iOpenShell » Electronic Structure and Quantum Chemistry » how can one know withdraw or donate groups?

how can one know withdraw or donate groups?

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Member
Registered: Jun 2010
Posts: 6
I have many functional groups . how can I know the withdrawor / donate ones ? Is measuring ESP good choice ? is there other methods?
Administrator
Registered: Aug 2009
Posts: 43
Location: Cleveland, OH
Dear Karrar

in order to insight electron withdrawing/donating character of the group one do not need to run any calculations.
Just remember basic organic chemistry! Basically, there are 2 ways e donation/withdrawal can be made: through a resonance effect and through an inductive effect. Both of them could pretty much independently withdraw or donate. For example, it could be that a functional group donates through resonance and withdraws through induction or vice versa. Alternatively both of them could have the same nature.

Inductive effects occur due to sigma bond polarity, e.g. C-Cl bond. Chlorine is highly electronegative and thus withdraws electrons through the sigma bond with carbon. It will pull the electrons away from the ring.

Resonance effects occur due to pi orbital overlap between the substituent atom and the ring. Basically, the substituent will be forming a double bond with the ring as one or more of the resonance structures. For example, a substituent that has oxygen bonded to the ring (OCH3, for example) can have the oxygen form a double bond, thus transferring some of the electron density from the oxygen to the ring. Hence the e-donating property of OCH3.


There are tons of tabulated data that summarize nature of the groups and their relative strength. e.g. http://en.wikipedia.org/wiki/Inductive_effect

hope this will help!
Member
Registered: Sep 2007
Posts: 12
Location: University of Queensland
In physical organic chemistry, electodonation/withdrawal ability is quantified by linear free-energy relationships, a canonical example of which is the Hammett equation. These are all a form of surprisal analysis, where the surprisal calculated from the thermal equilibrium constants for a family of reactions can be parametrized as a line.

The fact that there are many parameter scales defined and used in such equations (each defined against a different reference reaction) implies that there are distinguishable channels for donation and withdrawal in molecules. One can identify the channel for a given reaction of interest by finding the parameter scale (reference reaction) that provides the best fit.

A really good, modern reference for further background is Addy Pross's book ""Theoretical and Physical... Of Organic Reactivity". For a cheaper, less modern review see Ernest & Grunvald's. "Rates and Equilibria of Organic Reactions" from Dover.
Member
Registered: Dec 2012
Posts: 1
jsadjadjasddlasd
Member
Registered: Mar 2013
Posts: 17
Location: Los Angeles, CA
Great info.

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