Preparation and reactions of alkyl halides, SN1 and SN2 reaction, uses of various alkyl halides

1. Alkyl halide
Mr. Mote G.D.
ADCBP, Ashta Dist: Sangli
Maharashtra

2. Alkyl halides
• Alkyl halides are the derivatives of alkanes in which hydrogen
atom is replaced by a halogen atom F, Cl, Br or I
• Alkyl halides are represented by R-X, R-alkyl group, X- halogen
like chloro, Fluoro, Bromo, and Iodo group.
C
H
H
H Cl
C
H
H
H C C
H
H
H C
H
H
Cl
H
H
C
H
H
Cl
Methyl chloride
ethtyl chloride propyl chloride

3. Preparation methods of alkyl halides
1. Direct Halogenations of alkanes.
2. Addition of hydrogen halides to alkenes and
alkynes.
3. Action of Hydrogen halides on alcohol.
4. Action of Phosphorus halides on alcohols.
5. Action of Thionyl chloride on alcohols.
6. Halogen Exchange.
7. The Hunsdiecker Reaction.

4. 1. Halogenation of alkane
• It involves the substitution of H-atoms of alkanes by as many halogen
atoms i.e., by chlorine (chlorination) ; by bromine (bromination) by iodine
(iodination) or by fluorine (fluorination)
• Methane reacts with chlorine in the presence of ultraviolet light or at high
temperature (300°C) to yield methyl chloride or chloromethane and
hydrogen chloride.
CH4
Cl2
uv light
CH3Cl HCl
Methane chlorine methyl chloride
4Mr. Mote G.D

5. 2. Addition of hydrogen halides
1. Alkenes react with hydrogen halides (HCl, HBr or HI) to form
alkyl halides.
H2C CH2 HBr CH3 CH2 Br
ethyl bromide
ethylene
C
H
C
H HBr
2-Bromo butane2-Butene
H3C CH3
C
H
C
H
H3C CH3
BrH
5Mr. Mote G.D

6. 3. Action of halogen acids on alcohol
• Alcohols react with HBr or HI to produce alkyl bromides or
alkyl halides.
6Mr. Mote G.D
H3C
H2
C HBr CH3 CH2 Br
ethyl bromide
OH
H2O
ethyl alcohol
Mechanism:
HBr H+ + Br-
H3C
H2
C OH
CH3 CH2 Br
ethyl bromide
H2O

7. 4. Action of Phosphorus halides on alcohol
• Alcohols react with phosphorus halides to produce alkyl
halides.
7Mr. Mote G.D
H3C
H2
C PCl5
CH3 CH2 Cl
ethyl bromide
OH
HCl
ethyl alcohol
POCl3
P
Cl
Cl
Cl
Cl
Cl
H3C
H2
C OH
ethyl alcohol
+
-HCl
H3C
H2
C O-
P
Cl
Cl
Cl
Cl+
CH3 CH2 Cl
ethyl bromide
POCl3
Mechanism

8. 5. Action of thionyl chloride on alcohol
• Alcohols react with thionyl chloride in the presence of
pyridine to produce alkyl halides.
8Mr. Mote G.D
H3C
H2
C SOCl2
CH3 CH2 Cl
ethyl chloride
OH
SO2
ethyl alcohol
HClpyridine
S OCl
Cl
H3C
H2
C OH
ethyl alcohol
-HCl
CH3 CH2 O- + S O
Cl
CH3 CH2 Cl
ethyl chloride
SO2+
Mechanism

9. 6. Halogen exchange reaction
• The alkyl bromide is heated with concentrated solution of
sodium iodide in acetone to form alkyl iodide.
9Mr. Mote G.D
H3C
H2
C NaI CH3 CH2 I
ethyl iodide
Br
NaBr
ethyl bromide
pyridine

10. 7. Hunsdiekar reaction
• Silver salt of carboxylic acid react with halogen to give unstable
intermediate which is decarboxylated to form alkyl halides.
10Mr. Mote G.D
CH3 C
O
OAg
Silver acetate
Br2
CCl4
CH3 Br + AgBr + CO2
Methyl bromide
Mechanism
CH3 C
O
OAg
Silver acetate
Br2
-AgBr
CH3 C
O
OBr CH3 C
O
O + Br
-CO2
CH3 + Br
CH3 Br
Methyl bromide

11. Reactions of alkyl halides
1. Nucleophilic substitution reactions
a) Substitution by hydroxyl group
b) Substitution by amino group
c) Substitution by alkynyl group
d) Substitution by alkoxy group
e) Substitution by thiol group (SH)
f) Substitution by thioether group (—S—).
g) Substitution by an ester group. (RCOO—).
h) Substitution by a Cyano or Isocyano group
2. Reduction: Reduction of Alkyl halides
3. Elimination reactions
4. Wurtz reaction
5. Reactions with Metals.

12. Nucleophilic substitution reaction
• Nucleophile: any reagent that donates an unshared pair of
electrons to form a new covalent bond.
• Nucleophilic substitution reaction in which one nucleophile is
substituted for another.
• Two types of nucleophilic substitution reactions
1) SN1 : Unimolecular Nucleophilic Substitution
2) SN2 : Bimolecular Nucleophilic Substitution
12Mr. Mote G.D
Nu: C X
Nucleophilic
substitution C Nu :X
Nucleophile
Alkyl halide

13. I. Substitution by hydroxyl group
• Alkyl halides react with aqueous potassium hydroxide to form
alcohol.
13Mr. Mote G.D
H3C
H2
C NaOH CH3 CH2 OH
Ethanol
Br
NaBr
ethyl bromide
H2O

14. SN1 : Unimolecular Nucleophilic Substitution
• In this reaction, bond breaking between carbon and leaving
group is complete before bond formation with nucleophile.
• This type of reaction is classified as unimolecular because
only the alkyl halide is involved in rate determing step.
• Rate: k[alkyl halide]
14Mr. Mote G.D
C
CH3
H3C
CH3
Br
Nucleophilic
substitution C
CH3
H3C
CH3
OCH3
Methanol
CH3OH
HBr
2-methoxy-2-methylpropane2-bromo-2- methylpropane

15. I. Mechanism of SN1
1. Ionization of a C-X bond gives a 3° carbocation intermediate
2. Reaction of methanol from either face of the planra carbocation intermediate gives an oxonium ion.
3. Proton transfer to give tert- butyl methyl ether
15Mr. Mote G.D
C
CH3
H3C
CH3
Br
slow, rate
determining C
CH3
H3C
CH3
Br-
C
CH3
H3C
CH3
fast
HOCH3 C
CH3
H3C
CH3
OCH3
H
C
CH3
H3C
CH3
OCH3
H
Br-
C
CH3
H3C
CH3
OCH3 HBr
2-methoxy, 2-methyl
propaneoxonium ion
oxonium ionCarbocation
ter butyl carbocation
ter butyl bromide

16. Energy profile diagram of SN1
Two peak will arises
1. One is formation transition when leaving group is removing.
2. Second peak arises when nucleophile forming pair with carbocation.
3. Carbocation lies in between two peak.
4. Product has lower energy
16Mr. Mote G.D

17. SN2 : bimolecular Nucleophilic Substitution
• It is concerted process, bond breaking and bond formation
takes place simultaneously
• This type of reaction is classified as bimolecular because the
alkyl halide and nucleophile are involved in rate determining
step.
• Rate: k[alkyl halide][Nucleophile]
17Mr. Mote G.D
C
H
H
H
Br
Nucleophilic
substitution C
H
H
H
OH
NaOH
NaBr
methanolmethyl bromide

18. Mechanism of SN2
1. The nucleophile attacks the reactive centre from the opposite side of leaving group, Backside attack by
nucleophile.
2. SN2 reaction is driven by the attraction between negative charge of nucleophile and positive charge on
leaving group.
18Mr. Mote G.D
NaOH OHNa
C
H
H
Na
H
BrOH-
C
H
H
H
OH NaBr
C
H
H
H
BrHO

19. energy profile of SN2
1. Only one rate determining step because one transition step is generated
2. Initially reation is slow and after transition reaction will faster
19Mr. Mote G.D

20. SN1 versus SN2
20Mr. Mote G.D
Sr.No SN1 SN2
1 Unimolecular Bimolecular
2 Most favorable alkyl
halide 3°>2°>1
Most favorable alkyl halide
1°>2°>3°
3 Reacting nucleophile
weak
Reacting nucleophile strong
base
4 Solvent is protic like
alcohol
Solvent is aprotic DMSO
and acetone
5 Stereochemistry is
mixed retention as well
as inversion
Stereochemistry is
inversion

21. Factor affecting on SN1 and SN2
21Mr. Mote G.D
Sr.
no
SN1 SN2
1 Good leaving group- alcohol Better leaving groups faster the reaction
2 Reactivity order: 3°>2°>1° Reactivity order: 3°>2°>1°
3 Stable carbocation-tertiary
carbocation
4 Weak bases-Water or Alcohol Favored by strong base
5 Polar protic solvents Polar aprotic solvents
6 Only substrate concentration
fasters the reaction
substrate concentration as well as strong
base fasters the reaction
7 First order kinetics Second order kinetics

22. II. Substitution by amino group
• Alkyl halides react with alcoholic solution of ammonia to form
amines.
22Mr. Mote G.D
NaBr
NH3
NH2 + H+
H3C
H2
C Br
δ+
δ−
CH3 CH2 NH2
HBr
Mechanism
H3C
H2
C NH3
CH3 CH2 NH2
ethyl amine
Br
HBr
ethyl bromide
Ethanol
ethyl amine
ethyl bromide

23. III. Substitution by alkynyl group
• Alkyl halides react with sodium acetylides to form higher
alkynes.
23Mr. Mote G.D
H3C
H2
C C CH3 CH2 CBr
NaBr
ethyl bromide
Ethanol
CHNa CH
but-1-yne
CH + Na+
H3C
H2
C Br
δ+
δ−
CH3 CH2 C
Mechanism
ethyl bromide
CH
C
NaBr
CH CNa
but-1-yne

24. IV. Substitution by alkoxy group
• Alkyl halides react with sodium methoxide to form ethers.
• Sodium alkoxide is prepared by dissolving sodium in alcohol.
• This method of making alcohol is called as williamson ether
synthesis
24Mr. Mote G.D
H3C
H2
C CH3 CH2 OBr
NaBr
ethyl bromide
CH3ONa CH3
methoxyethanesodium methoxide
CH3ONa
CH3O + Na+
H3C
H2
C Br
δ+
δ−
CH3 CH2 O CH3
NaBr
Mechanism

25. V. Substitution by thiol group
• Alkyl halides react with potassium hydrosulfide to form thiols.
25Mr. Mote G.D
+ K+
H3C
H2
C I
δ+
δ−
CH3 CH2 SH
Mechanism
H3C
H2
C KSH CH3 CH2 SH
ethanethiol
I
KI
ethyl iodide
Ethanol
SH
KI
ethanethiol
KSH

26. VI. Reaction with potassium sulphide
• Alkyl halides react with potassium sulphide to form dialkyl
sulfides
26Mr. Mote G.D
K2S
ethyl iodide
CH3 CH2 I
S CH2CH3H3CH2C
diethylsulphide
2
2KI+
CH3 CH2 I SK K+ + I CH2 CH3
S CH2CH3H3CH2C
diethylsulphide
2KI+
Mechanism

27. VI. Reaction with silver salt of carboxylic acid
• Alkyl halides react with silver salt of carboxylic acid to form
ester
27Mr. Mote G.D
CH3COOAg
ethyl bromide
CH3 CH2 Br
silver acetate
CH3 CH2 COOCH3 AgBr
ethyl acetate
ethanol
CH3COOAg
CH3COO + Ag+
H3C
H2
C Br
δ+
δ−
AgBr
Mechanism
CH3 CH2 COOCH3
ethyl acetate

28. VII. Reaction with sodium cyanide
• Alkyl halides react with sodium cyanide in ethanol to form
alkyl cyanides.
28Mr. Mote G.D
NaCN
ethyl iodide
CH3 CH2 I
ethanol
CH3 CH2 CN
ethyl cyanide
NaI
NaCN
+ Na+
H3C
H2
C I
δ+
δ−
NaI
Mechanism
CH3 CH2 CN
ethyl cyanide
CN-

29. 2. Reduction of alkyl halides
• Alkyl halide undergo reduction with nascent hydrogen in
presence of reducing agent like Zn/HCl to form alkanes.
R X
Alkyl halide
H2
R H HX
Alkane
Zn/HCl
H3C I
Methyl iodide
H2
H3C H HI
methane
Zn/HCl
29Mr. Mote G.D

30. 3. Elimination reaction: dehydrohalogenation of alkyl
halides
• When alkyl halide is heated with alcoholic solution of sodium
hydroxide to form alkene and hydrogen halide.
R
H
C
H
CH2
x
NaOH in alcohol
∆
R
H
C CH2
alkene
H3C
H
C
H
CH2
Cl
NaOH in alcohol
∆
H3C
H
C CH2
Propene
alkyl halide
Propyl chloride
H2O
H2O
HX
HCl
30Mr. Mote G.D

31. 4. Wurtz reaction
• Higher alkanes are produced by heating an alkyl halide with
sodium metal in dry ether. Two molecules of alkyl halide lose
their halogen atoms as NaX. These net result is the joining of
two alkyl group to yield symmetrical alkane having even
number of carbon atoms.
R X 2Na R X
Alkyl halide
ether
R R
Symmetrical
alkane
2NaX
H3C Br 2Na H3C Br
Methyl halide
ether
H3C CH3
ethane
2NaBr
31Mr. Mote G.D

32. 5. Reaction with metal: formation of Grignard
reagents
• Alkyl magnesium halides(Grignard reagent) are obtained by
treating alkyl halides with magnesium in anhydrous ether to
give alkanes.
RX Mg
ether
RMgX
Alkyl halide Alkyl magnesium halide
RMgX H OH R H MgX(OH)
Alkane
CH3MgI H OH 3HC H MgI(OH)
Methane
Methyl magnesium
iodide
32Mr. Mote G.D

33. Uses of alkyl halides
Sr.
No
Name Uses
1 Ethyl chloride Used to prevent pain caused by injection, muscle pain, injury
2 Chloroform Solvent for fats, waxes and rubber, preparation of chloropicrin and chloretone.
3 Trichloroethylene To remove grease from metal parts, solvent
4 tetrachloroethylene To remove grease from metal parts, solvent
Dry cleaning
5 Dichloro methane Strong solvent, flammability suppressant, vapour pressure depressant, Viscosity
thinner
6 tetra chloromethane Propellants for aerosol, dry cleaning, degreasing, fire extinguisher, pesticide
7 iodoform Disinfectant, antiseptic,
CH3 CH2 Cl
ethyl chloride
CHCl3
chloroform
C C
Cl
Cl
H
Cl
trichloro ethylene
C C
Cl
Cl
Cl
Cl
tetrachloroethylene
CH2
Cl
Cl
dichloro
methane
C
Cl
ClCl
Cl
tetrachloro
methane
CHI3
iodoform