Meaningful Numbers for Mass Spectrum and Other Things To Remember

Methyl

15

Ethyl

29

Propyl

43

Butyl

57

Pentyl

71

Chlorine

35

Bromine

79

With each addition of a carbon chain how much do you add to the previous MS number? (Ex: ethyl 29 + ? = propyl)

14

What does the y-axis represent in a MS graph?

Abundance of fragments

What does the x-axis represent on a MS graph?

Molecular Weight(MW)

What is the molecular ion peak?

The highest molecular weight fragment that is the most significant(tells the molecular weight for the original compound)

What is the base peak?

Tallest peak (can be important, just depends)

What determines fragmentation abundance?

Stability and similar MW despite a different fragment piece.

What does an odd molecular ion peak mean?

Presence of Nitrogen

What does it mean if the M+2 peak has one that’s 1/3 the size of the M peak?

Potentially the presence of chlorine

What if the M+2 peak is the same size of the M peak?

Potential presence of Bromine

Phenyl

77

Formyl

29

Acetyl

43

Which fragment will be shown in the MS graph?

Positive ones

What happens when the beam hits an electron?

The electron gets knocked out of orbit and turns the atom positively charge.

What is the first step after injecting your sample in Mass Spectroscopy?

A neutral sample molecule gets shot with an electron beam and an electron will get ejected and turn the molecule positively charged.

What happens after a molecule is positively charged in Mass Spectroscopy.

Molecular Ion (radical cation) gets fragmented into smaller pieces and the positive fragments gets read in the instrument.

Ionization Energy

E it takes to lose an electron

Which electron will be removed?

Highest energy electron

What are the two types of fragmentation for Alkyl Chloride/Bromide and Ethers?

Heterolytic Fragmentation and alpha cleavage(homolytic fragmentation)

What is heterolytic fragmentation?

fragmentation of the X or O bond. (results in a peak we CAN see)

Draw the mechanism for a heterolytic fragmentation for CH3-CH(Cl)-CH3 and CH3CH2CH(CH3)-O-CH(CH3)-CH3.

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Draw the mechanism for an alpha cleavage for CH3-CH(Cl)-CH3 and CH3CH2CH(CH3)-O-CH(CH3)-CH3.

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What two types of fragmentation do Alcohols do?

Dehydration (heterolytic fragmentation) and alpha cleavage.

Draw a mechanism for dehydration of CH3CH2CH(H)CH2-CH(OH)-CH3

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How much mass do you lose and which hydrogen is used for dehydration?

lose 18 and the 3rd hydrogen

Why are some fragments more abundent?

Increased stability, different fragments but same MW