1. Synthesis and Properties of Alkene
ORGANIC CHEMISTRY- 1
Synthesis and Properties of Alkene By
Dr. Sulaiman Al Sulaimi
Assistant Professor
UNIVERSITY OF NIZWA

2. Gasoline
Gasoline: The Kingdom of Earth runs on alkanes or runs on oil. The distillation of crude oil is the first step in gasoline production, Straight-chain gasoline are poor fuel in automobiles because of engine knock, an uncontrolled combustion that can occur in a hot engine.

3. CH3-CH2- CH2- CH2- CH2- CH2- CH3
Gasoline
The octane number of a fuel is the measure by which its antiknock properties are judged. straight-chain hydrocarbons are more prone to induce engine knock than are highly branched compounds.
Heptane, a particularly bad fuel, is assigned a base value of 0 octane number while 2,2,4-trimethylpentane (iso-octane) has a rating of 100.
CH3-CH2- CH2- CH2- CH2- CH2- CH3
Heptane
Octane Number = 0

4. ALKENES
Alkenes are also called Olefins (CnH2n) “unsaturated” hydrocarbons. Alkenes occur abundantly in nature. Ethylene (H2C=CH2) is a plant hormone that induces ripening in fruit.
Functional group = carbon-carbon double bond
sp2 hybridization => flat, 120o bond angles
σ bond & π bond => H2C=CH2
No rotation about double bond!

5. ALKENES
Examples:
C3H6 propylene
C4H8 butylenes

6. Cis/Trans Isomerism:
When disubstituted (two substituents other than hydrogen) alkenes contain two substituents on the same side of the double bond they are called cis alkenes and when substituents are attached to opposite side are called trans alkenes.
Two methyl groups on the same side of double bond = Cis

7. (CH3)2C=CHCH3 CH3CH=CCH2CH3
Cis–trans isomerism is not possible If one of the double-bond carbons (vinyl carbon) is attached to two identical groups.
CH3CH=CHCH CH3CH2CH=CH2
Yes No
CH3
(CH3)2C=CHCH3 CH3CH=CCH2CH3
No Yes

8. E and Z Alkenes
For trisubstituted (three substituents) and tetrasubstituted (four substituents) double bonds, a more general method is needed for describing double-bond geometry.
If the Higher mass ranked groups are on the same side, then alkene has Z geometry (German zusammen, meaning “together.”) If the higher-ranked groups are on opposite sides, while the alkene has E geometry (German entgegen, meaning “opposite).

9. E and Z Alkenes

10. Examples:

11. Stability of Alkenes
The trans isomer is more stable than the cis isomer by 2.8 kJ/mol (0.66 kcal/mol) at room temperature, corresponding to a 76:24 ratio. Cis alkenes are less stable due to steric strains between two larger substituents on the same side of the double bond.

15. Industrial Preparation of Alkenes
Ethylene, Propene and butene are synthesized industrially by cracking of light alkanes. The process is complex and involves radical reactions. The high-temperature reaction conditions cause spontaneous homolytic breaking of C-C and C-H bonds and as results smaller fragments are formed.

16. Preparations Dehydration of Alcohols
Removal of water is called dehydration. Con. H2SO4 is used.
The order of dehydration is ter. alcohol > sec. alcohol > prim. alcohol

17. Mechanism of the acid-catalyzed dehydration of a tertiary alcohol to yield an alkene. The process is an E1 reaction and involves a carbocation intermediate.

18. Zaitsev’s rule: According to this rule, major product is the most substituted alkene i.e., major product is obtained by elimination of H from that β-carbon which has the least number of hydrogen.

19. By dehydrohalogenation of Alkyl halide
Alcoholic solution of KOH
By Dehalogenation of Vicinal dihalides
Having two halogen atoms on adjacent carbon atom is called vicinal
dihalides

20. Hydrogenation
The addition of hydrogen in the presence of catalyst e.g. Pd (palladium) in BaSO4 .