism as the tertiary carbocation is stabilised by the electron releasing methyl groups around it.
Also the bulky methyl groups prevent the hydroxide ion from attacking the halogenoalkane in the same way as the mechanism above.
The Br first breaks away from the halogenoalkane to form a carbocation intermediate. The hydroxide nucleophile then attacks the positive carbon
Nucleophilic substitution with cyanide ions
Change in functional group: halogenoalkane —> nitrile
Reagent: KCN dissolved in ethanol/water mixture
Conditions: Heating under reflux
Mechanism: Nucleophilic substitution
Type of reagent: Nucleophile, :CN-
This reaction increases the length of the carbon chain (which is reflected in the name) In the above example butanenitrile includes the C in the nitrile group
Naming nitrates
Nitrile groups have to be at the end of a chain. Start numbering the chain from the C in the CN.
Nucleophilic substitution with ammonia
Change in functional group: halogenoalkane —> amine
Reagent: NH3 dissolved in ethanol
Conditions: Heating under pressure (in a sealed tube)
Mechanism: Nucleophilic substitution
Type of reagent: Nucleophile, :NH3
Naming amines
In the above example
propylamine, the propyl shows
the 3 C’s of the carbon chain.
Sometimes it is easier to use the
IUPAC naming for amines e.g.
Propan-1-amine
Further substitution with amonia
Further substitution reactions can
occur between the halogenoalkane
and the amines formed leading to a
lower yield of the amine. Using
excess ammonia helps minimise this.
Elimination with alcoholic hydroxide ions
Elimination: removal of small molecule (often water) from the organic molecule
Change in functional group: halogenoalkane —> alkene
Reagents: Potassium (or sodium) hydroxide
Conditions: In ethanol ; heat
Mechanism: Elimination
Type of reagent: Base, OH-
Why is the solvent important to the type of reaction
Aqueous - substitution
Alcoholic - elimination
The structure of the halogenoalkane also has an effect on the degree to which substitution or elimination occurs in this reaction. Primary tends towards substitution
Tertiary tends towards elimination
Uses of halogenoalkanes
Chloroalkanes and chlorofluoroalkanes can be used as solvents. CH3CCl3 was used as the solvent in dry cleaning.
Halogenoalkanes have also been used as refrigerants, pesticides and aerosol propellants
Many of these uses have now been stopped due to the toxicity of halogenoalkanes and also their detrimental effect on the atmosphere.
Why is ozone beneficial
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.
Where are chlorine radicals formed
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 CF2Cl + Cl
The
Equation of chlorine catalysing the decomposition of ozone
Cl. + O3 —> ClO. + O2
ClO. + O3 —> 2O2 + Cl.
Overall equation
2 O3 —> 3 O2
The regenerated Cl radical means
that one Cl radical could destroy
many thousands of ozone molecules.
What do free radical chlorine atoms catalyse?
The chlorine free radical atoms 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
What is an alternative for chloroflurocarbons?
Legislation to ban the use of CFCs was supported by chemists and that they have now developed alternative chlorine-free compounds.
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.
