halogenalkanes

Nucleophilic substitution involving halogenalkanes

We met the halogenalkanes earlier. The halogenalkanes form a homologous series of compounds with the general formula CnH2n+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 postive 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 vunerable 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 a nucleophilic substitution reaction the halogen atom is replaced or substituted with another atom or group for example hydroxide, cynaide and ammonia are often used as nucleophiles to attack the C-X bond. The other commnon reaction that halogenalkanes undergo are 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 three 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 or at its simplest three hydrogen atoms.

examples of primary halogenalkane molecules

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:

secondary halogenalkane examples example of a tertiary halogenalkane molecule

The image opposite shows 2-chloro-2methylpropane, a tertiary halogenalkane. In a tertiary halogenalkane molecule the carbon atom attached to the halogen has three carbon atoms or alkyl groups directly attached, 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 spieces, that is nucleophiles with lone pairs of electrons. In the diagram shown below a nucleophile uses a 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.

SN1 or SN2

The nucleophlic substitution reactions that halogenalkanes undergo can follow one of two different mechanisms or routes. These mechanisms are called SN1 and SN2. This is short for substitution nucleophilic 1 and substitution nucleophilic 2. The mechanism, either the SN1 or SN2 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 very brief outline just to help you get the idea of what the terms SN1 and SN2 relate to.

Primary halogenalkanes and SN2

The image below shows the hydroxide ion (OH-) using one of its lone pairs of electrons to attack the δ+ carbon atom in bromomethane, a primary halogenalkane 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, 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 SN2, 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 group 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:

There are a number of factors that affect how quickly these SN2 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 groups 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.

SN1 reactions

Nucleophilic substitution reactions involving primary halogenalkanes readily take place by an SN2 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 halogenalanes 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 SN2 mechanism. We can summarise this as shown below:



However by changing the reaction conditions, for example the solvent used, we can make tertiary halogenalkanes undergo nucleophilic subsitition reactions. SN2 reactions work best with polar protic solvents, that is solvents that will release H+ ions or can form hydrogen bonds, whereas SN1 reactions prefer polar aprotic solvents ( that is solvents which do not release H+ but are polar, e.g. acetone) The mechanism which has been suggested for these reactions involving tertiary halogenalkanes is called a SN1 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 SN2 mechanism where the reaction rate depends onboth the concentration of the nucleophile and the halogenalkane. The mechanism for the SN1 is outlined below. The reaction of tertiary halogenalaknes occurs in two steps:

Since step1 is the slow step, then this will hold up the reaction, step 2 can be as fast as "it likes" but it will make no difference to the overall rate of the reaction, this will depend only on the slow step - the rate limiting step.

Key Points

Practice questions

Check your understanding - Questions on Nucleophilic substitution reactions

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