Year 12s: what will be the first way you research your future options? >>

Inductive / mesomeric effects and resonance stabilisation...help please?

Hi I'm in need of some chemistry help! So far I understand that an inductive effect is the push or pull of electrons in the sigma bonds in molecules. Eg. when an electronegative atom is attached to a carbon atom in a molecule via a sigma bond this causes electrons to be pulled away from the carbon atom. Is this correct? I understand that some atoms / groups have negative inductive effects(-I ie.electron withdrawing) and positive inductive effects (+I ie.electron donating).
Could anyone maybe answer these questions?
1. How is an inductive effect different from a mesomeric effect? or what is a simple definition of a mesomeric effect?
2. Why do some atoms / groups have a negative inductive effect but a positive mesomeric effect? eg. the OH group?
3. What is the definition of resonance stabilisation and how is it different to a mesomeric effect? or is it the same thing?

Any help would be much appreciated! Thank you :] :confused:

Reply 1

shengoc

Original post by Bonjour

This can be simple or difficult depending on the textbook or other sources where you read from. Inductive effect is only active over a short range, one or two atoms away; mesomeric effect can extend over a whole molecule - i.e. highly conjugated system (highly alternating, unsaturated system) - usually responsible for their colour, despite not bearing a transition metal.

Resonance stabilisation is a consequence of mesomeric effect. Essentially, they are the same. i.e. you can draw different structures for carbonate, sulfate, phosphate, nitrate because the pi bonds are conjugated, hence you get that mesomeric effect (due to the conjugation, pi bonds aligned/oriented correctly for delocalisation of electrons) which results in different structures of the same energy (resonance); rule of thumb is if you compare a particular family of compounds, i.e. the -ate ions of carbon, sulfur, phosphorous, nitrogen, etc. - the more resonant structures you can draw, the more stable is that species.

This might help for supplementary reading.
http://www.chemguide.co.uk/mechanisms/eladd/carbonium.html