# 16.4 Preparation of Alkenes Alkenes can be synthesized through various methods. This section focuses on common laboratory preparations: the dehydration of alcohols, dehydrohalogenation of halogenoalkanes, vicinal dehalogenation, and controlled hydrogenation of alkynes. These methods are primarily types of **elimination reactions**. --- ## 1. Dehydration of Alcohols *Dehydration* is an elimination reaction that involves the removal of a water molecule ($H_2O$) from an alcohol. This reaction is also classified as a **β-elimination reaction**, where a hydrogen atom from the β-carbon (the carbon adjacent to the carbon bearing the -OH group) and the hydroxyl group are removed, forming a double bond. **Key Features:** - **Reactant:** An alcohol (e.g., ethanol). - **Product:** An alkene (e.g., ethene) and water. - **Conditions:** Requires a catalyst and heat. - **Catalysts:** The reaction is typically catalyzed by passing the alcohol vapors over hot **aluminium oxide** ($Al_2O_3$), which also acts as a dehydrating agent. Other common catalysts include concentrated **sulfuric acid** ($H_2SO_4$) and **phosphoric acid** ($H_3PO_4$). **Ease of Dehydration:** The ease of dehydration follows the order: **Tertiary (3°) > Secondary (2°) > Primary (1°)**. Tertiary alcohols dehydrate most readily because they form more stable carbocation intermediates. **General Reaction:** $$ \text{Alcohol} \xrightarrow[\text{Heat}]{\text{Catalyst}} \text{Alkene} + H_2O $$ **Example: Preparation of Ethene from Ethanol** When ethanol vapor is passed over hot aluminium oxide, it dehydrates to form ethene and water. Note that temperature matters; at $170^\circ C$ with $H_2SO_4$, ethene is the major product. $$ \underset{\text{Ethanol}}{CH_3CH_2OH} \xrightarrow[Heat]{Al_2O_3} \underset{\text{Ethene}}{CH_2=CH_2} + H_2O $$ --- ## 2. Dehydrohalogenation of Halogenoalkanes *Dehydrohalogenation* is an elimination reaction involving the removal of a hydrogen atom and a halogen atom from adjacent carbon atoms in a halogenoalkane. This is another example of a **β-elimination reaction**. **Key Features:** - **Reactant:** A halogenoalkane (e.g., bromoethane). - **Reagent:** A strong base dissolved in ethanol, such as hot ethanolic **potassium hydroxide** ($KOH$). The alcoholic medium favors elimination over substitution. - **Product:** An alkene, a salt, and water. **General Reaction:** $$ \text{Halogenoalkane} + \text{Base} \xrightarrow[\text{Heat}]{\text{Ethanol}} \text{Alkene} + \text{Salt} + H_2O $$ **Example: Preparation of Ethene from Bromoethane** Ethene can be prepared by heating bromoethane with a hot, ethanolic solution of potassium hydroxide. $$ \underset{\text{Bromoethane}}{CH_3CH_2Br} + KOH \xrightarrow[\text{Heat}]{\text{Ethanol}} \underset{\text{Ethene}}{CH_2=CH_2} + KBr + H_2O $$ --- ## 3. Dehalogenation of Vicinal Dihalides Vicinal dihalides are compounds where two halogen atoms are attached to adjacent carbon atoms. When treated with **Zinc dust** in an anhydrous solvent like methanol or acetic acid, they undergo dehalogenation to form an alkene. **Reaction:** $$ \underset{\text{Vicinal dihalide}}{R-CHX-CHX-R} + Zn \rightarrow R-CH=CH-R + ZnX_2 $$ --- ## 4. Controlled Hydrogenation of Alkynes Alkenes can be prepared by the partial reduction of alkynes. The stereochemistry of the product depends on the reducing agent used: 1. **Cis-alkenes:** Produced using hydrogen gas with **Lindlar's catalyst** ($Pd/BaSO_4$ poisoned with quinoline). 2. **Trans-alkenes:** Produced using **Sodium in liquid ammonia** ($Na / \text{liq. } NH_3$). --- ## Concept Assessment Exercise 16.2 Prepare **but-1-ene** by the following preparatory methods. **i. Dehydration of an alcohol** **ii. Dehydrohalogenation of a halogenoalkane** ### Solution **i. Preparation of But-1-ene by Dehydration of an Alcohol** To produce but-1-ene ($CH_2=CHCH_2CH_3$), we must start with an alcohol where the -OH group is on carbon 1, which is **butan-1-ol**. **Reaction:** $$ \underset{\text{Butan-1-ol}}{CH_3CH_2CH_2CH_2OH} \xrightarrow[\text{Heat}]{\text{conc. } H_2SO_4} \underset{\text{But-1-ene}}{CH_2=CHCH_2CH_3} + H_2O $$ **ii. Preparation of But-1-ene by Dehydrohalogenation of a Halogenoalkane** To produce but-1-ene, we must start with a halogenoalkane where the halogen is on carbon 1, such as **1-bromobutane**. **Reaction:** $$ \underset{\text{1-Bromobutane}}{CH_3CH_2CH_2CH_2Br} + KOH \xrightarrow[\text{Heat}]{\text{Ethanol}} \underset{\text{But-1-ene}}{CH_2=CHCH_2CH_3} + KBr + H_2O $$