Making a Halogenoalkane
Aim
The aim of this activity is to prepare a sample of 1-bromobutane, an example of a halogenoalkane. This activity will involve learning how to use new techniques and equipment, firstly to carry out the reaction and form the product, and secondly to separate and purify the halogenoalkane.
I will attempt to obtain the maximum yield possible of 1-bromobutane. I shall examine the product that I have obtained and comment on the amount. I shall also evaluate any practical difficulties I experience in obtaining the halogenoalkane.
Background Information
The overall process involved in making a halogenoalkane is simply to replace the -OH group in an alcohol by a halogen atom.
The actual process to produce a halogenoalkane is slightly more complex, involving more than one single reaction. Butan-1-ol is heated in a mixture of concentrated sulphuric acid and sodium bromide. There are two main reactions. Firstly, the sulphuric acid reacts with the sodium bromide to form hydrogen bromide. Secondly, the hydrogen bromide reacts with the butan-1-ol to form 1-bromobutane and water.
. Making HBr:
The reaction for formation of HBr can be summarised by the following equation:
NaBr + H2SO4 ? NaHSO4 + HBr
There is also an unwanted side reaction that takes place at this stage. That is the oxidation of the product we want, the HBr into water and bromine:
2HBr + [O] ? H2O + Br2
(It does serve some purpose in colouring the halogenoalkane, which later on will be useful).
The oxidising agent is the sulphuric acid, which is reduced to sulphur dioxide amongst other things.
As I will explain later, the conditions of the first reaction will have to be configured so that production of HBr is maximised, and so that the oxidation of HBr is minimised.
2. Production of 1-bromobutane:
The formation of 1-bromobutane can be summarised by the following equation:
CH3CH2CH2CH2OH + HBr ? CH3CH2CH2CH2Br + H2O
butan-1-ol 1-bromobutane
I will now describe the exact mechanism involved in the reaction.
The butan-1-ol picks up a H+ ion (from the HBr), it is protonated:
This leaves the butan-1-ol positively charged. The Br- which is left, is negatively charged, carrying four lone pairs of electrons. Such species are called nucleophiles. They are deficient in positive charge and will attack anything positively charged, such as the protonated butan-1-ol.
This is an example of a nucleophilic attack. One of the lone pairs on the bromide ion begins to make a bond to the carbon atom, and at the same time the C-+OH2 bond begins to weaken.
A transition state is reached in which the C atom is partially bonded to both
-+OH2 AND Br-. This state only exists for a moment, before it is rapidly converted into the products.
The transition state is made from two molecules. The reaction is said to have a molecularity of 2. We also know that it involves a substitution, and a nucleophilic attack. The mechanism is described as an SN2 reaction.
So far I have explained the reaction for making 1-bromobutane. There are however, many impurities in the liquid product (I shall explain in more detail about unwanted substances in my evaluation). To obtain a pure yield of the halogenoalkane, it is necessary to put the product through a few stages of purification.
The four main experimental stages are ...
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The transition state is made from two molecules. The reaction is said to have a molecularity of 2. We also know that it involves a substitution, and a nucleophilic attack. The mechanism is described as an SN2 reaction.
So far I have explained the reaction for making 1-bromobutane. There are however, many impurities in the liquid product (I shall explain in more detail about unwanted substances in my evaluation). To obtain a pure yield of the halogenoalkane, it is necessary to put the product through a few stages of purification.
The four main experimental stages are the following:
. Carrying out the reaction
2. Separating the required product from the reaction mixture
3. Purifying the product
4. Testing the product to check that it is a pure sample of 1-bromobutane.
List of Apparatus:
* 10 cm3 measuring cylinder
* butan-1-ol (7.5 cm3)
* access to a balance
* 50 cm3 pear-shaped flask
* sodium bromide (10 g)
* tap funnel and stopper (doubles as separating funnel)
* distillation head
* clamps and stand
* 250 cm3 beaker (for water bath)
* concentrated sulphuric acid (10 cm3)
* anti-bumping granules
* reflux condenser
* guard tube containing soda lime
* small Bunsen burner
* long teat pipette
* concentrated hydrochloric acid (10 cm3)
* sodium hydrogencarbonate solution, 5% (10 cm3)
* anhydrous sodium sulphate
* test tubes (2)
* small funnel with cotton wool plug
* glass rod
* small beaker
* 0-110 oC thermometer and holder
* specimen tube
Safety:
Both acids are corrosive. Butan-1-ol is harmful and flammable.
Eye protection must be worn.
Vapours produced in this practical are harmful. Work in a well ventilated area.
Method
Part 1: Carrying out the reaction
. Into a 10 cm3 measuring cylinder, pour about 7.5 cm3 of butan-1-ol. (CARE Flammable. Keep bottle stoppered when not in use and well away from naked flames. Avoid skin contact and do not breathe vapour.) This will be approximately 6 g of the alcohol. Weigh the measuring cylinder and contents and then pour the butan-1-ol into a 50 cm3 pear-shaped flask. Weigh the empty measuring cylinder and record the exact mass of butan-1-ol you have added to the flask.
2. Add 10 g of sodium bromide and 10 cm3 of water to the flask.
3. Set up the apparatus shown in Figure 1, which will allow you to cool the mixture while making additions to it. (Clamp the neck of the flask.) It is important to cool the mixture whilst adding the sulphuric acid because, this will stop the volatile reactants from being lost. This also stops most of the oxidation of HBr
4. Place 10 cm3 of concentrated sulphuric acid in the tap funnel (CARE Extremely corrosive. Avoid skin contact). Do not stopper the tap funnel. Gradually add the acid to the reaction mixture over a period of about 5 minutes. After each addition, place a stopper in the funnel; grasp the funnel with one hand and the stand with the other. Gently swirl the assembly to help mix the contents of the flask. Remove the stopper before attempting to add more acid.
Figure 1: Apparatus for addition of concentrated sulphuric acid
5. Remove the tap funnel and distillation head from the top of the flask, and the cooling bath from around the flask. Dry the outside of the flask, and add a few anti-bumping granules. These will act as nuclei and will ensure that the liquid boils steadily and safely. Set up the apparatus in Figure 2, which will heat the mixture under REFLUX. Attach a guard tube filled with soda lime (CARE Irritant) to the top of the condenser. The soda lime within the guard tube absorbs any escaping gases, e.g. HBr and SO2.
6. Adjust the apparatus until the flask rests on a gauze and tripod. Make sure the condenser water supply is on. Gently heat the reaction mixture with a small flame until the mixture starts to boil and then allow it to REFLUX for between 30 minutes and 40 minutes. Condensed droplets should be falling back into the flask during reflux at a rate of about 1 drop per second. Covalent bond making and breaking takes a considerable amount of time. Refluxing solves the problem by raising the temperature, so that more reactant molecules are above their activation energies and are more likely to form the product.
Process of Reflux:
Figure 2 Apparatus for heating under reflux
Part 2: Separating the product from the reaction mixture
7. Allow the reaction mixture to cool. The flask now contains 1-bromobutane and probably the following substances too:
* Sodium bromide
* Sodium hydrogensulphate
* Sulphuric acid
* Butan-1-ol
* Hydrogen bromide
* Bromine
* Water
* But-1-ene
8. Carefully use a long pipette to suck off the upper organic layer from the reaction mixture, and transfer it to a small separating funnel. The mixture now only contains: C4H9Br, C4H9OH, HBr, Br2 and H20.
Part 3: Purifying the product
9. Add 10 cm3 of concentrated hydrochloric acid. (CARE Corrosive. Avoid skin contact) Shake the mixture vigorously in the stoppered funnel, releasing the pressure from time to time. When left for a while, two layers will form. This is because the HCl protonates the butan-1-ol and water, making them more ionic, and therefore soluble in the aqueous layer rather than the organic layer. The 1-bromobutane has a higher density than the butan-1-ol, the HCl and the water. The halogenoalkane will therefore be the lower layer.
0. Run off the 1-bromobutane layer into a clean test tube. Clean out the separating funnel and pour the 1-bromobutane back into it. Add 10 cm3 of 5% sodium hydrogencarbonate solution. Stopper the funnel and shake the contents gently and then more vigorously. The addition of this alkali neutralises the acid. It forms an ionic salt, and CO2 gas (the production of this gas will increase the pressure, so gently remove the stopper from time to time to release the pressure). The ionic salt dissolves in the aqueous layer. Once again, the lower layer will contain 1-bromobutane. This needs to be transferred into a clean test tube.
1. To the liquid in the test tube, add a small amount of anhydrous sodium sulphate, which acts as a drying agent and removes the last traces of water. Add the drying agent in small quantities, swirling after each addition, until the liquid is totally clear.
Part 4: Identifying the product and testing its purity
2. Most of the unwanted substances should now have been removed, except small amounts of butoxybutane and but-1-ene, which will be dissolved in the bromobutane. Carrying out a distillation can separate these.
3. Set up clean and dry apparatus for a simple distillation, complete with thermometer (see Figure 3). Transfer the dried bromobutane into the distillation flask by pouring it through a small funnel, fitted with a small cotton wool plug to catch the drying agent. Press the cotton wool gently with a glass rod to extract as much liquid as possible. Add a few anti-bumping granules to the flask.
4. Weigh a clean, dry specimen tube. Gently heat the liquid in the distillation flask using a small hand-held Bunsen flame. At first you will see but-1-ene coming out of solution, but nothing will condense in the receiver. Then the temperature of the vapour will rise. The boiling point of 1-bromobutane is 102 ?C so you need to collect the fraction which distils in the range 100 ?C - 104 ?C. This fraction should be collected in the weighed specimen tube. Stop the distillation when the thermometer rises above 104 ?C. Stopper the specimen tube, and record the mass of 1-bromobutane collected. (Butoxybutane boils at 142 ?C and will remain as a residue in the flask.)
Figure 3:Apparatus for a simple distillation
Results
Mass of cylinder
6.56g
Mass of cylinder and buta-1-ol
2.41g
MASS OF BUTAN-1-OL
5.85g
Mass of specimen test tube
3.28g
Mass of 1-bromobutane and test tube
5.99g
MASS OF 1-BROMOBUTANE
2.71g
Analysing and Evaluating Results
From my results, the total mass of 1-bromobutane obtained was 4.30g. This was from 5.85g of butan-1-ol.
I correctly analysed the product by distilling it (see Part 4 of my method). I collected the fraction that distilled at 102 ºC, the 1-bromobutane. I observed during this process, that there was only a very small amount of residue (butoxybutane) left. This implied that my purified mixture contained very little impurities.
I will now comment on the amount of halogenoalkane obtained. As previously stated, the overall chemical equation for the reaction is:
CH3CH2CH2CH2OH + HBr ? CH3CH2CH2CH2Br + H2O
butan-1-ol 1-bromobutane
I started with 5.85 g of butan-1-ol.
? Number of moles of butan-1-ol = 5.87 (g)
74 (g/mol)
= 0.0791 mol
There is a 1 : 1 mole ratio of butan-1-ol to 1-bromobutane.
? MAXIMUM MASS OF 1-BROMOBUTANE:
= 0.0791(mol) * 136.9(g/mol)
= 10.82 g
So, from calculations, the maximum yield possible if 1-bromobutane is 10.82 g. The actual yield was considerably lower than this: 2.71 g.
The percentage yield obtained was:
2.71 * 100 = 25%
10.82
This means that some of the reactant or product must have been lost during the process.
In calculating the maximum yield, I have assumed all of the HBr reacts with the butan-1-ol. In fact some of the HBr is oxidised into H2O and Br2 by the sulphuric acid. Also, some of the volatile reactants evaporate when the acid is added. I tried to keep this to a minimum by cooling the liquid whilst adding the acid. During the reflux reaction I have assumed that all of the butan-1-ol forms 1-bromobutane. The protonated butan-1-ol can eliminate a molecule of water and a H+ ion to form but-1-ene:
This is called dehydration or elimination.
Another way 1-bromobutane could have been lost is during the separation stages. I had to extract as well as I could the organic layer, taking no impurities from the other layer. This meant deliberately leaving some 1-bromobutane behind to avoid contamination of the product. The bromine that was produced made the separation easier because it coloured the organic layer, containing the halogenoalkane.
Another practical difficulty was that the amount of mixture that I was dealing with, compared to the size of the apparatus, was extremely small. If I had to transfer the mixture from a piece of apparatus, then 100% of the mixture wasn't transferred. A very small amount remained to 'wet' the internal surface of the apparatus due to attractive forces. This was most pronounced when I had to transfer the product through a piece of cotton wool (see method stage 13). I lost a considerable amount of product, because cotton wool is so absorbent. I had to apply a fair amount of pressure to try and maximise the amount of liquid product.
There aren't many ways to improve the experimental techniques in this practical. The main objectives would be to increase the production of the halogenoalkane, rather than the production of unwanted impurities, and also to improve the filtration techniques. Industry would have developed more efficient ways of obtaining 1-bromobutane.
Pritchard Benjamin
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