(OCHEM) Does BR use questionable logic to make science questions more difficult?

[MCAT guy]

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[BerkReviewTeach]

This question is for those who have gone through BR passages.

I'm doing an ochem passage and I've seen this trend multiple times, curious if any of you have ran into the same thing.

I am answering a question, I know the concept they are testing and there are 2 choices which really could be interpreted correct depending on your point of view.

One specific example... we are adding EWG and EDGs to a diels alder rxn and there are all kinds of rxn rates. Lets say with no EDGs the rate is 1, with a methyl EDG the rate is 8, and with 2 methyl groups that are sterically hindering the rxn, the rate drops to 0.0005.

Now the question posed:

How would the rate change if you used ethyl groups instead of methyls? (testing if I know steric hinderance can lower the rate...)

• The rate would be about 0.0005 (nearly the same)
• The rate would be significantly lower than 0.0005

So as I'm answering, you could reason... well the steric hinderance is there with the methyls, a little extra hinderance compounded with EDG on the conjugated diene would either drop it some or stay close to the same (EDG + conj. diene speeds the rxn). So I answer that it would be "about 0.005" reasoning that could mean a little less, but not necessarily significantly less.

Well it was wrong. Sometimes I run into questions like this where I know the concept, know the reasoning and can't decide b/t two answer choices because they are kind of vague. I hope the AAMC questions don't do stuff like this...

This question isn't testing your understanding of the concept as much as it's testing you on how you interpret data in the table. Trials 4 and 8 are significantly lower than comarable trials, which indicates that steric hindrance is a big factor (bigger than adding an extra carbonyl group, according to the numbers in the table). When ethyls replace methyls, the groups are essentially twice as large, which means sterics will be greater. The table tells us sterics is significant, so the rate should drop to a notably lower value.

Fir their questions, try to use the data in the passage more than background information. This will help with their questions and ultimately help you on the MCAT.

 

[MCAT guy]

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[MCAT guy]

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