Date | November 2018 | Marks available | 1 | Reference code | 18N.2.hl.TZ0.9 |
Level | HL | Paper | 2 | Time zone | TZ0 |
Command term | State | Question number | 9 | Adapted from | N/A |
Question
Halogenoalkanes undergo nucleophilic substitution reactions with sodium hydroxide.
State a reason why most halogenoalkanes are more reactive than alkanes.
Classify 1-bromopropane as a primary, secondary or tertiary halogenoalkane, giving a reason.
Explain the mechanism of the reaction between 1-bromopropane with aqueous sodium hydroxide using curly arrows to represent the movement of electron pairs.
State, giving your reason, whether the hydroxide ion acts as a Lewis acid, a Lewis base, or neither in the nucleophilic substitution.
Suggest two advantages of understanding organic reaction mechanisms.
Markscheme
polarity/polar «molecule/bond»
OR
carbon–halogen bond is weaker than C–H bond ✔
primary AND Br/bromine is attached to a carbon bonded to two hydrogens
OR
primary AND Br/bromine is attached to a carbon bonded to one C/R/alkyl «group» ✔
Accept “primary AND Br/bromine is attached to the first carbon in the chain”.
curly arrow going from lone pair/negative charge on O in HO– to C ✔
curly arrow showing Br leaving ✔
representation of transition state showing negative charge, square brackets and partial bonds ✔
formation of organic product CH3CH2CH2OH AND Br– ✔
Do not allow curly arrow originating on H in HO–.
Accept curly arrow either going from bond between C and Br to Br in 1-bromopropane or in the transition state.
Do not penalize if HO and Br are not at 180° to each other.
Do not award M3 if OH–C bond is represented.
«Lewis» base AND donates a pair of electrons ✔
Any two of:
choose «most» appropriate reaction «for preparing the target compound» ✔
design/discover new reactions/reagents ✔
apply this knowledge to other areas of chemistry/science ✔
«retro-»synthesis «more effective» ✔
control/predict «desired» products ✔
control rate of reaction «more effectively» ✔
satisfy intellectual curiosity ✔
predicting how changing reagents/conditions might affect reaction ✔
suggesting intermediates/transition states ✔
Accept other reasonable answers.