March 24, 2023

had to do a molecular assessment of gabapentin for my organic chemistry lecture. So here it is.

Looking at the stereochemistry of gabapentin you can see that the only carbon that looks like it would be a chiral center is this one right here however because of this ring it is identical if you take this pathway and identical if you take this pathway and because that does not form four different groups attached to that carbon there is no chiral center and

Because there is no chiral center than the r and the s rotations do not matter and looking at the c nmr of gabapentin you can see there are five different carbon groups there’s um the ones around the ring here that are the ch2 the carbon right here at the top that is attached to these two different substituent’s the and then each substituent has this one has two

Different carbon groups and then this one only has one different carbon group there there are five signals expected and the chemical shifts included on each one are the the ring group right here is 50 to 0 ppm because of the ch2 bonds the carbon right here is a 50 to 0 ppm because it is sp3 hybridized carbon the carbon up here because of that bond with the nitrogen

Has a shift of 100 to 50 ppm the carbon right here because it is another ch2 with different connected groups it is 50 to 0 ppm shift and then the last carbon right here attached that double oxygen bond and then the oh h makes it just the sp2 hybridize so because of that double bond you expect the 220 to 150 ppm shift and looking at the h in amar’s you can see that

There are five different hydrogen groups to equal the five different signals you’ve got the ring here all those ch two carbons along there make the the h a group then the protons attached up here is another ch2 group so it is hb because of the different connectivities then that nitrogen has two hydrogen’s attached to it so it’s a third group and then over on this

Substituent that carbon is also a ch2 but it’s connected to different groups so it’s its own one and then the last hydrogen is down here connected to that oh h so it is h ii and then here is the the chart for those different age groups h a which has 10 hydrogen’s in the group has a expected chemical shift of about 1.2 each because of the methylene groups and because

They are all the same here without these neighbors are too far away these neighbors are too far away it the splitting would be singlet and then this group up here that hb there are two there and so you’re it’s an ethyl another methylene group which a with the about a one point 2 ppm shift and because of these hydrogen’s right beside it these it has so this group

Has two neighboring hydrogens so you’re expecting a triplet signal and the hc those are these nitrogen’s hydrogen’s right here once again it is because it’s connected to that nitrogen about 0.5 to 5 ppm shift and it does have these two neighbors right here so you would get another triplet signal and then the hd another methylene with two hydrogen’s it’s a singlet

There’s no neighbors beside it and then the last one down here attached to the o is because it’s a carboxylic group with this double o bond it’s about a 12 ppm shift however there are no other hydrogen neighbors so it is just a singlet

Transcribed from video
Gabapentin stereochemistry and NMR By Ashley Kawaii