CHEM 313 Chapter Notes -Dipeptide, Dimethyl Sulfoxide, Phenyl Group
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H2n(s)hco2hh2nco2hhnh2ho2c(r)nh2iho2c(s)nh2(r)hoo(r)nh3o(s)h3ccf3(r)ho2cnh2shs(l)r(d)r(l)s(l)sidechainrr(d)sidechainsr(l)hhhhhhpka ~ 50pka ~ 15nnhnnpka ~ 9nhnpka ~ 5this question asks why the sp3 hybridized h atom of the molecule on the right (cyclopentadiene) is way more acidic - 1035 times! The cyclopentane is a general alkyl- anion that is not stabilized by anything. The cyclopentadiene upon losing an h+ becomes sp2 hybridized and the lone pair interacts with the double bonds generating an aromatic species. The second part asks why the pyridinium is more acidic then the 4- dimethylaminopyridinium. Something stabilizes the conjugate acid on the right relative to the pyridinium. Both are in the same hybridization, have octets, and are aromatic. Better resonance structures exist for 4-dimethylaminopyridinium because it spreads out the positive charge of conjugate acid and therefore stabilizes it relative to pyridine: provide the relative concentration of each species at ph 7. 4: Provide a base titration curve for the dipeptide shown below. Also, assign the following pka values to the appropriate groups: 2, 4, 8,10.
