simple O-chem question reg: R/S priority

[dan123]

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I seemed to have forgotten but when "D" is a substituent of a chiral center, is that the lowest priority or highest? My ochem teacher never used this but I have seen it in just about all mcat prep books. Thanks in advance for your help

 

[salim271]

Do you mean deuterium? Its higher than hydrogen because it has a greater mass, but lower priority than everything else.

 

[phant0m0o0o]

D is deuterium, or a hydrogen with a neutron giving it a weight of 2

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[dan123]

yes that is what I meant...thank you for the quick replies. Just so I am clear though and dont have to ask anymore....

So D gets lowest if no H in molecule, but If D and H are in the same chiral molecule, H will get lowest priority?

Thanks again for the quick replies and for all the help

 

[DavidTheGreek]

Correct, H would be considered the lowest priority, always. Now, if there is only deuterium then it will get the lowest (after all, it's basically H). If they are both present, H will take the lowest, followed by D

 

[dan123]

Got it, thank you so much.

 

[salim271]

I've seen D turn up more when looking at the stability of axial vs. equatorial, in case you run into that sometime, know that because D is heavier, it is also bulkier and more likely to be placed in the equatorial position than a H.

 

[BerkReviewTeach]

I've seen D turn up more when looking at the stability of axial vs. equatorial, in case you run into that sometime, know that because D is heavier, it is also bulkier and more likely to be placed in the equatorial position than a H.

You actually have this backwards. D is more likely to be axial than H, because it has a greater mass than H which results in a short bond length on average for C-D than C-H. Shorter bonds get pushed to axial because the longer bonds assume equatorial orientation.

It comes down to the realization that heavier means its nucleus weighs more, not that its electron cloud takes up more space. A perfect example is a CH3 group versus an F. CH3 weighs 15 g/mole while F weighs 19 g/mole, but CH3 is significantly larger (bulkier) than F.

 

[kehlsh]

You actually have this backwards. D is more likely to be axial than H, because it has a greater mass than H which results in a short bond length on average for C-D than C-H. Shorter bonds get pushed to axial because the longer bonds assume equatorial orientation.

It comes down to the realization that heavier means its nucleus weighs more, not that its electron cloud takes up more space. A perfect example is a CH3 group versus an F. CH3 weighs 15 g/mole while F weighs 19 g/mole, but CH3 is significantly larger (bulkier) than F.

wow that was insightful 👍

 

[salim271]

You actually have this backwards. D is more likely to be axial than H, because it has a greater mass than H which results in a short bond length on average for C-D than C-H. Shorter bonds get pushed to axial because the longer bonds assume equatorial orientation.

It comes down to the realization that heavier means its nucleus weighs more, not that its electron cloud takes up more space. A perfect example is a CH3 group versus an F. CH3 weighs 15 g/mole while F weighs 19 g/mole, but CH3 is significantly larger (bulkier) than F.

Whoops lol thx for the correction I must be slipping post-MCAT... haha.