Nucleophilic substitution involving halogenalkanes

The halogenalkanes form a homologous series of compounds with the general formula C_nH_2n+1X, where X is one of the halogens (F, Cl, Br or I). Some simple examples of primary, secondary and tertiary halogenalkane molecules are shown below:

3d model of primary, secondary and tertiary halogenalkane molecules.

The halogens are generally electronegative elements; which 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^-).

Electrophiles and nucleophiles

Cartoon image showing definitions for nucleophiles and electrophiles

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, that is an electrophile. 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 electron rich species; that is nucleophiles with lone pairs of electrons. The image below shows how a nucleophile can use its lone pair of electrons to remove and ultimately take the place of the halogen in a halogenalkane molecule; that is the nucleophile substitutes for the halogen atom in the halogenalkane molecule.

Outline of the mechanism for nucleophilic substitution reactions to show how the reaction takes place.

For more details on the mechanisms of these nucleophilic substitution reactions visit the page on S_N1 and S_N2 reactions which describes in detail the type of reactions undergone by primary, secondary and tertiary halogenalkane molecules.

Reaction of primary halogenalkanes with water - Preparation of alcohols by nucleophilic substitution

It is possible to prepare alcohols by hydrolysing halogenalkanes with water; however the reaction is slow simply because water is not a particularly good nucleophile and the halogenalkanes are barely soluble or not soluble at all in water, so an aqueous ethanol mixture is a better choice as a solvent since the halogenalkanes are soluble in it. This increases the rate of the hydrolysis reaction.

halogenalkane_(aq) + water_(l) → alcohol_(aq) + acid_(aq)

RX_(aq) + H₂O_(l) → ROH_(aq) + HX _(aq)

The acid produced (HX_(aq)) will be either hydrochloric acid (HCl), hydrobromic acid (HBr) or hydroiodic acid (HI) depending on which halogen is present in the halogenalkane used. It is also worth mentioning that the (aq) state symbol used after the halogenalkane does not mean it is dissolved in water since the halogenalkanes are largely insoluble in water it simply means it is in solution with ethanol as the solvent. This hydrolysis reaction can be thought of as occurring in two steps, an example of this nucleophilic substitution reaction is shown below using the primary halogenalkane bromoethane:

Step 1:

Here the water molecule uses its lone pair of electrons to act as a nucleophile and attack the partially charged carbon atom (C^δ+). In the example below bromoethane is being hydrolysed with water. An intermediate is formed which contains an oxygen atom with a positive charge- an oxonium ion.

The mechanism for the hydrolysis of a halogenalkane by water.

Step 2:

To complete the reaction the intermediate ion formed above has a proton (H⁺) removed from it by a water molecule. Here the water molecule is acting as a base to accept a hydrogen ion (H⁺) and form the alcohol ethanol and also the oxonium ion or hydroxonium ion (H₃O⁺). This is outlined below.

Note: the hydroxonoium ion (H₃O⁺) is often simply shown as H⁺ in many equations.

Measuring the rate of hydrolysis of a halogenalkane

The colours of silver halide precipitates after addition of silver nitrate to solutions containing halide ions.

In the hydrolysis reaction of a halogenalkane as shown above we can easily measure the rate of reaction by making use of a reaction you will have seen before, that is the reaction of silver nitrate solution with halide ions (Cl^-, Br^-, I^-) to form white, cream and yellow precipitates of the silver halide. This can be shown as:

X^-_(aq) + Ag⁺_(aq) → AgX_(s)

For example you may recall that chloride ions (Cl^-) will react with a solution of silver nitrate to form a white precipitate of silver chloride.

NaCl_(aq) + AgNO_3(aq) → AgCl_(s) + NaNO_3(aq)

Similarly bromide ions (Br^-) will give a cream coloured precipitate of silver bromide and iodide ions (I^-) will form a yellow precipitate of silver iodide.
The experimental set-up to measure the rate of hydrolysis of the halogenalkanes is shown below. You will need 3 labelled test-tubes, each test-tube will contain:

Test-tube 1; 5ml of hot aqueous ethanol and 0.5ml of 1-chlorobutane.
Test-tube 2; 5ml of hot aqueous ethanol and 0.5ml of 1-bromobutane.
Test-tube 3; 5ml of hot aqueous ethanol and 0.5ml of 1-iodobutane.

You will also need three other test-tubes; each of these should contain 5ml of acidified silver nitrate solution. The basic set-up is shown below although I have only shown two test-tubes in the water bath whereas in reality there should be six test-tubes; 3 test-tubes of the acidified silver nitrate and 3 test-tubes containing each of the halogenalkanes dissolved in hot ethanol.

Pour the acidified silver nitrate solution into each of the test-tubes and start the stop-clock. Time how long it takes for each of the coloured white, cream and yellow precipitates of the silver halide to form.

experimental set-up for the hydrolysis of a halogen alkane

Results

As mentioned above the hydrolysis reaction of halogenalkanes with water is a slow one. However due to the decrease in bond enthalpy of the C-X bond as we descend group 7, we can order the rate of reaction as:

iodo > bromo > chloro > fluoro

We can use primary, secondary or tertiary halogenalkanes in this hydrolysis reaction. Here the order of the rate of reaction is:

tertiary > secondary > primary

The reasons for this are due to differences in the mechanisms of these reactions. Though it does not matter if you use primary, secondary or tertiary halogenalkanes similar trends are seen in the order that iodo react faster than bromo which reacts faster than chloro halogenalkanes.

Key Points

Halogenalkanes can be hydrolysed by water to form alcohols. The reaction is slow because water is a poor nucleophile and the halogenalkanes are almost insoluble in water.
The main factor in the rate of this hydrolysis reaction is the strength of the C-X bond.
The rate of the reaction can be measured by timing how long it takes a coloured precipitate of silver halide to form. An outline of the experimental procedure is:
- warm a solution containing a few cm³ of ethanol, water and acidified silver nitrate solution.
- record the time it takes for the coloured precipitate of silver halide to appear
- The quicker the coloured precipitate appears the faster is the hydrolysis reaction and the weaker the C-X bond.

Nucleophilic substitution involving halogenalkanes

Electrophiles and nucleophiles