We met the halogenalkanes earlier. The halogenalkanes form a homologous series of compounds with the general formula CnH_2n+1X; where X is one of the halogens (F, Cl, Br or I). The C-X bond in a halogenalkane is a polar bond with the carbon atom bearing a slight positive charge (δ⁺) and the halogen having a slight negative charge (δ^-). The partial positive charge on the carbon atom attached to the halogen makes it vulnerable to attack by electron rich species (nucleophiles). The two most common reactions you will meet that halogenalkanes undergo are nucleophilic substitution and elimination reactions.

In nucleophilic substitution reactions the halogen atom is replaced or substituted with another atom or group for example hydroxide ions (OH^-), cyanide ions (CN^-) and ammonia (NH₃) are often used as nucleophiles to attack the C-X bond. The other common reaction that halogenalkanes undergo is an elimination reaction; these elimination reactions are a good method of preparing alkenes.

The way in which halogenalkanes react can in part be down to their structure. That is whether they are primary, secondary or tertiary halogenalkanes. So let us start by looking at some structural isomerism involving halogenalkanes. The image below shows two primary halogenalkane molecules. In a primary halogenalkane molecule the carbon atom attached to the halogen atom (X) will have one alkyl group and two hydrogen atoms attached to it. The last molecule in the image below is simply a methane molecule where on of the hydroegn atoms has been replaced by a halogen, in this case an iodine atom. Simple halogenalkane molecules where the carbon atom attached to the halogen is bonded to only hydroegn atoms are called methyl halogenalkanes. Two primary halogenalkane and a methyl halogenalkane are shown in the diagram below:

In a secondary halogenalkane molecule the carbon atom joined directly to the halogen will have 2 other alkyl groups and one hydrogen atom attached to it; two examples of secondary halogenalkane molecules are shown below:

The image opposite shows 2-chloro-2methylpropane, a tertiary halogenalkane. In a tertiary halogenalkane molecule the carbon atom attached to the halogen has three alkyl groups directly attached to it; this means that there are no hydrogen atoms attached.

The halogens being electronegative elements means that the C-X bond in the halogenalkane molecules will be a polar one, with the carbon atom in the C-X bond having a partial positive charge (δ⁺) and the halogen atom having a partial negative charge (δ^-). A nucleophile is an electron rich species. Nucleophiles can be neutral molecules or ions with lone pairs of electrons which they can donate to an electron deficient molecule/atom (electrophiles); such as the carbon atom in a C-X bond. Examples of nucleophiles include hydroxide ions (OH^-), cyanide ions (CN^-), ammonia (NH₃) and water (H₂O). All these molecules whether charged or neutral are able to act as nucleophiles simply because they have lone pairs of electrons.

The electron deficient carbon atom attached the halogen in a halogenalkane molecule is susceptible to attack by these electron rich species, that is nucleophiles with lone pairs of electrons. In the diagram shown below a nucleophile uses its lone pair of electrons to form a covalent bond to the carbon atom attached directly to the bromine atom (the halogen) and at the same time the carbon-bromine bond breaks and a bromide ion (Br^-) leaves. The end result is that the bromine atom is replaced or substituted by the nucleophile, this is nucleophilic substitution.

S_N1 or S_N2

The nucleophilic substitution reactions that halogenalkanes undergo can follow one of two different mechanisms or routes. These mechanisms are called S_N1 and S_N2. This is short for substitution nucleophilic 1 and substitution nucleophilic 2. The mechanism, either the S_N1 or S_N2 that is followed by the halogenalkane when it reacts with a nucleophile will depend on whether it is a primary, secondary or tertiary halogenalkane.

Reaction rates

Later in your A-level course; probably in year 13 you will study a unit or module on kinetics. Kinetics is a topic that investigates the factors that affect the rate or speed of a chemical reaction and you will be able to work out a mathematical relationship between the variables such as temperature and the concentration of the reacting chemicals and the reaction rate. I will not attempt to explain this topic here but to give a very brief outline just to help you get the idea of what the terms S_N1 and S_N2 relate to.

Primary halogenalkanes and S_N2

The image below shows the hydroxide ion (OH^-) using one of its lone pairs of electrons to attack the δ⁺ carbon atom in bromomethane to form the alcohol methanol and a bromide ion (Br^-). Now at any given temperature and concentration of the hydroxide ion and the bromomethane the reaction occurs at a certain rate. If we were to say double the concentration of the hydroxide ion then the rate of reaction would also double. Similarly if we double the concentration of the bromomethane then the rate of reaction also doubles. This tells us that the rate of reaction depends on the concentration of both starting reagents; that is it is a second order reaction. The rate of the reaction will change if we alter the concentration of either the hydroxide ion or the bromomethane.

The mechanism put forward to explain this observation is called the S_N2, substitution nucleophilic bimolecular. The main features of this mechanism are outlined below. The nucleophile uses its lone pair of electrons to attack the δ⁺ carbon atom in the C-X bond. This forms a transition state in which the new bond between the carbon atom and the incoming nucleophile is forming at the same time as the C-X, the bond between the carbon and the leaving halogen atom is breaking. The final step is the complete formation of the C-Nu bond and the breaking of the C-X bond to form the products of the reaction; this is outlined below:

There are a number of factors that affect how quickly these S_N2 reactions proceed at, one of the most obvious factors is anything which can block or at least slow or hinder the incoming nucleophile as it attacks the δ⁺ carbon atom in the C-X bond is perhaps the most obvious. Since the incoming nucleophile (Nu) needs to approach the δ⁺ C atom at an angle of 180⁰ then any large bulky groups attached to the carbon atom could slow its approach or even block it. In the example given above the the δ⁺ carbon atom in the C-X is bonded to three very small hydrogen atoms; what would happen to the rate of the reaction if one of these hydrogen atoms was replaced by a larger group such as a methyl group (-CH₃) or an ethyl group (-C₂H₅) or even larger groups? Well as you might expect as the groups become larger they do indeed block the incoming nucleophile and slow the reaction rate. This is summarised in the diagram below which starts with a methyl halogenalkane molecule which undergoes a rapid SN2 reaction and the final image shows a tertiary halogenalkane which does not undergo a SN2 reaction with nucleophiles.

S_N1 reactions

Nucleophilic substitution reactions involving primary halogenalkanes readily take place by an S_N2 mechanism; however as the size of the groups attached to the carbon atom bonded to the halogen increase the rate of reaction slows down simply due to the fact that these larger groups block the incoming nucleophile from approaching at 180⁰. Secondary halogenalkanes have two alkyl groups attached to the carbon bonded to the halogen, here these two groups are more effective at blocking the incoming nucleophile and so secondary halogenalkanes react slower than primary halogenalkanes. What about tertiary halogenalkanes? Well no doubt you will probably have figured out that tertiary halogenalkanes do not react with nucleophiles by a S_N2 mechanism. We can summarise this as shown below:

So how do tertiary halogenalkanes react with nucleophiles? The mechanism which has been suggested for these reactions involving tertiary halogenalkanes is called a S_N1 reaction. For example if we use 2-bromo-2-methylpropane as an example of a tertiary halogenalkane (as shown above) then in its reaction with a nucleophile the rate of reaction depends only on the concentration of the halogenalkane and NOT the nucleophile. This is different from the S_N2 mechanism where the reaction rate depends on both the concentration of the nucleophile and the halogenalkane. The mechanism for the S_N1 is outlined below. The reaction of tertiary halogenalkanes occurs in two steps:

• Step 1- This is the rate limiting or slow step. The C-X bond in the tertiary halogenalkane breaks to form a stable tertiary carbocation. The concentration of the intermediate carbocation at any one time is likely to be low.
• Step 2- The nucleophile attacks the carbocation. This is a fast step

Solvent choice

Key Points

• Halogenalkanes form structural isomers to give primary, secondary and tertiary structures.
• Halogenalkanes undergo nucleophilic substitution reactions with nucleophiles such as hydroxide ions, cyanide ions and ammonia.
• There are two different mechanisms for these nucleophilic substitution reactions:
• Primary halogenalkanes react by a S_N2 mechanism. Here the reaction rate depends on the concentration of the halogenalkane and the nucleophile.
• Tertiary halogenalkanes react by a S_N1 mechanism. Here the reaction rate depends only on the concentration of the halogenalkane.
• Secondary halogenalkanes can react by both mechanisms but the S_N1 mechanism gives faster reaction rates and this will be the main mechanism followed by secondary halogenalkanes.

Check your understanding - Questions on Nucleophilic substitution reactions

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