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EDIT: I posted in the wrong forum! Can someone please move it to MCAT Study Material? )
Hello,
This is the basic structure of penicillin.
The 'R' group is variable.
Some penicillins are more acid stable, and some are less.
The acid-unstable penicillins are hydrolyzed at the amide bond between the 'R' group and the other half of the molecule.
Some side groups make penicillin more acid resistant than others.
This is explained (search for 'decreases the electron density of the side chain') by the idea that electron withdrawing side groups remove electron density from the carbonyl, making it less likely to be protonated by the acid. This protonation begins the acid hydrolysis.
TPR has a question (4) that asks why the amide bond in dicloxacillin and nafcillin is stable at low pH.
The answer is:
"amide bonds do not easily undergo hydrolysis"
because
"amides are stabilized by the ability of the nitrogen lone pair to donate electron density to the electrophilic carbonyl carbon. Amides do not easily undergo hydrolysis"
Does this make sense? Because there is an amide bond in all penicillins, not just in dicloxacillin and nafcillin. Wouldn't that make them all equally acid stable then?
Seems like their reasons would work only if some penicillins didn't have an amide bond between the R group and the 6-APA (other half).
Hello,
This is the basic structure of penicillin.
Some penicillins are more acid stable, and some are less.
The acid-unstable penicillins are hydrolyzed at the amide bond between the 'R' group and the other half of the molecule.
Some side groups make penicillin more acid resistant than others.
This is explained (search for 'decreases the electron density of the side chain') by the idea that electron withdrawing side groups remove electron density from the carbonyl, making it less likely to be protonated by the acid. This protonation begins the acid hydrolysis.
TPR has a question (4) that asks why the amide bond in dicloxacillin and nafcillin is stable at low pH.
The answer is:
"amide bonds do not easily undergo hydrolysis"
because
"amides are stabilized by the ability of the nitrogen lone pair to donate electron density to the electrophilic carbonyl carbon. Amides do not easily undergo hydrolysis"
Does this make sense? Because there is an amide bond in all penicillins, not just in dicloxacillin and nafcillin. Wouldn't that make them all equally acid stable then?
Seems like their reasons would work only if some penicillins didn't have an amide bond between the R group and the 6-APA (other half).