Session 38

Today we’re going to break down some organic chemistry discrete questions, especially focused on SN1 vs SN2 reactions, so you can crush your MCAT!

The MCAT has been re-balanced with this new format exam where they really toned down the organic chemistry. It used to comprise 40% of one of the sections of the exam, but now it has been reduced to a lowly 16% of one of four sections. But even though the mighty has fallen, we can’t ignore this entirely.

Listen to this podcast episode with the player above, or keep reading for the highlights and takeaway points.

[01:55] SN1 vs SN2 MCAT Question #1:

Which of the following alkyl chlorides is most likely to undergo SN1 reaction:

• (A) Chloromethane
• (B) Chloroethane
• (C) 2-Chloropropane
• (D) 2-Chloro-2-Methylpropane

Bryan’s Insights:

Very few of these will show up in the MCAT but by far, the most common is our classic SN1 vs SN2 reactions, nucleophilic substitution both unimolecular and bimolecular.

Remember that a tertiary carbon or a secondary carbon is going to be favored by SN1, whereas a methyl carbon or a primary carbon goes to SN2. In this case, for SN1, we’re looking for a tertiary or a secondary carbon that would get attacked by the nucleophile.

The right answer here is (D) 2-Chloro-2-Methylpropane because the chloro is on the second carbon and there’s also a methyl group on that second carbon so that’s what we call a tert butyl group in the kind of common nomenclature. It’s a classic substrate for SN1.

[Related episode: Are There Hidden MCAT Prereqs I Should Take?]

[03:35] A Quick Breakdown of SN1 vs SN2 Reactions for the MCAT

The key to understanding this is starting with the mechanism itself. SN2 is the classic backside attack. The nucleophile comes in, attacks the carbon, and kicks off the living group all in a single step.

Just google SN1 versus SN2 and you will find a million charts out there. Rather than just memorizing, go down the list of all the different factors and figure out why this would favor a backside attack.

Why would a methyl or a primary carbon favor a backside attack? And that’s because there is less stearic hindrance. Or why would a polar aprotic solvent prefer SN2? So the nucleophile attacks the carbon instead of attacking the solvent. And so on and so forth.

Go down all the various factors and ask yourself how does this help the SN2 backside attack? As opposed to the SN1 mechanism, which is a two-step process, where first the living group just leaves. It’s totally on its own and forms the carbocation intermediate. That’s the big phrase you want to remember with SN1.

Again for SN2, remember the backside attack, and for SN1, remember the carbocation intermediate. And when you review these charts, don’t just bluntly memorize them but try to understand how does this solvent or whatever favor forming that carbocation intermediate.

If you can plug everything back into the mechanism, you will be much more likely be able to apply it correctly under pressure on test day as opposed to having memorized generic stuff in the abstract.

[06:30] Organic Chemistry MCAT Question 31:

If 3,000 molecules of triglycerides are hydrolyzed into their component molecules, what would the resulting mixture contain?

• (A) 3,000 fatty acid molecules and 3,000 glycerol molecules
• (B) 9,000 fatty acid molecules and 3,000 glycerol molecules
• (C) 3,000 phospholipid molecules and 3,000 glycerol molecules
• (D) 9,000 phospholipid molecules and 3,000 glycerol molecules

A triglyceride is your lipid, your classic energy storage that the body uses. Remember the nomenclature triglyceride, if you drew this, is E-shaped where there’s these big long tails making up the three prongs of the E. So that’s three fatty acid molecules and all of them are just plugged together in these little ester linkages to a glycerol backbone.

So if you take 3,000 fats and chop them up, you’re going to get 3,000 glycerol backbones and 9,000 fatty acids. Simple math.

[08:15] SN1 vs SN2 MCAT Question #44:

For the reaction below, which solvent will best promote an SN2 mechanism of reaction? (Then it shows us a figure of an alcohol being attacked by a hydrochloric acid and the chloride is attaching on and H2O is the byproduct.)

• (A) H2O
• (B) Methanol
• (C) Acetone
• (D) Toluene

Bryan’s Insights:

You should be able to walk into the MCAT knowing what kind of solvent favors SN2 and you really don’t have to look at the reaction.

On the MCAT, anytime two answer choices say the same thing, you can throw them both out. In this case, methanol has an OH group attached to a CH3. That alcohol, OH on the methanol and for H2O, the OH is in water, those are chemically very similar as far as their solvent behavior goes.

Both of them have hydrogen bonding so you can throw out both choices (A) and (B) together because if it were H2O then methanol would be the same answer as they both behave the same way.

Now, we just have to decide between acetone and toluene. Keep in mind that in order for the reaction to go, to dissolve the hydrochloric acids, dissolve the propanol pictured here, you actually have to have a polar solvent. So the right answer here is acetone, which is your classic polar, aprotic solvent, that SN2 is favored by.

For toluene, you wouldn’t even be able to dissolve the reagents because toluene is just a purely organic solvent.

[Related episode: Why Do You Want to Be a Doctor? You Need to Know This!]

[10:10] Polar and Nonpolar Molecules

And by polar, remember that a molecule can be either polar or nonpolar depending on the bonds found in the molecules. If you had a really electronegative atom bonded into a much less electronegative atom, like oxygen and hydrogen, that creates a polar bond. But that’s just a polar bond.

For a molecule to be a polar molecule, you have to add up all the polar bonds it has and they add as vectors. This has a little bit of physics as you have to know how to add vectors in physics. And if all of the polar bonds add up to an overall polar molecule then that’s what you’ve got.

Whereas for example in carbon tetrachloride CCl4, (where chlorine is a lot more electronegative than carbon so a single bond from carbon to chlorine would be a polar bond) you have essentially a four-way tug-of-war with no winner. Therefore, a molecule ends up being non-polar because no one gets to win that tug-of-war on the electrons.

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Links and Other Resources

• Khan Academy: SN1 versus SN2.
• Check out my book about the MCAT, co-written with Blueprint MCAT (formerly Next Step Test Prep): The Premed Playbook: Guide to the MCAT.
• Related episode: Deep Dive into a Mix of Discrete MCAT Questions.
• Related episode: Breaking Down an O-Chem MCAT Passage.
• Need MCAT Prep? Get tutoring, classes, and full-length practice tests at Blueprint MCAT (formerly Next Step Test Prep)!