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Grade: 12th pass


please i need good and great explanaton of the mechanism of preparation of alkene

4 months ago

Answers : (2)

312 Points
there are many ways but best is dehydrohalogenation see below
1. Dehydration: loss of H and OH (water) from adjacent carbons of an alcohol to form an alkene 2. Dehydrohalogenation: loss of H and X from adjacent carbons of an alkyl halide to form an alkene C. Addition reactions: two reactants add to form a product - no (or few) atoms are left over. Opposite of an elimination reaction
. Rearrangement: a reactant undergoes bond reorganization to give a product which is an isomer of the reactant
4 months ago
24737 Points

Dehydration of Alcohols

Heating most alcohols is a general method of preoarationof alkenes. Heating alcohols with a strong acid causes them to lose a molecule of water (to dehydrate) and form an alkene.

The reaction is an elimination and is favoured at higher temperatures.  The most commonly used acids in the laboratory are Bronsted acids - proton donors such as sulphuric acid and phosphoric acid.  Lewis acids such as alumina (Al2O3) are often used in industrial, gas phase dehydrations.

Dehydration reactions of alcohols show several important characteristics which shall be explained.

 ?The experimental conditions-temperature and acid concentration-that are required to bring about dehydration are closely related to the structure of the individual alcohol.  Alcohols in which the hydroxyl group is attached to a primary carbon (primary alcohols) are the most difficult to dehydrate.  Dehydration of ethanol, for example, requires concentrated sulphuric acid and a temperature of 180°C.

CH_{3}-CH_{2}-OH\xrightarrow[180^{0}C]{conc. H_{2}SO_{4}}CH_{2}=CH_{2} +H_{2}O

Secondary alcohols usually dehydrate under milder conditions.  Cyclohexanol, for example, dehydrates in 85% phosphoric acid at 165-170°C. 

Tertiary alcohols are usually so easily dehydrated that extremely mild conditions can be used, ter-butyl alcohol, for example, dehydrates in 25% H2SO4 at a temperature of 85°C.

Thus, overall, the relative ease with which alcohols undergo dehydration is in the following order

Ease of  Dehydration:  3° Alcohol >  2° Alcohol > 1° Alcohol

This behaviour, is related to the stability of the carbocation formed in each reaction.

Some primary and secondary alcohols also undergo rearrangements of their carbon skeleton during dehydration.

Mechanism of Alcohol Dehydration :  An E1 Reaction

The mechanism is an E1 reaction in which the substrate is a protonated alcohol (or an alkyloxonium ion).  We consider the dehydration of CH3CHOHCH3 that proceeds through a carbonium ion intermediate.  A catalytic role is assigned to the acid and O in ROH is a basic site.

Instead of HSO4-, a molecule of alcohol could act as a base in step 3 to give ROH2+.

Because step 2 is then, the rate determining step, it is the step that determines the reactivity of alcohols toward dehydration.  The formation of a tertiary carbocation is easiest because the free energy of activation for step 2 of a reaction leading to a tertiary carbocation is lowest.  The order of reactivity of the alcohols reflects the order of stability of the incipient carbonium ion (3°>2°>1°). 

4 months ago
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