University:
Amherst College
Course:
CHEM 221 | Organic Chemistry I
Academic year:

2021
Views:

346

Pages:

4
Author:

jbroieduroseba4

Lecture outline Position of hydration equilibrium depends mainly on stability of carbonyl cmpd — Thinking about how the "R" groups affect the δ+ C, explain the following variations in the equilibrium %s of carbonyl compound and hydrate. R1 R1 H+ or HO– C O + H2 O OH C R2 R2 % ket or ald at equilibrium OH R1 R2 % hydrate H H 0.01 % 99.99 % H CH3 42 % 58 % CH3 CH3 99.99 % 0.01 % Ph Ph ≈ 100 % ≈0% H CCl3 ≈0% ≈ 100 % What do the first three entries tell us about the thermodynamic stability of ketones vs aldehydes with respect to carbonyl addition reactions? Now for a reaction that's closely related to hydration… b. Acetal formation (reversible, acid-catalyzed) H+ C O + 2 RO–H OR C OR acetal + H2 O mech: R2C O mech: + H+ 2 CH3 OH R2C R2C + H2 O OCH3 H R2C OCH3 –H OH CH3–OH R2C OH OCH3 R2C OCH3 – H2O CH3–OH –H R2C hemiacetal OH O–CH3 H R2C OH2 O–CH3 No, it's not that complicated — (1) CH3OH adds, (2) H2O leaves, (3) CH3OH adds — i.e., that's one already familiar reaction, happening three times (forward, backward, and forward) — plus a bunch of H+-transfers Go back and find each of these parts. Now let's review… how would you describe this mechanism? (pick one) (a) OMG — Do we really have to memorize this?! It's like a zillion steps. That is like totally impossible. (wrong! — go back and look again) (a) Basically O bonding to C+ and the reverse, with a bunch of H+s coming and going — I've seen that stuff like a zillion times already — is that all there is to orgo? That's like totally boring. I feel a nap coming on…. Zzzzzzzzz…. (correct!) This equilibrium can be driven either direction (acid-catalyzed both ways, of course) — alcohol solvent (no water!) ketone or aldehyde H+ (usually H2 SO4 or RSO3 H) H+, H2 O acetal "hydrolysis" + (or "H 3O ") e.g., (fill in the acetals) O H+ Ph H3O+ EtOH O Ph O H+ Ph HO H3O+ OH Recall that hydration is catalyzed by acid or base. But acetal formation and hydrolysis only work with an acid catalyst. WHY? This is important. (1) Why is an acetal stable in aqueous base? More generally, why do strong bases and strong nucleophiles not bother acetals? (2) Why can an acetal not be formed with a base catalyst (e.g. with ROH and RO– catalyst)? Why can the reaction now make it past the hemiacetal stage? Because acetals do not react with bases, nor undergo attack by nucleophiles, and because they're sooooooo very easy to make and hydrolyze, they're excellent carbonyl protecting groups e.g., How would you carry out the following transformation (without worrying about protecting group strategy, at least to start)? OH O Br Also, as mentioned in the text (p 779), because aldehyde carbonyls are less stable with respect to addition than ketone carbonyls, an aldehyde can be selectively protected in the presence of a ketone. Perhaps that would help us here… O O O A variety of protecting groups have been developed for just about every common functional group. The choice depends on the rxn conditions that the protecting group needs to withstand, and the conditions necessary to put it on and take it off.

Lecture Outline Position of Hydration Equilibrium

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