Download α-cleavage of alkenes

α-cleavage of alkenes
R
R'
R
R
When there is a choice of R•,
more substituted tends to form
R'
R'
Resonance stabilized
Inductive Cleavage
Unlike α-cleavage (single electron movement), inductive cleavage
fragments molecules via two electron movements
Results from greater electronegativity of heteroatoms versus carbon.
X
R
R
R
R'
X
R'
X
X
X
X
C
C
C
C
R'
R
R'
R
R'
R'
Fragmentation of fragments
R
YH2
R
YH2
formed by α-cleavage
R'
Y
R
R
Y R'
Examples of inductive cleavage
R
O
R'
H2C
R'
O
H2C
O
R'
M+
O
C
O
R
m/z = 77
common fragment for monosubstituted benzene derivatives
β-cleavage
• occurs with carbonyl compounds with suitably long alkyl chains
O
R'
O
R
R'
enolate-like
stable fragment
R
MacLafferty Rearrangement
For carbonyl compounds with long (≥ 3C):
R
R
H
R
O
H
O
R
H
O
H
O
Loss of neutral
fragment
McLafferty: 2-hexanone
O
H
O
m/z=58
Note that product from McLafferty rearrangement form from
loss of neutral fragment: follows N-rule.
2-hexanone
O
α
McLafferty
inductive
β
α
M+
Identifying isomers
O
Cannot undergo
McLafferty
McLafferty beyond ketones
R
H
R
R
H
O
O
H
R
R
H
O
O
H
R
R
H
H
O
O
Fragmentation patterns for associated
with functional groups
Alkanes: α-cleavage
linear: peaks at 15, 29, 43, 57, etc.
branched: predominate cleavage at branch points
M+ often weak
Alkenes: distinct M+
α-cleavage, fragments with mass CnH2n-1
and CnH2n (McLafferty)
Cycloalkanes: prominent M+
fragments from loss of side groups
Fragmentation patterns (continued)
Benzene derivatives: prominent M+
-loss of H from benzylic position (M-1)
- side chain cleavage
- long chain McLafferty
alcohols: α-cleavage next to C-O bond
M-1 peak (loss of H)
M+ peak strong for aromatic alcohols (phenols)
Ethers:
α-cleavage, inductive cleavage,
successive α-cleavage+inductive
Fragmentation: carbonyls
ketones: pronounced M+
aliphatics: α-cleavage
McLafferty (long chain
β-cleavage
strong ArCO+ peak
aromatics: prominent M+, strong ArCO+ peak
aldehydes: as above, distinct M-1
aromatics: loss of CO to give Ar+
carboxylic acids: M+ usually weak
McLafferty prominant (m/z=60)
R
H
H
O
OH
O
OH
Fragmentation esters
esters: distinct M+, even at high m/z
McLafferty: cleavage can occur from either side:
R
H
H
O
OR'
O
OR'
for aromatics, strong M+, loss of OR predominates
amides:
aliphatic: M+ usually discernable
McLaffery (when permitted)
α-cleavage gives m/z=44 for primary amides
(NH2CO+)
aromatic: prominent M+, ArCO+ (α-cleavage), Ar+
amines:
aliphatics: α-cleavage
M-1 visible
nitriles:
M-1 (α-cleavage of H)
McLafferty (where permitted): base peak
for aliphatic often m/z=41
nitro: aliphatic: M+ usually weak
m/z=46 (NO2+)
aromatic: strong M+, loss of NO2 (M-46)
halides: aliphatic: M+ often very weak, α-cleavage
aromatic: M+ prominent
Ar+ peak apparent (loss of halide)