[yes9111]

Hello!  We were ranking the activation strengths of different substituents for benzene.  According to our textbook (other online resources agree to this) ranks the -OH group greater than an -OR group in terms of activation. 
Why would the -OR group be less activating?  For example, with an -OCH3 group, the CH3 group would donate electron density to the oxygen which would in turn donate electrons to the benzene ring and overall the OCH3 group would be more electron donating than a simple OH group.

[orgopete]

Hello!  We were ranking the activation strengths of different substituents for benzene.  According to our textbook (other online resources agree to this) ranks the -OH group greater than an -OR group in terms of activation. 
Why would the -OR group be less activating?  For example, with an -OCH3 group, the CH3 group would donate electron density to the oxygen which would in turn donate electrons to the benzene ring and overall the OCH3 group would be more electron donating than a simple OH group.

I agree with this analysis, but if one compares actual rates and finds the HO-phenyl reacts faster, then the prediction must fail. I can only guess at a suitable reason that electron donation by an OH group might be greater. If we were to draw an intermediate in which a more complete donation of electrons might be required, then we might consider drawing an hyperconjugative intermediate with the oxygen. If so, then the oxygen might be written in the keto-form with a neighboring proton. A similar structure would require a neighboring methyl cation. At least this would seem a plausible reason for the reverse in reactivity.

[opsomath]

Two things:

Methyl is only electron donating through a hyperconjugation type mechanism, and that works much more effectively in the case of an electron deficient center such as a carbocation. In the case of like a O-CH3 group, the lone pairs of oxygen and the C-H bonds of the methyl group will repel each other so that they don't overlap, essentially minimizing the ability of methyl to donate electrons through hyperconjugation. This is similar to the way that antiaromatic molecules distort so that they're actually "non-aromatic," if you've studied that.

To compare to the other situation, a methyl group next to a carbocation will be *attracted* so that it's rotated into a conformation where the C-H bonding orbital overlaps well with the empty p orbital of the carbocation. That allows the electron donation to happen.

In other words, methyl really only donates electron density to things that "need" it.

Methyl groups are actually a little bit electron withdrawing if you just look at inductive effect. This is because carbon is more electronegative than hydrogen. You can also see this trend in the chemical shift of CH2 vs. CH3 protons, the methyl protons are more shielded.

Okay, then the other effect;

In phenol-type compounds, there can be a concerted mechanism where something pulls the proton off the OH, and the electron pair enters the ring and pushes another pair out to react with the electrophile. In other words, it reacts like a phenolate and not just a phenol, which is way faster.

Hope this helps. Like a lot of things in chemistry, it is complicated.

[Babcock_Hall]

How much faster is phenol than anisole?  We have sometimes used phenol as a carbocation scavenger, but it is tough to remove from the product (we extract it into ethyl acetate or other organic solvents because our product is water-soluble).  Anisole would be easier to remove, but I wonder whether or not it would be effective.

[opsomath]

Ooh, an interesting problem. Can you tell me any more about the chemical system involved, or is it all secret industry stuff?

My experience suggests that the difference is substantial, maybe tenfold or so. A quick exercise of google-fu doesn't turn up direct numbers.

 Why not try N,N-dimethylaniline? Should be nice and nonpolar, and it's a great reducing agent.

[Babcock_Hall]

We are an academic lab, and we make 4-methyoxybenzylesters out of carboxylic acids as protecting groups.  We remove under acid, which also removes the BOC group.  The methoxybenzyl cation needs to be picked up by something.  Besides a free ammonium group, we often have a pyridinium ring present.

[opsomath]

Oh...never mind on the NDMA, then, unless it's very weak acid. Maybe hydroquinone dimethyl ether?

[Babcock_Hall]

Thanks; that's a promising suggestion. 

[opsomath]

We used to have a jar of that stuff around. It smelled like a busy public restroom. Turns out that's because urinal cakes are made of p-dichlorobenzene, and your nose can't distinguish between the dichloro stuff and the dimethoxy stuff very well.