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Chemistry

Test for Alcoholic Group

Test for Alcoholic Group

Q: 2. How does the Lucas test help in distinguishing between primary, secondary, and tertiary alcohols?

The Lucas test uses a reagent made of concentrated hydrochloric acid (HCl) and anhydrous zinc chloride (ZnCl₂). The distinction is based on the speed at which turbidity (a cloudy appearance) forms due to the creation of an insoluble alkyl chloride:Tertiary Alcohols: React immediately, causing instant turbidity.Secondary Alcohols: React slowly, showing turbidity after 5–10 minutes.Primary Alcohols: Do not show turbidity at room temperature and may only react upon heating.

Q: 6. How is the fruity smell of an ester used as a confirmatory test for alcohols?

This reaction, known as esterification, involves heating the alcohol with a carboxylic acid (like acetic acid) and a few drops of a strong acid catalyst (like concentrated H₂SO₄). The alcoholic -OH group and the carboxylic acid's -COOH group react to form an ester and water. Esters are known for their distinct, pleasant fruity smells (like that of bananas or apples), and the production of this aroma confirms the presence of the alcohol.

Reviewed by:

Ritika Singla

Alcohols

In organic chemistry, functional groups play a crucial role. Alcohols are referred to as the organic compound with -OH functional group. Compounds containing a -OH group bonded to a tetrahedral carbon atom are alcohols. For alcohol, the general formula is R-OH, where R consists of an alkyl group. Ethanol is one of the common alcohols. It is present in the drugs and is the main component of alcoholic drinks. Alcohol denotes an entire class of compounds of which methanol and ethanol are the primary members. The higher alcohols and the primary alcohols are expressed generally with the formula C_nH_2n+1OH.

The primary, secondary, and tertiary alcohol is included as the simplest mono alcohol that is the prime subject of the article. Here, we will study the identification test for alcohol.

Qualitative Test For Alcohol

Aim

To identify the present functional group (test for Alcoholic group) in a given organic compound.

Theory

Qualitative test for alcohol- there are various tests to detect the alcoholic group present-

Ester test
Sodium metal test
Ceric ammonium nitrate test
Acetyl chloride test
Iodoform test

Ester Test

Carboxylic acids react with alcohols resulting in the formation of fruit-smelling ester. The reaction is called the esterification reaction between an alcohol and a carboxylic acid. This reaction is a concentrated sulphuric acid-catalyzed reaction.

R-OH + R-COOH → R-COOR + H2O

CH3OH + CH3-COOH → CH3-COOCH3 + H2O

Sodium Metal Test

It is based on the appearance of brisk effervescence when alcohol reacts with active metals such as sodium because of the release of hydrogen gas.

Below, the chemical reaction is given.

2R-OH + 2Na → 2R-O-Na + H2↑

2CH3-OH + 2Na → 2CH3-O-Na + H2↑

Ceric Ammonium Nitrate Test for Alcohol

Due to the formation of a complex compound and ammonium nitrate, alcohol or ceric ammonium nitrate reactions form a pink or red-colored precipitate.

(NH4)2 [Ce(NO3)6] + 3ROH → [Ce(NO3)4(ROH)3] + 2NH4NO3

(NH4)2 [Ce(NO3)6] + 3CH3OH → [Ce(NO3)4(CH3OH)3] + 2NH4NO3

The formation of ester and hydrogen chloride results from alcohol reactions with acetyl chloride

R-OH + CH3-CO-Cl → CH3-COOR + HCl

HCl + NH4OH → NH4Cl + H2O

Iodoform

This test is conducted with secondary alcohols, acetaldehyde, and ketones. The compound is heated in the presence of a sodium hydroxide solution and iodine. The presence of alcohol is shown by the formation of a yellow iodoform precipitate.

CH3-CH(OH)-CH3 + I2 + 2NaOH → CH3-CO-CH3 + 2NaI + 2H2O

CH3-CO-CH3 + 3I2 + 4NaOH → CHI3(Iodoform) + CH3COONa + 3NaI + 3H2O

Luca’s Test

This test is used to distinguish between primary, secondary, and tertiary alcohols. Lucas reagent is a mixture of concentrated hydrochloric acid and zinc chloride. Tertiary alcohols on treatment with Luca’s reagent result in the formation of white ppt immediately. Secondary alcohol on treatment with Lucas reagent forms white ppt after 5 minutes. Primary alcohol on treatment with Luca’s reagent does not form white ppt at room temperature.

The chemical reactions are given below.

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Materials Required

Sodium metal
Acetic acid
Ceric ammonium nitrate
Acetyl chloride
Ammonium hydroxide
Iodine
Sodium hydroxide
Calcium sulfate
Lucas reagent (zinc chloride and Con.HCl)
Test tubes
Test tube holder
Filler

Procedure and Observations (Identification Test for Alcohol)

Tests	Procedure	Observation	Interference
1. Ester Test	1. In a clean, dry test tube, take 1ml of the organic liquid to be tested. 2. Add 1 ml of glacial acetic acid and 2-3 drops of concentrated sulfuric acid. 3. For 10 minutes heat the mixture in a water bath. 4. The hot mixture is poured into a cold water-containing beaker. 5. In the beaker, smell the water. 6. The alcoholic group's existence is confirmed by a fruity scent	Fruity smell indicates the presence of the alcoholic group.	Presence of an alcoholic group
2.Sodium Metal Test	1. Take the organic compound into a dry test tube to be tested. 2. To remove excess water, add 1gm of anhydrous calcium sulfate and shake well. 3. To another clean test tube, decant the solution. 4. Add a small piece of sodium metal. 5. If brisk effervescence appears due to the evolution of hydrogen gas indicate the presence of the alcoholic group.	Release of Brisk effervescence	Brisk effervescence is due to the evolution of hydrogen gas indicating the presence of the alcoholic group.
3. Ceric Ammonium Nitrate Test	1. In a dry test tube, take 1ml of the given compound. 2. Add a few drops of ceric ammonium nitrate and shake the solution well. 3. Observe the solution. 4. If red precipitate occurs, it conforms to the involvement of the alcoholic group.	The appearance of wine red colour precipitate	Presence of an alcoholic group
4. Acetyl Chloride Test	1. In a clean test tube, take 2ml of the organic compound given 2. Add 1gm of sulfate of anhydrous calcium and shake well. 3. Solution Filtering. Add 3 to 4 drops of acetyl chloride to the filtrate, then shake well. 4. Take a rod of a glass dipped in a solution of ammonium hydroxide. 5. Put the glass rod next to the mouth of the test tube. 6. The involvement of the alcoholic party is changed if white fumes occur.	White fumes formation	Presence of an alcoholic group
5. Iodoform Test	1. In a clean, dry test tube, take 1ml of the given organic compound. 2. Add 1ml of 1 percent solution of iodine to it. 3. Add dilute sodium hydroxide solution dropwise until the brown colour of iodine is discharged. 4. In a water bath, heat the mixture gently. 5. The presence of either ethanol or acetaldehyde or methyl ketone is suggested by the formation of yellow precipitates.	Yellow precipitate formation.	Presence of an alcoholic group
6. Lucas Test	Take about 1 ml of dry alcohol in a clean dry test tube and add 8-10 drops of Lucas reagent. Shake the mixture well.	1. If cloudiness appears immediately. 2. If cloudiness appears within five minutes. 3. If cloudiness appears only upon heating.	1. Presence of tertiary alcohol. 2. Presence of secondary alcohol. 3. Presence of primary alcohol.

Precautions

Sodium metal should be carefully treated, as it responds violently to water.
As it can cause nasal irritation, iodine should not be inhaled.

Did you Know?

The boiling points of alcohols of equal molecular weights are much higher than those of alkanes. Ethanol, for example, has a boiling point of 78 °C (173 °F) with a molecular weight (MW) of 46, while propane (MW 44) has a boiling point of −42 °C (−44 °F). Such a large difference in boiling points means that ethanol molecules are much more highly attracted to each other than propane molecules are. The ability of ethanol and other alcohols to form intermolecular hydrogen bonds results in much of this disparity. (For a discussion of hydrogen bonding, see chemical bonding: Intermolecular forces.)

FAQs on Test for Alcoholic Group

1. What are the common laboratory tests for identifying the alcoholic functional group?

Several tests can confirm the presence of an alcoholic (-OH) group. The most common ones include:

Sodium Metal Test: Produces brisk effervescence due to the release of hydrogen gas.
Esterification Test: Forms a compound with a characteristic fruity smell (an ester) when heated with a carboxylic acid.
Ceric Ammonium Nitrate Test: The solution turns from yellow to red or pink upon adding the alcohol.
Lucas Test: Used to distinguish between primary, secondary, and tertiary alcohols based on the rate of reaction.

2. How does the Lucas test help in distinguishing between primary, secondary, and tertiary alcohols?

The Lucas test uses a reagent made of concentrated hydrochloric acid (HCl) and anhydrous zinc chloride (ZnCl₂). The distinction is based on the speed at which turbidity (a cloudy appearance) forms due to the creation of an insoluble alkyl chloride:

Tertiary Alcohols: React immediately, causing instant turbidity.
Secondary Alcohols: React slowly, showing turbidity after 5–10 minutes.
Primary Alcohols: Do not show turbidity at room temperature and may only react upon heating.

3. What is the chemical principle behind the Ceric Ammonium Nitrate test for alcohols?

The Ceric Ammonium Nitrate test works on the principle of ligand exchange. The alcohol molecule, acting as a nucleophile, displaces a nitrate ligand from the yellow-coloured [Ce(NO₃)₆]²⁻ complex. This results in the formation of a new, red or pink coloured complex, [Ce(OR)(NO₃)₅]²⁻, which serves as a positive indication for the presence of an alcoholic group.

4. Why does the sodium metal test produce effervescence with an alcohol?

This test demonstrates the weakly acidic nature of alcohols. The hydrogen atom of the hydroxyl (-OH) group is slightly acidic. An active metal like sodium is reactive enough to displace this hydrogen atom, leading to the formation of a sodium alkoxide and the liberation of hydrogen gas (H₂). The bubbles of hydrogen gas escaping from the solution are observed as brisk effervescence.

5. Which specific structural feature in an alcohol is required to give a positive Iodoform test?

A positive Iodoform test is a specific test given only by alcohols that contain the CH₃-CH(OH)- group. This means the alcohol must be either ethanol or a secondary alcohol where the hydroxyl group is attached to the second carbon atom. The reaction forms a characteristic yellow precipitate of iodoform (CHI₃).

6. How is the fruity smell of an ester used as a confirmatory test for alcohols?

This reaction, known as esterification, involves heating the alcohol with a carboxylic acid (like acetic acid) and a few drops of a strong acid catalyst (like concentrated H₂SO₄). The alcoholic -OH group and the carboxylic acid's -COOH group react to form an ester and water. Esters are known for their distinct, pleasant fruity smells (like that of bananas or apples), and the production of this aroma confirms the presence of the alcohol.

7. Are alcohols considered acidic or basic, and how does this property influence their chemical tests?

Alcohols are amphoteric, meaning they can act as both very weak acids and very weak bases. However, they are more commonly regarded as weak Brønsted acids because the hydrogen of the hydroxyl group can be donated. This acidic character, though weaker than water's, is fundamental to tests like the sodium metal test, where an active metal can displace this acidic proton. Their ability to act as a weak base (Lewis base) is due to the lone pairs on the oxygen atom, which is crucial in reactions like the Lucas test.