hmm.. the explanation by
bromidewind seems irrelevant to both of us, because i haven't studied mass spectrometry yet, and i guess the same applies for you.
I was able to dig into my class notes, and this is what I came out with:
Ortho Effect:
when some unusual property is observed due to the presence of an ortho substituent, then it's known as the ortho effect.
may be due to:
1) SIR
2) SIP
3) sometimes intramolecular H-bonding as in o-nitrophenol.
SIR (Steric Inhibition of Resonance)
It arises due to the steric repulsion between a bulkier group with it's ortho substituent. This effect is usually seen in the bulkier groups like
-COOH, -SO3H, -NO2, NMe2Ortho substituents present with these bulkier groups throws these groups out of the plane, because of which the resonance connection with benzene ring vanishes, that's why it is refereed as inhibition of resonance due to steric repulsion(SIR).
It's not applicable on smaller groups like
-NH2, OH, -CN, but they
can impose their effect on the bulkier groups.
Eg:
comparison on acidic nature b/w a normal benzoic acid(NBA), and an ortho-substituted benzoic acid(OSBA),
the NBA experiences +M effect from the benzene ring which destablises the COO
-, while in OSBA, the COO
- has been turned out of ring's plane hence minimizing any destabilizing mesomeric effects on it from the ring.
Thus acidic nature of OSBA > NBA.
SIP (Steric Inhibition of Protonation)
A similar effect acting on aniline-type structures, where protonation of -NH
2 is inhibited because the cation formed is less stable due to the steric repulsion.
so otho substituted aniline would be less basic than aniline, as well as it's meta & para isomers.
Exception: SIP doesn't apply in case of -OMe and -OH. (why? we haven't been taught yet..
)
UV