[3.3.3] Halogenoalkanes Flashcards
Nucleophilic Substitution, Elimination & Ozone Depletion
How do you name halogenoalkanes?
- Based on original alkane, with a prefix indicating halogen atom:
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Fluoro for F; Chloro for Cl; Bromo for Br; Iodo for I.
- Substituents are listed alphabetically.
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Fluoro for F; Chloro for Cl; Bromo for Br; Iodo for I.
Name this molecule.
1-bromopropane.
Name this molecule.
2-chloro-2-methylbutane.
What is a primary halogenoalkane?
A halogenoalkane with only one carbon attached to the carbon atom adjoining the halogen.
What is a secondary halogenoalkane?
A halogenoalkane with two carbon atoms attached to the carbon atom adjoining the halogen.
What is a tertiary halogenoalkane?
A halogenoalkane with three carbon atoms attached to the carbon atom adjoining the halogen.
Classify each of the molecules below as either being: a primary, secondary or tertiary halogenoalkane.
What reactions do halogenoalkanes undergo?
- Nucleophilic substitution.
- Elimination.
Which structure of halogenoalkane favours nucleophilic substitution reactions?
Primary halogenoalkanes.
Which structure of halogenoalkane favours elimination reactions?
Tertiary halogenoalkanes.
What is a nucleophile?
- An electron pair donator - they always have a lone pair.
- e.g. :OH⁻, :NH₃, CN⁻.
Explain why the carbon-halogen bond enthalpy influences the rate of reaction.
- The rate of substitution reactions depends on the strength of the C-X bonds (X = halogen atom).
- The weaker the bond (i.e. the lower its bond enthalpy) the easier it is to break and the faster the reaction.
- The iodoalkanes are the fastest to substitute, then the bromoalkanes, the the chloroalkanes and finally the slowest of the four being the fluoroalkanes.
Describe and explain a method by which you could carry out test-tube hydrolysis of halogenoalkanes to compare their relative rates of reaction.
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Aqueous silver nitrate is added to a halogenoalkane and the halide leaving group combines with a silver ion to form a silver halide precipitate.
- e.g. Ag⁺ (aq) + I⁻ (aq) -> AgI (s)
- 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.
- The quicker the precipitate is formed, the faster the substitution reaction and the more reactive the halogenoalkane.
- The rate of substitution reactions depends on the strength of the C-X bond (X being the halogen atom). The weaker the bond, the easier it is to break and the faster the reaction.
- As you go down group 7, the formation of the precipitate increases as the strength of the C-X bond becomes weaker so:
- AgI, a yellow precipiate, forms the fastest
- Then AgBr, a cream precipitate
- And AgCl, a white precipitate forms the slowest
What is the change in functional group, reagent, role of reagent & conditions when a halogenoalkane undergoes a nucleophilic substitution reaction with hydroxide ions ?
CHANGE IN FUNCTIONAL GROUP
- Halogenoalkane -> alcohol.
REAGENT
- Potassium (or sodium) hydroxide
ROLE OF REAGENT
- Nucleophile, :OH⁻
CONDITIONS
- In aqueous solution; Heat under reflux
- The aqueous conditions needed are important. If the solvent changed to ethanol, an elimination reaction would occur.
Draw and state the mechanism when bromoethane reacts with aqueous potassium hydroxide.
Name the product.
- MECHANISM: Nucleophilic substitution.
- PRODUCT: Ethanol.
(The carbon has a small positive charge because of the electronegativity difference between the carbon and the halogen)
What is the change in functional group, reagent, role of reagent & conditions when a halogenoalkane undergoes a nucleophilic substitution reaction with cyanide ions?
CHANGE IN FUNCTIONAL GROUP
- Halogenoalkane -> nitrile.
REAGENT
- Potassium cyanide/KCN dissolved in ethanol/watermixture
ROLE OF REAGENT
- Nucleophile, :CN⁻
CONDITIONS
- Heating under reflux.
Draw and state the mechanism when bromoethane reacts with potassium cyanide/KCN.
Name the product.
- MECHANISM: Nucleophilic substitution.
- PRODUCT: Propanenitrile.
- This reaction increases the length of the carbon chain which is reflected in the name.
(The carbon has a small positive charge because of the electronegativity difference between the carbon and the halogen)
How do you name nitriles?
- The suffix -nitrile is used.
- Nitrile groups have to be at the end of the chain.
- Start numbering the chain from the C in the CN.
- Nitrile groups increase the length of the carbon chain which needs to be reflected in the name.
Name this molecule.
Propanenitrile.
Name this molecule.
3-methylbutanenitrile.
(Note the naming: butanenitrile and not butannitrile)
What is the change in functional group, reagent, role of reagent & conditions when a halogenoalkane undergoes a nucleophilic substitution reaction with ammonia?
CHANGE IN FUNCTIONAL GROUP
- Halogenoalkane -> amine.
REAGENT
- NH₃ dissolved in ethanol.
ROLE OF REAGENT
- Nucleophile, :NH₃
CONDITIONS
- Heating under pressure (in a sealed tube).
Draw and state the mechanism when bromopropane reacts with ammonia.
Name the product.
- MECHANISM: Nucleophilic substitution.
- PRODUCT: Propan-1-amine.
(The carbon has a small positive charge because of the electronegativity difference between the carbon and the halogen)
What is elimination?
Removal of small molecule (often water) from an organic molecule.
What is the change in functional group, reagent, role of reagent & conditions when a halogenoalkane undergoes an elimination reaction with hydroxide ions?
CHANGE IN FUNCTIONAL GROUP
- Halogenoalkane -> alkene.
REAGENT
- Potassium (or sodium) hydroxide
ROLE OF REAGENT
- Base, :OH⁻
CONDITIONS
- In ethanol ; heat.
Draw and state the mechanism when bromoethane reacts with alcoholic potassium hydroxide.
Name the product.
- MECHANISM: Elimination.
- PRODUCT: Propene.
Often a mixture of different structural isomers forms as a result of elimination and nucleophilic substitution reactions.
What types of halogenoalkane structures allow for this to happen?
With unsymmetrical secondary and tertiary halogenoalkanes, two or sometimes three different structural isomers can be formed.
Name the products formed from the elimination of 2-methyl-2-chlorobutane.
Describe the uses of halogenoalkanes.
Why have many of these uses now been stopped?
USES OF HALOGENOALKANES
- Chloroalkanes and chlorofluoroalkanes can be used as solvents.
- CH₃CCl₃ was used as the solvent in dry cleaning.
- Halogenoalkanes have been used as refrigerants, pesticides and aersol propellants.
WHY HAVE MANY OF THESE USES STOPPED?
- Many of these uses have now been stopped due to the toxicity of halogenoalkanes and also their detrimental effect on the atmosphere.
- Man-made chlorofluorocarbons (CFCs) can cause a hole to form in the ozone layer.
How is ozone beneficial?
- Naturally occurring ozone (O₃) layer in the upper atmosphere is beneficial as it filters out much of the sun’s harmful UV radiation.
What is the issue with ozone in the lower atmosphere?
A pollutant and contributes towards the formation of smog.
How do chlorine free radicals form in the upper atmosphere?
- 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·
Explain, using equations, how chlorine free radical atoms catalyse the decomposition of ozone and contribute to the hole in the ozone layer.
- Cl· + O₃ -> ClO· + O₂
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ClO· + O₃ -> 2O₂ + Cl·
- The regenerated Cl radical means that one Cl radical could destroy many thousands of ozone molecules which can lead to a hole in the ozone layer.
OVERALL EQUATION
- 2O₃ -> 3O₂
Legislation to ban the use of CFCs was supported by chemists and they have now developed alternative chlorine-free compounds.
What alternatives have been developed?
- HFCs (hydrofluorocarbons) 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.
