Alkyl Halide: Structure & Reactions

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson we will learn about alkyl halides and common alkyl halides in the world. We'll also learn how they are formed, why they are important, and what further reactions can be done with an alkyl halide.

Alkyl Halide

What do seafood toxins; chloroform; certain antibiotics, such as Chloromycetin; and some hormones, such as the thyroid hormone T4, all have in common? They all have alkyl halides in their chemical makeup. They each have very different functions, but the alkyl halides in each of these help them perform their individual functions.

A carbon has space for 4 substituents (or bonds); the most common substituent on carbon is hydrogen. An alkyl is when one (or more) of these spaces is taken up with something other than a hydrogen. So an alkyl halide is when that space is taken up by a halogen instead of a hydrogen. A halogen is any element in the group VIIA of the periodic table (the elements that are circled on this periodic table image). These elements are: fluoride, chlorine, bromine, iodine, and astatine.

The halogens are in group VIIA and are circled on this periodic table.
Periodic table

Examples of Alkyl Halides

An alkyl halide can be as simple as a chloromethane, which is a common solvent previously used as a refrigerant:

Chloromethane is the simplest alkyl halide.

Or they can be like chloroform, which doctors used in the past to help perform surgeries:

The alkyl halide chloroform.

Or they can be complex, like this toxin that can be found in mussels. Since the ocean has many halogens in it, toxins, such as mussel toxin, often form. Mussel toxin is toxic to humans when they eat the mussels, partly due to the excess chlorine it introduces into our systems.

Mussel toxin is a complex alkyl halide.
Seafood toxin


Another complex alkyl halide is the thyroid hormone T4, which is important for our body to properly function:

The thyroid hormone T4 is an alkyl halide.

You will notice that the mussel toxin, chloromethane, and chloroform all have chloride as the halide while the thyroid hormone uses iodine as the halide. The antibiotic Chloromycetin also uses chloride as the halide (as is apparent by the name 'chloro'):

The alkyl halide Chloromycetin is used as an antibiotic.

Any of the halides can be used, but since chlorine and fluorine are the most common (in the earth's crust) of the halogens they are found more frequently in products. Iodine and bromine are somewhat rare, but they are still found frequently. Astatine is extremely rare (probably the rarest element on the earth).

How to Make an Alkyl Halide

You can chose from many ways to make an alkyl halide. The starting reactant can be an alkene (a carbon double bonded to another carbon), an alkyne (a carbon triple bonded to another carbon), or an alcohol (a carbon connected to an oxygen-hydrogen group).

In this example, we see the alkene turning into an alkyl halide:

An alkyl halide can be made by starting with an alkene and adding H-X to it
Making an alkyl halide

The blank lines coming off of the carbons can be either another carbon or a hydrogen, but since they are not important to the reaction they are not shown. Halogens are frequently denoted as an 'X' to refer to any of the halogens.

This is similar to how all alkyl halides are formed. If X-X is used instead of H-X as the reactant, then both carbons will be halogenated instead of just one, and we will end up with two alkyl halides.

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