1. Chapter 15 Structure Determination by Spectroscopy (II): UV-Vis & Mass
Organic Chemistry II
Fall 1999
Chapter 15 Structure Determination by Spectroscopy (II): UV-Vis & Mass
Ultraviolet-Visible: electronic transitions
200~800 nm (143~36 kcal/mol):  610 Figure 15.1
A = e cl at lmax (e max, solvent );  611 Figure 15.2
chromophore (absorption part):  613 Figure 15.3
alkanes: s  s*, lmax< 200 nm, used as solvents
allowed: p  p* (e >1,000), forbidden: n  p* (e <1,000)
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2. Interpretation of UV-Vis Spectra
Organic Chemistry II
Fall 1999
Interpretation of UV-Vis Spectra
color: absorption in the visible region
complementary color:  614 Table 15.1 & bottom
indication of conjugated double bonds
similar absorption for the same chromophore: for structural identification:  616 top
UV-B/A: 295~320/ nm (sun-burn)
sunscreen: ozone & PABA,  617 Focus On
practice:  616 Problem 15.6
OrgChem-Chap15
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3. log emax l (nm) Organic Chemistry II Fall 1999 Chapter 13
OrgChem-Chap15
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4. Mass Spectrometry: Mw & Formula
Organic Chemistry II
Fall 1999
Mass Spectrometry: Mw & Formula
molecular weight: molecular ion (M•+),  618
spectrometer: only cation,  618 Figure 15.4
spectrum: m/z & base peak,  619 Figure 15.5
molecular formula: HRMS or isotope intensity
HRMS: atomic wt. vs exact mass,  619 Table 15.2
Mw 44: CO2, C2H4O, C3H8 - different exact mass
relative intensity of isotope peaks:  620 Table 15.3
%[M+1]  1.1C + 0.4N; %[M+2]  (1.1C)2/ O
practice:  & 623 Problem 15.10
OrgChem-Chap15
Chapter 13

5. Fragmentation Pattern (I): Stability
Organic Chemistry II
Fall 1999
Fragmentation Pattern (I): Stability
alkanes: [R-R’]•+  [R]+ + [R’]• or [R]• + [R’]+
octane/2-methylpentane:  Figure
alkenes: allyl cation (m/z=41),  629 Figure 15.10
benzylic cleavage (m/z=91):  629 Figure 15.11
practice:  631 Practice 15.2
OrgChem-Chap15
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6. Fragmentation Pattern (II): Stability
Organic Chemistry II
Fall 1999
Fragmentation Pattern (II): Stability
alcohols: M-18(H2O) peak,  625 top
a-fission: 2-butanol,  630 middle & Figure 15.12
carbonyls: a-fission & McLafferty rearrangement,  630 & 631 & Figure 15.13
practice:  632 Problem 15.14
Focus On: GC/MS,  633
OrgChem-Chap15
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