Mass Spectrometry

Question 1

What does an odd m/z (mass:charge ratio) indicate about the composition of the compound?

Answer 1

It indicates that the compound contains an odd number of nitrogen atoms. (Nitrogen rule: states that the molecular ion of a compound with an odd number of nitrogen atoms has an odd m/z ratio; if there are zero or an even number of nitrogen atoms, the molecular ion has an even m/z ratio.)

Question 2

What are the axis labels on a mass spectrum?

Answer 2

x-axis - m/z (mass to charge ratio) y-axis - relative abundance (0 to 100)

Question 3

What is a molecular ion (M+)?

Answer 3

The radical cation formed by removal of a single electron from a parent molecule in a mass spectrometer.

Question 4

What effect do common isotopes have a mass spectrum?

Answer 4

They give rise to peaks shifted at higher m/z values. An isotope two atomic mass units heavier will give rise to an M+2 peak that will have a value 2 m/z higher than the lower mass isotope.

Question 5

What are the most common elements giving rise to M+2 peaks?

Answer 5

Chlorine, bromine, and oxygen. Chlorine (3:1), bromine (1:1), Oxygen (low relative abundance of isotope, but additive effect of many oxygens can contribute significantly to the M+2 peak).

Question 6

How to calculate the relative intensity of an M+1 peak.

Answer 6

% (M+1) = Σ (% abundance of heavier isotope x # of atoms in the formula) Hydrogen and carbon both have M+1 isotopes so you would obtain the sum of the two probabilities.

Question 7

If you have the relative abundance of an M+1 peak what conclusions can you draw about the molecule?

Answer 7

Most likely the M+1 peak is dominated by 13C isotopes (relative abundance of 1.11%). Using the relative abundance on the spectrum compared with 12C you can divide out how many carbons there are. (Ex: an M+1 peak 5.74% of the molecular ion: 5.74% / 1.11% = ~ 5 carbon atoms)

Question 8

What is characteristic of alkene and alkyne mass spectrums?

Answer 8

They show a strong peak for the molecular ion (probably formed by removal of one pi electron from the double/triple bond)

Question 9

Aside from the molecular ion, what is one of the most prominent peaks in the mass spectrum of most alkynes?

Answer 9

The delocalization stabilized 3-propynyl (propargyl) cation (m/z 39).

Question 10

What is the m/z of a ethyl cation?

Answer 10

29

Question 11

What is a common fragment observed in the mass spectrum of alkenes?

Answer 11

An resonance-stabilized allylic cation (m/z 41)

Question 12

What is characteristic of the mass spectrum of an alcohol?

Answer 12

There will be a low intensity molecular ion peak from primary and secondary alcohols, and no molecular ion peak detectable from tertiary alcohols.

Question 13

What common fragmentation pattern is observed in the mass spec of alcohols?

Answer 13

The loss of a water molecule (M -18) - this is useful when the M+ (molecular ion) peak is detectable. Another common pattern is loss of an alkyl group from the carbon bearing the -OH group (forms an alkyl radical and a resonance-stabilized oxonium ion).

Question 14

What are the typical alkane fragmentation patterns?

Answer 14

Fragmentation tends to occur toward the middle of unbranched chains forming a series of fragments differing by 14 amu (CH2). Branched chains give fragments at the branches to generate secondary and tertiary cations.

Question 15

What are the characteristic fragmentation patterns of aliphatic (non-aromatic) aldehydes and ketones?

Answer 15

α-cleavage (cleavage of one of the bonds to the carbonyl group; fragments will occur for loss of either group), and McLafferty rearrangement.

Question 16

What is a McLafferty rearrangement?

Answer 16

Occurs in aldehydes and ketones - the carbonyl oxygen abstracts a hydrogen five atoms away to give an alkene and a new radical cation. Molecules at even m/z will give a fragment at even m/z due to McLafferty rearrangement b/c it involves cleavage of 2 bonds. Results in enol of acetone (C=COHOH)

Question 17

How do you distinguish between an aldehyde and ketone on a mass spectrum?

Answer 17

α-cleavage of an aldehyde proton will give an often quite distinct M-1 peak, a ketone will not have this.

Question 18

What are the most common fragmentation patterns of a carboxylic acid?

Answer 18

α-cleavage of the carboxyl group [COOH]⁺, and McLafferty rearrangement.

Question 19

What is the m/z ratio resulting from α-cleavage of a carboxylic acid?

Answer 19

45 - [COOH]⁺

Question 20

What are the common fragmentation patterns observed on the mass spectrum of esters and amides?

Answer 20

α-cleavage, and McLafferty rearrangement

Question 21

What m/z is generally observed for α-cleavage of an ester?

Answer 21

59 [O=C-O-CH3]⁺

Question 22

What are the characteristics of an aromatic hydrocarbon mass spectrum?

Answer 22

Strong molecular ion peak, and most alkylbenzenes fragment to give the tropylium cation.

Question 23

What peak is observed for most alkylbenzenes?

Answer 23

m/z 92 - due to fragmentation to give the tropylium cation.

Question 24

What characteristics are common on the mass spectrum of amines?

Answer 24

Usually a weak molecular ion peak for aliphatic amines (though valuable if present: if the m/z is odd, there is an odd # of nitrogens in the compound), and an even m/z resulting from β-cleavage.

Question 25

What fragmentation pattern is commonly observed in the mass spectrum of amines.

Answer 25

β-cleavage of the C-C bond with the largest R group around the nitrogen.

Question 26

What peak is commonly seen on the mass spec of amines?

Answer 26

m/z 30 from all amines (primary, secondary, tertiary) due to complex rearrangement and fragmentation.

Question 27

What fragment corresponds to peak at m/z 57?

Answer 27

tert-butyl cation

Question 28

What fragment corresponds to a peak at m/z 43?

Answer 28

propyl/isopropyl cation

Question 29

What fragment corresponds to a peak at m/z 41?

Answer 29

allyl cation [H²C=CHCH²⁺]

Question 30

What fragment corresponds to a peak at m/z 45?

Answer 30

Carboxyl cation {COOH]⁺

Question 31

What fragment corresponds with a peak at m/z 60?

Answer 31

McLafferty rearrangment of carboxylic acid [C=COHOH] - becomes enol of acetone.

Question 32

What does an M-31 peak indicate?

Answer 32

Loss of a methyl ester radical (OCH₃ radical)

Question 33

What fragment is indicated by a peak at m/z 30?

Answer 33

A primary amine most likely (can be secondary or tertiary). [H²C=NH² - resonance stabilized]

Question 34

What would cause an M-17 for a carboxylic acid?

Answer 34

Loss of -OH; fairly common, strong peak