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Functional groups refer to the atom or the group of atoms that have replaced the hydrogen atoms from the existing hydrocarbons (organic compounds). They include, but are not limited to, -COOH (carboxylic acid), -OH, -Cl (halo-alkanes), -CHO (aldehydes), -COOR, etc.; functional groups are responsible for the characteristics of the molecule. Organic compounds that have -OH as a functional group attached to the carbon chain are known as alcohols whose general formula is given by R-OH; R being an alkyl group. They can be of a primary, secondary, or tertiary kind.
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Key Terms: Alcohol, Effervescence, Colouration changes, Precipitation, Complex formation, Halo-alkanes, Aldehydes, Organic compounds, Hydrocarbons, Carboxylic acid
Aim
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To identify the presence of alcohol (-OH) as a functional group in a given organic compound.
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Theory
The following tests can be performed to detect the presence of alcohol as a functional group:
- Sodium metal test
- Ester test
- Ceric Ammonium Nitrate test
- Acetyl Chloride test
- Iodoform test
Sodium metal test
When alcohol reacts with active metals like sodium brisk effervescence appears which is a result of the release of the hydrogen gas due to the chemical reaction.
Given below is the chemical equation for the chemical reaction:
2R-OH + 2Na → 2R-O-Na + H2↑
e.g. 2CH3-OH + 2Na → 2CH3-O-Na + H2↑
Sodium metal test
It is important to note that the alcohol being tested should be dry because when sodium reacts with water, the reaction is known to be highly exothermic in nature.
Ester test
When alcohols are made to react with a carboxylic acid, the reaction results in the formation of esters. The reaction is known as esterification and the asters have a distinct fruity smell. It is known to be a slow reaction that is aided by the catalyst concentrated sulphuric acid.
Given below is the chemical equation for the chemical reaction:
R-OH + R-COOH → R-COOR + H2O
e.g. CH3OH + CH3-COOH → CH3-COOCH3 + H2O
Ceric Ammonium Nitrate test
Upon making alcohols react with ceric ammonium nitrate, a complex compound is formed along with the formation of ammonium nitrate. The complex formed is a pink or red coloured precipitate.
Given below is the chemical equation for the chemical reaction:
(NH4)2 [Ce(NO3)6] + 3ROH → [Ce(NO3)4(ROH)3] + 2NH4NO3
e.g. (NH4)2 [Ce(NO3)6] + 3CH3OH → [Ce(NO3)4(CH3OH)3] + 2NH4NO3
Ceric Ammonium Nitrate test
Acetyl Chloride test
When alcohol is made to react with acetyl chloride, the reaction results in the formation of an ester and hydrogen chloride. The resulting hydrogen chloride reacts with the ammonium hydroxide to form white fumes of ammonium chloride and water.
Given below is the chemical equation for the chemical reaction:
R-OH + CH3-CO-Cl → CH3-COOR + HCl
HCl + NH4OH → NH4Cl + H2O
Iodoform test
The iodoform test is a test that is conducted successfully with secondary alcohols, acetaldehyde, and ketones. The given organic compound is heated in the presence of a sodium hydroxide solution and iodine and the presence of alcohol is detected by the formation of a yellow iodoform precipitate.
Given below is the chemical equation for the chemical reaction:
CH3-CH(OH)-CH3 + I2 + 2NaOH → CH3-CO-CH3 + 2NaI + 2H2O
e.g. CH3-CO-CH3 + 3I2 + 4NaOH → CHI3(Iodoform) + CH3COONa + 3NaI + 3H2O
Lucas test
Used to distinguish between primary, secondary, and tertiary alcohols, the test makes use of the Lucas reagent to achieve the same. The Lucas reagent is a mixture of concentrated hydrochloric acid (conc. HCl) and zinc chloride (anhydrous ZnCl2).
- Tertiary alcohols upon being treated with the Lucas reagent result in the formation of a white ppt almost immediately.
- Secondary alcohol upon being treated with the Lucas reagent forms a white ppt after 5 minutes.
- Primary alcohol upon being treated with the Lucas reagent do not form white ppt at room temperature.
Lucas test
Requirements
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- Sodium metal
- Acetic acid
- Ceric ammonium nitrate
- Acetyl chloride
- Ammonium hydroxide
- Iodine
- Sodium hydroxide
- Calcium sulphate
- Lucas reagent (anhydrous ZnCl2 and conc.HCl)
- Test tubes
- Test tube holder
- Filler
Procedure and observations
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Preparation of Reagents
- Ceric Ammonium Nitrate
Add 20gm of orange crystals of ceric ammonium nitrate to 200ml of warm dilute nitric acid.
- Lucas Reagent
To 135gm of anhydrous zinc chloride and dissolve it in 100ml of concentrated hydrochloric acid.
- Iodine Solution
Dissolve 5gm of potassium iodide in 40ml of water. Further, add 1gm of solid iodine to that solution and dissolve it completely.
Tests and Observations
| Test | Procedure | Observation | Conclusion |
|---|---|---|---|
| Sodium metal test | Take about 1 ml of the given organic compound in a dry test tube, to it add about 1gram of anhydrous calcium sulphate and shake well to remove all water. Filter the liquid into another clean dry test tube and add a small piece of sodium metal. | Brisk effervescence is produced. | A brisk effervescence is produced due to the evolution of hydrogen gas indicating the presence of an alcoholic group. |
| Ester test | Take about 1 ml of the given organic compound in a clean dry test tube, to it add about 1 ml glacial acetic acid and 2-3 drops of conc. sulphuric acid. Warm the mixture over a water bath for about 10 minutes. Pour it into about 20 ml of cold water taken in a beaker and take in the smell produced. | A fruity smell is produced. | Presence of alcoholic group. |
| Ceric Ammonium Nitrate test | Take about 1 ml of the given organic compound in a clean dry test tube and add a few drops of ceric ammonium nitrate reagent and shake well. | A pink or red coloured precipitate is observed to have developed. | Presence of alcoholic group. |
| Acetyl Chloride test | Take about 2 ml of the given organic compound in a clean dry test tube, add 1 g of anhydrous calcium sulphate and shake well to remove all water. Filter the liquid into another clean dry test tube and to the filtrate, add 2-3 drops of acetyl chloride. Shake the test tube well and bring a glass rod dipped in ammonium hydroxide solution near the mouth of the test tube. | White fumes are produced. | Presence of alcoholic group. |
| Iodoform test | Take 1ml of given organic compound in a clean dry test tube. To it add 1ml of the prepared 1% iodine solution to it. Add dilute sodium hydroxide solution dropwise until the brown colour of iodine is discharged. Heat the mixture gently over a water bath. The formation of a yellow precipitate indicates the presence of either ethanol or acetaldehyde or methyl ketone. | Formation of a yellow precipitate. | Presence of alcoholic group. |
| Lucas test | Take about 1 ml of the given organic compound in a clean dry test tube and add 8-10 drops of Lucas reagent. Shake the mixture well. | Possible observations and inferences:
| |
Result
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From the above tests, it has been observed that the given organic compound is an alcohol of _________ degree (i.e. primary alcohol, secondary alcohol or tertiary alcohol).
Precations
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- Sodium metal should be handled carefully as it reacts with water violently and the reaction is highly exothermic in nature
- Iodine should not be inhaled as it can possibly cause irritation to the nose.
Sources of error
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- Rushed judgment without the wait time required for the necessary precipitation/ colouration changes to occur.
- Incorrect use of the water bath.
Things to Remember
- Fruity smell indicates the presence of the alcoholic group in Ester Test.
- Ceric Ammonium Nitrate Test determines the presence of wine red colour precipitate.
- Alcohols are slightly acidic in nature and can be detected by their reaction with sodium. When sodium is added to alcohol, sodium alkoxide is formed along with the release of hydrogen gas that burns with a pop sound.
- Functional groups are important in chemistry because they are the portion of a molecule that is capable of characteristic reactions. They, therefore, determine the properties and chemistry of many organic compounds. An organic compound is any compound that contains carbon and another element.
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Sample Questions
Ques. What is spurious alcohol? (1 Mark)
Ans. Spurious alcohol is illicit liquor that has been made by improper distillation or by using methylated spirit which is not fit for human consumption.
Ques. Are alcohols acidic or basic? (1 Mark)
Ans. Alcohols are weak acids and bases when dissolved in water. Due to the polarization of the O–H bond making the hydrogen atom partially positive. Additionally, the electron pairs on the oxygen atom of alcohol make it both basic and nucleophilic.
Ques. What is Lucas reagent? (1 Mark)
Ans. The mixture of anhydrous zinc chloride prepared in 100ml of concentrated hydrochloric acid is called Lucas reagent. It is used to help distinguish between primary, secondary and tertiary alcohols.
Ques. What precaution must be taken before carrying out the sodium metal test? (1 Mark)
Ans. The alcohol should be dried completely before carrying out the sodium metal test because sodium itself reacts with water vigorously.
Ques. Of the two hydroxy organic compounds ROH and R’OH, the first one is basic and the other is acidic in behavior. How is R different from R’? (Compartment Delhi 2013) (1 Mark)
Ans. When R is alkyl, R-OH will behave as a bronsted base and when R’ is an aryl, R’-OH behaves as a bronsted acid.
Ques. Give a chemical test to distinguish between 2-Pentanol and 3-Pentanol. (Compartment Delhi 2013) (1 Mark)
Ans. 2-pentanol upon undergoing the iodoform test gives a yellow ppt. of iodoform while 3-pentanol does not give this test.
Ques. Give simple chemical tests to distinguish between the following pairs of compounds: Benzoic acid and Phenol (All India 2017) (1 Mark)
Ans. We make use of the ferric chloride test. Adding neutral FeCl3 in both the solutions, phenol reacts with neutral FeCl3 to form an iron-phenol complex giving violet colour but benzoic acid does not.
Ques. Give a separate chemical test to distinguish between the following pairs of compounds:
(i) Ethanol and Phenol
(ii) 2-Pentanol and 3-Pentanol (Compartment Delhi 2012) (2 Marks)
Ans. (i) Ethanol on reacting with I2 in NaOH gives yellow ppt of iodoform whereas phenol does not respond to this test.
(ii) 2-Pentanol on reacting with I2 in NaOH gives yellow ppt of iodoform whereas 3-pentanol does not respond to this test.
Ques. Give chemical tests to distinguish between the following pairs of compounds:
(i) Pentan-2-ol and Pentan-3-ol
(ii) Methanol and Phenol (Compartment All India 2013) (3 Marks)
Ans. (i) 2-pentanol gives iodoform test with yellow ppt. of iodoform while 3-pentanone does not give this test.
(ii) Distinction between Methanol and Phenol.
By making use of the FeCl3 test:
Phenol gives a violet coloured solution with FeCl3 while methanol does not.
Ques. O-nitro phenol is more acidic than o-methoxy phenol. Explain why. (Compartment All India 2013) (1 Mark)
Ans. O-nitro phenol is more acidic than o-methoxy phenol due to the presence of the NO2 group which has a -I effect. It weakens the O-H bond of phenol by withdrawing their electrons and thus releases H+ ion easily while due to the +I, +R effect of OCH3, o-methoxy phenol is less acidic.
Ques. Arrange the following compound groups in the increasing order of their property indicated:
(i) p-nitrophenol, ethanol, phenol (acidic character)
(ii) Propanol, Propane, Propanal (boiling point) (Delhi 2017) (2 Marks)
Ans. (i) P-nitrophenol > phenol > ethanol
(ii) Propanol > propanal > propane
Ques. Write the equation involved in Reimer-Tiemann reaction. (Delhi 2014) (2 Marks)
Ans. Reimer-Tiemann reaction proceeds as the reaction of phenol with CHCl3 in presence of aqueous NaOH at 340K followed by hydrolysis; giving salicylaldehyde.
Reimer-Tiemann reaction
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