By:Dr. Siham Lahsasni

1

Unsaturated Hydrocarbons 1 Alkenes

Unsaturated hydrocarbons that contain one or more double carbon-carbon bonds. Alkenes with just one double C-C bond form an homologous series with the general formula CnH2n. The first few in this series are ethene, propene, butene, and pentene.

2

HYBRIDISATION OF ORBITALS - ALKENESHYBRIDISATION OF ORBITALS - ALKENES

2s22p2 2s12p3 3 x sp2 2p

The electronic configuration of a carbon atom is 1s22s22p2

If you provide a bit of energy you can promote (lift) one of the s electrons into a p orbital. The configuration is now 1s22s12p3

Alternatively, only three orbitals (an s and two p’s) combine or HYBRIDISE to give three new orbitals. All three orbitals are equivalent. The remaining 2p orbital is unchanged.

Bonding in Ethylene : Bond length =1.34 Å with the atoms separated by bond angles of 120°.

4

Hybridization Sp2

Nomenclature of alkenes

5

1. The ene suffix indicates an alkenes or cycloalkenes.

2. The longest chain chosen for the root name must include both carbon atoms of the double bond.

3. The root chain must be numbered from the end nearest a double bond carbon atom. If the double bond is in the center of the chain, the nearest substituent rule is used to determine the end where numbering starts.

4. In cycloalkenes the double bond carbons are assigned ring locations C1 and C2. Which of the two is C1 may be determined by the nearest substituent rule.

5. Substituent groups containing double bonds are:     H2C=CH–   Vinyl groupH2C=CH–CH2–   Allyl group

6

CH2 CH2 CH3 CH2 CH2 BrCH2

Cl

CH2

Ethylene

Ethene

Common

IUPAC

Propylene

Propene

Vinyl bromide

Bromoethene

Allyl chloride

3-Chloro-1-propene

Vinyl cyclohexane

Cyclohexyl ethene

CH3CH3

CH3

CH3 CH3

CH3

CH3

CH3

Geometric (cis-trans) isomerism

7

The prefix cis- is used when the two arms of the longest chain are on the same side of the double bond.

The prefix trans- is used when they are on opposite sides of the double bond.

E-Z NOTATION FOR GEOMETRIC ISOMERISM

8

If the two groups with the higher molecular weight are on the same side of the double bond, that is described as the (Z)- isomer. So you would write it is (Z)-name of compound.

If the two groups with the higher molecular weight are on opposite sides of the double bond, then this is the (E)- isomer.

E Z

Physical Properties of Alkenes

9

Alkenes are non polar compounds.Insoluble in water.Soluble in non polar organic solvents.They are less dense than water.Range of physical states: ≤ 4 C's are gases5 - 17 C's are liquids ≥ 18 C's are solidsThe alkenes has a boiling point which is a

small number of degrees lower than the corresponding alkanes.

Preparation of alkenes

1- Dehydration of alcohols:

CH3CH2OHH

+/ heat

CH2 CH2 + OH2

OH

H

+ OH2

H+/ heat

cyclohexanol cyclohexene

Ethanol Ethene

10

CH3 CH3

OH

CH2CH3

CH3 CH3

+ OH2

+ OH2

1-Butene minor

2-Butene major

11

Saytzeff s Rule: In every instance in which more than one alkene can be formed, the major product is the alkene with the most alkyl substituents attached on the double bonded carbon.

2- Dehydrohalogenation of Alkyl Halides

CH2 CH2

XH

+ KOHAlcohol

heatCH2 CH2 + KX + OH2

an alkyl halideAlkene

CH3CH3

Br

+ KOHalcohol / heat CH3

CH3+ CH3

CH2

2-Butene major 1-Butene minor

Br

CH3

H

HH

+ KOHAlcohol / heat

CH3+

1-methylcyclohexene 3 methylcyclohexene

CH3

major minor

12

Reaction of alkenes

13

1- Additions to the Carbon-Carbon Double Bond

1-1- Addition of hydrogen: Hydrogenation (reduction)

A

A

A

A

+ H2

H H

A

A

A

A

An alkeneAn alkane

Pt or Ni or Pd

CH2 CH2 + H2Pt CH3 CH3

CH3CH2

+ H2Pt

CH3 CH3

14

1-2- Addition of Halogens: Halogenation

A

A

A

A

+ X2

X X

A

A

A

A (X= Cl or Br)

CH3CH3

+ Cl2CCl4 CH3 CH3

Cl

Cl

+ Br2

CCl4

Br

Br

1-3- Addition of Hydrogen Halides: Hydrohalogenation

A

A

A

A

+H X

A

A

A

A (X= Cl or Br or I)HX

CH3 CH3 + ClHCCl4 CH3 CH3

Cl

H

+ HICCl4

I

H

15

CH3 CH2

CH3 CH3

Br

CH3 CH2Br

major

minor

Markonikov’s rule: In addition of HX to unsymmetrical alkenes the hydrogen halide adds to the double bonded carbon that bears the greater number of hydrogen atoms and the negative halide ion adds to the other double bonded carbon.

CH3CH3

CH3+ ClH

CH3CH3

CH3 Cl

16

1-4- Addition of HOX: Halohydrin formation

17

A

A

A

A

+OH X

A

A

A

A (X= Cl or Br)H2O, X2

CH3

+ OH2 , Cl2

CH3

OH

Cl

CH3 CH2 + H2O, Br2 CH3 CH2Br

OH

A

A

A

A

+H OH

A

A

A

A

CH3 CH3 + CH3 CH3

OH

H

CH3

+CH3

H

OH

OH2H

+

H+

OH2

OH2H

+

1-5- Addition of H22O: Hydration

18

19

1-6- Addition of HCN

A

A

A

A

+H CN

A

A

A

A

CH3 CH3 + CH3 CH3

CN

H

CH3

+CH3

H

CN

HCNH

+

H+

HCN

HCNH

+

2- OZONOLYSIS

A

A

A

A

+O O

A

A

A

A

O

CH3 CH3

O3

H2O, Zn O

A

A+ O

A

A

1) O3

2) H2O, Zn

CH3

O+ CH3

O

CH3CH3

1) O3

2) H2O, Zn

CH3O

2

20

21

3- Oxidation

4- Epoxidation

KMnO4 / OH-

OH

OH

RCO3HO