EI Fragmentation

Question 1

What are the EI rules for fragmenting?

Answer 1

1 —> straight chain alkanes are not commonly broken
2 —> the molecular ion decreased in intensity as size of molecule increases
3 —> cleavage is more likely at points of higher substitution
4 —> Double bonds, rings and aromatic systems resist cleavage
5 —> double bonds like cleavage at allylic positions
6—> saturated rings tend to cleave at alpha positions
7 —> unsaturated (but non aromatic) rings like reverse deils-alder
8 —> alkyl-substituted aromatic systems form tropillium ions
9 —> cleavage often occurs at the c-c bond next to the heteroatom
10 —> elimination of small stable molecules is most common

Question 2

When do you know which fragment to do?

Answer 2

Look at mass spec fragments!

Question 3

What is the mclafferty rearrangement?

Answer 3

Specific for ketones. Alpha carbon is one away from normal alpha. Sever bond between alpha and beta. alpha becomes radical. and ketone turns into double bonded OH. Double bond forms between beta and gamma

Question 4

Alpha Cleavage

Answer 4

Bond connected to alpha carbon is broken. Alpha carbon becomes double bonded to halogen or oxygen.

Question 5

Heterolytic Cleavage

Answer 5

Bond connecting to ion breaks! radical on functional group. Charge on broken C