Organic Chemistry Study Tips: Learn the Trends

by James

in Organic Chem Study Tips, Organic Chemistry 1, Organic Chemistry 2

This is the first in a series of occasional articles about useful strategies and tactics for studying organic chemistry. 

Here is a general template for a short exam question in organic chemistry:

As you change [increase/decrease] variable X, you change [increase/decrease] variable Y. Why?

This sounds a little abstract, so let’s make it more specific.

 Example #1: As you increase the chain length of a hydrocarbon, you increase the boiling point. Why? Because the increased surface area allows for greater Van Der Waals attraction between molecules. 

 Example #2: As you increase the number of alkyl groups attached to a carbocation, you increase its stability. Why? Because alkyl groups are electron-releasing, and they stabilize the positive charge. [Technically, hyperconjugation provides a much more satisfying answer, but we'll stick with simple explanations for now].

Professors in organic chemistry prize the ability to understand the relationships between properties much more than the ability to remember rote facts, and they design their tests accordingly. This is one of the key differences between studying for orgo and studying for physiology or biology. For example, it’s not so important that you know that the pKa of water is 15.7 (versus, say 13 or 17) but it is extremely important that you know that it is less acidic than acetic acid and more acidic than acetylene.

If you looked at my first summary sheet, you will see that it has all kinds of schemes like this:

I put a lot of these types of things in, because if there’s one thing that 10 years of organic  chemistry has nailed into my head, it’s an appreciation for the importance of learning the trends. 

Now. Here’s 10 more or less random examples I pulled off the top of my head (click for larger image):

These are further examples of concepts, or relationships between properties. If you look at practice exam problems, you will see questions that depend on these types of relationships again and again.

You should find as many examples of these types of relationships as you can. Collect them like fine wines, and appreciate them – they are the distilled essences of organic chemistry, and from these simple concepts flow all the beautiful complexity of the discipline.  Two  reasons why this is important:  1) They are perfect exam fodder, especially for multiple choice exams  2) Knowing these relationships will help you in solving problems you have never seen before.

Remember: Every problem you will encounter was designed with a purpose in mind, and one of the main purposes of problems is to test your understanding of these concepts. Learn them.

A final note: make your knowledge less fragile. How? Here’s one way. Switch every instance of “increase” in the above table for “decrease”, and vice versa.  Example: Decreasing substitution at a carbocation decreases the stability of the carbocation. Try to look at these relationships from as many angles as you can. Push the envelope of the variables. Do thought experiments.

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{ 8 comments… read them below or add one }

MJ Arayata September 3, 2010 at 5:42 am

I am so grateful to have found you website.
Thank you. You are a live saver!

Best wishes.

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James September 17, 2010 at 2:15 am

Thanks! Let me know what else I can write that would be useful.

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Kim November 2, 2010 at 7:57 am

Can I just say THANK GOD for you!?!?!?
wow I feel like by reading your blogs and summary sheets I’m getting a much better understanding of these concepts. It’s nice to have it broken down like that. Thank you sooo much! You really saved me.
:)
Kimia

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James November 3, 2010 at 3:59 am

Thanks! Glad I could help.

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jaspreet kaur February 9, 2012 at 10:49 am

gud to see such type of website.thanks to u .

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Nima Omid-Fard March 10, 2012 at 9:30 pm

Doesn’t increasing the S-character of a bond decrease acidity of the bond? I.e C-H bond is less acidic than C(triple bond)H?? This little summary sheet says opposite?

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james March 12, 2012 at 12:16 am

Alkynes are sp hybridized (50% s character) and are more acidic C-H bonds. Alkanes are sp3 hybridized (25% S character) and are less acidic.

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Nima Omid-Fard March 13, 2012 at 4:32 am

O right, I mistakenly thought S-character meant “single bond” character. thanks!

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