[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)