Organic structural spectroscopy 2nd edition lambert solutions manual

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Chapter 8

Fragmentation and Ion Activation Methods 8-1 (a)

For 1-phenyl-3-heptanone, only a single McLafferty is possible, which involves the loss of carbons 5-7 as propene. The product has m/z 148.

(b)

For ethyl 2-methylpentanoic acid, there are two possible single McLaffertys.

Loss of ethene from the ethyl ester gives m/z 116, and loss of propene from the acid component gives m/z 102. The double McLafferty gives m/z 74.

(c)

For 2,2-dimethyl-6-phenylhexanoic acid, only a single McLafferty is possible.

Loss of 4-phenyl-1-butene gives m/z 88.

(d)

For 4,6-dimethyl-5-decanone, loss of propene or butene is possible to give m/z 142 or 128. A double McLafferty would give m/z 86.

8-2 (a)

The ion at m/z 44 is the typical aldehyde McLafferty ion. The ion at m/z 56 is the radical cation of butene. These are complementary ions (the sum of their masses is the molecular weight of hexanal).

OH H m/z 56

m/z 44 (b)

In this case, the ionization potential of butene is close to that of the enol of acetaldehyde (this observation indicates that the EAs of the cations are similar).

8-3 (a) 100

Ethyl 4-Methylpentanoate EI MW = 144 O

88

(a)

43

80 101

O

99

60

40

20

0 0

10

20

30

40

50

60

70

80

90 100 110 120 130 140 150 160 170

m/z

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Chapter 8

m/z 144 O

m/z 99

O

α-cleavage

m/z 43 O γ-cleavage products

O m/z 88

O

OH m/z 101

O

(potentially cyclized)

(b) 100

(b) t-Butyl Ether EI MW = 130

80

57

115

59

O

60

40

20

0 0

10

20

30

40

50

60

70

80

90

100 110 120 130 140 150

m/z

O O

3° cation

m/z 115

m / z 57 m/z 130 H O β-elimination

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m/z 59

α-cleavage

McLafferty

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Fragmentation and Ion Activation Methods

(c) 100

162

(C) N,N-Dibutylaniline EI MW = 205

80 N

60 106

120

40

20

0 0

10

20

30

40

50

60

70

80

90 100 110 120 130 140 150 160 170 180 190 200 210

m/z

N

α-cleavage

m/z 162 N

m/z 205

N

NH m/z 106

m/z 120

(d) 100

43

(d)

3,5-Dimethyl-2-octanone EI MW = 156

72

80 O

60

40

85

20

113

0 0

10

20

30

40

50

60

70

80

m/z

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90

100

110

120

130

140

150

160

ene reaction

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Chapter 8

m/z 72 m/z 113

McLafferty OH

α-cleavage products

γ-cleavage

m/z 43

O

(potentially cyclized)

O

m/z 156 O

m/z 85

(e) 100

43

(e)

2,3-Dimethyl-1-butanol EI MW = 102

71

80

OH

60

69 84

40

20

0 0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

m/z

d e h y d ra t i o n

OH

m/ z 4 3 2° cation

m/z 102

m/z 71 α-cleavage (charge stays with alkyl group)

8-4 (a)

m/z 84

m/z 69 allylic cation

For 2-hexanone and 3-methyl-2-pentanone, there would be a difference in the McLafferty ions. 2-Hexanone gives m/z 58 and 3-methyl-2-pentanone gives m/z 72. In the α-cleavages to give acyl and alkyl cations, one expects 3methyl-2-pentanone to give a greater yield of the alkyl cation component (sec-

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Fragmentation and Ion Activation Methods

67

butyl, m/z 57) than 2-hexanone, which yields a 1˚ cation (butyl). The former difference is the most useful. (b)

For ethyl heptanoate and propyl hexanoate, the McLaffertys are a key. Ethyl heptanoate has McLaffertys potentially at m/z 88 (single on alkyl), 130 (single on ethoxy), and 60 (double). Propyl hexanoate has McLaffertys potentially at m/z 102 (single on alkyl), 116 (single on propoxy), and 60 (double). In the observed spectrum, propyl hexanoate gives a significant signal at m/z 117, which corresponds to a hydrogen atom transfer to the McLafferty ion. αcleavages are the other key. One expects loss of ethoxy from ethyl heptanoate to give m/z 113 and loss of propoxy from propyl pentanoate to give m/z 99.

(c)

For ethyl benzoate and methyl 4-methylbenzoate, α-cleavages also are important. Ethyl benzoate loses ethoxy to give m/z 105 and methyl 4methylbenzoate loses 31 to give m/z 119. Ethyl benzoate is capable of βelimination with loss of ethene to give m/z 122. Both can give α-cleavages to produce different phenyl cations (m/z 77 for ethyl benzoate and m/z 91 for methyl 4-methylbenzoate).

(d)

For N-propylbutanamine and N,N-diethylpropanamine, the α-cleavages are important. A key difference is that N-propylbutanamine can lose propyl in an α-cleavage (from the butyl group) to give m/z 72. This mass is not likely in N,N-diethylpropanamine. The other difference is that N,N-diethylpropanamine can give an α-cleavage via methyl loss, so m/z 100 is likely with it.

Both give loss of ethyl to give m/z 86.

8-5 (a)

In the radical cation of tributylamine, loss of propyl via α-cleavage is likely.

In the protonated amine, loss of butene and potentially of butane are likely.

(b)

In the radical cation of dibutyl ether, loss of propyl via α-cleavage is likely. In the protonated ether, loss of butene and potentially of butane are likely.

(c)

In the radical cation of butyl benzoate, loss of propyl via α-cleavage on the butoxy group, loss of butoxy, or formation of the phenyl cation are likely. In the protonated ester, loss of butanol is likely, and loss of butene is a possibility.

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Chapter 8