Problem 87 - Chemistry and Chemical Reactivity

Question

Addition of water to alkene \(\mathbf{X}\) gives an alcohol \(\mathbf{Y}\). Oxidation of \(\mathbf{Y}\) produces 3,3 -dimethyl-2-pentanone. Identify \(\mathbf{X}\) and \(\mathbf{Y}\), and write equations for the two reactions.

Step-by-Step Solution

Answer

\( \mathbf{X} \) is 3,3-dimethyl-1-pentene; \( \mathbf{Y} \) is 3,3-dimethyl-2-pentanol.

Step 1: Identify Alcohol Y

To determine the structure of alcohol \( \mathbf{Y} \), we start by analyzing the 3,3-dimethyl-2-pentanone. The alcohol that is oxidized to form a ketone is typically a secondary alcohol. Therefore, \( \mathbf{Y} \) must have the structure with an OH group located on the 2nd carbon of 3,3-dimethylpentane, which would be 3,3-dimethyl-2-pentanol.

Step 2: Determine Alkene X

As we have identified \( \mathbf{Y} \) as 3,3-dimethyl-2-pentanol, the corresponding alkene \( \mathbf{X} \) would be obtained by removing the alcohol group and introducing a double bond between the second and third carbons in the longest chain. Hence, \( \mathbf{X} \) is 3,3-dimethyl-1-pentene.

Step 3: Write Reaction Equations

Now, write the chemical equations for the reactions:1. **Hydration of Alkene \( \mathbf{X} \):**\[ \text{3,3-dimethyl-1-pentene} + \text{water} \rightarrow \text{3,3-dimethyl-2-pentanol} \]2. **Oxidation of Alcohol \( \mathbf{Y} \):**\[ \text{3,3-dimethyl-2-pentanol} \xrightarrow[\text{oxidizing agent}]{} \text{3,3-dimethyl-2-pentanone} \]

Step 4: Verify Consistency

Ensure that both the structural formulas and the transformations are chemically feasible and that the product of each reaction is correct. The addition of water to 3,3-dimethyl-1-pentene to yield a secondary alcohol and its oxidation to yield a ketone are consistent with known chemical reactions.

Key Concepts

Hydration Reaction

Hydration reactions are a type of chemical reaction where water is added to a molecule. In the case of alkenes, the process involves breaking a double bond and adding an OH (hydroxyl group) from water across the double bond. This reaction typically requires an acidic catalyst to proceed.
In our original exercise, the hydration of alkene \( \mathbf{X} \) involves such a process. By adding water to 3,3-dimethyl-1-pentene, we form the alcohol 3,3-dimethyl-2-pentanol.

• Initial Molecule: Alkene with a double bond
• Reactant Added: Water (H₂O)
• Product: Addition of a hydroxyl group to form the alcohol

One of the key outcomes of hydration is the creation of an alcohol, in our case, a secondary alcohol specifically.

Oxidation Reaction

Oxidation reactions involve the increase in the oxidation state of a molecule, typically by the loss of electrons. In organic chemistry, this reaction often includes the loss of hydrogen or the gain of oxygen. When we oxidize an alcohol, it usually transforms into a carbonyl compound.
The exercise provided involves oxidizing the alcohol 3,3-dimethyl-2-pentanol (\( \mathbf{Y} \)) to form 3,3-dimethyl-2-pentanone. This process shows:

• The conversion of a secondary alcohol to a ketone.
• Requirement of an oxidizing agent, like potassium dichromate (\( K_2Cr_2O_7 \)) or PCC.

The key result of this reaction is the transformation of the hydroxyl group into a carbonyl group, making it a ketone.

Secondary Alcohol

Secondary alcohols are alcohols where the hydroxyl group (OH) is attached to a carbon atom which is itself connected to two other carbon atoms. This positioning influences the properties and reactions of the alcohol.
In the case of the exercise, 3,3-dimethyl-2-pentanol is a secondary alcohol. This positioning is significant because secondary alcohols typically oxidize to form ketones, unlike primary alcohols, which form aldehydes. Some key characteristics are:

• Has a hydroxyl group on a carbon atom bonded to two other carbons.
• Undergoes oxidation to form ketones.

Secondary alcohols like 3,3-dimethyl-2-pentanol give rise to compounds such as ketones upon oxidation, highlighting their importance in synthetic organic chemistry.

Chemical Equations

Chemical equations represent the changes occurring during a chemical reaction. These equations show the reactants, products, and relevant conditions such as catalysts or heat. They are both a symbolic representation and a critical tool for understanding molecular interactions.
In the provided solution, two key chemical equations demonstrate the transformations:

• Hydration Equation: \[ \text{3,3-dimethyl-1-pentene} + \text{H}_2\text{O} \rightarrow \text{3,3-dimethyl-2-pentanol} \]
• Oxidation Equation: \[ \text{3,3-dimethyl-2-pentanol} \xrightarrow[\text{oxidizing agent}]{} \text{3,3-dimethyl-2-pentanone} \]

Understanding these equations involves recognizing the structure of reactants and the pathways leading to products, all while balancing atoms and charges where applicable.