To solve this problem, you need to read the description of the...

Question

To solve this problem, you need to read the description of the Hammett $σ$ , $ρ$ treatment given in [Chapter 15, Problem 92]. When the rate constants for the hydrolysis of several morpholine enamines of para-substituted propiophenones are determined at pH 4.7, the $ρ$ value is positive; however, when the rates of hydrolysis are determined at pH 10.4, the $ρ$ value is negative.
b. What is the rate-determining step of the hydrolysis reaction when it is carried out in an acidic solution?
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b. What is the rate-determining step of the hydrolysis reaction when it is carried out in an acidic solution?

Accepted Answer

All right. Hi, everyone. So for this problem, let's review the heme equation consider the reaction shown below when R equals a fennel group, the row value for the reaction is negative 5.4. While when R equals hydrogen, the row value for the reaction is minus 0.5, what accounts for the difference in the row values? So the question is asking us to review the Hammitt equation specifically. Now recall that the Hammett equation describes a linear relationship between reaction rates and equilibrium constants for reactions that involve derivatives of benzo acid, right. So the Hammond equation itself says that the log of the equilibrium constant over a reference equilibrium constant equals Sigma rope. Right now recall that Sigma is a constant that represents a specific substituent on the ring. While depends only on the type of reaction that's taking place and not on the substituent, right? So for this question, we're going to ignore the row, sorry, the Sigma value and focus only on the row value in this case. And as for our answer choices, right, there are multiple different reasons as to the difference in the real value that are being proposed, right? Option A says the, that the inductive effect accounts for the difference. Option B says that the reaction mechanism accounts for the difference. Option B option C sorry says that the difference in X accounts for the difference. While option D says that all of the above plays a role. So here, the reaction in question is the conversion of chlorine to a hydroxy group, right? So here on the bottom, what I've done is I've actually gone ahead and drawn both of our starting materials there with the different substitutes for our, our value or our group. I should say. So if we focus our attention on our benzo acid derivative that contains a phenyl group, what I'd like to bring to your attention is that when we go ahead and introduce our hydroxide for a substitution reaction, this would actually proceed via an S N one mechanism because here, right, when our frontal group is attached, we can go ahead and actually form a secondary benzyl Carbocaine, right. So after that occurs, and chlorine detaches itself from the rest of the molecule, we get a relatively stable secondary benzyl Carbocaine. So that is what's favorite to form, right. So recall that because we're forming a carbo CAD during an S N one process that leads to a positive charge on a carbon, right? And that positive charge is stabilized greatly by its proximity to the benzene rate, right? Because it can participate in resonance. So when that's the case, right, when we have a positive charge and it's stabilized, that leads to a large and negative real value. And specifically, if we go ahead and scroll upwards, we'll notice that our row value when R is a final group is negative 5.4, right? So anything beyond negative five, it's is considered to be large and negative. Therefore, this positive charge accounts for the large positive real value when R is a group. Now, on the other hand, right, when R is hydrogen, we have a primary substrate, right, our aha in this case is primary, which means that substitution is going to take place via an S M two mechanism, right? Because recall that primary carbo ions are not stable. So an S N two process is favored instead. So now like I mentioned before, this is in fact, an S and two process, right? So here, what I'd like to bring to your attention is that in both our studying material and our product, our primary carbon here actually has a partial positive charge, right? Because both our O H group and our chlorine have partial negative charges associated to them due to greater electron negativity, right? But here it's not a full positive charge, unlike our S N one mechanism that had a carbo cat ion instead, it's only a partial positive, it's a partial positive and therefore recalls that that corresponds to a relatively small negative row value. And here if I go ahead and scroll up, right? When R was equal to hydrogen, the row value was negative 0.5, which is relatively small and it's negative, right. So the core reason behind this difference is the fact that there are different mechanisms, right? Because our large negative row value is explained by our S N one mechanism. And our small negative role value is explained by our S N two mechanism. So in this case, your answer is actually going to be option B because it's the difference in the reaction mechanisms that accounts for the difference in the row values that we see. So with that being said, if you suck around to the end, I thank you very much for watching and I hope you found this helpful.

Key Concepts

Hammett Equation

Substituent Effects

pH Influence on Reaction Rates

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