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
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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Once the carbocation is formed, instead of a nucleophile adding to the carbon bearing the positive charge, a base comes in and pulls off a hydrogen on a carbon adjacent to the carbocation.
Second step of the E1 reaction to form an alkene
The E2 Reaction
The E2 reaction and the SN2 reaction also share similarities in common. The E2 reaction happens all in one step and so only one mechanistic step needs to be drawn. In the E2 process, a base comes in and pulls off a hydrogen atom while simultaneously, the halogen is ejected from the substrate.
Mechanism of the E2 reaction
The Utility of Haloalkanes
As you can see, haloalkanes are a very special class of organic compound that can be extremely versatile. For that reason, these types of substrates find wide applications in the field of organic synthesis. In fact, compounds that contain at least one halogen are often the main starting point for even the most complex synthesis projects of pharmaceuticals and pesticides, just to name a couple.
In this lesson, we learned that haloalkanes are a class of organic compound in which a halogen is bonded to an alkane. We talked about that these types of compounds can have what we called a 'dual nature' or 'split personality' because they can undergo two types of reactions. Substitution is when a nucleophile exchanges itself for the halogen, and elimination is when an alkene is formed after a base pulls off a hydrogen adjacent to the carbon bearing the halogen. We saw two types of both substitution and elimination reactions:
The SN1 reaction involves the formation of a carbocation, followed by addition of a nucleophile to the intermediate.
The SN2 reaction happens in one step, and still results in the halogen being replaced by a nucleophile.
The E1 reaction, like the SN1 reaction, involves the formation of a carbocation intermediate, but instead of substitution, a base pulls off a hydrogen and the product is an alkene.
The E2 reaction also results in an alkene product, and happens in one step mechanistically, just like the SN2 reaction.
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