Question

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

Short answer

Answer: The reaction of propenoic acid with HCl yields only 3-chloropropanoic acid due to Markovnikov's rule, which governs the regioselectivity of the addition reaction. In this case, the hydrogen from the HCl adds to the second carbon atom of the double bond and the chlorine atom adds to the first carbon atom, resulting in the formation of 3-chloropropanoic acid.

Step-by-step solution

1. Understanding the structure of propenoic acid (acrylic acid)

Propenoic acid, also known as acrylic acid, is an unsaturated carboxylic acid with the molecular formula C3H4O2. Its structure consists of a double bond between the first and second carbon atoms, and a carboxyl group (-COOH) attached to the third carbon atom. The double bond introduces reactivity in the molecule and can participate in addition reactions.

2. Treating propenoic acid with HCl

When propenoic acid is treated with HCl, the reaction is an addition reaction where the hydrogen atom from the HCl is added to one of the carbon atoms, and the chlorine atom is added to the other carbon atom.

3. Markovnikov's rule

Markovnikov's rule is an important principle that helps predict the major product in addition reactions involving unsymmetrical alkenes and hydrogen halides (like HCl). According to this rule, the hydrogen atom will be added to the carbon atom with the greater number of hydrogen atoms already attached, while the halogen atom (in our case, Cl) will bond to the carbon atom with the fewer number of hydrogen atoms attached.

4. Applying Markovnikov's rule to the reaction of propenoic acid with HCl

In the case of propenoic acid, the double bond is between the first and second carbon atoms. The first carbon atom has two hydrogen atoms attached to it, while the second carbon atom has only one hydrogen atom attached. Therefore, according to Markovnikov's rule, the H from the HCl will add to the second carbon atom (which has more hydrogens), and the Cl will add to the first carbon atom (which has fewer hydrogens). The result is a 3-chloropropanoic acid molecule.

5. Conclusion

The reason propenoic acid (acrylic acid) treated with HCl gives only 3-chloropropanoic acid is due to the Markovnikov's rule, which governs the regioselectivity of the addition reaction. In this specific case, the hydrogen from the HCl adds to the second carbon atom of the double bond, and the chlorine atom adds to the first carbon atom, resulting in the formation of 3-chloropropanoic acid.