R1, R2 and R3 maybe an alkyl or aryl group or even hydrogen. Most alcohols are similar to alkanes, the only difference being that a hydrogen atom is replaced by an -OH group. The most common alcohols are methanol (CH3OH), ethanol (C2H5OH) and propanol (CH3CH2CH2OH).
Through this practical I shall try to investigate the different chemical properties of the alcohols. This will include an analysis of the reaction of alcohols with reactive metals such as sodium, the role of alcohols in the esterifcation processes and the different oxidation steps and products for different primary and secondary alcohols.
1. Rack with 4 clean test tubes.
2. Bunsen burner
3. Sodium metal
4. Dilute sulphuric acid
5. Dilute potassium dichromate
6. Salicylic acid
7. Glacial ethanoic acid
1. Take 1 ml of ethanol in a test tube and add a small grain of sodium.
2. Try to identify the gas produced.
3. Repeat step 1, this time using 10 ml of water instead of ethanol.
4. Observe what happens, and after the reaction is complete keep the test tubes aside.
5. Now take a clean test tube and add 10 drops of ethanol to it followed by 10 drops of glacial acetic acid followed by 6 drops of concentrated sulphuric acid.
6. Warm the mixture without boiling it for 5 minutes.
7. Pour the contents into 50 ml of water in a beaker and smell cautiously.
8. Repeat the steps 5 – 7 but this time using 2-hydroxybenzoic acid (salicylic acid) instead of glacial ethanoic acid. After observing keep the test tubes aside.
9. Now take a clean test tube and 10 drops of dilute sulphuric acid and 5 drops of potassium dichromate to 5 drops of ethanol.
10. Mix the mixture thoroughly and warm it with caution.
11. Repeat the steps 9 – 10 once with methanol and another time with propan-2-ol.
Test Tube Contents
1 ml ethanol + sodium metal
Sodium metal sinks into the ethanol. Steady evolution of a gas, which extinguishes burning splint with ‘pop’ sound.
10 ml of water + sodium metal
The metal melts to silvery balls and moves quickly over to the water surface. Rapid evolution of a gas that extinguishes a burning splint with a ‘pop’ sound.
10 drops of ethanol + 10 drops of glacial ethanoic acid + 6 drops of concentrated sulphuric acid
A colourless compound with a fruity smell is produced.
10 drops of ethanol + 10 drops of 2-hydroxybenzoic acid + 6 drops of concentrated sulphuric acid
A colourless compound is produced that gives a balm type odour.
10 drops of dilute sulphuric acid + 5 drops of potassium dichromate + 5 drops of ethanol
Initially a colourless solution is present. Upon heating the solution turns gradually to a dark green colour.
In the reaction between sodium and ethanol the products are sodium ethoxide and hydrogen gas. The presence of hydrogen gas is corroborated by the burning splint, extinguishing with a pop sound when inserted in Beaker 1.
The reaction is given below:
2Na (s) + 2C2H5OH (aq) –> 2C2H5ONa (aq) + H2 (g)
The fact that ethanol reacts with sodium liberating hydrogen indicates that the alcohol contains some hydrogen ions and these are being reduced by sodium.
It is interesting to note that the reaction between sodium and water is much more vigorous than the reaction between sodium and alcohols. The equation for the reaction between sodium and water is given below:
2Na (s) + 2H2O (l) –> 2NaOH (aq) + H2 (g)
As observed, the metal melts to silver balls due to heat energy that is released by the highly exothermic reaction. The sodium metal shoots to the surface of the water because it has a density less than 1 g/cm3.
The fact that the sodium and ethanol reaction was less vigorous than the sodium and water reaction indicates that the concentration of hydrogen ions in water is greater than concentration of hydrogen ions in ethanol. Thus water is a stronger acid than ethanol and this is corroborated by the Ka values. The Ka value of ethanol is 10-18 mol dm-3 and the Ka value for water is 10-16 mol dm-3.
The fact that ethanol is weaker acid can be further corroborated by the equilibria of the two reagents :
C2H5OH + H2O C2H5O- + H3O+
H2O + H2O OH- + H3O+
The ethoxide ion is more basic than the hydroxide ion, because of the electron pushing tendency of the alkyl group, known as the +I effect. The C2H5 group pushes electrons to the O, increasing the negative charge density on the atom making it ready to accept protons. Thus the equilibrium above is further to the left in the case of ethanol than water. Hence, ethanol is the weaker acid.
When glacial ethanoic acid, concentrated sulphuric acid and ethanol are mixed, an esterification reaction occurs the product of which is ethyl ethanoate. The fruity smell that is obtained is due to the formation of the ester and the role of concentrated sulphuric acid here is that of a catalyst. This is because without the use of H+ ions as catalysts, it will take days to establish the equilibrium at room temperature. The reaction is given below:
CH3COOH (aq) + C2H5OH (aq) –> CH3COOCH2CH3 (aq) + H2O (l)
While the mixture should be heated to allow the equilibrium to be established fast, it is also important to note that boiling the mixture will lead to unsuccessful results because ethanol has a low boiling point of 780 C, and hence heat might cause it to vapourize thus hindering the esterification process. Moreover, ethyl ethanoate itself is very volatile having a boiling point of 770 C and hence the ester produced might be completely lost in the heating process. Moreover, ethanol vapour is extremely flammable and hence once it boils off it might catch fire.
It was also interesting to note that the smell of the ester became more prominent after the heated mixture was added to water. This is because initially a certain amount of the alcohol was in excess but when the mixture was added to water the excess alcohol dissolved and as a result the ester’s smell became more prominent.
Ethy ethanoate has many important uses. It is used in the preparation of artificial perfumes and essences. It serves as solvents for oils, fats, gums and varnishes. Ethyl ethnoate is also used to treat many skin diseases.
However, When 2-hydroxybenzoic acid (salicylic acid) reacts with ethanol and concentrated sulphuric acid, yet again an esterification reaction occurs. But this time the product is different. It gives a balm type smell and can be identified as ethyl salicylate. The smell of this ester is typical of commercial materials such as ‘oil of wintergreen’ which is often used in balms and ointments.
When dilute sulphuric acid and dilute potassium dichromate is added to ethanol, a primary alcohol, oxidation occurs. The oxidation is a two step process as shown by the structure of ethanol:
H – C – C – O – H
Since two hydrogen atoms are attached to the carbonyl carbon (ie. the carbon to which the -OH group is attached) therefore the oxidation occurs in two steps. In the first step, the product is ethanal and then on the following step ethanoic acid is produced. This is shown in the diagram below:
H H H H
H – C – C – O – H –> H – C – C = O –> H – C – C = O
H H H H H OH
On heating the green coloured solution formed is due to the presence of excess Cr3+ ions. It is worth mentioning here that because the potassium dichromate was in excess that the oxidation proceeded completely and the final product was ethanoic acid. Had the potassium dichromate been in a limited amount then the final product would have been ethanal and not ethanoic acid.
If methanol had been used instead of ethanol then the oxidation product would have been methanoic acid and the two step oxidation would occur as follows:
H -C – OH –> H – C = O –> H – C = O
H H OH
Here because the carbonyl carbon has only hydrogen attached to it thus the reaction occurs by only one step.
Similarly, if propan-2-ol were used instead of ethanol then the final product would have again been propanone because in propan-2-ol is a secondary alcohol – where one hydrogen is attached to the carbonyl carbon. Thus the reaction would have proceeded as follows:
H OH H H O H
H – C – C – C – H –> H – C – C – C – H
H H H H H
Here the oxidising agent was potassium dichromate, another equally apt oxidising agent would have been potassium permanganate. It is also worth noting that if a tertiary alcohol such as 2-methylpropan-2-ol had been used with these oxidising agents in the laboratory conditions, then the reaction would not have occurred.
Through this experiment we can confidently comment on the properties of the alcohols. Yet our entire experiment is just based on the two alcohols – ethanol and methanol. For greater insight into the properties of the alcohols the entire experiment should be repeated with more alcohols such as propanol and butanol. Along with that for investigating properties such as oxidation of alcohols – the alcohols should be reacted with more than one oxidising agent.
Hence along with potassium dichromate, one could react the alcohols with oxidising agents such as potassium permanganate and observe the results. Furthermore, for checking the reactivity of the reactive metals with the alcohols one could have used other reactive metals such as calcium, magnesium and potassium and reacted them with ethanol and other alcohols such as propanol and methanol. This would help to get a better view of the reactions of the alcohols, in general, with the reactive metals.
Moreover, the experiment could be conducted at different atmospheric conditions such as high temperature and pressure to determine how the reactions and properties of the alcohols changes with atmospheric conditions.
When reacting the sodium metal with ethanol and water it is important to take only a small grain of sodium for the reaction. This is because a very large amount of sodium will surely react vigorously with these reagents and lead to an explosion. It is for this reason that the sodium metal should be always stored under kerosene, because on exposure to air it will react explosively.
The esterification reaction between the ethanoic acid and ethanol should be heated but not boiled, because boiling will affect the esterification process and lead to erroneous results.
Through this investigation I was able to gain an insight into the chemical properties of alcohol. The conclusions from this investigation were that:
(i) Reactive metals such as sodium react with ethanol to produce hydrogen gas. However, the reaction is not as vigorous as the reaction between the reactive metals and water.
(ii) Acids react with alcohols in the presence of an acid catalyst to produce an ester. The ester produced might have a fruity smell such as ethyl ethanoate. It is because of the smell of esters that they are found to be of immense use in commercial goods.
(iii) Alcohols undergo oxidation depending on the number of hydrogen atoms attached to the carbonyl carbon. While, a primary alcohol, in the presence of excess oxidising agent, undergoes a complete two step oxidation to form an alkanoic acid – secondary alcohols undergo one step oxidation to form alkanones. Tertiary alcohols however do not undergo oxidation at all.