Pyrrole and the [CsHs ion are isostructural (same structures), isolobal (same nu
ID: 586706 • Letter: P
Question
Pyrrole and the [CsHs ion are isostructural (same structures), isolobal (same number and approximate symmetry of atomic and molecular orbitals), and isoelectronic (same number of valence electrons). Pyzrole has Car symmetry. Note the geometric implications of this fact.) a) What is the (approximate) hybridization of orbitals at the N atom? b) The five MOs of pvr role are shown in the diagram on the next page. Which is the HOMO pyrrole In pyrrole, the HOMO and the HOMO-1 correspond to two orbitals that are degenerate in [CsHs With reference to the atomic orbitals used to form them, why are they not degenerate in pyrrole? Also shown on the next page are two electron density maps showing the HOMO electron density of both pyrole and dimethylamine density.) In terms of the MO dia smaller than typical amines. (Consider that the HOMO of any amine will resemble that of NHs.) c) d) these diagrams, blue corresponds to a region of high electron gram, explain why pyrrole has a K, fifteen orders of magnitude e) Protonation of pyrrole does not occur at the N atom to give an ammonium ion. Examine carefully the nature of the pyrrole HOMO and the electron density map. Draw a curved-arrow mechanism for and the product of the addition of H to pyrrole NExplanation / Answer
(a) The Nitrogen atom of pyrrole is sp2 hybridized. This is because the lone pair of nitrogen is involved in delocalization causing the pyrrole aromatic.
(b) The HOMO of pyrrole is 1a2. HOMO stands for highest occupied molecular orbital and here it is 1a2.
(c) In case of pyrrole, the orbitals are formed by the overlap of orbitals of four carbon atoms and one nitrogen in which the overlapping orbitals are of different energy. Hence, the HOMO of pyrrole and HOMO-1 is non-degenerate.
(d) In pyrrole, the hybridization of N atom is sp2 whereas in amine it is sp3. With the increase in s-character increases the acidity. Hence, pyrrole has more s-character and it is more acidic than amine and conversely has less Kb
(e) The protonation happens at the 2 and 5 positions of pyrrole and not at the nitrogen atom.
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