topic 6D

Question 1

primary halogenoalkane

Answer 1

• one carbon attached to the carbon atom adjoining the halogen

Question 2

secondary halogenoalkane

Answer 2

• two carbons attached to the carbon atom adjoining the halogen

Question 3

tertiary halogenoalkane

Answer 3

• three carbons attached to the carbon atom adjoining the halogen

Question 4

substitution

Answer 4

swapping a halogen atom for another atom or groups of atoms

Question 5

nucleophile

Answer 5

electron pair donator e.g. OH-, NH3, CN-

Question 6

what does the rate of substitution reactions depend on

Answer 6

• on the strength of the C-X bond
• the weaker the bond the easier it is to break and the faster the reaction

Question 7

is water a good or poor nucleophile

Answer 7

• poor nucleophile but it can react slowly with halogenoalkanes in substitution reactions

Question 8

nucleophilic substitution with aqueous hydroxide ions
- change in group
- reagent
- conditions
- mechanism
- role of reagent

Answer 8

• change in group: halogenoalkane to alcohol
• reagent: potassium or sodium hydroxide
• conditions: in aqueous solution; heat under reflux
• mechanism: nucleophilic substitution
• role of reagent: nucleophile, OH-

Question 9

SN2 mechanism for halogenoalkanes

Answer 9

• primary halogenoalkane
• show the transition state

Question 10

SN1 mechanism

Answer 10

• tertiary halogenoalkanes
• first the halogen breaks away from the halogenoalkane to for a carbocation intermediate
• the OH- nucleophile then attacks the positive carbon

Question 11

nucelophilic substitution with ammonia
- change in group
- reagent
- conditions
- mechanism
- role of reagent

Answer 11

• change in group: halogenoalkane to amine
• reagent: NH3 dissolved in ethanol
• conditions: heat under pressure in sealed tube
• mechanism: nucleophilic substitution
• role of reagent: nucleophile

Question 12

elimination with alcoholic hydroxide ions
- change in group
- reagent
- conditions
- mechanism
- role of reagent

Answer 12

• change in group: halogenoalkane to alkene
• reagent: potassium/sodium hydroxide
• conditions: in ethanol; heat
• mechanism: elimination
• role of reagent: base, OH-

Question 13

hydrolysis reaction with aqueous silver nitrate

Answer 13

• aqueous silver nitrate is added to a halogenoalkane
• halide leaving group combines with a silver ion to form a silver halide precipitate
• the quicker the precipitate is formed the faster the substitution reaction hence the more reactive the haloalkane

Question 14

rates of hydrolysis of iodo, bromo and chloroalkanes - fastest to slowest & colour of precipitate

Answer 14

• AgI - yellow precipitate
• AgBr - cream precipitate
• AgCl - white precipitate

Question 15

rates of hydrolysis of primary, secondary & tertiary halogenoalkanes

Answer 15

• primary halogenoalkanes are the least reactive so will take the longest to produce a precipitate.
• then it’s secondary halogenoalkanes.
• tertiary halogenoalkanes are the most reactive so will produce a precipitate almost instantaneously

Question 16

trend in reactivity of primary, secondary & tertiary halogenoalkanes

Answer 16

• primary halogenoalkanes have the highest boiling point and therefore are the least reactive
• as more carbon atoms are connected to the carbon atom the halogen is connected to, the more reactive the molecule is.
• reactivity goes tertiary > secondary > primary (most to least)

Question 17

trend in reactivity of chlor, bromo & iodoalkanes in terms of bond enthalpy

Answer 17

• iodoalkanes have the highest melting and boiling points due to greater London forces
• C-I bonds are weakest hence why iodoalkanes are the most reactive