Identify the indicated hydrogens in the following molecules...

Key Concepts

Pro-R and Pro-S Designation

Pro-R and pro-S designations are used to differentiate between two identical substituents, such as hydrogen atoms, that are attached to an otherwise chiral center. If replacing one of the two identical hydrogens with another substituent leads to a configuration that is R according to the CIP rules, that hydrogen is labeled as pro-R; conversely, if it leads to an S configuration, it is labeled as pro-S. This concept is important for understanding the subtle stereochemical nuances in molecules that appear symmetric but can lead to different stereochemical outcomes upon substitution.

Chirality

Chirality refers to the geometric property of a molecule having a non-superimposable mirror image. This phenomenon typically arises when a carbon atom is bonded to four distinct substituents, creating a center of asymmetry known as a stereogenic center. Understanding chirality is fundamental to the study of stereochemistry because many molecules exist in enantiomeric forms, which can have significantly different chemical and biological properties.

CIP Priority Rules

The Cahn–Ingold–Prelog (CIP) priority rules provide a systematic method for ranking the substituents attached to a stereogenic center based on atomic number and connectivity. This ranking is crucial for determining the absolute configuration (R or S) of a chiral center. These rules are applied step by step to compare substituents, and even in cases where two substituents are identical, the application of these rules helps in distinguishing between them when considering prochiral hydrogens.

Transcript

00:01 We want to identify which of these hydrogens are going to be pro r and pro s.

00:07 Now, what we are going to do, since on this carbon here, we have the two hydrogens.

00:16 We're going to replace one of the hydrogens with its, or one of its isotopes, deuterium.

00:23 So let's first replace this one facing forward with a deuterium, and then figure out if that now is going to be r or s.

00:33 So remember that deuterium has the same atomic number as hydrogen, since they're just isotopes of each other.

00:42 So in the tiebreaker of what has higher priority for r &s, since it goes atomic number, then atomic weight, deuterium would have the larger atomic weight, so that would be our higher priority group.

00:58 So we're going to have 4 -3, and then we need to decide, well, what has a higher priority? this oxygen or this carbon? well, oxygen has a higher atomic number, so we'd have one going to the left, and then two going towards our carbon there.

01:11 And notice that this rotates counterclockwise, and so that's going to be s.

01:20 Because remember when we draw an s, that first curve we draw goes counterclockwise.

01:25 At least that's the way i remember it.

01:28 So this is going to be pro s.

01:31 And since these are the same groups, if you were to just move that deuterium to the, the other side, it would give us the other orientation.

01:40 So pro r.

01:44 Now we can come up here and do the same thing...

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