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Alcohol is an organic compound with at least one or more than one hydroxyl (-OH) groups attached to a saturated carbon atom. In everyday life, alcohol is found in drinks such as wine, beer, liquor etc. However, in Chemistry, the term ‘alcohol’ is generally used for primary alcohol, also called ethyl alcohol or ethanol. It is used for medicine, liquor, fuels, perfumes, and many different purposes.
Alcohol is sometimes used in its pure form, and sometimes it is mixed in different, large or little proportions. While giving the IUPAC name to an alcohol organic compound, the suffix -ol is added if the alcohol group is of first priority. But if the alcoholic group is not of high priority, then the prefix hydroxy- is added to the IUPAC name of that compound.
Key Terms: Alcohol, Lucas Test, Victor Meyer Test, Schiff’s Reagent, Jones Test, Iodoform Test, Esterification, Primary Alcohols, Secondary Alcohols
Types of Alcohols
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Alcohols are classified on the basis of the number of carbons attached to the alcohol group. Alcohols are classified into three major types which are as follows:
- Primary Alcohols: When the alcohol group is attached to a carbon that is bonded with only one other carbon (1° carbon), such alcohols are known as primary alcohols.
- Secondary Alcohols: When the alcohol group is attached to a carbon bonded with two other carbons (2°carbon), such alcohols are known as secondary alcohols.
- Tertiary Alcohols: When the alcohol group is attached to a carbon bonded with three other carbons (3° carbon), such alcohols are known as tertiary alcohols.
Types of Alcohols
Properties of Alcohols
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The major properties of alcohols are listed below:
- Alcohols are colourless in nature.
- Alcohols are highly flammable and give a blue flame without smoke when burned.
- Alcohols are mostly liquid at room temperature.
- Solubility of alcohols in water is high, but due to their hydrophobic nature, the solubility of alcohols decreases when the size of alkyl/aryl groups increases.
- Boiling point of alcohol increases with an increase in carbon atoms since the Van der Waals force increases.
Chemical Tests for Identification of Alcohols
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For the identification of alcohols, certain tests are performed. Some of them are explained here in detail:
Victor Meyer Test
- Given solution is first treated with phosphorus and iodine to get iodoalkane, and then treated with silver nitrate to get nitroalkane.
- The nitroalkane hence obtained is treated with nitrous acid and the resulting solution is finally made alkaline with potassium hydroxide.
- The colour of the solution is observed.
- If the resulting colour is red, then the alcohol is a primary alcohol.
- If the resulting colour is blue, then the alcohol is a secondary alcohol.
- Otherwise, if there is no colour and the resulting solution is colourless, then the alcohol is a tertiary alcohol.
Lucas Test
- A solution formed of equal amounts of anhydrous zinc chloride and concentrated HCl is mixed with the sample liquid.
- The time taken in the appearance of cloudiness after mixing the solution is observed.
- If there is no cloudiness at room temperature, then the sample solution is a primary alcohol.
- If cloudiness appears after 5 minutes of mixing the solution, then the sample solution is a secondary alcohol.
- If cloudiness appears right after mixing the solutions, then the sample solution is a tertiary alcohol.
Iodoform Test
- Potassium iodide or iodine is added to the solution to be experimented with. Then the sodium hydroxide is added until the dark colour of the solution turns pink.
- The reaction is observed.
- If a yellow-coloured precipitate is formed, then it indicates a positive result.
- If the results are negative, there will be no reaction at all.
Tests for Identification of Alcohol
Oxidation Test
- The given alcohol is added to potassium or sodium dichromate to any strong acid.
- The colour of the solution is observed.
- If the alcohol is primary or secondary, the orange solution will turn green coloured immediately or after heating.
- If the alcohol is tertiary, the solution will remain orange.
Ester Test / Esterification
- Mix an organic acid (RCOOH) with an alcohol (ROH).
- Fruity smell is observed.
- The fruity smell indicates the presence of an alcoholic group.
Schiff’s Reagent
- It is a red-coloured dye whose colour disappears when sulphur dioxide is passed by it.
- If even a very little amount of aldehyde is present, the reagent's colour changes to a blend of red and violet colour.
- Generally the reagent is used in cold form because in the presence of heat the colour changes faster and makes the reaction of the ketone confusing.
- So, when the Schiff's reagent vapour is passed by these reactions, the red-violet colour appears if an aldehyde is formed.
- But if no aldehyde or primary alcohol is present in it, then a little shade of pink or no colour change is observed within a minute.
- To identify secondary alcohols, acidic potassium dichromate (K2Cr2O7) can be used.
Jones Test
- The main purpose of this test is to differentiate between primary alcohols and secondary alcohols.
- Chromium trioxide is used in this test since it is an oxidising agent.
- If primary alcohol is present in the sample compound, then in the presence of the Jones reagent, the primary alcohol is converted into aldehyde and the carboxylic acid will be converted to ketone.
- If secondary alcohol is present in the sample compound then it will convert to carboxylic acid.
Uses of Alcohols
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Some of the major uses of alcohols are as follows:
- It is used for sterilisation as it is a good sterilising agent.
- It is easy to remove stains from clothes with the help of alcohol.
- It has a lot of medicinal uses and is also used in cosmetics, as in perfume.
- It is used as a preservative in libraries.
- Used for drinking in various varieties.
Things to Remember
- Alcohol is an organic compound attached to a hydrocarbon chain with one or more hydroxyl groups.
- It is used in cosmetics, fuel, cleansers, alcoholic beverages, etc.
- Primary alcohol (1° alcohol) is the type of alcohol whose carbon atom is bonded with one alkyl group.
- Similarly, in secondary (2° alcohol) and tertiary groups (3° alcohol), the carbon atoms are bonded with two and three alkyl groups respectively.
- Alcohols are colourless, highly flammable and soluble in water.
- There are various tests performed for the identification of alcohols such as Lucas Test, Jones Test, Victor Meyer Test, Schiff’s Reagent, etc.
Previous Years’ Questions (PYQs)
- Jones reagent is…
- Victor Meyers test is not given by…
- In Victor-Meyer's test, the colour given by… (JEE Main 2014)
- Primary, secondary and tertiary alcohols can be distinguished by… (KEAM)
- Which one of the following reactions is easily possible… (AMUEEE 2013)
- Lucas test is done for…
- Lucas reagent is… (JCECE 2007)
- The reaction of Lucas reagent is fastest with…
- The alcohol that produces turbidity immediately with Lucas reagent…
- Arrange the following alcohols in increasing order of their…
- Which of the following does not turn Schiff's reagent to pink…
- The order of a reaction for an esterification process is… (GUJCET 2008)
- Esterification of acid chloride with ethanol is usually carried…
- Which compound gives iodoform test…
Sample Questions
Ques. Are alcohols acidic or basic? (3 Marks)
Ans. By concept, alcohols are neither acidic nor basic. But generally, they are very weak acids with pH values somewhere between 15-20. If alcohol is combined with any other base such as NaOH then the alcohol acts like a base. And similarly if combined with an acid, then it becomes acidic.
Ques. What is Lucas's reagent? (3 Marks)
Ans. Lucas reagent is a solution of anhydrous zinc sulphate and concentrated HCl, which is used for the identification of alcohols that have low molecular weights. According to the time taken by turbidity to appear, the alcohols are classified as primary, secondary or tertiary alcohols. The time taken by the types of alcohol is shown as primary>secondary>tertiary.
Ques. Why is there a difference in reactivity of 1°, 2° and 3° alcohols with Lucas Reagent? (5 Marks)
Ans. The reaction of 1°,2° and 3° alcohols with Lucas reagent take place through a reaction mechanism. Lucas reagent forms carbocation with all three types of alcohol. But the stability of carbocation is different for all these three reactions. Tertiary alcohol gives results instantly with Lucas reagent because its carbocation is very stable. While secondary alcohol (2° alcohol) gives results with Lucas reagent after taking a few minutes as its carbocation is less stable and in the case of primary alcohol (1° alcohol) at room temperature, it doesn't give any result with Lucas reagent at all, because it's carbocation is not stable at all.
Thus, we can see that the stability of carbocation of primary, secondary and tertiary alcohol can be written as 3°>2°>1°. It’s maximum in 3° alcohol and minimum in 1° alcohol.
Ques. In the Iodoform test, why is a yellow precipitate formed? (3 Marks)
Ans. When sodium hydroxide and Iodine are mixed with a compound that contains either secondary alcohol with a methyl group or a methyl ketone in the main position, a light yellow precipitate of triiodomethane or iodoform is formed, which gives the smell of an antiseptic. It can be used to identify aldehydes or ketones.
Ques. How is alcohol used for medicinal purposes? (3 Marks)
Ans. Since ancient times, alcohol has served medicinal purposes. Physicians prescribe them for snake bites to disease control. If taken in a prescribed amount, alcohol can be considered a miracle in the medical field. Alcohols are used in medicine as sterilisation, disinfectant, antiseptic and antidotes. There are also other uses for alcohol, such as in liquor, beverages, gasoline, phenyl, etc.
Ques. Why does the ester give a fruity smell? (3 Marks)
Ans. The aroma of an ester is produced when alcohol and carboxylic groups are combined. Esters smell fruity because they exhibit weak intermolecular forces. Since esters are volatile in nature, their smell reaches our nose easily in comparison with other smells. An example of this is Ethyl Butyrate, which gives a smell that resembles a pineapple.
Ques. What is the process of making Schiff's reagent? (3 Marks)
Ans. Schiff’s reagent, also known as leuco fuchsin. Where leuco means white or discoloured and fuchsin is a dye. It is made by combining fuchsin with sodium bisulfite and then it is dissolved in HCl. From time to time, the solution is shaken and then it is decolourised with charcoal. The mixture thus formed is filtered until it becomes colourless.
Ques. What is Jones reagent? (3 Marks)
Ans. Jones reagent is a solution made by mixing chromium trioxide (or potassium dichromate) with aqueous or diluted sulphuric acid. After dissolving it, the resulting solution becomes acidic and is added to the acetone solution of an organic substrate. Jones test also known as Jones oxidation is mainly used for the oxidation of primary alcohols to carboxylic acids or aldehydes and secondary alcohols to ketones.
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