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

In this reaction, we have propenoic acid (acrylic acid) reacting with hydrochloric acid (HCl) to form S-chloropropanoic acid. The chemical structures of these compounds are as follows: Propenoic acid (acrylic acid): \(\mathrm{CH_2=CHCOOH}\) Hydrochloric acid (HCl): \(\mathrm{HCl}\) S-chloropropanoic acid: \(\mathrm{CH_3CHClCOOH}\) (S stereoisomer)

The reaction between propenoic acid and HCl is an electrophilic addition reaction. In this kind of reaction, a nucleophile (electron-rich species) attacks an electrophile (electron-poor species). 1. The nucleophile in this case is the double bond in propenoic acid, and the electrophile is the hydrogen atom in HCl. 2. The nucleophile attacks the electrophile, forming a bond to the hydrogen atom and the chloride ion detaches from the H-atom. 3. A carbocation intermediate is formed at the more substituted carbon (Markovnikov's rule): \(\mathrm{CH_3CH^{+}COOH}\) 4. Finally, the chloride ion (a good nucleophile) attacks the carbocation, forming S-chloropropanoic acid as a product.

The stereochemistry of this reaction is governed by the approach of the nucleophile (chloride ion) to the carbocation intermediate. The chloride ion can attack the carbocation from either the front or the back, leading to different stereoisomers. However, in this specific case, only the S enantiomer of chloropropanoic acid is formed. This selectivity can be explained by considering that the carbonyl oxygen atom in the carbocation intermediate's neighbouring ester motif can donate its lone electron pair, forming a resonance structure in which the positive charge is distributed between two carbon atoms. The resonance structure allows the carbonyl oxygen to interact with the carbocation carbon through delocalization of electron density, creating a combined electrostatic cloud at the electrophilic site. As a result, the chloride nucleophile's approach is sterically hindered from the front side, where the oxygen atom is present, and the nucleophilic attack takes place from the backside, exclusively leading to the formation of the S enantiomer of chloropropanoic acid. By understanding the reaction mechanism and the stereochemical factors involved in the reaction, we can account for the exclusive formation of the S enantiomer of chloropropanoic acid when propenoic acid (acrylic acid) is treated with HCl.