Summary of halogeno-compound
by Kwok Man Ki
Characteristics of halogeno-compound:
High polarity of C-Hal due to electronegative difference(EN)
Two types of reactions:
1. Nucleophilic substitution(SN)
Nucleophile: molecule or anion that contains unshared pair of electrons to seek
positive center
[Molecularity is the no. of molecules in transition state in rate determining step]
2. Elimination(E)
Base: attack the positive –carbon
E may compete with SN1
∵Same r.d.s.
CH3X  CH2+ + X∴C-X  C-Nu (unimolecular nucleophilic substitution)
 Alkene (elimination)
1 : SN2
2o : SN2 with E
3o : SN1 with E
E dominant SN -- high temp
--strong base
Haloalkane
Reactions/explanations
Conditions
SN2(Bimolecular)
CH3CH2Br + OH
Methyl or 1o
[HO---CH2CH3---Br]CH3CH2OH + BrSN1(Unimolecular)
r.d.s.: CH3CH2Br CH3CH2+ + BrMainly 3o
o
Factors affecting rate of SN
I. structure of haloalkane
SN1:stability of
carbocation
e.g.alkyl group -electron
donating group
intermediate-stabilized by
resonance
relatively unhindered
Order of nucleophilic ability:
RO->HO->>ROH>H2O
(negative chargeuncharged
molecule)
SN2:steric effect
II. Nature of nucleophile
SN1:no effect not
involve in r.d.s.
SN2:concentration of
nucleophile and
nucleophilicity
III. Nature of leaving group
ease of leaving(agree
I->Br->Cl->Fwith order of bond
enthalpy)
uncharged molecule
-better leaving group
Protonation(alcohol in
acid)
Rate of reaction:
Unreactivity of Halobenzene
∵C-X bond has multiple
bond character ∴ no SN2
∵Sp2 overlap  C-X bond
shorter and bond enthalpy
larger
∵If SN1 loss of aromaticity
Elimination
Halide
Dihalides (-2mole Br)
Formation of bond
C-C: ↑1 carbon atom
C-O bond: alcohol
ether
C-N bond
Conc.NH3  mixture of
products may be formed
∵alkyl group bonded to N
act as electron donating
group→nucleophilic
power↑
large excess of NH3
CN- as base(↑1 C)
H2O>>OH->ORHX + R-OH+  R-X + H2O
(neutral; good leaving group)
SN1+SN2>SN1>SN2
e.g. Ar–CH2X
Ar-COCl (Both O and Cl are
electronegative to withdraw
electrons form C, making C
become more positive)
H-C-C-Br  -C=CBr-C-C-Br  -C≡C-
Alcoholic NaOH
Excess
KOH/C2H5OH with
reflux
R-C-Br +  R-C-CN  COOH KCN/alcohol;H+
≠HCN
∵weak base→ poor
nucleophile
toxic gas
R-X  R-OH
NaOH/reflux
R-X  R-O-R
CH3CH2O-Na+/reflux
(CH3CH2-OH + Na
 CH3CH2O- + Na+
+ H2
Mixture of products
CH3CH2-X + NH3 
CH3CH2-NH2  (CH3CH2)2-NH
 (CH3CH2)3-N
 (CH3CH2)4-N+XNH3 + CH3CH2I 
CH3CH2-NH2 (monosubstituted)
CH3CH2I + CN- 
LiAlH4/ether or
CH3CH2CH2-NH2
2H2/Ni