Table of Contents
What is Preparation of Alkenes?
The preparation of alkenes is the process of converting an alkane into an alkene. The most common method of preparing alkenes is through the use of olefin metathesis. In olefin metathesis, two alkenes are converted into two new alkenes. This process is typically catalyzed by a metal catalyst, such as ruthenium.
Methods of Preparation of Alkenes
There are a number of ways to prepare alkenes, but the most common methods involve the use of alkyl halides and alkali metals.
One method involves the use of alkyl halides and alkali metals. Alkyl halides are compounds that contain a halogen atom (F, Cl, Br, I) bonded to an alkyl group (a group of atoms that includes carbon and hydrogen). Alkali metals are elements that include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs).
When alkyl halides and alkali metals are combined, the alkali metal reacts with the alkyl halide to form an alkene and a salt. The salt is a compound that contains the elements sodium (Na) and halogen (F, Cl, Br, I).
The reaction can be represented using the following equation:
RCI + Na → RCH=CH2 + NaCl
Another method of preparing alkenes involves the use of vinyl compounds and Grignard reagents. Vinyl compounds are compounds that contain a vinyl group (a group of atoms that includes carbon and hydrogen). Grignard reagents are compounds that contain a magnesium (Mg) atom bonded to a radical (R).
When vinyl compounds and Grignard reagents are combined, the magnesium atom in the Grignard re
Preparation of Alkenes From Alkynes
Alkenes may be prepared from alkynes by several methods. One method is the hydrohalogenation of alkynes, which is the addition of a hydrogen halide (HX) to an alkyne. In the presence of a catalyst, hydrogen halides add to alkynes to form alkenes. The most common catalyst used for this reaction is platinum, although other metals such as palladium and rhodium can also be used.
The hydrohalogenation of alkynes occurs in two steps. The first step is the addition of the hydrogen halide to the alkyne to form a Markovnikov product. The second step is the elimination of the hydrogen halide to form the alkenes.
The following reaction illustrates the hydrohalogenation of an alkyne:
In this reaction, the alkyne is reacted with hydrogen chloride to form the alkenes. The hydrogen chloride adds to the alkyne to form the Markovnikov product, which is a molecule with a hydrogen atom on the carbon atom that is attached to the triple bond. The hydrogen chloride is then eliminated to form the alkenes.
Preparation of Alkenes From Alkyl Halides
The preparation of alkenes from alkyl halides involves the elimination of a halogen atom from the alkyl halide molecule. The halogen atom is eliminated by the addition of a hydrogen atom to the alkyl halide molecule. This addition of a hydrogen atom to the alkyl halide molecule is called a hydrogenation reaction.
The hydrogenation reaction is a two-step process. The first step is the addition of a hydrogen atom to the alkyl halide molecule. The second step is the elimination of the halogen atom from the alkyl halide molecule.
The addition of a hydrogen atom to the alkyl halide molecule is a reversible reaction. The addition of a hydrogen atom to the alkyl halide molecule is called a hydrogenation reaction. The addition of a hydrogen atom to the alkyl halide molecule is also called a reduction reaction.
The addition of a hydrogen atom to the alkyl halide molecule is a chemical reaction. The addition of a hydrogen atom to the alkyl halide molecule is also called a hydrogenation reaction. The addition of a hydrogen atom to the alkyl halide molecule is also called a reduction reaction.
The addition of a hydrogen atom to the alkyl halide molecule is a redox reaction. The addition of a hydrogen atom to the alkyl halide molecule is also called a reduction reaction.
Preparation of Alkenes From Vicinal Dihalides
A vicinal dihalide is a compound containing two adjacent halogens. The two halogens can be on the same side of the carbon chain (syn) or on opposite sides of the carbon chain (anti). Alkenes can be prepared from vicinal dihalides by the elimination of a halogen atom.
The elimination of a halogen atom from a vicinal dihalide can be accomplished by heating the compound with a strong base. The base attacks the carbon-halogen bond and breaks it, resulting in the elimination of a halogen atom. The product of this reaction is an alkene.
The following reaction illustrates the preparation of an alkene from a vicinal dihalide.
2-bromo-2-chloropropane → propene
Carbocation Rearrangement.
The rearrangement of the carbocation is the most important step in the mechanism. The rearrangement can be either through a chair conformation or a boat conformation. In the chair conformation, the hydrogen atoms on the carbon adjacent to the carbocation move to the opposite side of the carbon atom. This rearrangement is more stable because it minimizes the electron-density on the carbon atom. In the boat conformation, the hydrogen atoms move to the same side of the carbon atom. This is less stable because it increases the electron-density on the carbon atom.
A Hydride Ion (H: – ) Movement
A hydrogen ion moves by diffusion down its concentration gradient.
Alkyl Halide Dehydrohalogenation
In organic chemistry, the dehydrohalogenation of alkyl halides is the removal of a hydrogen halide from an alkyl group. The alkyl group is typically a methyl, ethyl, or propyl group. This process is used to produce alkanes by removing a hydrogen from an alkyl halide.
The general mechanism for the dehydrohalogenation of an alkyl halide is as follows:
The alkyl halide is first converted into an alkene by the removal of a hydrogen halide. This is then converted into an alkane by the removal of a second hydrogen halide.
The dehydrohalogenation of an alkyl halide can be catalyzed by a variety of different substances, including aluminum chloride, titanium tetrachloride, and zinc chloride.
s About Alkenes
An alkene is a hydrocarbon that has at least one carbon-carbon double bond. Alkenes are named for the alkene functional group, which is composed of a carbon atom double-bonded to an oxygen atom.
