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بسم هللا الرحمن الرحيم
ماجدة عبد العزيز السيد / د
Lecture-6
Orientation of Electrophilic Addition: Markovnikov’s Rule
In an unsymmetrical alkene, HX reagents can add in two different
ways, but one way may be preferred over the other.
If one orientation predominates, the reaction is regiospecific.
Markovnikov’s rule: When an unsymmetrical reagent adds to an
unsymmetrical alkene, the hydrogen of the reagent bonds to the
carbon of the double bond that has the greater number of hydrogen
atoms attached to it. (i.e. addition will be regiospecific, leads to the
formation of only one product.
In presence of peroxides (like H2O
2, and per acids (peroxy
acid) as HMNO4, HClO
4, H
3PO
5, H
2SO
5, RCOOOH), addition
will follow anti-Markonikov’s rule.
Halohydrin formation
In aq. Sol. Cl2, Br2 and I2 react with alkenes to form compounds known
as vicinal halohydrins, which have halogen and OH group on adjacent
carbons.
Syn-Hydroxylation of alkenes
Hydroxylation is the addition of OH group to each end of the double
bond. The most common reagents are: Osmium tetroxide and
potassium permanganate.
➢Like permanganate, ozone cleaves double bonds to give ketones and
aldehydes.
➢However, ozonolysis is milder, and both ketones and aldehydes can be
recovered without further oxidation.
➢Ozonolysis is used to locate the position of a double bond.
Ozonolysis of alkenes
General reactions of alkynes
Addition reaction: is similar to alkenes, but occurs slower and mainly
trans.
a) Addition of halogens
b) Addition of hydrogen
Catalytic hydrogenation takes place in two steps, with an alkene
intermediate.
With efficient catalysts such as Pt. Pd or Ni, it is usually impossible to
stop the reduction at the alkene stage. Hydrogenation of an alkyne can be
stopped at the alkene stage by using Lindlar's cartalyst
Lindlar's cartalyst (it is a poisoned palladium catalyst,
composed of powdered barium sulfate (BaSO4)coated with
palladium poisoned with quinoline).
c) Ozonolysis of alkynes
Osonolysis, followed by hydrolysis give carboxylic acids.
A-Alkyl halides
General formula: RX (R: alkyl or substituted alkyl group, X: F, Cl, Br, or I).
Alkyl halides
Monohaloalkanes
CnH
2n+1X
Dihalogenated
compounds CnH2nX2
Polyhalogenated
alkyl halides
Primary 1o
RCH2X
Secondary 2o
R2CHX
Teriary 3o
R3CX
Vicinal
(alkylene dihalides)
Geminal
(alkylidene dihalides)
α,ω-Dihalides
(polymethylene dihalides)
11
Primary 1o
RCH2X
Secondary 2o
R2CHX
Teriary 3o
R3CX
Monohaloalkanes
CnH
2n+1X
12
Geminal
(alkylidene dihalides)
Vicinal
(alkylene dihalides)
α,ω-Dihalides
(polymethylene dihalides)
Dihalogenated compounds
Halogen atoms are attached to
terminal carbon atoms and are
separated by 3 or more carbon
atoms.
X-CH2-CH
2-CH
2-CH
2-X
tetra methylene dihalide or 1,4
dihalobutane13
Nomenclature of alkyl halides
Two methods:
1- IUPAC system: alkane with a halo-substituent: a systematic halo
alkane names
(fluorine is fluoro-, chlorine is chloro-, bromine is bromo-, and iodine is
iodo-) eg. 1-chlorobutane
2- Common or trivial names: naming the alkyl group and then the
halide eg. isopropyl bromide
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Physical character of alkyl halides
➢ The increase in M.Wt. causes an increase in the boiling points.
➢ Atomic weight order: I > Br > Cl > F
CH3I CH
3Br
43oC 5
oC
a) Boiling point
15
Structure of alkyl halides
C X
H
H
H
+
➢ Order of electronegativities : F > Cl > Br > I.
c) Miscibility
Halogenated hydrocarbons do not form strong hydrogen bonds with
H2O , thus are immiscible with water.
CH3Cl
Methyl chloride
CH2Cl
2
Methylene chloride
CHCl3
Chloroform
CCl4
Carbon tetrachloride
77oC 61oC 40oC -24oC
(Generally the boiling point of 1ry alkyl halides is greater than that of
3ry alkyl halides).
➢ In case of isomeric alkyl halides, n-alkyl halides have the greatest
boiling points.
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From alkanes
From alkenes
and alkynes
Hunsdiecker
reaction
From alcohols
Preparation of
alkyl halides
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1- From alkanes
Free radical halogenation (rarely effective method ) Why?
✓ As mixtures of products are produced due to:
1- there are different kinds of hydrogen atoms that can be
abstracted.
2- more than one halogen atom may react giving multiple
substitution.
CH3-CH2-CH3CH3CH2CH2Cl + CH3CHCH3 and others
Cl
Cl2
light, 25
o
C
Exception: Free radical bromination is exceptionally highly
selective, gives good yields with alkanes that have one type of
H atom that is more reactive than the others.
CH3
C
CH3
CH3 H + Br2
CH3
C
CH3
CH3 Br
isobutane t-butyl bromide (90 %)
h
18
2- From alcohols
Alcohols react with halogenating agents like hydrogen halides
(HX) , phosphorous halides (PX3
or PX5) (X = I, Br, Cl) or thionyl
chloride (SOCl2).
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There's Inversion of configuration at the carbon (using pyridine
as a solvent) since the reaction proceeds through a backside
attack (SN2).
3- From alkenes and alkynes
Br
CH2 CH2 + Br2 CH2CH2Br (vic)
C C2 HX
C C
H
H
X
X
gem dihalide
C C
X
X
X
X
polyhalogenated
alkyl halide
X2
This addition obeys Markovinov's rule.
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4- Hunsdiecker reaction
Basis: decraboxylative halogenation of carboxylic acid
(converts heavy metal salts of carboxylic acids to alkyl halides
with the loss of one carbon atom)
R C
O
OAg + Br2
heatR Br + CO2 + AgBr
Ag , Hg , or Pb salt
COOHI2 , Pb(OAc)4
cyclobutanecarboxylic iodocyclobutane
acid
I
+ CO2
The Hunsdiecker reaction is usually carried out by treating
the carboxylic acid with a heavy metal base such as Ag2O,
HgO, or Pb(Oac)4
to form the heavy-metal salt. Bromine or
iodine is added, and the reaction mixture is heated.
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