Ochem Destroyer Questions (2011)

[Vsno123]

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A couple questions:

1) What is going on in #80? From the answer I can see that hydrogens are being replaced by deuterium but why?

2) For #82, I know that vinylic hydrogens aren't acidic but in this case, doesn't the D position have more resonance structures and "s" character than A?

3) #103: Why is the reaction with Br2 and light an exothermic reaction? Doesn't the reaction require UV energy to break the stable molecule reaction apart into 2 reactive radicals making it endothermic?

any help on these questions would be really appreciated 🙂

 

[Vsno123]

anyone?

 

[Dental2000]

Bump...

 

[Dew517]

1) Alpha hydrogens are replaced with Deuterium when treated with D2O with acidic or basic catalyst (in this case DCl is our acid catalyst, like HCl). This is similar to when you add something like Cl2 and Acid or base to halogenate the alpha position of a ketone or aldehyde. The mechanism involves protonation the carbonyl oxygen and then the cl- comes in and rips off an alpha proton. Then the newly formed enol attacks the d20 to pick up the deuterium

2) This has been talked about many times, yes allylic protons are acidic, but the ease of access to the outer-lying proton would win in this case. This is a kinetics vs. thermodynamics question where the kinetics would win (at least according to romano.) If you meant the vinylic proton, they are by definition less acidic than ketone alpha protons. Not sure what resonance you are referring to with vinylic protons...they are very unstable.

3)it is not an exothermic reaction. Delta H is is positive meaning the reaction needs energy to proceed, or is endothermic. Yes, splitting the Br2 requires energy.

 

[Vsno123]

Thank you for answering! Here is why I can't understand Destroyer's reasoning:

1) #80- There needs to be a base for the alpha hydrogen on the carbonyl group to be removed. Under acidic conditons, the conj base of acetic acid is usually used (since it is a weak acid and therefore a strong base). But in this case, Cl- comes from a very strong acid and remains relatively stable in the solution as an anion.

3) #103- Sorry, I knew it was an endothermic reaction, I mean to ask-->Why is it a spontaneous reaction when it requires energy to happen in the first place? The answer in the back says delta G is negative. I see 2 correct answers among the choices (A and E).

 

[Dew517]

1) sorry, I didn't mean to write cl- rips off the proton. Cl- is a very weak base and will stay dissacoiated in solution. The D20 will rip off the proton instead. Yes the conjugate base of acetic acid would also tear off the proton because it is a strong base.

2) Just because delta G is negative, it does not imply that delta H is negative or positive. If delta S and delta H are both positive (like in our case), then the reaction can be spontaneous at HIGH temperatures. Reactions are always spontaneous with negative delta H and positive delta S, but there are other combinations that can lead to sponantiety. (negative delta S,H and low temperatures). There's nothing wrong with a spontaneous reaction that has a positive delta H. You just need high temps to fuel the reaction. Remember delta G = delta H - T(deltaS).

 

[Vsno123]

Oh yeah, I don't know why I didn't apply it to the formula..duhh. Ok, so the reaction says it happens with hv light <--- I believe this just means UV light. Is that the same thing as under high temperatures, or do we just assume somehow?

Are you by any chance good at gen chem too? 🙂 I have another 3 questions from destroyer lol but I'll just do another post