How To Simplify Substitution And Elimination

These two reactions seem to take up a lot of space in your organic chemistry textbook.

Solvents, substitution patterns, leaving group….whew! Look – I get it…I’ve been there.

However, I have found a much simpler way to learn this material.

Instead of thinking about 4 different reactions, let’s start by breaking these reactions down into “1’s” and “2’s”…as I like to think of them.

So…we’ll put SN2 and E2 together (2’s) and SN1 and E1 together (1’s). This will allow us to place these into two general categories.

Start by classifying your reagents into negative or neutral

Before you start, see if your reagent is negatively charged or neutral. This will help you classify the reagent into one of these two categories.

This will make it much easier on yourself!

Metal/Negative Charge and Acid/Neutral

Anytime you see a metal atom or a negative charge, you should think SN2 or E2. Now, saying metal atom or negative charge is really saying the same thing, right?

If you have a metal atom (positively charged), then you must a negative charge.

In order to do a “2-reaction” (SN2 or E2), you need a negative charge. That is just how these reactions roll.

But if you see an acid or a neutral compound, you should be getting ready to write SN1 or E1 on your exam or quiz. This is because the “1-reactions” (SN1 and E1) involve a carbocation.

OK…so let’s further expand on the “2-reactions.”

The Negative charge – The “2-reactions”

If you have a negatively charged oxygen, then think E2. Oxygens with a negative charge tend to act as bases. However, there is one exception.

Anyone heard the name Williamson…The Williamson Ether Synthesis? Here is the reaction just in case is slipped your mind.

As we know, negatively charged oxygen will react SN2-style with a methyl or primary alkyl halide (see * in above image).

When we move to a secondary carbon, it tends to go E2. And with tertiary carbons…well…we can’t do SN2. So…the only other option is E2.

And to wrap it up…if you have any other negative charge, think SN2.

Neutral and acids – The “1-reactions”

What you will be looking for here are acids and alcohols.

When you see alcohols, you should be thinking SN1. Now, I tend to lean towards E1 with acids.

But this can be a bit tricky. We know that acids can react with alcohols to generate alkyl halides via SN1.

There can be so overlap. So – one thing to think about…

If you see neutral/acid and heat…. you will more than likely get E1 mechanism. Heat is a tip off for elimination.

Let’s look at few examples to see how to use this

Example #1

Let’s take a look at the following reaction….

In the first one, we can see that we have a metal/negative charge. The cyanide (CN-) is our negative charge (nucleophile)…therefore it will go SN2.

Example #2

Ok – onto the next example…

In the above reaction, we have an alcohol…and only an alcohol. This must mean that we have an SN1 reaction.

Example #3

In this next example below, we have an oxygen minus. But wait…

We also have an alcohol. So – what do we do?

Don’t worry…this one is easy – Negative always beats neutral! So, it must be E2 since we have an oxygen minus.

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