Problem 67 - Chemistry An Atoms First Approach

Question

How would you synthesize each of the following? a. 1,2-dibromopropane from propene b. acetone (2-propanone) from an alcohol c. tert-butyl alcohol ( 2 -methyl-2-propanol) from an alkene (See Exercise 62.) d. propanoic acid from an alcohol

Step-by-Step Solution

Answer

a. Brominate propene using Br2 to form 1,2-dibromopropane. \[ \text{Propene + Br}_2 \rightarrow \text{1,2-dibromopropane}\] b. Oxidize propan-2-ol using K2Cr2O7 and H2SO4 to form acetone (2-propanone). \[ \text{Propan-2-ol + K}_2\text{Cr}_2\text{O}_7 + \text{H}_2\text{SO}_4 \xrightarrow[]{\Delta} \text{2-propanone (acetone)}\] c. Hydrate 2-methylpropene using H2O and H2SO4 to form tert-butyl alcohol (2-methyl-2-propanol). \[ \text{2-Methylpropene} + \text{H}_2\text{O} + \text{H}_2\text{SO}_4 \rightarrow \text{tert-butyl alcohol (2-methyl-2-propanol)}\] d. Oxidize propan-1-ol using K2Cr2O7 and H2SO4 to form propanoic acid. \[ \text{Propan-1-ol} + 2 \, \text{K}_2\text{Cr}_2\text{O}_7 + 2 \, \text{H}_2\text{SO}_4 \xrightarrow[]{\Delta} \text{Propanoic acid}\]

Step 1: Bromination of propene

Add one equivalent of bromine (Br2) to the propene molecule. The bromination of an alkene is an electrophilic addition reaction. Br2 will add across the double bond of propene, forming 1,2-dibromopropane as the product. \[ \text{Propene + Br}_2 \rightarrow \text{1,2-dibromopropane}\] b. Synthesis of acetone (2-propanone) from an alcohol

Step 1: Identify the precursor alcohol

Determine the alcohol precursor from which we can synthesize acetone. It is propan-2-ol, given that oxidation of this secondary alcohol will result in a ketone.

Step 2: Oxidation of propan-2-ol

Oxidize propan-2-ol using an appropriate oxidizing agent, such as potassium dichromate (K2Cr2O7) and concentrated sulfuric acid (H2SO4), and then heat the solution. This reaction will transform the secondary alcohol into 2-propanone (acetone). \[ \text{Propan-2-ol + K}_2\text{Cr}_2\text{O}_7 + \text{H}_2\text{SO}_4 \xrightarrow[]{\Delta} \text{2-propanone (acetone)}\] c. Synthesis of tert-butyl alcohol (2-methyl-2-propanol) from an alkene

Step 1: Identify the precursor alkene

Determine the alkene precursor from which we can synthesize tert-butyl alcohol. It is 2-methylpropene, given that hydration of this alkene will result in a tertiary alcohol.

Step 2: Hydration of 2-methylpropene

Hydrate 2-methylpropene using water (H2O) in the presence of an acid catalyst, such as concentrated sulfuric acid (H2SO4) or phosphoric acid (H3PO4). This reaction will result in the formation of tert-butyl alcohol (2-methyl-2-propanol). \[ \text{2-Methylpropene} + \text{H}_2\text{O} + \text{H}_2\text{SO}_4 \rightarrow \text{tert-butyl alcohol (2-methyl-2-propanol)}\] d. Synthesis of propanoic acid from an alcohol

Step 1: Identify the precursor alcohol

Determine the alcohol precursor from which we can synthesize propanoic acid. It is propan-1-ol, given that oxidation of this primary alcohol will result in a carboxylic acid.

Step 2: Oxidation of propan-1-ol

Oxidize propan-1-ol using an appropriate oxidizing agent, such as potassium dichromate (K2Cr2O7) and concentrated sulfuric acid (H2SO4), and then heat the solution to reflux. This reaction will transform the primary alcohol into propanal (an aldehyde) first, followed by further oxidation into propanoic acid. \[ \text{Propan-1-ol} + 2 \, \text{K}_2\text{Cr}_2\text{O}_7 + 2 \, \text{H}_2\text{SO}_4 \xrightarrow[]{\Delta} \text{Propanoic acid}\]

Key Concepts

Bromination

Bromination is a fascinating process where bromine atoms are added to a molecule. This type of reaction is commonly seen in organic synthesis, especially with alkenes. Alkenes, which are hydrocarbons with double bonds, readily react with halogens like bromine. When you add bromine ( Br_2) to an alkene like propene, an electrophilic addition reaction occurs. The double bond in the alkene opens up, allowing two bromine atoms to attach to the molecule, converting it to 1,2-dibromopropane.

• Bromination is useful for adding complex groups to simple molecules.
• It changes physical properties like reactivity and solubility.

When conducting bromination, you must be cautious, as bromine is highly reactive and needs to be handled carefully in a controlled environment. The reaction serves as a fundamental stepping stone in learning about how alkenes can be functionalized in organic chemistry.

Oxidation Reactions

Oxidation reactions are pivotal in transforming alcohols into different functional groups like ketones or carboxylic acids. These reactions involve increasing the oxidation number of the carbon atom, typically with an oxidative agent like potassium dichromate ( K_2Cr_2O_7) in the presence of an acid like sulfuric acid ( H_2SO_4) .

• For converting secondary alcohols to ketones, mild oxidation is used.
• For turning primary alcohols into carboxylic acids, strong oxidants are required.

Consider the scenario where propan-2-ol is oxidized into acetone. Here, the secondary alcohol loses hydrogen atoms, resulting in the formation of a ketone—acetone ( CH_3COCH_3) . Oxidation reactions are crucial as they facilitate myriad synthetic applications in creating more complex molecules from simpler ones.

Hydration Reactions

Hydration reactions involve the addition of water to a compound. Alkene hydration is a classic method for producing alcohols. Alkenes, when treated with water in an acidic environment, undergo an electrophilic addition to form alcohols. For instance, when 2-methylpropene is hydrated, it results in the formation of tert-butyl alcohol (also known as 2-methyl-2-propanol). Hydration is very useful for:

• Producing alcohols sustainably.
• Modifying the functional groups of hydrocarbons.

Utilizing catalysts like concentrated sulfuric acid ( H_2SO_4) enhances the hydration process, making it more efficient. This reaction shows the versatility of converting simple alkenes into more valuable alcohol products, especially in industrial applications.

Alkene Reactions

Alkenes are versatile molecules in organic chemistry, primarily because of their double bonds. These double bonds are sites of reactivity where various reactions, including halogenation, hydration, and oxidation, can take place. The reactivity of alkenes makes them a central focus in the making of significant chemical materials. Key reactions include:

• Bromination: Adding bromine across double bonds, leading to dibromoalkanes.
• Hydration: Introducing water molecules to form alcohols.
• Oxidation: Transforming alkenes into alcohols or carboxylic acids, depending on the starting material.

Alkene reactions provide a foundation for building complex chemical structures in organic synthesis. Through careful control of reaction conditions and choice of reagents, chemists can exploit alkene versatility to synthesize a wide range of useful compounds.