6.3 - Halogenoalkanes and alcohols
Halogenoalkanes (alkyl halides/haloalkanes)
● General formula: CnH2n+2-mXm (where m is number of halogen atom substitutions)
● Primary, secondary, tertiary...
Properties
● Solubility - even though halogenoalkanes are polar molecules, their CANNOT
hydrogen bond ∴ insoluble in water (thus soluble in non-polar\organic solvents)
● Boiling temperatures - higher than corresponding alkanes as they have higher Mr
leading to greater London forces, ALSO they are polar ∴ have permanent
dipole-dipole
● Non-flammable
Preparation of halogenoalkanes, from...
Alcohols
● Chloroalkanes
1. CnH2n+1OH + PCl5 → CnH2n+1Cl + POCl3 + HCl
Observations: steamy fumes
2. NaCl + H2SO4 → NaHSO4 + HCl, CnH2n+1OH + HCl → CnH2n+1Cl + H2O
Conditions: concentrated H2SO4
● Bromoalkanes
KBr + H2SO4 → KHSO4 + HBr, CnH2n+1OH + HBr → CnH2n+1Br + H2O
Conditions: 50% H2SO4
● Iodoalkanes
P + 1½I2 → PI3, 3CnH2n+1OH + PI3 → 3CnH2n+1I + P(OH)3
Conditions: react iodine with damp red phosphorus
●
Alkenes
● Electrophilic addition of halogen (diatomic)/hydrogen halide to C＝C bonds
1
, Alkanes
● Free-radical halogenation with halogen (diatomic)
○ This method is less controlled than others, and is only of preparative use for
very short or specifically activated alkanes
● Conditions: UV light
Reactions
● Reactivity of halogenoalkanes can be rationalised by considering the polarity of the
C—X bond
○ (in decreasing electronegativity) F > Cl > Br > I > At
●
●
Nucleophilic substitution
●
(where X = Cl, Br, I — won't work for F as C—F is strong due to F’s small radius)
○ When nucleophile is H2O, reaction is hydrolysis; trend in rate of hydrolysis:
chloroalkanes < bromoalkanes < iodoalkanes (C—I bond is the weakest)
● Eg CH3I + OH- → (hydrolysis | KOH (aq)) → CH3OH + I-
2
Halogenoalkanes (alkyl halides/haloalkanes)
● General formula: CnH2n+2-mXm (where m is number of halogen atom substitutions)
● Primary, secondary, tertiary...
Properties
● Solubility - even though halogenoalkanes are polar molecules, their CANNOT
hydrogen bond ∴ insoluble in water (thus soluble in non-polar\organic solvents)
● Boiling temperatures - higher than corresponding alkanes as they have higher Mr
leading to greater London forces, ALSO they are polar ∴ have permanent
dipole-dipole
● Non-flammable
Preparation of halogenoalkanes, from...
Alcohols
● Chloroalkanes
1. CnH2n+1OH + PCl5 → CnH2n+1Cl + POCl3 + HCl
Observations: steamy fumes
2. NaCl + H2SO4 → NaHSO4 + HCl, CnH2n+1OH + HCl → CnH2n+1Cl + H2O
Conditions: concentrated H2SO4
● Bromoalkanes
KBr + H2SO4 → KHSO4 + HBr, CnH2n+1OH + HBr → CnH2n+1Br + H2O
Conditions: 50% H2SO4
● Iodoalkanes
P + 1½I2 → PI3, 3CnH2n+1OH + PI3 → 3CnH2n+1I + P(OH)3
Conditions: react iodine with damp red phosphorus
●
Alkenes
● Electrophilic addition of halogen (diatomic)/hydrogen halide to C＝C bonds
1
, Alkanes
● Free-radical halogenation with halogen (diatomic)
○ This method is less controlled than others, and is only of preparative use for
very short or specifically activated alkanes
● Conditions: UV light
Reactions
● Reactivity of halogenoalkanes can be rationalised by considering the polarity of the
C—X bond
○ (in decreasing electronegativity) F > Cl > Br > I > At
●
●
Nucleophilic substitution
●
(where X = Cl, Br, I — won't work for F as C—F is strong due to F’s small radius)
○ When nucleophile is H2O, reaction is hydrolysis; trend in rate of hydrolysis:
chloroalkanes < bromoalkanes < iodoalkanes (C—I bond is the weakest)
● Eg CH3I + OH- → (hydrolysis | KOH (aq)) → CH3OH + I-
2
