# Mass Spec- breaking the benzene ring

leah3000

## Homework Statement

the molecular formula of the compound under analysis is C8H8O2 = Mr of 136
the mass spectrum shows the peak at 91 to be the most stable. I'm supposed to obtain structures representing the peak at 91 as well as at 118 and 65

## The Attempt at a Solution

I represented 91 as the benzene ring with H-C(-H) attached

@ 118 H-C-C(=O) is attached

I'm having trouble getting the structure at 65

C8H8O2 would be represented as the benzene ring with H-C(-H)-C(=O)-O-H attached.
The only way i'm seeing to obtain 65 is if i remove the attached group from C8H8O2 along with a C from the benzene ring. Is this even possible? Because I know the benzene ring is very stable...

PhaseShifter
Are you positive this compound contains a benzene ring?

Could it be a cyclopentadiene or fulvene derivative?

leah3000
Are you positive this compound contains a benzene ring?

Could it be a cyclopentadiene or fulvene derivative?

well...those groups aren't on the syllabus I'm studying. Benzene and phenol are the only aromatic compounds I'm studying. Can benzene be an option?

Homework Helper
Gold Member

## Homework Statement

the molecular formula of the compound under analysis is C8H8O2 = Mr of 136
the mass spectrum shows the peak at 91 to be the most stable. I'm supposed to obtain structures representing the peak at 91 as well as at 118 and 65

## The Attempt at a Solution

I represented 91 as the benzene ring with H-C(-H) attached

@ 118 H-C-C(=O) is attached

I'm having trouble getting the structure at 65...

It's not benzene anymore... it's tropylium (m=91).

leah3000
It's not benzene anymore... it's tropylium (m=91).

I don't understand

maybe i explained it wrong? i was trying to describe the structure i assumed the fragments to be. But what I'm not clear on is if it's ever possible to break the benzene ring. I know the ring is stable due to the delocalised electrons...so I don't understand why the benzene ring itself would fragment. I have no idea as to what other solution it could be.

Homework Helper
Gold Member
...I'm not clear on is if it's ever possible to break the benzene ring.

That's why I gave you a hint... tropylium ion. Very interesting fragmentary pattern for tropylium ion... nuff said.

I know the ring is stable due to the delocalised electrons...so I don't understand why the benzene ring itself would fragment. I have no idea as to what other solution it could be.
See above.