Question

Identify the type of organic reaction that would best accomplish each conversion. \(\begin{array}{ll}{\text { a. alkyl halide } \rightarrow \text { alkene }} & {\text { c.alcohol }+\text { carboxylic acid } \rightarrow \text { ester }} \\\ {\text { b. alkene } \rightarrow \text { alcohol }} & {\text { d. alkene } \rightarrow \text { alkyl dihalide }}\end{array}\)

Short answer

The best organic reactions for each given conversion are as follows: a. Alkyl halide to alkene: Dehydrohalogenation, using alcoholic potassium hydroxide (KOH) or sodium ethoxide (NaOEt). b. Alkene to alcohol: Hydroboration-oxidation, using borane reagent (BH3) followed by hydrogen peroxide (H2O2) and a hydroxide ion source. c. Alcohol and carboxylic acid to ester: Esterification, using sulfuric acid (H2SO4) or hydrochloric acid (HCl) as a catalyst. d. Alkene to alkyl dihalide: Halogenation, using a halogen molecule such as chlorine (Cl2) or bromine (Br2).

Step-by-step solution

a. Alkyl halide to alkene

One of the best ways to convert an alkyl halide to an alkene is through an elimination reaction. In this case, dehydrohalogenation is the specific type of elimination reaction used, which involves the removal of a hydrogen and a halogen from the molecule. The most common reagents used for dehydrohalogenation are alcoholic potassium hydroxide (KOH) or sodium ethoxide (NaOEt).

b. Alkene to alcohol

One of the most common ways to convert an alkene to alcohol is through an addition reaction known as hydroboration-oxidation. This reaction involves the following steps: 1. Hydroboration: A borane reagent (BH3) adds across the carbon-carbon double bond of the alkene. 2. Oxidation: Peroxide, an oxidizing agent (usually hydrogen peroxide, H2O2) and a hydroxide ion source are used to replace the boron atom with a hydroxyl group (-OH) to form the alcohol.

c. Alcohol and carboxylic acid to ester

To convert an alcohol and a carboxylic acid to an ester, a condensation reaction called esterification is employed. The reaction involves the -OH group of the carboxylic acid and the -OH group of the alcohol to form a new ester bond. The most common reagent used for this reaction is sulfuric acid (H2SO4) or hydrochloric acid (HCl), which acts as a catalyst for the reaction. The byproduct formed during this process is water.

d. Alkene to alkyl dihalide

To convert an alkene to an alkyl dihalide, we use a type of addition reaction called halogenation. In this reaction, a halogen molecule, such as chlorine (Cl2) or bromine (Br2), is added across the carbon-carbon double bond in the alkene to form the alkyl dihalide product. The halogen addition happens on both carbons involved in the double bond.

Key concepts

Elimination Reaction

An elimination reaction involves removing atoms or groups from a molecule, leading to the formation of a double bond or a new functional group. This type of reaction plays a critical role in organic chemistry, especially when converting alky halides into alkenes. In particular, dehydrohalogenation is a specific elimination where both a hydrogen atom and a halogen atom are removed. The typical reagents found in elimination reactions include alcoholic potassium hydroxide (KOH) and sodium ethoxide (NaOEt). These substances promote the removal process, resulting in the creation of a more saturated structure, like an alkene, characterized by its double-bond features. The reaction conditions, such as heat, often influence the outcome of these reactions.

Addition Reaction

Addition reactions are a central part of transforming unsaturated molecules, like alkenes, into saturated ones by adding atoms across double or triple bonds. One common method is hydroboration-oxidation, used to turn alkenes into alcohols. This specific type of addition reaction occurs in two main stages:

• Hydroboration: A borane reagent, such as BH3, adds to the carbon-carbon double bond, facilitating further reaction.
• Oxidation: Next, using hydrogen peroxide (H2O2) and a hydroxide ion, the boron atom is replaced with a hydroxyl group (-OH), forming alcohol.

This gentle method allows for syn-addition, meaning both the boron and hydrogen add to the same side of the alkene, ensuring a straightforward addition process for specific alcohol derivatives.

Condensation Reaction

Condensation reactions are vital for creating bigger molecules by joining smaller one with the elimination of a small molecule like water. One classic example is esterification, an organic process that transforms alcohols and carboxylic acids into esters. The reaction mechanism includes the alcohol contributing a hydroxyl group that combines with the carboxylic acid’s hydroxyl group. Catalysts like sulfuric acid (H2SO4) or hydrochloric acid (HCl) often speed up this reaction. The byproduct of esterification is water, which emphasizes the concept of losing a small molecule during the transformation that offers a rich source for ester linkages in both biological and industrial chemistry.

Hydroboration-Oxidation

Hydroboration-oxidation is a multi-step chemical process designed to convert alkenes into alcohols efficiently. The procedure highlights the importance of addition reactions in organic synthesis, delivering alcohols with excellent regioselectivity due to its syn-addition mechanism.

• Step 1 – Hydroboration: This involves the borane (BH3 or its derivatives) adding across the double bond, focusing on the less substituted carbon due to steric and electronic factors.
• Step 2 – Oxidation: The boron atom is eventually displaced by a hydroxyl group courtesy of oxidizing agents like hydrogen peroxide (H2O2). This transition is noted for being clean and predictable, which helps in synthesizing valuable alcohols.

By maintaining control over the conditions throughout both stages, this method provides a consistent pathway for specific functionalization in a variety of organic frameworks.

Halogenation

In organic chemistry, halogenation is the addition of halogen atoms to an organic molecule, typically involving alkenes and alkynes. It's a form of addition reaction that saturates double or triple bonds with halogens like chlorine (Cl2) or bromine (Br2). This process is crucial for generating alkyl dihalides, where two halogen molecules attach to each side of an originally unsaturated alkene. This reaction not only adds a valuable functional group but also impacts the physical and chemical properties of the compound. Halogenation facilitates the generation of various commercially and industrially important organohalogen compounds by adjusting conditions to manage rate and selectivity effectively. By comprehending halogenation, chemists can customize the characteristics of molecules for a wide range of applications.