[novice]

A question regarding the Inductive & Mesomeric Effect. I am quite CONFUSED. I'm studying these effects with regards to the effect exerted by substituents on a benzene ring.

I'll start by stating what i think i understand:

Electron withdrawing group (EWG) are deactivating and eletron donating groups (EDG) are activating, makes it nice and easy to understand (what i learnt last year). However this year, they've introduced the inductive and mesomeric effect to explain why a substituent is activating or deactivating.

This is what im told as a general rule:

EDG= +I effect & +M effect = activating
EWG= -I effect & -M effect = deactivating

However, there are obviously heaps of exceptions or its wrong.

For example lets use NH2 as an example first, apparantly it is -I and +M. Why? NH2 is an EDG, so i thought it would be +I and +M?

Then for Cl, its apparantly -I, +M. Its an EWG, so i thought it would be -I, -M???

Then for CF3, its apparantly -I and no M effect. Now whys there no M effect?

Finally, then theres another story of how one effect overrides/is stronger then another effect?

Sorry for the long post, and i really appreciate any explanations in advance!!!

Thankyou!

[macman104]

For example lets use NH2 as an example first, apparantly it is -I and +M. Why? NH2 is an EDG, so i thought it would be +I and +M?

NH2 is -I because nitrogen is more electronegative than carbon.  However, NH2 also has a lone pair of electrons availabel to donate into the ring.  So while there is some amount of -I going on, there is a full lone pair that can donate and resonate into the ring for a much more powerful +M effect.

Then for Cl, its apparantly -I, +M. Its an EWG, so i thought it would be -I, -M???

Halogens are kinda funny, check out the link I've posted below.

Then for CF3, its apparantly -I and no M effect. Now whys there no M effect?

It can't resonate, it just has a very very strong electron pull because the of the three fluorines on the carbon

Finally, then theres another story of how one effect overrides/is stronger then another effect?

True, and this is going to rely on you understanding the basis for these effects.  For example, being able to reason that while the amine (NH2) is a -I, +M, it is still electron donating.

This page seems to have some good explanations for the different effects, including some explaining some of the conflicts you are trying to resolve.  Including halogens.  I'd recommend reading it.

[novice]

thanks for your help mate, bout to sleep but my main problem is im still confused with what the mesmoric effect is....i know inductive effect is transmission of charge, but is their a simple way of explaining mesomeric effect...i understand the stuff about resonance stabalisation but.. =/

also, with NH2, if its an EDG, how can it have a -I effect, i thought it transmits a charge "inwards"?  In our lecture the first few slides basically say:

EDG= +I effect & +M effect = activating
EWG= -I effect & -M effect = deactivating

i think its a generalisation, but there seems to be LOTS of exceptions?



EDIT - Am i correct in saying inductive effect is just ability to transmit a charge (with + being "pushing" a charge in and - "pulling" a charge out) and mesomeric affect is the ability to resonate?

[macman104]

Inductive is electronegative pull.  It is the ability of an atom, or a group to "pull" on the electron density. A -I effect is pulling electron density and a +I effect is donating electron density (usually relative to carbon)

Mesomeric effect is synonymous with resonance effect.  In the US, resonance is the more commonly encountered term, but to describe this concept, the term mesomeric is usually used.  If something has a +M effect then when it resonantes, the resonance adds electron density into the ring, usually resulting in an increased negative charge being experienced by some of the carbons in the ring.  If something has a -M effect, it's resonance takes electron density out of the ring, usually resulting in some of the ring carbons experiencing a slightly positive charge.

Try drawing the resonance of NH2 on benzene to see the +M effect, and then try drawing the resonance of acetophenone to see the -M effect

[novice]

Once again thanks, i think i have a better grasp of it now, though i have still have more questions. I think the main reason for me still being slightly confused is that im trying to classify everything exactly (ie. if it is EDG then it is activating thus it must be O/P directing...etc etc) with so many variables (I effect, M effect, EDG/EWG, Activating/Deactivating, Meta directing/Ortho-Para directing) which is probably impossible (always exceptions) plus my level of knowledge isnt that great haha.

I'll throw up a few more questions just to see if i can get a bit more clarification:

1) Halogens still confuse me, the site you referred me to said they are an exception as they are both inductive electron withdrawing (electronegativity) and resonance donating (lone pair donation).  However a substituent like NH2 is also electronegative and has a loine pair to donate, yet it is still activating and O/P directing.

2) Are any substituents on a benzene ring ever going to be +I (well at least out of the more common groups that all have N, O or a halogen which mean the substituent is more electornegative then C).
Are alkyl groups +I?

3) The site made me realise groups that have double bonds to an electronegative group (eg C=O, O=N-O) are -M, groups with a lone electron pair (halogens, NH2...) are +M. What about all the other groups that dont fit in these two categories such as alkyl groups and CF3 ect?

I've spent so much time on this, probably wasnt worth it >.<


EDIT - Its been over a year since i learnt about hybridisation...so, with mesomeric effect, it all has to do with pi electrons right, what about lone pair electrons, they arent necessarily pi electrons right? How come a +M effect is usually a result of substituents with an atom adjacent to the benzene ring that has a lone pair of electrons?

[macman104]

Once again thanks, i think i have a better grasp of it now, though i have still have more questions. I think the main reason for me still being slightly confused is that im trying to classify everything exactly (ie. if it is EDG then it is activating thus it must be O/P directing...etc etc) with so many variables (I effect, M effect, EDG/EWG, Activating/Deactivating, Meta directing/Ortho-Para directing) which is probably impossible (always exceptions) plus my level of knowledge isnt that great haha.

You have I effect and M effect which dictate the rest of the variables.  Understand the base mechanics, and you don't need to remember all the other "variables".

1) Halogens still confuse me, the site you referred me to said they are an exception as they are both inductive electron withdrawing (electronegativity) and resonance donating (lone pair donation).  However a substituent like NH2 is also electronegative and has a loine pair to donate, yet it is still activating and O/P directing.

Reread the site, Halogens are O/P directing.

2) Are any substituents on a benzene ring ever going to be +I (well at least out of the more common groups that all have N, O or a halogen which mean the substituent is more electornegative then C).
Are alkyl groups +I?

Yup, they are.  They're right in the middle of the list on that other side under "weakly activating".  Also, I misspoke earlier, the reference is not C, the reference is H.  So, an alkyl group is more electron donating than if a hydrogen were at that position.

3) The site made me realise groups that have double bonds to an electronegative group (eg C=O, O=N-O) are -M, groups with a lone electron pair (halogens, NH2...) are +M. What about all the other groups that dont fit in these two categories such as alkyl groups and CF3 ect?

They don't exhibit a M effect, as they cannot resonate.  Their activity and effect on substitution patterns is a result of their inductive effect and their steric effect (search that previous link for the phrase "steric" to jump to that part).

I've spent so much time on this, probably wasnt worth it >.<

It is important you understand this.  If you can get a firm grasp on how inductive, resonance and steric effects influence these things, you will be able to reason substitution patterns for pretty much any case.

EDIT - Its been over a year since i learnt about hybridisation...so, with mesomeric effect, it all has to do with pi electrons right, what about lone pair electrons, they arent necessarily pi electrons right? How come a +M effect is usually a result of substituents with an atom adjacent to the benzene ring that has a lone pair of electrons?

Correct, lone pairs are not necessarily pi electrons.  And +M effects are about pi electrons or lone pairs, as those are the electrons with an ability to resonate.  +M is usually found with lone pairs, because those the most typical case where electrons have the ability to resonate into the ring.  However, you can imagine a system, like this except with a double bond conjugated to the ring.  This system is really just a fancy NH2, because the conjugation donates into the ring, but it at least has you realize that it doesn't have to be a straight NH2 to qualify.

[novice]

thanks again 

with the halogens, they are ortho/para directing because they can donate their lone electrons and increase electron density at the ortho and para sites? but arent they deactivating because they have a dominating -I effect that withdraws electrons? Isnt this contradictory?

How are groups such as methyl able to direct ortho/para without lone electrons?

I just keep coming up with more queries >.<

[venkatjohnny]

methyl group is O,P- directing coz of hyperconjugation( Bakernathan effect). It is also called as no bond resonance...

[Ahmed Abdullah]

methyl group is O,P- directing coz of hyperconjugation( Bakernathan effect). It is also called as no bond resonance...

Could you explain how it works?