In the molecule ${\mathrm{CH}}_2\mathrm{CHBrCl}$, which of the two hydrogens on the methylene carbon is pro-$R$?

• A. The hydrogen that, if replaced by a deuterium atom, gives the ($R$)-enantiomer at the methylene carbon
• B. The hydrogen that, if replaced by a deuterium atom, gives the ($S$)-enantiomer at the methylene carbon
• C. Neither hydrogen is pro-$R$
• D. Both hydrogens are pro-$R$

Answer: (A)

Verified step by step guidance

1. Identify the stereogenic center in the molecule. Here, the methylene carbon (\( \mathrm{CH}_2 \)) is attached to two hydrogens, a bromine (Br), and a chlorine (Cl) atom, making it a potential stereocenter upon substitution of one hydrogen.
2. Understand the concept of pro-R and pro-S hydrogens: these are the two hydrogens on a methylene carbon that, if one is replaced by a different group (e.g., deuterium), the resulting stereocenter will have either the R or S configuration.
3. Assign priorities to the substituents attached to the methylene carbon according to the Cahn-Ingold-Prelog (CIP) rules: Br > Cl > the other carbon chain > hydrogen (or deuterium when substituted).
4. For each hydrogen, imagine replacing it with a deuterium atom (D) and determine the absolute configuration (R or S) of the resulting stereocenter by arranging the substituents in order of priority and applying the CIP rules.
5. The hydrogen whose replacement by deuterium leads to the (R)-configuration is the pro-R hydrogen; the other is pro-S. Thus, identify which hydrogen is pro-R by this method.