5. Halogen Derivatives Flashcards
is chloromethylbenzene a halogenoalkane or halogenoarene?
halogenoalkane. the methyl group (not the halogen) is directly attached to
does halogenoalkanes or alkanes have a higher boiling point?
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.
the boiling points of halogenoalkanes increase/decrease down group 17.
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.
solubility of halogenoalkanes and halogenoarenes
- soluble in polar solvents
- insoluble in water
preparation of halogenoalkanes [3]
- nucleophilic substitution of alcohols
- electrophilic addition of alkenes
- free radical substitution of alkanes
why is FRS not a recommended method of obtain a halogenoalkane?
a mixture of mono substituted and poly substituted halogenoalkanes will be obtained (wastage)
preparation of halogenoarenes [1]
- electrophilic substitution of benzene
explain the term nucleophilic substitution for halogenoalkanes
a halogen atom is substituted by a nucleophile
what is the reaction rate of SN2 mechanism?
rate = k[Nu][RX]
for energy profile diagrams, transition state for nucleophilic sub mechanism is the maximum/minimum?
maximum
briefly state the steps for SN2 mechanism
- nucleophilic attack
- transition state
- product formation
which halogenoalkanes go through which mechanism?
- 1° halogenoalkane - SN2
- 2° halogenoalkane - SN1 / SN2
- 3° halogenoalkane - SN1
what is the reaction rate of SN1 mechanism?
rate = k[RX]
briefly state the steps of SN1 mechanism
- formation of carbocation (slow)
- attack by Nu- (fast)
factors affecting nucleophilic sub
- C-X bond strength
- SN1 vs SN2
order of reactivity of halogenoalkanes towards nucleophiles in group 7
R-I > R-Br > R-Cl > R-F
C-F bond is so strong that fluoroalkanes are _____ & seldom undergo NS / elimination.
chemically inert
explain steric factors for SN1 vs SN2
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.
explain electronic factors for SN2
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
explain the stability of carbocation for SN1
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+
name the NS reactions of halogenoalkanes
- hydrolysis (HX to alcohol)
- formation of nitriles
- formation of 1° amines
reagent & conditions for hydrolysis (formation of alcohol from HX)
NaOH(aq)/KOH(aq), heat
reagent & conditions for formation of nitriles from HX
ethanolic KCN/NaCN, heat
reagent & conditions for formation of 1° amines from HX
excess concentrated NH3, heat
what’s a step up reaction
carbon chain lengthened by 1
reactions of nitriles & state the reagent and conditions [3]
- acid hydrolysis [form COOH]
- H2SO4(aq), heat - basic hydrolysis [form COO- salts]
- NaOH(aq), heat - reduction [form 1° amines]
- LiAlH4 in dry ether / H2(g), Ni, heat
why is NH3 used in excess for nucleophilic sub of HX w NH3?
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
reagents and conditions of elimination of HX from halogenoalkanes
ethanolic NaOH / KOH , heat
why must the solvent be aqueous to form alcohol from HX ?
if the solvent is not aqueous, elimination will occur instead of hydrolysis to form alkene
does halogenoarenes undergo nucleophilic substitution? why?
halogenoarenes are unreactive towards NS
1. partial double bond character of C-X bond
2. steric hindrance
3. electronic factors
name the reactions of electrophilic sub of halogenoarenes [3]
- nitration (conc. HNO3, conc H2SO4, >55°C)
- halogenation (X2, AlX3/FeX3, heat)
- friedel-craft alkylation (R-X, AlX3/FeX3, heat)
steps to distinguish between halogenoalkanes and halogenoarenes [3]
- add NaOH(aq) to both compounds and warm
- cool and acidify with HNO3(aq)
- 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
