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

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!

ALKENES Examples: C3H6 propylene C4H8 butylenes

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

(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=CHCH3 CH3CH2CH=CH2 Yes No CH3 (CH3)2C=CHCH3 CH3CH=CCH2CH3 No Yes

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).

E and Z Alkenes

Examples:

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.

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.

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

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.

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.

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

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