Why OH is electron donating

[reising1]

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Imagine an OH group on a benzene ring. This OH group is electron donating. Why is this? Isn't O more electronegative than the benzene carbon it's attached to?

 

[HesitantManatee]

Oxygen is more electronegative than carbon but in phenol the lone pairs on oxygen will go into the ring system of benzene. Since oxygen donating its lone pairs its electron donating.

 

[gettheleadout]

Yup, the most accurate resonance hybrid for phenol includes some contribution of oxygen electrons into the pi system. Oxygen does pull against this inductively due to its greater electronegativity, but the resonance donation outweighs this. Same goes for alkoxy substituents on benzenes.

 

[Synapsis]

Yup, the most accurate resonance hybrid for phenol includes some contribution of oxygen electrons into the pi system. Oxygen does pull against this inductively due to its greater electronegativity, but the resonance donation outweighs this. Same goes for alkoxy substituents on benzenes.

Yea, and it's also found with many nitrogen substituents that have available lone pairs, like amines. But not ones like the nitro group.

Another interesting thing is while substituents like chlorine are electron withdrawing, they can actually still act as electron donating through resonance. The effect is just too small because first, it gives a minor resonance form with a positive charge on a chlorine, and second, the chlorine's 3p orbitals are bigger than those of a benzene's 2p orbitals, giving weaker overlap.

But I'm sure that's out of the scope of the MCAT.