Ch. 13: Mass Spectrometry, IR Spectroscopy, UV/Vis Spectroscopy

Question 1

What is mass spectrometry? What kind of information does it give us?

Answer 1

It allows us to determine the molecular mass and molecular formula of a compound, as well as some of its structural features. A mass spectrum gives us structural information about the compound because the m/z values and relative abundances of the fragments depend on the strength of the molecular ion’s bonds and the stability of the fragments.

Weak bonds break in preference to strong bonds, and bonds that form more stable fragments break in preference to those that form less stable fragments.

Question 2

What is Infrared spectroscopy?

Answer 2

tells us the kind of functional groups a compound has

Question 3

What is UV/Vis spectroscopy?

Answer 3

provides information about organic compounds with conjugated double bonds

Question 4

What is Nuclear magnetic resonance (NMR) spectroscopy?

Answer 4

provides information about the C–C and C–H framework of an organic compound.

Question 5

What are the classes of organic compounds?

Answer 5

Question 6

What are the qualifications for a fragment to be accelerated through the analyzer tube in mass spectrometry?

Answer 6

must be positively charged

Question 7

What is an m/z value on a mass spectrum?

Answer 7

Each m/z value is the m/z value of one of the fragments to the nearest whole number.

Question 8

Where is the molecular ion on a mass spectrum?

Answer 8

The molecular ion is the fragment that results when the molecule loses an electron as a result of being bombarded by the electron beam. The peak with the highest m/z value is the molecular ion.

Question 9

How do you find the molecular mass of a compound from a mass spectrum?

Answer 9

The m/z value of the molecular ion gives the molecular mass of the compound.

Question 10

What do peaks beside the highest m/z one represent?

Answer 10

Peaks with smaller m/z values, called fragment ion peaks, represent positively charged fragments of the molecular ion.

Question 11

What is the base peak of a mass spectrum?

Answer 11

the tallest peak, it has the greatest relative abundance. It has the greatest relative abundance because it is generally the most stable positively charged fragment. The base peak is assigned a relative abundance of 100%, and the relative abundance of each of the other peaks is shown as a percentage of the base peak.

Question 12

What m/z value represents the presence of a propyl cation?

Answer 12

m/z = 57

Question 13

What m/z value represents the loss of a methyl radical?

Answer 13

m/z = 71

Question 14

Name one method used to identify fragment ions.

Answer 14

take the difference between the m/z value of a given fragment ion and that of the molecular ion.

Question 15

What m/z value represents to presence of a methyl cation?

Answer 15

15

Question 16

What m/z value represents the presence of a ethyl cation?

Answer 16

29

Question 17

What is the rule of 13?

Answer 17

You can use it to calculate the molecular formula of a compound from the m/z value pf the molecular ion.

1) The base value must be determined. To do this, divide the m/z value of the molecular ion by 12. The answer gives the number of carbons in the compound. The number of hydrogens is determined by adding the number left over to the number of carbons.

2) If the compound has one oxygen, then one O (16amu) must be added to the base value and one C and four Hs (16amu) must be subtracted from it. If the compound has two oxygens, the process must be repeated. (Notice that to maintain the m/z value, you need to subtract the same number of atomic mass units that you add).

Question 18

What is the nitrogen rule?

Answer 18

If a compound has a molecular ion with an odd-numbered mass, then the compound contains an odd number of nitrogen atoms.

Question 19

What is the M + 1 peak?

Answer 19

The M + 1 peak owes its presence to the fact that there are two naturally occurring isotopes of carbon: 12C and 13C (98.89% and 1.11%). Because mass spectrometry records individual molecules, any molecule containing a 13C will appear at M+1.

Question 20

What is the M + 2 peak?

Answer 20

M + 2 peak due to 18O having two heavy isotopes in the same molecule (like 13C and 2H, or two 13Cs). These situations are unusual, so M + 2 peaks are generally very small. The presence of a large M+2 peak is evidence of a compound containing either Cl or Br, because each of these elements has a high percentage of naturally occurring isotope that is two units heavier than the most abundant isotope.

If the M+2 peak is 1/3 the height of the M peak, then the compound contains a Cl

If the M and M+2 peaks are about the same height, then the compound contains a Br

Question 21

What is the base peak of the C–Cl bond and how does it break?

Answer 21

It breaks heterolytically and the base peak is at m/z = 43

Question 22

What is heterolytic bond cleavage?

Answer 22

one atom gets both of the shared electrons

Question 23

What is homolytic bond cleavage?

Answer 23

the two electrons in the bond are divided equally between the products

Question 24

What is a molecular ion?

Answer 24

When an electron beam hits a molecule, it knocks out an electron and produces a molecular ion, aka a radical cation, a species with an unpaired electron and a positive charge.

Question 25

What kinds of fragments from a mass spectrometer are accelerated through the analyzer tube

Answer 25

only positively charged fragments

Question 26

How many oxygens does an ester have?

Answer 26

2 Os

Question 27

If a compound contains an amide, what elements does it have beside C and H?

Answer 27

O and N

Question 28

Describe the fragmentation of an alkyl halide

Answer 28

either cleavage of halogen or alpha cleavage

Question 29

What is alpha cleavage?

Answer 29

An electron is knocked off an atom (usually by electron collision) to form a radical cation. Electron removal generally happens in the following order:
1) lone pair electrons,
2) pi bond electrons,
3) sigma bond electrons.

Question 30

Describe the fragmentation of alcohols.

Answer 30

loss of water (M-18) or alpha cleavage

Question 31

Describe the fragmentation of ketones.