Elimination in Haloalkane Reactions

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

This lesson will focus on two very important processes in organic chemistry, substitution and elimination in the context of haloalkanes. We will focus on the mechanisms involved in these reactions and what reaction conditions favor each pathway.

Substrates with a Split Personality

Did you know that some organic compounds, or substrates as sometimes they are called, can be thought of as having a split personality? That's right! Depending on the chemical environment they are exposed to, these certain compounds can behave in one of two ways. The particular compounds we are talking about are called haloalkanes. Haloalkanes are organic compounds made of carbon, hydrogen (the alkane part), and either chlorine, bromine, or iodine. In their simplest form, you can always remember a haloalkane as just an alkane with a halogen tacked onto it!

Haloalkanes are special substrates, because they can do two types of reactions all based on what type of reaction conditions we expose them to. Haloalkanes will either undergo what we call substitution or elimination reactions. Let's dive in and learn what each type of reaction looks like and how they work!


Substitution is when a nucleophile (electron donor) reacts with a haloalkane, and when this happens the nucleophile replaces the halogen or 'substitutes' itself for the halogen.

General substitution reaction of a haloalkane with a nucleophile

In case our lives weren't complicated enough already, there are actually two mechanistic pathways (ways the reaction can happen) that substitutions can undergo. Let's talk about each one briefly.

The SN1 Reaction

That looks like a weird symbol, but it's really quite simple. The 'S' stands for substitution, the 'N' stands for nucleophilic, and the '1' stands for the word unimolecular. So literally, SN1 means substitution that's nucleophilic and unimolecular. The term unimolecular simply means that the rate (how fast) the reaction happens only depends on the haloalkane substrate and nothing else we throw into the reaction pot.

If a substitution reaction happens by an SN1 mechanism, a few steps are involved. Let's see how the reaction happens by using 2-bromopropane as our model substrate and water as the nucleophile. In the first step, the bromine pops off of the haloalkane substrate, resulting in a bromide anion and a carbocation (a carbon with a positive charge).

First step in the S N1 reaction mechanism

After we form our carbocation, the next step is when a water molecule attacks the carbocation. Note the positive charge on the oxygen atom after it bonds to the carbocation. That's because when oxygen has three bonds, it always has a +1 charge.

Nucleophilic attack of a water molecule on the carbocation intermediate

In the final step of the reaction, another water molecule acts as a base to pull off a hydrogen atom which results in our final organic product. Notice how at the end of the reaction, we've substituted a bromine atom for an OH group.

Final step of the S N1 reaction mechanism in which the neutral organic product is formed

The SN2 Reaction

The other type of substitution process that ultimately gives the same product as the SN1 reaction, but just goes by a different mechanism is known as the SN2 reaction. The good news about this mechanism is it happens all in one step so we only need to remember one thing: in any SN2 reaction, the bond making and bond breaking events both happen at the exact same time.

Another way of thinking about it, is the nucleophile comes in and attacks the carbon that the halogen is bonded to, while at the same time the halogen pops off as our leaving group. A leaving group is anything that 'leaves' the substrate, as its name implies. Notice how when we draw the mechanism, both arrows are drawn in one fell swoop to give the substitution product.

Mechanism of the S N2 reaction


Now that we've learned how haloalkanes can participate in substitution reactions, let's take a look at the other side of their personality. A haloalkane can also undergo a type of process known as an elimination reaction. An elimination reaction is easy to pick out, because the product is always an alkene (carbon-carbon double bond). Just like in our cases of substitution, an elimination reaction can proceed in one of two distinct mechanistic pathways. Let's talk about each individually.

The E1 Reaction

The E1 reaction is very similar to the SN1 reaction in that they follow the same kinetics and they share the first step in the mechanism in common. Just like in the SN1, reaction the first step in the E1 reaction is popping off the bromine from the substrate to form a carbocation intermediate.

First step in the E1 reaction

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