Chemistry · Ch 9 — Hydrocarbons
Preparation
Preparation
9.3.4 Preparation of Alkenes
Alkenes are prepared by four main methods in the laboratory. Each method gives you control over the structure and sometimes even the geometry of the double bond formed. Understanding the mechanism behind each method helps you predict which alkene will form and whether it will be cis or trans.
1. From Alkynes (Partial Reduction)
Alkynes contain a triple bond. By adding exactly one molecule of dihydrogen (), you can reduce the triple bond to a double bond, producing an alkene. The key is to stop the reaction at the alkene stage — if you add too much hydrogen, you get the alkane.
Using Lindlar's Catalyst — gives cis alkenes
When an alkyne is treated with a calculated amount of in the presence of palladised charcoal that has been partially deactivated with poisons such as sulphur compounds or quinoline, the reduction stops at the alkene. This partially deactivated catalyst is called Lindlar's catalyst.
The hydrogen atoms add to the same side of the triple bond (syn addition), producing the cis alkene.
Examples from the textbook:
Ethyne gives ethene. (Ethene has no geometrical isomers because both carbons have two identical hydrogen atoms.)
Propyne gives propene.
Propene () does not show geometrical isomerism. Why? One of the doubly bonded carbons has two identical hydrogen atoms attached. For cis-trans isomerism, each carbon of the double bond must have two different substituents.
Using Na in liquid ammonia — gives trans alkenes
When an alkyne is reduced with sodium metal dissolved in liquid ammonia, the addition of hydrogen occurs from opposite sides (anti addition), producing the trans alkene.
This is a powerful method when you specifically need the trans isomer.
2. From Alkyl Halides (Dehydrohalogenation)
Alkyl halides () on heating with alcoholic potash (potassium hydroxide dissolved in ethanol) lose one molecule of hydrogen halide () to form an alkene. This reaction is called dehydrohalogenation — removal of hydrogen and halogen from adjacent carbon atoms.
This is an E-elimination reaction. The hydrogen atom is eliminated from the -carbon atom — the carbon atom next to the carbon bearing the halogen. The -carbon is the one attached to the halogen; the -carbon is adjacent to it.
Factors affecting the rate:
The rate of dehydrohalogenation depends on two things:
- Nature of the halogen atom: The rate follows the order:
Iodine is the best leaving group; chlorine is the poorest among these.
- Nature of the alkyl group: The rate follows the order:
Tertiary alkyl halides react fastest because the resulting alkene is more substituted (more stable).
When a secondary or tertiary alkyl halide can eliminate in two different ways (giving two different alkenes), the more substituted alkene (Saytzeff product) is the major product. This is the Saytzeff rule — the alkene with more alkyl groups on the double bond carbons is more stable and forms preferentially.
3. From Vicinal Dihalides (Dehalogenation)
Dihalides in which the two halogen atoms are attached to two adjacent carbon atoms are called vicinal dihalides (from Latin vicinus, meaning neighbour).
When a vicinal dihalide is treated with zinc metal, the zinc removes both halogen atoms as , and a double bond forms between the two carbons. This reaction is called dehalogenation.
Examples:
1,2-dibromoethane gives ethene.
1,2-dibromopropane gives propene.
The reaction works because zinc has a strong affinity for halogens. The two halogen atoms are eliminated simultaneously, and the zinc atom bridges them, forming a cyclic intermediate that collapses to give the alkene.
4. From Alcohols by Acidic Dehydration
Alcohols () on heating with concentrated sulphuric acid () lose a water molecule to form an alkene. Since a water molecule is eliminated in the presence of an acid, this is called acidic dehydration of alcohols. …