Recognition of Molecular Ion Can use CI to verify (usually not convenient) Nitrogen rule: No N present M+ is even N present M+ is odd M-15 present? M-18? M-31? M-29? M-3  M-14 peaks indicate not

Nitrogen Rule

Spectrum of an Amine

Spectrum of an Alcohol

M+ present?

Recognition of Molecular Ion Can use CI to verify (usually not convenient) Nitrogen rule: No N present M+ is even N present M+ is odd M-15 present? M-18? M-31? M-29? M-3  M-14 peaks indicate not Go back to slides 3, 4, and 5

Deducing molecular formula For small molecules don’t need HRMS Use %m+1 to get # of Cs Use Nitrogen rule M-18 peak indicates oxygen Doublet M+ peaks indicate Cl (3:1) and Br (1:1) Peak at 127 = I Pek at 77 is phenyl group (C6H5) M+2 peak = 4-5% indicates S Deduce molecular formula!

Molecular Formula?

Molecular Formula?

Molecular Formula? m/z %abund 20.2 1.2

Index of Hydrogen Deficiency Units of Unsaturation = r + db CnHmXxNyOz r + db = n – m/2 – x/2 + y/2 + 1

Fragmentation of M+ Draw structure of M+ if possible and then draw possible heterolytic cleavages (curved arrows) or homolytic cleavages (fish hooks) Alternatively, show homolytic or heterolytic cleavage cleavage of the molecule and then remove an electron from one of the fragments. Fragmentation to give neutral molecule and a radical cation must involve some kind of rearrangement.

General Rules Probablity of bond cleavage depends on stability of fragments produced. (3º > 2º > 1º > CH3) (oxonium > carbocation) (allylic, benzylic > no resonance) R on cycloalkanes α cleave. Cyclohexenes do retro Diels-Alder. R on aromatic rings β-cleave to benzylic Bonds β to heteroatoms cleave Loss of small stable molecules, H2O, NH3, C2H4, HCN, CO, H2S,etc often occur Increase branching  decrease M+ Add carbons  decrease M+ Cleave more likely at higher º C Unsubst cyclic have strong M+ (especially aromatic) Alkenes cleave to allylic species