The document discusses SN1 and SN2 reaction mechanisms of alcohols. Tertiary alcohols undergo SN1 reactions with hydrogen halides faster than secondary or primary alcohols due to their ability to form stable carbocation intermediates. Primary alcohols favor SN2 reactions. Polar solvents can stabilize carbocations and favor the SN1 pathway. Common reagents used to convert alcohols to alkyl halides include thionyl chloride, phosphorus halides, and halogenation using sodium halides.

1. Done by
SHOBANA.N.S
QUEEN MARY’S COLLEGE

3.  Possible to make tertiary chloroalkanes
 But to make primary or secondary ones you
really need to use a different method.
Tertiary alcohol + HCl  Tertiary chloroalkanes

4.  Alcohols that form stable carbocations will react
faster
 Tertiary chloroalkane formed is insoluble and so it
looks cloudy

5.  It’s just the SN1 reaction of an alcohol with
HCl.
 The Lucas reagent is ZnCl2 in concentrated
HCl.

6. For comparison :

7. OH
primary
OH
secondary
OH
tertiary
ZnCl2, HCl
Cl
>10 minutes
(if at all)
Cl
<5 minutes
Cl
1-2 seconds

8. LUCAS TEST

9.  Instead of hydrobromic acid, usually alcohol is
treated with a mixture of sodium or potassium
bromide and concentrated sulfuric acid.
 This produces hydrogen bromide which reacts with
the alcohol.

10.  Alcohol reacts with mixture of sodium or
potassium iodide and conc. phosphoric(V)
acid, H3PO4.

11.  The most frequently used of these reagents is HF-pyridine
(Olah’s Reagent)

12. GENERAL FORMAT

13. • SN1indicates substitution, nucleophilic, unimolecular
reaction
• It involves two steps
STEP 1
• Loss of the leaving group, LG, to generate a
carbocation intermediate

14. STEP 2
• Rapid attack of a nucleophile on the electrophilic
carbocation to form a new sigma bond
REACTIVITY ORDER :
(CH3)3C- > (CH3)2CH- > CH3CH2- > CH3-

15.  Polar solvents which can stabilize carbocations
which can favor the SN1 reaction
• Hydrogen halide reactivity order :
HI > HBr > HCl > HF
• Secondary and tertiary alcohols will proceed via
SN1 reaction
HO
HBr
Br
+
Br
cis & trans

16. HO
H-Br
Br
+
Br
cis & trans
H
HO
H
- H2O
Br
Br

17.  SN2 indicates
a substitution, nucleophilic, bimolecular reaction.
 This is a concerted process (single step).

18.  REACTIVITY ORDER:
CH3- > CH3CH2- > (CH3)2CH- > (CH3)3C-
 Primary alcohol will proceed via SN2 Mechanism

19. PX3
PX5
SOCl2
SOCl2
PCl3
PCl5

20.  3R - OH + PX3 3R - X + H3PO3
 R - OH + PX5 R - X + POX3 + HX

21.  3R - OH + PCl3 3R - Cl + H3PO3
 R - OH + PCl5 R - Cl + POCl3 + HCl
C2H5OH
PCl5
C2H5Cl
POCl3
HCl
+ + +
C2H5OH
+
+
PCl3

27. REACTION MECHANISM

28.  It involves the exchange of one halogen atom for
another
The reaction works well for primary halides, allyl, benzyl,
and α-carbonyl halides.
Secondary halides are far less reactive.
Vinyl, aryl and tertiary alkyl halides are unreactive.
LiCl
Acetone
RT, 72 h
73 %
EXAMPLE 1: Synthesis of Taxol

29. Synthesis of Strychnine
O
O
N
O
N
N
H3C
CH3
O
Ms
t-Bu
O
O
N
O
N
N
H3C
CH3
O
H
t-Bu
O
O
N
Br
N
N
H3C
CH3
O
t-Bu
MsCl LiBr

30. Synthesis of Doliculide
NaI
MEK
85 °C, 15 h
Synthesis of Cristatic Acid
MsCl, LiBr
CH2Cl2, THF
0 °C, 3 h
76 %

31. SYNTHESIS OF HIRSUTENE
n-Bu4N+ I-PhH
Reflux, 2 h
75 %
SYNTHESIS OF AIGIALOMYCIN
NaI
MEK
Reflux, 24 h
99 %

35. • This is a useful reaction, because the resulting alkyl
halides are versatile compounds that can be converted
into many compounds that are not directly accessible
from the alcohol itself.