1 of 21

Lab Skills Seminar

Analysis of Unknowns - Spectroscopy

2 of 21

Mass Spectrometry

  • Three peaks:
    • Molecular Ion Peak
      • Molecular weight is even? Or odd?
    • Base Peak
    • Any other major peak (eg. halogens)
      • Can you get information from the M+1 or +2 peaks?
  • Calculate the unsaturation number

​

3 of 21

Infrared (IR) Spectroscopy

    • Functional group region vs. Fingerprint region

4 of 21

Infrared (IR) Spectroscopy

  • Functional Group Region – look at this region first and focus on the following peaks:
    • C=O
    • -OH
    • C-H → both sp2 & sp3
    • -NH2
  • Fingerprint Region – typically only for C-X peaks; use as confirmation for C-C (sp2 & sp3)
    • Typically for this course, the halogens you need to watch out for are Br, Cl, or I

5 of 21

Infrared (IR) Spectroscopy

  • Peaks you should be looking for right away:

3010-3100cm-1 sp2 C-H stretch (aromatic and alkenes)

2850-2960cm-1 sp3 C-H stretch (alkane)

2500-3500cm-1 (broad) –OH stretch H-bonded

~1700cm-1 C=O carbonyl stretch

~1600, ~1450cm-1 benzene C=C and ring breathing

​

  • Peaks that may be useful in fingerprint region:

800-500cm-1 C-X stretch

700-900cm-1 aromatic ring substitution peaks

​

  • These peaks are very obvious and tells you a lot about the functional groups present

​

6 of 21

Nuclear Magnetic Resonance (NMR)�Spectroscopy

  • Shielding/Deshielding due to inductive effects by EN atoms/groups
  • Downfield – Deshielded → the e- are farther from p+ b/c of EN groups; EN atom or group is attached to or near the C-H, thus larger chemical shift values and higher energy are observed
  • Upfield – Shielded → the e- are closer to p+; no EN atom or group is near nor attached to C-H, thus smaller delta values and lower in energy

​

- Tetramethylsilane

(CH3)4Si

7 of 21

  • For the midterm, a table of values for IR and NMR will not be given.
    • Idea is to test your understanding of MS, IR, and NMR.
    • Memorizing the values is not enough!
  • However, for the final exam, this table will be provided by Dr. Lana.

8 of 21

How to approach these problems

1) Find the chemical formula of your compound

  • This will also give you the unsaturation number

2) Look at Mass Spec

  • This will confirm the chemical formula and may reveal functional groups if fragmentation occurred

3) Look at IR spectrum

  • The functional region will give you the functional groups in the compound, and also hints to how the compound is organized
  • The fingerprint region is useful to determine whether it’s cis or trans alkene, number of substitutions and positions on benzene rings and C-X bonds

4) Look at 1H-NMR

  • This will give you the arrangement of the atoms in your compound

​

Following these steps should give you enough info to solve the structure.

9 of 21

A Simple IR Example…

http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi

ORTHO

META

PARA

10 of 21

A Simple IR Example…

http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi

ORTHO

META

PARA

11 of 21

A Simple IR Example…

http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi

ORTHO

META

PARA

12 of 21

A Harder Example...

NO Donald. Try again...

13 of 21

14 of 21

15 of 21

Example 3

  • Based on 13.56a of the 5th ed. Bruice textbook

http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi

88

45

The Chemical formula is C5H12O

​

16 of 21

17 of 21

Example 4

  • Based on 13.56b of the 5th ed. Bruice textbook

http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi

116

57

The Chemical formula is: C6H12O2

​

18 of 21

19 of 21

20 of 21

Example 5

  • Based on 13.60 of the 5th ed. Bruice textbook

http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi

58

31

57

The Chemical formula is: C3H6O

​

21 of 21

http://riodb01.ibase.aist.go.jp/

sdbs/cgi-bin/cre_index.cgi