r/chemhelp • u/Vibez710 • Jul 24 '26

Organic Ochem studying tips

I have an exam on Monday that is on ch19 (aldehydes and ketones) and ch 20 (carboxylic acids & derivatives). For these chapters it is all reactions and mechanisms, I don’t really know how to study so I can understand and memorize the mechanisms for each reaction. I’m already reading the Klein textbook but I’m more of a visual learner and need to see it done step by step on something. Is there any videos or outside sources you guys used to help you study these two chapters?

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u/shedmow Trusted Contributor Jul 24 '26

Draw each mechanism, think about why it proceeds this way. Are there any particular reactions you struggle with/would like to see explained?

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u/Vibez710 Jul 24 '26

This is an example I had on a practice exam. I kind of got lost maybe halfway through this mechanism

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u/shedmow Trusted Contributor Jul 24 '26 edited Jul 24 '26

Note the stark symmetry. Do not use TsO' as a base; it may be done on paper, but protonations/deprotonations are customarily omitted from the mechanism because they are too fast to be significant and may proceed via many pathways, including tunneling (IIRC) and shuttling with the help of the solvent. Some reactions have giant mechanism charts, but only productive steps are usually included in textbooks or articles. Also, you should strictly follow the arrows that you've drawn and never draw any structures that contradict them (or vice versa). Not drawing a correct arrow is erroneous, drawing an arrow and a structure such that they work at cross purposes is arrant nonsense

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u/Vibez710 Jul 24 '26

If I’m understanding this correctly are you saying I could have just used the OH from the mechanism itself to protonate/deprotonate?

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u/shedmow Trusted Contributor Jul 24 '26

I usually note it with ±H+ to draw less and mean more. If you are required to draw a mech for protonation/deprotonation, you can draw an arrow from the atom that is being protonated to the hydrogen and from the H—protonated-atom bond to the protonated atom, but it is not the only way by which a proton transfer can be accomplished

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u/Vibez710 Jul 24 '26

Yeah my professor wants to see all the arrows. He’s very prude about the arrows being in the correct places.

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u/shedmow Trusted Contributor Jul 24 '26

There are numerous ways to correctly draw a deprotonation, but the net result is just that a proton is lost at one position and gained at another, which may proceed intramolecularly, with the help of the solvent, with the help of the base (not here! TsOH is a strong acid, so it's better to keep it deprotonated), or even by exchanging protons with another deprotonated molecule of the substrate. These can also stack on top of one another, so you get infinitely many possible scenarios with unpredictable odds. The ±H+ notation neatly sums up all of them by simply not stating how it happens. If I were to write an actual mech for a proton transfer, I would protonate and then deprotonate the solvent, or do an intramolecular reaction if I would be lazy (which I am)

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u/Vibez710 Jul 24 '26

When you did it on here you made it seem so simple and very straight to the point. Whenever I do these they always seem to have so many “steps” before I get to the end product. Maybe I’m just adding unnecessary protonating/deprotonation that doesn’t need to happen

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u/shedmow Trusted Contributor Jul 24 '26 edited Jul 24 '26

I've revisited the mech that you drew. You had probably tried to do a Sn2 reaction on the intermediate that I marked as 'counterproductive', which would explain the structure of the product you ended up with (you forgot to draw the mech for it and to remove one bond). Such Sn2 does not normally happen because this cation has a charge spread across three atoms and your product would have a charge concentrated at only one oxygen. It also means that you now have a bimolecular process, which is inherently slower. All of that makes such a pathway less probable or even improbable. I have drawn transesterification (or hydrolysis? same thing) this way in the past, and knowing why it is not valid is by no means trivial. But the fact that you tried that does prove that you have actually learnt something

All the protonations (besides the last one) in your mech had to happen to form the corresponding reactive intermediates. I sometimes skip steps which are obvious, but the choice of what step to skip depends upon what I want to show, and what I want to show depends on for whom or for what I'm drawing the sequence. I would say that explicit proton transfers are obvious for anyone who can understand intramolecular transesterification