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Which of these compounds is the hemiacetal that forms when ethanol reacts with propanal?

A.
[tex]\[ \text{OH} \][/tex]
[tex]\[ \begin{array}{c}
\vert \\
\text{CH}_3 - \text{CH}_2 - \text{C} - \text{O} - \text{CH}_2 - \text{CH}_3 \\
\vert \\
\text{H}
\end{array} \][/tex]

B.
[tex]\[ \text{CH}_3 - \text{CH}_2 - \text{C} - \text{O} - \text{CH}_2 - \text{CH}_3 \][/tex]
[tex]\[ \vert \][/tex]
[tex]\[ \text{H} \][/tex]

C.
[tex]\[ \text{CH}_3 - \text{C} - \text{O} - \text{CH}_3 \][/tex]
[tex]\[ \vert \][/tex]
[tex]\[ \text{H} \][/tex]

Sagot :

GinnyAnswer
To determine which compound is the hemiacetal formed when ethanol reacts with propanal, let's first understand the reaction mechanism between ethanol and propanal:

1. Identify the Reactants:
- Ethanol: [tex]\( CH_3CH_2OH \)[/tex]
- Propanal: [tex]\( CH_3CH_2CHO \)[/tex]
- A hemiacetal is formed when an alcohol adds to an aldehyde.

2. Mechanism of Hemiacetal Formation:
- The aldehyde group of propanal ([tex]\( CH_3CH_2CHO \)[/tex]) has a carbonyl carbon, which is electrophilic.
- The hydroxyl group [tex]\( -OH \)[/tex] of ethanol ([tex]\( CH_3CH_2OH \)[/tex]) is nucleophilic.

3. Step-by-Step Reaction:
- The nucleophilic oxygen in the ethanol attacks the electrophilic carbon in the carbonyl group of propanal.
- This results in the breaking of the [tex]\( \pi \)[/tex]-bond of the carbonyl group, forming a tetrahedral intermediate.
- Subsequently, a proton transfer occurs to stabilize the intermediate, forming the hemiacetal.

4. Structural Formation:
- The initial carbonyl compound (propanal) will have its carbonyl ([tex]\( C=O \)[/tex]) transformed.
- The [tex]\( C \)[/tex] atom originally part of the carbonyl will now have a single bond with [tex]\( O \)[/tex], resulting in an [tex]\( OH \)[/tex] group.
- The oxygen from ethanol will form an [tex]\( O \)[/tex]-[tex]\(CH_2CH_3 \)[/tex] group attached to this carbon.

5. Naming and Drawing the Hemiacetal:
- The final structure of the hemiacetal combining both propanal and ethanol components is:

[tex]\[ CH_3-CH_2-CH(\textcolor{red}{OH})-O-CH_2-CH_3 \][/tex]

- This structure indicates that the original methyl and ethyl groups remain unaltered, while the carbon (formerly part of an aldehyde) now has an additional hydroxyl group from the aldehyde and an ethoxy group from ethanol.

6. Identify the Correct Compound Among Given Options:

- The hemiacetal formed can be identified as matching the structural description of:

[tex]\[ CH_3-CH_2-CH(\textcolor{red}{OH})-O-CH_2-CH_3 \][/tex]

Therefore, the correct hemiacetal formed when ethanol reacts with propanal is:

[tex]\[ CH_3-CH_2-CH(\textcolor{red}{OH})-O-CH_2-CH_3 \][/tex]
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