Aryl Halide: Structure & Reactions

Instructor: Korry Barnes

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

The primary focus of this lesson will be on a class of organic compounds called aryl halides. We will briefly discuss their general structure, then get a short survey of the different reactions they participate in.

Hey, What's an Aryl Halide?

Amidst all of the chemical compounds out there that organic chemistry instructors ask you to become familiar with, I'm sure at some point you've experienced the overwhelming feeling that it's just too much. Am I right? I know when I was taking my first organic chemistry course, the list of functional groups and classes of compounds I was asked to memorize made it seem like I was learning a foreign language!

In many senses, learning the ''language'' of organic chemistry does involve memorizing the vocabulary first, just like you would any other language you're unfamiliar with. So in this lesson, we are going to add a vocabulary term to our organic chemistry repertoire. We are going to talk about a class of compounds called aryl halides. The first thing we need to do is define what an aryl halide is, then we'll explore their structure, and lastly we'll learn about a few of the most important reactions they can undergo.

Definition of an Aryl Halide

Let's break down what those two little words mean first and foremost. The word aryl means aromatic (benzene), ring and the word halide means a halogen of some sort: fluorine, chlorine, bromine, or iodine. When you're looking at an aryl halide, it's simply one of the four halogen atoms bonded directly to a benzene ring. Pretty simple right?

Examples of aryl halides

Aryl Halide Structure

As we saw in the previous section, an aryl halide is classified by its distinct bonding of a halogen directly to a benzene ring. From a structural standpoint, one of the more important things to realize is the trend in bond lengths of the four aryl halides. Since the trend for atomic size goes F < Cl < Br < I, (meaning fluorine is smaller than chlorine, which is smaller than bromine, etc.) it's probably not surprising that in terms of bond length, the observation is as follows:

Bond lengths (given in picometers) of the four aryl halides

The bond lengths here are measured in picometers, which is a small unit of measurement used because we are talking about chemical bonds on a microscopic scale. Notice that as we go from fluorine, to chlorine, to bromine, to iodine, the bond lengths get longer and longer. That's because as the size of the halogen gets bigger, the bond has to elongate to make room for the larger atom that's bonded to the benzene ring.

Reactions of Aryl Halides: Electrophilic Aromatic Substitution (EAS)

Now that we know what an aryl halide is and we've touched on an important structural aspect of them, let's explore some of the more important reactions they can undergo.

Aryl halides readily undergo a reaction called electrophilic aromatic substitution or EAS for short. Due to the electronegative nature of the halogen, when another atom or group of atoms is added to the benzene ring, they will always go to the ortho (directly adjacent to the halogenated carbon) and para (directly across from the halogenated carbon) positions on the benzene ring. This is mainly due to the fact that these positions provide the most stability for the carbocation that will ultimately result when a carbon-carbon double bond breaks (from the aromatic ring).


Simply put, bromination just means adding one or more bromine atoms to a compound. Aryl halides can be brominated, and in order for this reaction to happen we simply need an iron (III) bromide catalyst FeBr3 and molecular bromine Br2.

Bromination of an aryl halide - notice how the bromine is on the ortho and para positions on the benzene ring in the products


Nitro groups (NO2 group) can be added to aryl halides. We call this nitration, and for this transformation, we can simply take our aryl halide and react it with concentrated nitric acid (HNO3) and sulfuric acid (H2 SO4). Since this process involves the use of concentrated acids, great care must be taken to minimize your exposure to these harmful chemicals.

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