Question

Account for the fact that treating propenoic acid (acrylic acid) with \(\mathrm{HCl}\) gives only 3 -chloropropanoic acid.

Short answer

Question: Explain why treating propenoic acid (acrylic acid) with HCl results in the formation of 3-chloropropanoic acid. Answer: Treating propenoic acid with HCl involves an electrophilic addition reaction mechanism, in which a more stable 2° carbocation is formed. The chloride ion then attacks the positively charged carbon atom, resulting in the formation of 3-chloropropanoic acid. Other products, such as 2-chloropropanoic acid, are not formed because they involve less stable carbocation intermediates.

Step-by-step solution

1. Identify the structure of propenoic acid

Propenoic acid, also known as acrylic acid, has the following structure: CH2=CHCOOH. It is an unsaturated carboxylic acid, with a double bond between its first and second carbons and a carboxylic acid group (-COOH) at its third carbon.

2. Understand the electrophilic addition reaction mechanism

Treating propenoic acid with HCl involves an electrophilic addition reaction mechanism. In such reactions, a nucleophile (in this case, the \(\pi\) electrons of the double bond) attacks an electrophile (in this case, the hydrogen atom of the HCl molecule). This attack results in the formation of a positively charged intermediate, known as a carbocation, and a negatively charged halide ion (\(\mathrm{Cl^{-}}\)).

3. Analyze the formation of carbocation intermediates

During the attack of the \(\pi\) electrons on the hydrogen atom of the HCl molecule, two possible carbocations could be formed: one with the positive charge on the second carbon atom (2° carbocation) and another one with the positive charge on the third carbon atom (1° carbocation). However, the more stable carbocation is the 2° carbocation, because it has more alkyl groups attached to the positively charged carbon atom, which can provide hyperconjugation and inductive effects to stabilize the carbocation.

4. Formation of 3-chloropropanoic acid

After the formation of the 2° carbocation, the negatively charged chloride ion (\(\mathrm{Cl^{-}}\)) will attack the positively charged carbon atom to form 3-chloropropanoic acid (CH2ClCH2COOH). This step completes the electrophilic addition reaction between propenoic acid and HCl.

5. Why only 3-chloropropanoic acid is formed

In this reaction, the only product formed is 3-chloropropanoic acid due to the preference for forming a more stable 2° carbocation intermediate. The less stable 1° carbocation would lead to the formation of 2-chloropropanoic acid, which is not observed because the energetic preference for the more stable intermediate dominates the reaction. Therefore, treating propenoic acid with HCl gives only 3-chloropropanoic acid.