Web Pages with Problems

Web pages with problems:

http://www.nd.edu/~smithgrp/structure/workbook.html (IR, 1H NMR, 13C NMR, MS; no answers available)

http://www.chem.ucla.edu/~webspectra/ (IR, 1H NMR, 13C NMR; answers are available)

http://webbook.nist.gov/chemistry/ (search for the IR and/or MS spectra for a known compound)

Abundance (abbreviated as abund.) is the same thing as intensity.

Base peak – the most intense peak in the spectrum

Weak bonds break preferentially

Bond-breakage resulting in more stable fragments is favored

Peaks are commonly observed at m/z values one and 2 units less than the m/z values of carbocations

When breaking bond between a carbon and a heteroatom (like halogen, oxygen and others), electrons go to more electronegative atom (the fragment to which electrons went is not present in the spectrum)

If an alkane chain is branched, there is increased chance for cleavage at a branch point because a more stable carbocation can form

Peaks corresponding to carbocations stabilized by resonance are usually quite intensive in the mass spectrum.

MASS SPECTRUM ——————→ MOLECULE

What is the molecular weight of my compound?

The peak with the greatest m/z value (molecular ion) in the spectrum gives molecular weight for the unknown compound. (There could be some smaller peaks after the molecular ion peak.) For the following discussion, M is the m/z value for the molecular ion.

How many carbons does my compound have?

Look for peak with m/z value one unit larger than m/z value for molecular ion (M+1). (For example, if M peak is at m/z = 114, M+1 peak will be at m/z = 115)

If needed, calculate relative abundances (rel. abund.) for M and M+1 peaks:

Relative Abundance for peak at Y m/z = (height of peak Y/ height of base peak) × 100%

Use one of this formula to find number of carbons (round off the result to the nearest whole number)

# of C = relative abundance of M+1 peak/(0.011 × relative abundance of M peak)

Do I have Br, Cl, or S in my compound?

YES, if there is a peak at m/z value two units larger than m/z value for molecular ion (M+2). (For instance, if M peak is at m/z = 114, M+2 peak will be at m/z = 116).

-Do the following calculation for M+2 peak (calculation 1):

Intensity of M+2 peak as % of M = (rel. abund. of M+ 2/ rel. abund. of M)× 100%

-Use the following table to distinguish between Br, Cl, and S

If there is NO peak at M+2, the molecule does not contain Br, Cl, or S.

Is my molecule an alcohol?

- Small molecular ion peak (because molecular ions of alcohols usually fragment quite readily in the mass spectrometer)

- A fragment ion peak at m/z = M-18 OR at m/z = M-17

(For example, if a molecule is an alcohol with MW = 118, it will have a small molecular ion M peak at 118, and a peak of some intensity at 100, which is M-18 peak).

Ketones usually have an intense molecular ion peak

Try to find the molecular formula by determining the number of hydrogen atoms and, if necessary, adding the appropriate number of oxygen and other atoms. (Make sure that the molecular formula agrees with the molecular weight determined from the spectrum)

Use tables on pages A-18 (common fragment ions) and A-19 (common fragment loss) to determine the structure of the compound.

A-18:

- Take m/z value for a fragment from the given spectrum

- Match it to the m/z value in the table

- Write down the possible ions for this m/z (if there are too many ions listed for the chosen m/z, try choosing another m/z. DO NOT write down the long list of possible ions for the chosen m/z)

A-19:

- Find the difference between two neighboring m/z (subtract the smaller m/z from the larger m/z), let’s call it Δ m/z

- Match Δ m/z to the value in the table

- Write down lost fragments corresponding to Δ m/z (Again, if there are too many possibilities, DO NOT spend time writing them down)

Other important tables: p. 488, table 13.2

Prepared by Natalia Zakharova