α-cleavage of alkenes

R R' R R'

R R'When there is a choice of R•, more substituted tends to form

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.

RX

R' R X R'

RC

R' R

X

RC

R'

X

CR'

X

CR'

X

Fragmentation of fragments

RYH2 R YH2

R'Y R YR R'

formed by α-cleavage

Examples of inductive cleavage

R OR'

M+

H2C OR'

H2C O R'

CO

R

O

m/z = 77 common fragment for mono-

substituted benzene derivatives

β-cleavage

•  occurs with carbonyl compounds with suitably long alkyl chains

R' RO

R'R

O

enolate-like stable fragment

MacLafferty Rearrangement For carbonyl compounds with long (≥ 3C):

OR H O

R H

OR H

OHR Loss of neutral

fragment

McLafferty: 2-hexanone

O OH

m/z=58

Note that product from McLafferty rearrangement form from loss of neutral fragment: follows N-rule.

2-hexanone O

M+

α

α

inductive β

McLafferty

Identifying isomers O

Cannot undergo McLafferty

McLafferty beyond ketones

R H R H R H R H

O

OR H

O

OR H

O

OR H

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)

OH

OR H

OH

OH

Fragmentation esters

OR'

OR H

OR'

OH

esters: distinct M+, even at high m/z McLafferty: cleavage can occur from either side:

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)