Preparation of Alkenes - Practice Questions & MCQ

Alkynes on partial reduction with calculated amount of dihydrogen in the presence of palladised charcoal partially deactivated with poisons like sulphur compounds or quinoline give alkenes. Partially deactivated palladised charcoal is known as Lindlar’s catalyst. Alkenes thus obtained are having cis geometry. However, alkynes on reduction with sodium in liquid ammonia form trans alkenes.

When alcohols are heated with dehydrating agents like H₂SO₄ 170^oC, H₃PO₄ 200^oC, ZnCl₂ 350^oC, BF₃, dry HCl, KHSO₄ etc alkenes are formed as follows:


It involves carbocation formation followed by H- or R- or Ring expansion to get more stable carbocation to get the major product.

Some examples include:

Saytzeff's rule
This rule states that in dehydrohalogenation reactions, the preferred product is always that alkene which is most stable or in other words which has more number of -hydrogen atoms.

Hoffmann's rule
This rule states that the alkene formed would be the least stable as the major product or in other words that alkene would be formed which has the least number of -hydrogen atoms.

Dehydration by Al₂O₃
Since the reagent used is Al₂O₃, thus the Saytzeff's rule will be applied and E₂ elimination will take place and no carbocation will form. When ethanol is passed over heated aluminium oxide then ethene is formed as the final product. The reaction occurs as follows:

Dehydration by ThO₂
Since the reagent used is ThO₂, thus the Hoffmann's rule will be applied and E₂ elimination will take place and no carbocation will form. The reaction occurs as follows:

When alkyl halides are heated with reagents like C₂H₅O^-, alc. KOH, NaNH₂, KNH₂, (Me)₃COK etc. then alkenes are formed as the product. The general reaction occurs as follows:

Mechanism

Some examples include:

(1)
 

(2) 

When vicinal dihalides are heated with Zn dust or NaI/Acetone, alkene having the same number of carbon is obtained. This reaction is known as dehalogenation. The reaction occurs as follows:

Mechanism

For example:

In this reaction, methylene triphenyl phosphorane or phosphorous ylide is treated with a carbonyl compound to prepare an alkene. There are two important components of this reaction:

• A carbonyl compound
• A species known as "ylide". The "ylide" is a species with opposite charges on adjacent atoms.

This reaction is named after George Wittig who was awarded the Nobel prize for this work in 1979. A principal advantage of alkene synthesis by the Wittig reaction is that the location of the double bond is absolutely fixed, in contrast to the mixtures often produced by alcohol dehydration.

Mechanism

For example:

Pyrolysis of quaternary ammonium salts follows the Hoffmann elimination. This means the less stable alkene will form. In this reaction, an amine reacts with 3 moles of methyl iodide and forms quaternary ammonium salt. Now heating this salt with moist Ag₂O or AgOH will form alkene.

The reaction occurs as follows:

Cope's reaction
When a tertiary amine oxide bearing one or more beta hydrogens is heated, it is converted to an alkene. The reaction is known as Cope elimination or Cope reaction. The net reaction is 1,2-elimination hence the name Cope elimination.

For example:

Pyrolysis of Esters
When esters are heated in presence of liquid N₂ and glass wool, then alkyl part of ester converts into respective alkene while alkanoate part of ester converts into respective acid.

For example: