Download CH CH Formula Weight = 86.1748114

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H3C
Formula Weight
H3C
Formula Weight
CH3
Drawing normal molecule
= 86.17536
+
CH2
= 85.1668714
Drawing normal ion – put + sign
ChemSketch automatically changes
to CH2
Drawing radical cation – put +. Sign on whichever atom
ChemSketch leaves it as CH3
H3C
Formula Weight
+
CH3
= 86.1748114
Mass spectrommetry – Characteristics of Functional Groups
Alkanes – see examples
Aromatic hydrocarbons – the tropylium cation
Alcohols – (1) dehydration
(2) Alpha cleavage
To do next week
Aldehydes/ketones – (1) McLafferty rearragnement
(2) Alpha cleavage
MS of straight chain alkanes
Mass Spectral Fragmentation of Hexane
• Hexane (m/z = 86 for parent) has peaks at m/z = 71, 57,
43, 29
3
MS of straight chain alkanes - heptane
Peaks separated by 14 mass units
Get smaller and smaller towards right
MS of alkanes
Branched-chain alkanes vs Straight chain alkanes
Peaks separated by 14 mass units
Get smaller and smaller towards right
Peaks separated by 14 mass units
Get smaller and smaller towards right but some
unusually high peaks
Tertiary and secondary carbocations are more stable so MS peaks may be
higher than usual
MS of branched-chain alkanes
Example of a branched-chain alkane
MS of Aromatic compounds
MS of Aromatic compounds – n-hexylbenzene
MS of Aromatic compounds – 1-methyl-4-hexylbenzene
12.3 Mass Spectrometry of Some
Common Functional Groups
Alcohols:
• Alcohols undergo α-cleavage (at the bond next to the
C-OH) as well as loss of H-OH to give C=C
10
MS of alcohols – 1-hexanol
MS of alcohols – 2-hexanol
Dehydration
product of α
cleavage product
69
MS of alcohols – 3-hexanol
Mass Spectral Cleavage of
Amines
• Amines undergo α-cleavage, generating
radicals
15
Fragmentation of Carbonyl
Compounds
• A C-H that is three atoms away leads to an internal
transfer of a proton to the C=O, called the McLafferty
rearrangement
• Carbonyl compounds can also undergo α cleavage
16