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.

Dehydration of Alcohols:
Alcohols undergo dehydration when allowed to react with concentrated acids in the presence of heat.

This reaction can be used to dehydrate all three types of alcohol viz. Primary, secondary and tertiary alcohols. Some examples are given below:

It is to be noted that the dehydration usually occurs via the Unimolecular elimination reaction (E_1) and involves a carbocation intermediate which can undergo rearrangement via hydride or alkyl shift and also undergo ring expansion for suitable substrates where the ring strain can be released. A drawback of this reaction is that a mixture of alkenes can be obtained due to the involvement of carbocation intermediates. Saytzeff’s alkene which is the more stable alkene is usually obtained as a major product.    

Consider the examples given below in which the carbon skeleton changes due to carbocation rearrangement and ring expansion respectively.

Case of Methyl shift

The mechanism of the reaction is given below:

Case of Ring expansion

The mechanism of the reaction is given below

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:

Dehydrohalogenation of Haloalkanes with Strong bases
Secondary and tertiary alkyl halides undergo dehydrohalogenation on reaction with a strong base to form Alkenes. The reaction is an elimination reaction. It is to be noted that primary haloalkanes form ether by the Williamson’s synthesis of Ethers. Some examples of the reaction are given below

This reaction is an example of elimination in which a is eliminated along with a halogen at the  carbon. The reaction occurs in a concerted mechanism and anti elimination takes place as shown below.

If there are different types of - hydrogen present in the substrate then usually the Saytzeff’s alkene is obtained as a major product. Please recall that Saytzeff’s alkene is the more substituted alkene having a greater number of hydrogens or greater alkylation around the double bond. 

However, in cases where bulky bases are used, the reaction usually takes place by the extraction of the least hindered hydrogen atom and often less substituted alkenes are obtained as a major product. Steric hindrance thus plays an important role in the reaction.

There is an anomaly shown in the reaction when Fluorine is present as the leaving group in the haloalkane and usually less substituted alkene is produced as a major product. This is explained by the poor leaving group ability of Fluorine and the reaction proceeds by a significant anionic character in the transition state.

The dehydrohalogenation occurs in an anti periplanar fashion and the hydrogen and the halogen should be in an anti orientation.

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:

In Cope's elimination, least hindered beta H is eliminated and Hoffman alkene is formed

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:

In pyrolysis of esters, least hindered beta H is eliminated and Hoffman alkene is formed