Relative vs. absolute stereochemistry

[DJtiesto]

One question I encountered on one of PR tests asked if a molecule underwent retention of configuration or inversion of configuration (I knocked out the other 2 answer choices).

The first molecule is 3-methyl cyclohexanol (the OH is drawn as going away and methyl coming toward you). THe second molecule is the exact same just replace the OH with a Deuterium (the methyl still coming at you and Deuterium away).

I reasoned that this is inversion of stereochemistry because the former molecule is 'S' while the second is 'R'. Am I getting relative and absolute stereochemistry mixed up?

The answer explained that the relative stereochemistry is the same but how can this be!?

---

Members don't see this ad.

 

[inaccensa]

One question I encountered on one of PR tests asked if a molecule underwent retention of configuration or inversion of configuration (I knocked out the other 2 answer choices).

The first molecule is 3-methyl cyclohexanol (the OH is drawn as going away and methyl coming toward you). THe second molecule is the exact same just replace the OH with a Deuterium (the methyl still coming at you and Deuterium away).

I reasoned that this is inversion of stereochemistry because the former molecule is 'S' while the second is 'R'. Am I getting relative and absolute stereochemistry mixed up?

The answer explained that the relative stereochemistry is the same but how can this be!?

Relative configuration is when you compare two compounds. Absolute can only be determined experimentally. As far as I understand, you place the 2 molecues side and side, just determine the configuration of the chiral compounds. The difference between OH and OD is D is heavier, but nonethess, methyl is still the lower priority molecule. So if you just replace the H with D, nothing happened to your configuration.

 

[DJtiesto]

Isn't deuterium (1 proton, 1 neutron) lighter than oxygen and carbon? Why would the priorities stay the same?

 

[inaccensa]

Isn't deuterium (1 proton, 1 neutron) lighter than oxygen and carbon? Why would the priorities stay the same?

You will only look at deuterium if the chiral carbon in question had 2 O or 2C. If there same element is attached to the carbon, they you look at the next atom attached to determine priority. Here, there are 2 different elements. You don't look at the OH as a group, you only look at O.