chevron_left A primary halogenoalkane has the halogen bonded to a carbon attached to one other carbon chevron_right

Primary Halogenoalkanes: The Simple Starters

What is a Halogenoalkane?

Let's start with the basics. A halogenoalkane, also known as an alkyl halide, is an organic compound in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms. The halogens are the elements found in Group 17 of the periodic table: fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). The general formula for a simple halogenoalkane with one halogen is C_nH_2n+1X, where X represents the halogen.

Think of an alkane like methane, CH₄. If we swap one hydrogen atom for a chlorine atom, we get chloromethane, CH₃Cl. This is the simplest example of a halogenoalkane. They are incredibly important in chemistry because the carbon-halogen bond is polar, making the carbon atom slightly positive and vulnerable to attack by other molecules, which leads to a wide range of chemical reactions. This is what makes them so useful for creating other, more complex chemicals.

Classifying Halogenoalkanes: Primary, Secondary, and Tertiary

Not all halogenoalkanes are the same. Their reactivity depends heavily on the carbon atom to which the halogen is attached. Chemists classify them into three main types based on this:

In a primary halogenoalkane, the carbon with the halogen is often at the end of a carbon chain. This carbon is less "crowded" because it only has one alkyl group (the -CH₃ or similar) attached to it, in addition to the halogen and two hydrogens. This lack of crowding is a key reason for their unique reactivity.

How to Make Primary Halogenoalkanes

There are several common methods for preparing halogenoalkanes in the laboratory. Here are two important ones that often yield primary compounds.

From Alkenes: Another method involves adding a hydrogen halide (HX) across the double bond of an alkene. This is called an addition reaction. For example, ethene (CH₂=CH₂) reacts with hydrogen bromide (HBr) to form bromoethane (CH₃-CH₂Br), another primary halogenoalkane. The equation is: $CH_2=CH_2 + HBr \rightarrow CH_3CH_2Br$

The Chemical Behavior of Primary Halogenoalkanes

The most important reactions of halogenoalkanes are nucleophilic substitution reactions. In these reactions, the halogen, called the leaving group, is kicked out and replaced by a different atom or group of atoms, called a nucleophile. A nucleophile is a "nucleus-loving" species that has a lone pair of electrons it can donate to form a new bond with the slightly positive carbon atom.

Primary halogenoalkanes primarily undergo a specific mechanism called S_N2^[1]. The name stands for Substitution, Nucleophilic, Bimolecular. Let's break down what this means:

• Bimolecular: The rate of the reaction depends on the concentration of both the halogenoalkane and the nucleophile. This means the two particles collide for the reaction to happen.
• Mechanism: The nucleophile attacks the carbon atom from the back, opposite the carbon-halogen bond. As the new bond forms, the old carbon-halogen bond breaks. This happens in a single, concerted step. The result is an inversion of the molecule's geometry, much like an umbrella turning inside out in the wind.

Because the carbon atom in a primary halogenoalkane is only attached to one other carbon and two small hydrogen atoms, it is relatively unhindered. This makes it easy for the nucleophile to approach and attack, which is why the S_N2 mechanism is favored.

Key Substitution Reactions in Action

Let's look at some specific examples of nucleophilic substitution reactions with primary halogenoalkanes.

From Labs to Life: Practical Uses of Primary Halogenoalkanes

While many halogenoalkanes are now restricted due to environmental concerns (like their effect on the ozone layer), they have been and still are used in various applications. Primary halogenoalkanes are particularly useful as building blocks in chemical synthesis.

Imagine a chemist wanting to make a new drug or a new type of plastic. They often start with simple molecules and react them together to build more complex ones. Primary halogenoalkanes are perfect for this. For instance, the reaction with cyanide ions (CN–) is a classic way to increase the length of a carbon chain. Bromoethane has 2 carbon atoms, but after reacting with KCN, the product, propanenitrile, has 3 carbon atoms. This is a powerful tool for creating larger organic molecules from smaller ones.

Chloromethane (CH₃Cl) was historically used as a refrigerant. Some primary halogenoalkanes are also used as solvents and as intermediates in the production of other chemicals, such as silicones and pesticides.

Important Questions

Footnote

^[1] S_N2: Stands for Substitution Nucleophilic Bimolecular. It is a specific reaction mechanism where a nucleophile substitutes a leaving group in a single, concerted step, and the rate of the reaction depends on the concentration of both reactants.