SYNTHESIS OF ALKENES VIA ELIMINATION REACTIONS

Preparation of alkenes 1- Dehydration of alcohols: Mineral acids (H2SO4, H3PO4) General equation:

Dehydration of a Primary Alcohol: An E2 reaction Dehydration of a Secondary or tertiary Alcohol: An E1 reaction

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. CH3CH2CH=CH2 + H2O 1-Butene (minor) CH3CH2CHCH3 2- butanol OH H+ ∆ CH3CH=CHCH3 + H2O 2- Butene (major)

Rearrangement during Dehydration of Secondary Alcohols

2- Dehydrohalogenation of Alkyl Halides General equation: Alcohol -C - C - + KOH H2C=CH2 + KX + H2O ∆ H X

i) By Zn in acetic acid or ethanol 3- Dehydrohalogenation of vicinal dihalides General equation: -CX – CX- + Zn -C=C- + Zn X2 Methanol ∆ i) By Zn in acetic acid or ethanol ii) By NaI in acetone

Reaction of alkenes 1- Additions to the Carbon-Carbon Double Bond General equation: H2C=CH2 +A-B H2A-C-C-H2B 1-1- Addition of hydrogen: Hydrogenation (reduction)

1-2- Addition of Halogens: Halogenation General equation: Inert solvent

1-3- Addition of Hydrogen Halides: Hydrohalogenation General equation:

MARKOVNIKOV’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.

1-5- Addition of H2O: Hydration The addition of water to a double bond usually follows Markovnikov’s rule.

2- OZONOLYSIS Alkene+O3 ozonoid + H2O Aldehyde+ Ketone

3- Oxidation 4- Epoxidation

Electrophilic Addition to Conjugated Dienes: 1,4-Addition The addition of a reagent to a pair of adjacent carbons is called 1,2-addition.

Treatement of a conjugated diene with bromine under the similar conditions gives, in addition to the exepected 1,2-addition product, an unexcepted 1,4-addition product.

Resonance The addition of bromine to 1,3-butadiene results in a secandary carbocation. This carbocation is also called an allylic carbocation.

The Diels-Alder reaction The reaction is one between a conjugated diene and a compound containing a double bond called a dienophile. The product of a Diels-Alder reaction is offten called an adduct.