5. Halogen Derivatives

Question 1

is chloromethylbenzene a halogenoalkane or halogenoarene?

Answer 1

halogenoalkane. the methyl group (not the halogen) is directly attached to

Question 2

does halogenoalkanes or alkanes have a higher boiling point?

Answer 2

halogenoalkanes. halogenoalkane has a simple molecular structure with pd-pd while alkanes has a simple molecular structure with id-id, thus more energy is needed to overcome the stronger FOA.

Question 3

the boiling points of halogenoalkanes increase/decrease down group 17.

Answer 3

increase. no. of electrons increases down the group, strength of id-id increase, hence more energy needed to overcome the stronger id-id, leading to the higher boiling point.

Question 4

solubility of halogenoalkanes and halogenoarenes

Answer 4

1. soluble in polar solvents
2. insoluble in water

Question 5

preparation of halogenoalkanes [3]

Answer 5

1. nucleophilic substitution of alcohols
2. electrophilic addition of alkenes
3. free radical substitution of alkanes

Question 6

why is FRS not a recommended method of obtain a halogenoalkane?

Answer 6

a mixture of mono substituted and poly substituted halogenoalkanes will be obtained (wastage)

Question 7

preparation of halogenoarenes [1]

Answer 7

1. electrophilic substitution of benzene

Question 8

explain the term nucleophilic substitution for halogenoalkanes

Answer 8

a halogen atom is substituted by a nucleophile

Question 9

what is the reaction rate of SN2 mechanism?

Answer 9

rate = k[Nu][RX]

Question 10

for energy profile diagrams, transition state for nucleophilic sub mechanism is the maximum/minimum?

Answer 10

maximum

Question 11

briefly state the steps for SN2 mechanism

Answer 11

1. nucleophilic attack
2. transition state
3. product formation

Question 12

which halogenoalkanes go through which mechanism?

Answer 12

1. 1° halogenoalkane - SN2
2. 2° halogenoalkane - SN1 / SN2
3. 3° halogenoalkane - SN1

Question 13

what is the reaction rate of SN1 mechanism?

Answer 13

rate = k[RX]

Question 14

briefly state the steps of SN1 mechanism

Answer 14

1. formation of carbocation (slow)
2. attack by Nu- (fast)

Question 15

factors affecting nucleophilic sub

Answer 15

1. C-X bond strength
2. SN1 vs SN2

Question 16

order of reactivity of halogenoalkanes towards nucleophiles in group 7

Answer 16

R-I > R-Br > R-Cl > R-F

Question 17

C-F bond is so strong that fluoroalkanes are _____ & seldom undergo NS / elimination.

Answer 17

chemically inert

Question 18

explain steric factors for SN1 vs SN2

Answer 18

SN2 is favoured for 1° H-X due to minimal steric hindrance as SN2 involves backside attack of Nu- thus the bulky alkyl groups bonded to C in 3° H-X will block the approach of Nu- from the back.

Question 19

explain electronic factors for SN2

Answer 19

order of reactivity of RX to SN2 :
3° RX < 2° RX < 1° RX < CH3X
the greater the no. of alkyl groups (e- donating) present, the less e- deficient C is, making it less susceptible to nucleophilic attack

Question 20

explain the stability of carbocation for SN1

Answer 20

order of reactivity of RX to SN1 :
3° RX > 2° RX > 1° RX > CH3X
the greater the no. of e- donating alkyl groups, the more dispersed the +ve charge on the C+ , the more stable the C+

Question 21

name the NS reactions of halogenoalkanes

Answer 21

1. hydrolysis (HX to alcohol)
2. formation of nitriles
3. formation of 1° amines

Question 22

reagent & conditions for hydrolysis (formation of alcohol from HX)

Answer 22

NaOH(aq)/KOH(aq), heat

Question 23

reagent & conditions for formation of nitriles from HX

Answer 23

ethanolic KCN/NaCN, heat

Question 24

reagent & conditions for formation of 1° amines from HX

Answer 24

excess concentrated NH3, heat

Question 25

what’s a step up reaction

Answer 25

carbon chain lengthened by 1

Question 26

reactions of nitriles & state the reagent and conditions [3]

Answer 26

1. acid hydrolysis [form COOH]
   - H2SO4(aq), heat
2. basic hydrolysis [form COO- salts]
   - NaOH(aq), heat
3. reduction [form 1° amines]
   - LiAlH4 in dry ether / H2(g), Ni, heat

Question 27

why is NH3 used in excess for nucleophilic sub of HX w NH3?

Answer 27

to prevent polyalkylation. since amines (products) are nucleophiles themselves, further substitution can occur which creates a mixture of products causing difficulty in separation and poor yield

Question 28

reagents and conditions of elimination of HX from halogenoalkanes

Answer 28

ethanolic NaOH / KOH , heat

Question 29

why must the solvent be aqueous to form alcohol from HX ?

Answer 29

if the solvent is not aqueous, elimination will occur instead of hydrolysis to form alkene

Question 30

does halogenoarenes undergo nucleophilic substitution? why?

Answer 30

halogenoarenes are unreactive towards NS
1. partial double bond character of C-X bond
2. steric hindrance
3. electronic factors

Question 31

name the reactions of electrophilic sub of halogenoarenes [3]

Answer 31

1. nitration (conc. HNO3, conc H2SO4, >55°C)
2. halogenation (X2, AlX3/FeX3, heat)
3. friedel-craft alkylation (R-X, AlX3/FeX3, heat)

Question 32

steps to distinguish between halogenoalkanes and halogenoarenes [3]

Answer 32

1. add NaOH(aq) to both compounds and warm
2. cool and acidify with HNO3(aq)
3. add AgNO3(aq)
   reasoning for steps 1-3
   1: RX will undergo NS
   2: HNO3 removes excess NaOH to prevent insoluble Ag2O/AgOH (brown ppt)
   3: Ag+ tests for presence of halide ions