1. 9. STRUCTURE DETERMINATION

2. OBJECTIVES 1. Use combustion analysis to determine empirical formula.
2. Determine molecular weight (and molecular formula) from mass spectrometry.
2. Calculate number of rings and double bonds from molecular formula.
3. Determine functional groups present from infrared spectroscopy.
4. Use 1H and 13C NMR spectroscopy to identify other structural features.
5. Combine conclusions from individual techniques to determine the structure of organic compounds.

3. COMBUSTION ANALYSIS CxHyOz + O2 xCO2 + (y/2)H2O
Measure mass of CO2 and H2O formed from a known mass of compound; data cited as mass % of each element present.
e.g., C H O
Mass%
Mole% =
Ratio

4. EMPIRICAL AND MOLECULAR FORMULAS; DETERMINATION OF “SODAR”
Molecular formula is an integral number of times the empirical formula.
Sum of double bonds or rings (”SODAR”)
For C,H,O: (2#C + 2 – #H) / 2
For C,H,O,N,Hal: (2#C + 2– #H – #Hal + #N) / 2
Each Hal replaces a H
Each N adds an extra H
If SODAR calculated from empirical formula is not an integral, this cannot be the molecular formula.

5. Examples:
CH3Br
CH5N
C3H8N
C6H6

6. MASS SPECTROMETRY Theory and Experiment M + e– (70 eV) M+ + 2e–
M lower mass ions

7. e.g., mass spectrum of ethane, CH3CH3
Data Available
e.g., mass spectrum of ethane, CH3CH3
Ions also fragment, which can give further clues about the structure (beyond scope of this course).
Knowledge of molecular weight (from mass spec) and empirical formula (from combustion analysis) allows determination of molecular formula.
abundance
m/e

8. SPECTROSCOPY: THE INTERACTION OF LIGHT AND MATTER

9. INFRARED SPECTROSCOPY
See Chapter 2
COOH
3100
sp2C–H
1600 C=C
3500, O-H
C–O
2900 sp3C–H
1700 C=O
4000
600
Wavenumber / cm-1

10. Problem
Empirical formula: C4H8O
IR; peak at 1720 cm-1; no broad peak at 3300, no strong peaks at 1000 to 1200 cm-1, no strong peaks at cm-1