Question

Indicate how you would synthesize each of the following compounds from any one of the given organic starting materials and inorganic reagents. Specify reagents and the reaction conditions. Starting materials: propylene, isobutylene.

Short answer

To synthesize 2-butanol from propylene, perform a hydroboration-oxidation reaction using diborane (B2H6) followed by oxidation with hydrogen peroxide (H2O2). To obtain 2-chloropropane from propylene, perform a hydrohalogenation reaction with hydrogen chloride (HCl). Lastly, to synthesize 2-methyl-1-butene from isobutylene, create a Grignard reagent from isobutylene and magnesium, react with formaldehyde, and then perform a dehydration reaction using concentrated sulfuric acid (H2SO4).

Step-by-step solution

Hydroboration-Oxidation of Propylene

To synthesize 2-butanol from propylene, we need to add water across the double bond to obtain an alcohol. We can use the hydroboration-oxidation reaction for this purpose. React propylene with diborane (B2H6) in the presence of tetrahydrofuran (THF) to form a trialkyl borane intermediate.

Oxidation with Hydrogen Peroxide

To complete the synthesis, treat the trialkyl borane intermediate with hydrogen peroxide (H2O2) in the presence of sodium hydroxide (NaOH) as a base. This will result in the formation of 2-butanol. #Example 2: Synthesis of 2-chloropropane from propylene#

Hydrohalogenation of Propylene

To form 2-chloropropane from propylene, we need to add a chlorine atom across the double bond. This can be done using the hydrohalogenation reaction. React propylene with hydrogen chloride (HCl) in the presence of an organic solvent like diethyl ether. The final product will be 2-chloropropane. #Example 3: Synthesis of 2-methyl-1-butene from isobutylene#

Grignard Reaction

First, prepare a Grignard reagent from isobutylene by reacting it with magnesium metal in the presence of anhydrous ether. This will result in the formation of the corresponding isobutyl magnesium halide (IsobutylMgX).

Reaction with Formaldehyde

Next, react the isobutyl magnesium halide (IsobutylMgX) with formaldehyde (HCHO) in the presence of anhydrous conditions. This reaction will form a primary alcohol after the addition of water or an aqueous acid.

Dehydration Reaction

In the final step, treat the primary alcohol formed in the previous step with a strong acid such as concentrated sulfuric acid (H2SO4) at a relatively high temperature, around 100-150 °C. The dehydration reaction will occur, resulting in the formation of 2-methyl-1-butene as the product.

Key concepts

Hydroboration-Oxidation

Hydroboration-Oxidation is a two-step reaction used to convert alkenes into alcohols. This reaction involves two main steps: hydroboration and oxidation. In the hydroboration step, the alkene reacts with diborane (B2H6) in the presence of a solvent such as tetrahydrofuran (THF). This interaction adds a boron atom to one of the carbon atoms in the double bond, forming a trialkyl borane intermediate. The beauty of this step is that it proceeds with anti-Markovnikov selectivity, meaning the boron atom attaches to the less substituted carbon atom of the double bond.

The next stage, oxidation, transforms the trialkyl borane into an alcohol. This is achieved by reacting it with hydrogen peroxide (H2O2) in alkaline conditions, usually in the presence of sodium hydroxide (NaOH). The result is an alcohol where the hydroxyl group (-OH) replaces the boron atom. This method allows for the formation of alcohols with precise control over their structure, especially useful in synthesizing compounds like 2-butanol from propylene.

Hydrohalogenation

Hydrohalogenation is an essential reaction for adding halogens to alkenes. This process involves the addition of a hydrogen halide (HX) to an alkene, where "X" represents a halogen like chlorine, bromine, or iodine. When propylene undergoes hydrohalogenation with hydrogen chloride (HCl), the chloride ion bonds with one of the carbon atoms from the double bond, resulting in the formation of 2-chloropropane.

One key aspect to remember is Markovnikov's rule, which often governs hydrohalogenation. According to this rule, the hydrogen atom from the hydrogen halide will add to the less substituted carbon atom, while the halogen will attach to the more substituted one. However, the solvent used and reaction conditions can influence the product distribution, allowing for further study and control of the outcome in more complex reactions.

Grignard Reaction

The Grignard Reaction is a powerful method for carbon-carbon bond formation in organic synthesis. This reaction starts with the formation of a Grignard reagent, produced by reacting an organohalide with magnesium in an anhydrous ether solvent. For instance, when isobutylene is transformed into isobutyl magnesium halide, this Grignard reagent is ready to participate in further reactions.

Grignard reagents are highly reactive and can add to carbonyl compounds like formaldehyde to form alcohols. The reaction involves nucleophilic addition where the carbon in the Grignard reagent attacks the electrophilic carbon in the formaldehyde, leading to the creation of a primary alcohol. After the reaction, careful addition of water or an acidic workup helps to stabilize and isolate the alcohol product.

These reactions provide a versatile toolkit not only for synthesizing alcohols but also for building more complex organic structures.

Dehydration Reaction

Dehydration Reaction involves removing water (H2O) from an alcohol to form an alkene. This process is typically achieved by heating the alcohol with a strong acid catalyst such as concentrated sulfuric acid (H2SO4). The reaction proceeds by protonation of the alcohol to form a good leaving group, followed by elimination to form the alkene.

The specific conditions, such as temperature and acid concentration, are crucial parameters that need precise control to avoid side reactions. For instance, converting a primary alcohol to an alkene like 2-methyl-1-butene requires heating at around 100-150 °C. Through careful adjustment of these conditions, the dehydration can preferentially form products with a specific structure, allowing chemists to synthesize desired alkenes efficiently.

Understanding the nuances of such reactions aids in constructing various organic molecules and helps achieve targeted synthetic pathways.