Organic Chemistry Reactions You Need To Know for the MCAT
- Jan 18, 2024
- MCAT Blog, MCAT Chemistry, MCAT Organic
- Reviewed By: Liz Flagge
Organic chemistry has, traditionally, been the subject of pain and anxiety among MCAT preppers. I blame our undergrad classes.
We’re thrown into a sea of nomenclature, bond angles, geometry, and electron pushing. We’re asked to divinate the product after being shown a handful of hexagons. And worst of all, when we ask questions, we’re often told to “just memorize it” for the MCAT.
Maybe I’m projecting a little bit, but this was my experience and it made studying organic chemistry MCAT topics way more stressful than it had to be.
Do you have to memorize every single reaction mechanism for the MCAT? Not a chance. Do you have to do any electron pushing and learning of product schemes? Not really.
There might be a little of that while you’re doing content review. However, you won’t be doing a lot of that during the actual test. Studying organic chemistry for the MCAT is not what we’ve been conditioned to expect from our undergrad classes.
Instead of memorizing every little thing, the MCAT spotlight shines on the big picture—overarching concepts that weave themselves into numerous topics.
It’s all about understanding, not cramming!
Organic Chemistry MCAT Reactions
Nucleophilic substitution is arguably one of the most high-yield organic chemistry reactions. It shows up all over the MCAT.
Part of its namesake, a fundamental part of this topic is identifying a nucleophile and an electrophile. Once you’re comfortable with electronegativity trends and charges, we can start determining “leaving groups”.
Imagine a dance floor where everyone is paired up. Suddenly, a new, more attractive dancer (the nucleophile) enters. This newcomer has an eye for a particular dancer (the electrophile) who is already engaged in a dance (bond) with a less exciting partner (the leaving group).
The nucleophile, being irresistible, sweeps in and forms a new dance partnership with the electrophile, causing the old, less exciting partner to be ‘kicked off’ the dance floor. This is the essence of the nucleophilic substitution reaction: a nucleophile replaces a leaving group in a molecule.
Just think of it as an exciting dance of molecules, where partners are constantly changing, creating beautiful new combinations. Again, it’s not about memorizing complex terms but understanding this molecular salsa happening on a microscopic dance floor.
Types of Nucleophilic Substitution
Specifically, there are two different kinds of substitution reactions students learn about: SN1 and SN2 reactions.
During your MCAT prep, you’ll get into much more detail about which reactions are favored under which conditions, but for now we’re going to keep dancing.
Picture a grand ballroom filled with molecules, all eager to perform their dance routines. On one side of the room, a performance is taking place that involves a single step. This is the dance known as an SN1 reaction.
In this dance, a molecule’s partner (the leaving group) decides to exit the dance floor before a new partner (the nucleophile) swoops in to take its place. It’s like a line dance where one dancer leaves the floor, creating an open spot that another dancer quickly fills.
On the other side of the room, the SN2 reaction is happening. The SN2 dance is a swift, synchronized move where the old partner (the leaving group) is pushed out at the exact moment the new partner (the nucleophile) comes in. Imagine a well-timed partner swap in a fast-paced tango.
Whether it’s the one-step groove of the SN1 or the coordinated twirl of the SN2, both substitution dances create fascinating new arrangements on our molecular dance floor.
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Elimination and Addition
In terms of MCAT relevance and likelihood of appearing on the test, elimination and addition reactions are right up there with substitutions.
Don’t worry though, you’ve pretty much already learned what elimination and addition reactions look like! Think of them as one-half of the substitution dances from above.
In an elimination reaction, two dance partners have had enough of each other. Too much stepping on each other’s toes and losing the beat, I suppose.
Once they choose to depart they leave behind a double bond as a keepsake of their time together. Unlike a substitution reaction, in an elimination reaction no nucleophile comes to take the place of a dance partner after the leaving group leaves, the double bond is what we’re left with.
Similarly, an addition reaction is the other one-half of the dance. In this case, a molecule with an existing double or triple bond decides it’s tired of dancing alone and invites some new partners to come join it.
There are no leaving groups with addition reactions. Everyone just has a great time and hangs out together.
Is this a comprehensive list of everything you’ll need to know about elimination and addition reactions? Unfortunately, no. Just like with substitution reactions, there are E1 and E2 versions of eliminations, rules to determine whether your product will be cis or trans, acid or based versions of addition reactions, and more.
However, just think of it like learning different variations of the same old song and dance!
Enolates, enolates, enolates! These chemical species, formed from aldehydes or ketones, present a negative charge on an oxygen atom and a double bond between two carbon atoms.
The presence of this negative charge makes enolates highly reactive, allowing them to participate in a variety of chemical reactions.
Enolates can exist in two forms: as anions, ready to donate electrons, or as neutral species, prepared to accept electrons. This duality adds a layer of complexity to enolate chemistry, but it’s just two sides of the same coin.
That said, don’t sleep on this organic chemistry MCAT topic! Enolate chemistry features prominently on the test, and the Aldol reaction in particular likes to pop up.
This reaction has a three-step mechanism: formation of the enolate, nucleophilic attack by the enolate, and protonation. Some of that should already look familiar to you just from our cursory review of nucleophiles above.
Lastly, we’ll briefly discuss carbs. No, not how we’re trying to cut them out of our diet. I’m thinking more along the lines of how organic chemistry and biology start to blur together.
Besides playing a significant role in the biochemistry of living organisms, carbohydrate reactions are also heavily tested on the MCAT. Here we’ll run down a quick list of the most common types of carb chemistry you’ll encounter.
Hydrolysis is the process of breaking down a carbohydrate using water. The flip side of that is dehydration synthesis, which is essentially the opposite of hydrolysis.
In this reaction, two smaller carbohydrate molecules combine to form a larger one, and a water molecule is produced as a byproduct. This is also incredibly similar to amino acids forming peptide bonds —again, big-picture concepts paying off in multiple topics.
Glycosylation is another crucial carbohydrate reaction. In this process, a carbohydrate attaches to a protein or a lipid, modifying its function. This reaction is vital for cell-cell communication, immune response, and protein stability.
Lastly, oxidation-reduction (redox) reactions are also common with carbohydrates. During these reactions, carbohydrates can be oxidized to produce energy or reduced to form other types of molecules. These reactions are crucial for metabolism and energy production in cells.
Why is all of this so important? Well, despite their bad rap, carbohydrates literally keep us alive. Reducing sugars, such as glucose, fructose, and galactose, can donate electrons and reduce other compounds and is used in the body for energy production.
The synthesis (formation) and hydrolysis (breaking) of glycosidic linkages are essential biological reactions. Glycosidic bonds connect monosaccharide units to form disaccharides and polysaccharides. Monosaccharides like glucose, fructose, and galactose are simple sugars and combine to form disaccharides and polysaccharides through dehydration synthesis reactions.
Don’t Give Up On OrganicChemistry
Organic chemistry might never be “fun” for you, but it doesn’t have to be miserable either! Plus, we don’t have to memorize every difference between a Wolff Kishner reduction and a Clemmensen reduction reaction, so that’s a tiny win in itself.
Instead, focus on big-picture concepts, notice common trends between reactions, and don’t let those molecular structures stress you out!
Once you know what you’re looking for, are comfortable with the nomenclature, and are familiar with these basic reactions, you’ll be ready to rock any organic chemistry MCAT passage.
But remember, you don’t have to conquer the MCAT alone. We break down organic chemistry MCAT concepts in our Blueprint MCAT modules and live classes to make them easy to understand and apply on the test.
Experience it for yourself by creating a free Blueprint MCAT account and starting a trial of our Self-Paced Course today!
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