Methods of Preparation of Alkenes in the Laboratory: Mechanism and Sample Questions

Alkenes, the hydrocarbons, contains a double bond between any two adjacent or adjoining carbon atoms. The structure of alkene characterizes the number of hydrogen atoms as twice the number of carbon atoms. Therefore, alkenes form a homologous series, the general formula of an alkene is represented as C_nH_2n. Ethene (C₂H₄) is the simplest alkene which has one double bond in its structure. Alkenes also have some important industrial usage, and also play an important role in our everyday lives. Here we will learn the methods of preparation of Alkenes in detail and discuss some important questions.

Key terms: Alkenes, alkynes,alkyl halides, vicinal halides, alcohols.

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Methods of Preparation of Alkenes

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Elimination processes, in which two atoms on adjacent carbon atoms are removed, result in the production of a double bond, are commonly used to make alkenes. Dehydration of alcohols, dehydrohalogenation of alkyl halides, and dehalogenation of alkanes are among the preparations.

Preparation of Alkenes from Alkynes

Alkynes can be used for the preparation of alkenes. Alkyne to alkene conversion is carried out by the reduction of alkynes with hydrogen in the presence of palladised charcoal. The charcoal used is moderately deactivated with the help of quinoline or sulphur compounds. This reaction results in the formation of alkenes. Palladised charcoal which is halfway deactivated is called Lindlar’s catalyst. The alkenes obtained from the above reaction have cis geometry. In order to form trans alkenes, alkynes are made to undergo reduction with sodium in liquid ammonia.

Preparation of Alkenes from Alkynes

Preparation of Alkenes from Alkyl Halides

Alkenes can be made from alkyl halides by heating them with alcoholic potash. Potassium hydroxide is dissolved in alcohol to make alcoholic potash. Dehydrohalogenation, or the removal of a single molecule of halogen acid, occurs in this reaction. The rate of reaction is determined by the type of the halogen group linked to the alkyl group.

Preparation of Alkenes from Alkyl Halides

Preparation of Alkenes from Vicinal Dihalides

Vicinal dihalides are dihalides with two neighbouring carbon atoms connected to two halogens, as specified by vicinal halides. These dihalides lose halogen molecules when they react with zinc metal, resulting in the creation of alkenes. Dehalogenation is the process of preparing alkenes from Vicinal dihalides in this way.

Preparation of Alkenes from Vicinal Dihalides

Dehydration of Alcohols

 In dehydration reactions, when an alcohol molecule is heated in the presence of a strong mineral acid, a molecule of water is removed. Between the neighbouring carbon atoms that have lost the hydrogen ion and hydroxide group, a double bond occurs.

The steps in this dehydration reaction's process are as follows.

1. Protonation of the alcohol:

This is a straightforward acid-base process that produces an oxonium ion, a positively charged oxygen atom.

2. The oxonium ion dissociates.

The oxonium ion splits into a carbocation, a positively charged carbon atom that is a volatile intermediate.

3. The carbocation is deprotonated.

The carbocation's positively charged end carbon draws electrons in the overlap area, which attach it to the next carbon. The carbon becomes slightly positive as a result of the electron movement, which attracts electrons in the overlap regions of all other atoms connected to it. As a result, the hydrogen on the carbon becomes only slightly acidic, allowing it to be extracted as a proton in the process.

Rearrangement of Carbocation

 In an alcohol dehydration, the carbocation may rearrange into more stable configurations. Two alkenes are produced when 2methyl3pentanol is dehydrated. The additional compound is formed due to a rearrangement of the carbocation intermediate, according to the mechanism of the reaction.

The intermediate carbocation is rearranged to produce the 2 methyl 1 pentene molecule.

The creation of a more stable carbocation is caused by the migration of a hydride ion (H:). Carbocations, like carbon atoms, are classed as primary, secondary, or tertiary. A primary carbocation is surrounded by one alkyl group; a secondary carbocation is surrounded by two alkyl groups; and a tertiary carbocation is surrounded by three alkyl groups.

Alkyl groups have the potential to "push" electrons away from themselves in theory. The inductive effect is the name for this phenomena. The intermediate carbocation will be more stable if there are more alkyl groups "pulling" electrons toward a positively charged carbon atom. The delocalization of charge density is responsible for this increase in stability. A charge on an atom causes it to be stressed. The more the tension is distributed across the molecule, the lower the charge density on any one atom gets, reducing stress. As he reduces his tension, the ion becomes more stable. As a result, tertiary carbocations are more stable than secondary carbocations, which only have two alkyl groups on which to delocalize the positive charge.

Alkyl groups, in actuality, do not "push" electrons away from themselves; rather, electrons are withdrawn from them. The electronegativity of an atom changes as it gains a positive charge and becomes an ion. The placement of the overlap zone relative to each carbon atom is determined in part by the electronegativity of the two atoms in the initial connection. The overlap zone moves closer to the more electronegative, positively charged carbon atom when the electronegativity of one of the carbon atoms increases owing to ion production. The surrounding carbon receives a partial positive charge as a result of the electron density rearrangement. The charge received by the second carbon atom is the same as the charge lost by the fully charged carbon atom. The charge is delocalized over the two carbons in this way.

Zaitsev Rule

Zaitsev is in charge. An alcohol dehydration reaction, in which hydrogen atoms are lost from two separate carbons on the carbocation, may be able to generate a double bond in some cases. The more highly substituted alkene, that is, the alkene with the most substituents on the carbon atoms of the double bond, is invariably the primary product, according to the Zaitsev rule. The following compounds are generated during the dehydration process of 2butanol.

The primary product, according to the Zaitsev rule, is 2butene. It's worth noting that each carbon atom in the double bond of 2butene is connected to one methyl group. In the case of 1butene, one of the double bond's carbon atoms contains one substituent (the ethyl group), while the other has none.

Alkyl Halides Dehydrohalogenated

Another elimination reaction is the dehydrohalogenation of alkyl halides, which involves the loss of a hydrogen and a halide from an alkyl halide (RX). The alkyl halide is generally dehydrohalogenated by reacting it with a strong base, such as sodium ethoxide.

Dehalogenation

Vicinal dihalides, which are alkane molecules with two halogen atoms on adjacent carbon atoms, react with zinc to generate alkenes.

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Things to Remember

• Solid, liquid, and gaseous alkenes occur in all three states. The first three alkenes are gaseous, while the last fourteen are liquids. They exist in the solid-state when the molar mass grows.
• Due to the presence of weak van der Waal forces, alkenes are not soluble in water.
• Alkenes, on the other hand, dissolve in organic solvents like benzene and acetone.
• The boiling point of an alkene is proportional to its molar mass. As a result, greater alkene boiling points are quite high.
• The polarity of an alkene is determined by the functional group linked to it.

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