TBR o-chem section 1 passage XI Question 73

[badmintondr]

The question says what would be the relative concentrations of the products at equilibirum? It also provides a table with pKa but their answer doesn't necessarily make sense to me.

I know having a high positive equilibrium constant will push it towards reaction completion while the other reactant has a smaller but still positive equilibrium constant. However, we are told that this reaction started with equimolar reactants and that the answer states that the equilibrium concentration of reactants will be 1:1.

I can understand 1:1 because they give a reaction equation but...how do the products end up having differing concentrations? without changing reactant concentrations

Thanks

PS. Random side note question: One answer stated that an 8 carbon diene would have a lower stability than a fully hydrogenated 8 carbon alkane but I thought pi bonding had precedence before the branching effect (which is a secondary effect)

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[EdCLS]

The products aren't in different concentrations. Consider this:

We know that the reactants were initially equimolar, and since one mole of H3CO- deprotonates one mole of H3CCH2SH, we know that the reactant concentrations must be equal to one another throughout the reaction. We can immediately rule out choices A and B, because they both show the concentration of one reactant being higher than the other at equilibrium.

What about the concentration of the products? Take a look at "Table 1" and note what the passage says about those values. The Keq values in the table are for each of those acids under the conditions of "Reaction 2." The Keq value for H3CCH2SH is much greater than one under those reaction conditions while the Keq value for H3COH is less than one. It's reasonable to assume from this that H3CCH2SH is the stronger acid of the two, and that the reaction as written will proceed to the right (toward H3CCH2S- and H3COH). Actually, we assume that the reaction not only proceeds to the right, but that Keq of that reaction is greater than one and the concentration of the products is higher than that of the reactants.

With that in mind, let's look at choices C and D. They both show the reactant concentrations being equal at equilibrium, so they pass that test. Choice C claims that [H3CO-] is greater than [H3COH]. That doesn't make sense, because it's claiming that a product is higher in concentration than one of the reactants. Cross out choice C and move onto D. D claims that [H3CCH2S-] > [H3CO-] and that [H3COH] > [H3CO-]. This makes sense because it claims that both of the products are higher in concentration than either reactant, so D is the right choice here.

To recap in a less long-winded way: the products do have the same concentrations in the answers. They're just presented in such a way that it seems confusing. I hope this helps.

Good luck!

P.S.: as for your side note, where did you see that in TBR? I would also tend to think that a diene would be more stable than an alkane.

 

[BerkReviewTeach]

PS. Random side note question: One answer stated that an 8 carbon diene would have a lower stability than a fully hydrogenated 8 carbon alkane but I thought pi bonding had precedence before the branching effect (which is a secondary effect)

I think the point of this is that hydrogenation of the diene is exothermic and exergonic, so given that energy is released by the reaction, the product alkane must be more stable than the reactant diene. Another way to look at it is that heat is necessary to carry out elimination, because the pi-bond you form is less stable than the sigma bonds you are breaking, meaning an alkene is more reactive and thereby less stable.