Mneumonic for OP and M directors?

[YA7ES]

Has anyone taken the time to make up a good one? thanks.

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[lrkoehle]

this is a fairly straight forward concept, just remember that meta directors withdraw electrons and ortho/para directors donate. This makes intuitive sense based on the carbocation intermediate in the mechanism. I know that doesn't answer your question the way you wanted, but this is one of those things where understanding what is happening is easier than memorizing stuff. Organic chemistry is not about memorizing stuff, because that makes it way more difficult than it should be.

 

[Celestine]

Correct me if I'm wrong, but I think the only exception to that is the halogens, which are weakly electron-withdrawing but ortho-para directing. Everything else follows the electron-withdrawing/meta and electron-donating/ortho-para pattern.

 

[unsung]

Has anyone taken the time to make up a good one? thanks.

I don't have a mnemonic, but I think it's easy to remember as long as you keep in mind the reasoning behind it.

Electron donators tend to be ortho/para directors (EXCEPTION: halogens). You can remember which ones are donators by thinking about whether it's got a lone pair available to potentially form a double bond into the ring & therefore push those electrons around, i.e. donating electron density into the ring.

For example, -OH has got those 2 lone pairs on oxygen, so one of the lone pairs could form a double bond w/ the ring. Don't get confused by the resulting positive charge on the oxygen. When I first learned this, I thought -OH would be a withdrawing group 'cuz of that positive charge. But in this case, the inductive effect (of the positive charge) is *weaker* than the resonance effect (of the lone pair). So the effect from the lone pair donating wins out and it's actually a donating group. Ethers -OR work the same way.

Electron withdrawers tend to be meta directors. Any group that doesn't have a lone pair to donate into the ring is probably a withdrawing group. You can always tell for sure if you can imagine a + charge on the atom that's connected right to the ring. For example a C=O group, you can imagine pushing the double bond electrons onto the oxygen, such that it becomes C-O, where the O has a negative charge and the C has a + charge. Here the inductive effect rules, and you know C=O is a withdrawing group, so it's meta-directing.

The only thing I would bother to MEMORIZE in all of this is the fact that electron donators tend to be o/p directors and electron withdrawers tend to be m directors. You can prove this to yourself once by drawing out the resonance structures, etc. and understanding why that's true. But once you do, just memorize those 2 facts, and you're good to go.

SOME degree of memorization is helpful, but I think it's more helpful to memorize the concepts behind why things work the way they do, rather than the derived results-- which just seem kind of random if you don't understand where they come from. It's always easier to memorize things that make some degree of sense and have some order rather than things that are just random where you don't see the relationships involved.

So to summarize, I would simply understand/memorize the following:


-- things with a lone pair tend to be electron donating
-- things without a lone pair & can carry a + charge tend to be electron withdrawing
-- electron withdrawing groups tend to be meta directors
-- electron donating groups tend to be ortho/para directors
-- EXCEPTION: halogens (have lone pair, but are o/p)
-- when in conflict, resonance stability wins out over inductive effect

 

[nontrdgsbuiucmd]

according to EK, benzene rings and Ortho Meta Para directors are no longer tested on the MCAT. I didn't run into any on the 7/10 mcat.

What was stressed as important for reactivity is if a substituent group is an electron withdrawer or donator (spelling?).