Methods of Preparation of Alkenes - JEE PDF Download

Ans. There are several methods to prepare alkenes, including: - Dehydration of alcohols: Alkenes can be prepared by heating alcohols with a strong acid catalyst, such as concentrated sulfuric acid or phosphoric acid. The acid removes a water molecule from the alcohol, resulting in the formation of an alkene. - Dehydrohalogenation of alkyl halides: Alkenes can be prepared by heating alkyl halides, such as alkyl chlorides or alkyl bromides, with a strong base, like potassium hydroxide (KOH) or sodium hydroxide (NaOH). The base removes a hydrogen atom from the alkyl halide, resulting in the formation of an alkene. - Decarboxylation of carboxylic acids: Carboxylic acids can be heated to high temperatures, leading to the loss of a carbon dioxide molecule and the formation of an alkene. - Dehalogenation of vicinal dihalides: Alkenes can be prepared by treating vicinal dihalides, which are organic compounds with two halogen atoms attached to adjacent carbon atoms, with zinc metal (Zn) in the presence of acid. - Wittig reaction: The Wittig reaction involves the reaction of a phosphonium ylide with a carbonyl compound to produce an alkene. This method is particularly useful for the synthesis of complex alkenes.

Ans. Dehydration of alcohols involves the removal of a water molecule from the alcohol molecule, resulting in the formation of an alkene. This process typically requires the presence of a strong acid catalyst, such as concentrated sulfuric acid or phosphoric acid. During the dehydration reaction, the acid protonates the hydroxyl group (-OH) of the alcohol, making it a better leaving group. The protonated alcohol molecule then undergoes elimination, where a water molecule is removed from the adjacent carbon atom. This elimination reaction leads to the formation of a carbocation intermediate. The carbocation is a positively charged carbon atom, which is then stabilized by rearrangement or resonance. Finally, a proton is abstracted from an adjacent carbon atom by the acid catalyst, resulting in the formation of an alkene.

Ans. Dehydrohalogenation of alkyl halides involves the removal of a hydrogen halide molecule (HX) from the alkyl halide, resulting in the formation of an alkene. This reaction is typically carried out in the presence of a strong base, such as potassium hydroxide (KOH) or sodium hydroxide (NaOH). The mechanism of dehydrohalogenation follows an E2 (elimination, bimolecular) pathway. The base deprotonates the hydrogen atom on the carbon adjacent to the halogen atom, generating a strong nucleophile. The nucleophile attacks the carbon atom, leading to the removal of the halide ion and the formation of a double bond. The E2 mechanism occurs in a concerted manner, meaning that the hydrogen and halide atoms are removed simultaneously. This results in the formation of a transition state with a partial negative charge on the carbon atom and a partial positive charge on the hydrogen atom. The transition state then collapses to form the alkene product.

Ans. Carboxylic acids can be converted into alkenes through a process called decarboxylation. However, direct conversion of carboxylic acids into alkenes is not commonly used due to the high energy requirements and low yields. Decarboxylation of carboxylic acids involves the heating of the acid to high temperatures, typically above 300°C. This process results in the loss of a carbon dioxide molecule (CO2) from the carboxylic acid, leading to the formation of an alkene. The decarboxylation reaction proceeds through a carboxylate anion intermediate, where the acid donates a proton to form a carboxylate ion. The carboxylate ion then undergoes a series of rearrangement and elimination steps, resulting in the formation of an alkene. However, it is important to note that the decarboxylation of carboxylic acids is not a practical method for alkene synthesis due to the high temperatures required and the limited control over the reaction conditions.

Ans. The Wittig reaction is a powerful method for the synthesis of alkenes. It involves the reaction of a phosphonium ylide with a carbonyl compound, typically an aldehyde or a ketone, to produce an alkene. The phosphonium ylide, which is a compound with a positively charged phosphorus atom and a negatively charged carbon atom, acts as a nucleophile. It attacks the carbonyl carbon of the aldehyde or ketone, forming a new carbon-carbon double bond. The reaction proceeds through a four-membered cyclic intermediate called the oxaphosphetane. The oxygen atom of the carbonyl group forms a bond with the phosphorus atom, while the carbon atom of the ylide forms a bond with the carbonyl carbon. The oxaphosphetane intermediate is then subjected to an intramolecular elimination, where the oxygen atom takes a proton from the adjacent carbon atom. This results in the formation of the alkene product and the regeneration of the phosphonium ylide, which can undergo further reactions.