Going through addition mechanisms of alkenes, and I've run into a stumper. I am wondering why bromination (which is normally an anti addition) of trans-anethole can sometimes produce minor amounts of syn addition, whereas cinnamic acid and trans-stilbene can not. All three of these molecules have resonance stability due to their phenyl groups. I cant see why trans-anethole would be favored for a syn rxn. Plz halp.
>implying when a reaction occurs it can only happen at one location on a chemical.
Reactions are all about rates and probabilities. Wanna know why people were so wet off of Heisenberg from Breaking Bad? Because his Meth was like 99% PURE. In high level chem synthesis labs its all about getting reactions to occur under conditions that allow for the highest amount of the product you want to form, form. Reactions can always form differing products.
OK, take this from a guy who only teaches Orgo and hasn't done it in the lab since... a long time.
Normal Trans addition involves a Br+1 (bromonium) ion, with a 3 membered ring and both carbons being neutral. However a carbocation can be more stable than a Br+1 ion. Anethole has an electron donating group (the other carbon) and an electron taking group (the phenyl with an oxygen that is para to our double bond) on either side of the double bond.
cinnamic acid has an electron donating in the unsubstituted phenyl, but an electron withdrawing in carboxylic acid
but stillbene has two electron donating phenyls and you've said it cannot
Well you've stumped me kid.
laws of thermodynamics and entropy, when properly understood reduce a lllloooot of problems in org chem and che and biol.
it gives u a frame work.
eg all side reactions are better termed the lowest energy level reached in the actual circumstances the molecules found themselves at that time.
Conjugation with the aromatic systems isn't relevant after the bridged intermediate forms, i.e. for anything but the olefin's nucleophilicity.
It really comes down to sterics. Draw the newmann projection of the intermediate. Of course the favored site for attack is on either carbon, anti to the bromonium, but the relative sterics of the methyl and aryl groups make attack on the beta carbon anti to the aryl group a competing minor side reaction. The bulkier groups in the other examples you provided disfavor this route.
Guys, all resonance isn't equal here...
The key to getting syn addition is opening the bromonium to form the carbocation, which a lot of people here understood. The other part, however, is how much resonance *stabilizes* the carbocation, which is *not* a binary yes/no energy. Rather, the electronic nature of the ring will matter quite a bit!
This guy (>>7638939) has the right idea. The methoxy is an *excellent* electron-donating group, and when you have an electron-rich aryl it better stabilizes a carbocation because -- to speak loosely -- it has electron density to give.
In cinnamic acid and stilbene, the naked phenyl more electron poor than the methoxy-studded arene. Accordingly, opening of the bromonium is more difficult and you don't get to observe syn addition.
It isn't sterics, a la (>>7639633)
Because all kind of reactions occur, not only the energetically best one. You will get most of the lowest energy one, yes but also the others.
That's why you also have to purify your compound.
No shit, Sherlock. Divergent reactivity ALWAYS prompts the question of "why" trends are observed, and THAT is what OP wants answered. I don't anyone gives a flying fuck about brominating trans-anethole.