Electron withdrawing and donating groups

[Entropy Happens]

What are electron withdrawing and electron donating groups? What makes a good donating/withdrawing group?

[mrsemmapeel]

Elements that are more electronegative e.g Cl are more withdrawing (they want electrons) elements that are more electropositive e.g H, Na are more likely to donate their electrons. As you go across the periodic table, left to right the elements become more electronegative.

[ChemSiddiqui]

Electron withdrawing group are one which produce the negative inductive effect while donating group produce the positve effects. Its means the same as what mrsemmapeel has mentioned. OH- and NO3- I think make a good withdrawing groups!

[big314mp]

Well...there are some things that have to be considered, and I'll use the hydroxyl group (-OH) to demonstrate:

 

First: there are two types of bonding electrons, sigma and pi. The electron withdrawing effects can be different between the two (i.e. a group may with draw sigma electrons and donate pi electrons).

 

Second: Sigma electrons are donated/withdrawn solely on the basis of electroneagativity, just as mrsemmapeel describes.

 

Third: Pi electrons are donated and withdrawn based on the resonance structure. If a resonance structure can be drawn that has multiple bonds to a group (and therefore formal charges) instead of a single bond, that group is electron donating.

 

For example, consider phenol:

 

The hydroxyl group has two lone pairs on the oxygen. One of these lone pairs can be donated to a bond with the hydroxyl group carbon. This pushes the electrons from the carbon-carbon double bond to the carbon that it ortho to the hydroxyl group carbon. This resonance structure has a formal plus charge on the oxygen, and a formal minus charge on the ortho carbons. The next double bond can also be pushed over, putting a minus charge at the para position.

 

As such, the hydroxyl group is an electron donating group, even though oxygen has a high electronegativity.

 

Other donating groups are alkoxides (-OR) and amines (-NR3). You will notice that both of these groups have one or more lone pairs to donate to a bond with an adjacent carbon. Thus by donating to a pi bond, these groups are electron donating (in spite of their electronegativities).

 

To make life easy, if your group has lone pairs, try drawing a resonance structure where that lone pair is in a bond.