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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)

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Primary 1o

RCH2X

Secondary 2o

R2CHX

Teriary 3o

R3CX

Monohaloalkanes

CnH

2n+1X

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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

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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

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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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