Question about Organic Chemistry

[Pre Opt Student]

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Hey!

This question is a little in depth, but in my Organic chemistry class we learned that acidity of a conjugate base increases as you go DOWN a group because atomic size is larger down a group. This effect on acidity, we learned, is stronger than the effect of electronegativity (which increases going up and across the periodic table). However, on my kaplan tests they seem to ignore the atomic size increase and this increase on acidity going down a group - but just look at electronegativity trends. Maybe this concept is too detailed for the OAT?

Does anyone know what the actual test would consider a stronger acid??

 

[blysssful]

I just looked it up:

Structural Effects on Acid Strength

• electronegativity
  HF > H[SIZE=-1]2[/SIZE]O > NH[SIZE=-1]3[/SIZE] > CH[SIZE=-1]4[/SIZE]
• weaker bond to H
  HI > HBr > HCl > HF

So it looks like you're right and the atomic size wins out over electronegativity if HI is is more acidic than HF. My issue with Kaplan (especially with my lecture portion) is that they try to oversimplify certain concepts so much that what they teach is borderline wrong.

 

[Pre Opt Student]

So what would you go by for the actual test?

 

[Gainesvillian]

All of my practice tests have seemed to only require the simplified version...I took Organic a few years ago and got a C, but my Kaplan knowledge is suiting me just fine

I'll post back tomorrow after the real thing if you please look at my gen chem questions posts! PLEASE : )

 

[blysssful]

So what would you go by for the actual test?

Well I would go by what I posted, and what we both learned in our actual organic courses. But I don't think you'll have a question asking you to choose between a whole bunch of different types of molecules where you'd have to integrate everything you know about acidity. It'll ask in a way to test if you know the electronegativity rule, or, possibly (but probably not), the atomic radius rule.

Just cause Kaplan didn't address it, doesn't mean it stops being true for the exam. It just probably isn't covered. Meaning: don't stress about it! I'm sure you have enough to stress about without worrying about little nuances like this! 🙂