Identification of Alcohols

What is Alcohol?

The term 'alcohol' refers to organic compounds with one, two, or more hydroxyl groups (-OH), which are attached to the carbon atom (alkyl group or hydrocarbon chain).

A derivative of water is defined as an alcohol where one hydrogen atom is replaced by an alkyl group, commonly represented by the letter R in the structure of an organic compound. There are various forms of alcohol as well.

Among most of the commonly occurring organic compounds, alcohols make up a significant portion. In addition to being used in sweeteners and perfumes, they can also be utilized as catalysts for the synthesis of related compounds, while others can be easily found in various organic chemicals found in various industries.

 

Types of Alcohol

There is a difference between alcohols depending on whether they contain hydroxyl groups. Alcohols also differ in their physical and chemical properties based on the location of the hydroxyl group.

Alcohol is divided into three types. There are three types of alcohol: primary, secondary, and tertiary alcohols.

An alkyl group is classified according to where its carbon atom is attached to the hydroxyl group. There are a number of alcohols that are described as colourless liquids or even solids at room temperature. Molecular weight means how soluble an alcohol is in water; the higher the molecular weight, the less soluble the alcohol and the greater the density, boiling point, vapour pressure, and viscosity.

 

Brief Overview of the Different Types of Alcohol

Primary Alcohols

Alcohols with only one single alkyl group attached to the carbon atom of the hydroxyl group (-OH) are considered primary alcohols. Methanol (propanol), ethanol, etc., are some examples of primary alcohols. Generally, the complexity of alkyl chains has no bearing on whether alcohol should be classified as primary or secondary. Primary alcohols are distinguished by the presence of just one linkage between the -OH group and the alkyl group.

Secondary Alcohols

Two alkyl groups are attached on either side of the carbon atom of a secondary alcohol. Alkyl groups present in these compounds may be structurally identical or different. Below are examples of secondary alcohols.

 Tertiary Alcohols

Alcohols with a tertiary structure contain hydroxyl groups attached to the carbon atom, which are connected to alkyl 3-groups. These alcohols have different physical properties depending on their structure. Alcohols form hydrogen bonds with neighbouring atoms when this -OH group is present. Alcohols have a higher boiling point than alkanes due to the weak bonds that form. A few examples are:

 

Methods of Identifying Alcohol

  1. Ferric Chloride Test

To determine whether an alcohol is aromatic or aliphatic, you can use iron (III) chloride. A reddish-orange color is created by the iron chloride compound. An aromatic alcohol, such as phenol, changes the coordination property of the central iron atom by replacing the chloride atoms with aromatic alcohol. The color changes from green to purple. In this case, the iron (III) chloride will not react with the amino alcohols, which is why the solution remains orange-red.

  1. Schiff's Reagent 

Schiff's reagent, which is a fuchsia dye that is discolored when sulfur dioxide is passed through it, can be used to distinguish between primary and secondary alcohols. It becomes bright magenta in the presence of aldehyde, even in very small amounts. Because ketones react slowly with it to impart the same color, it must be used absolutely cold. A faster color change occurs with heat, but the competition between the ketone reactions makes it confusing. The vapors produced by Schiff's reagent can be passed through this reaction mixture when heated in a bath of hot water.

  • Aldehyde is formed when Schiff's reagent turns magenta quickly.

  • A simple trace of pink color or no color change in the Schiff's reagent in a minute or so indicates that no aldehyde was formed, and no primary alcohol is present.

  • A color change in an acidified potassium dichromate (VI) solution can be used to identify secondary alcohols.

  1. Jones Test

As a powerful oxidizing agent, chromium trioxide is used in the Jones test to detect alcohol in the presence of sulfuric acid. A primary alcohol is converted into an aldehyde in the presence of Jones' reagent. In turn, the carboxylic acid will be transformed into a ketone, while the secondary alcohol will be converted into a carboxylic acid. This test is based on the chromium oxidation state. The Jones' reagent shows chrome to be in the oxidation state of +6. This reagent is bright reddish and orange due to the presence of Cr(VI) complexes.

The reaction process reduces chromium from Cr (VI) to Cr(III) at a reduced oxidation state of +3. To form the chromate ester, the chromic acid and the alcohol acid are combined. Following that, \[H_{2}O\] forms the carbonyl group while the Cr(VI) is reduced to Cr(IV) thereby cleaving the C-H alcohol bond. Two electrons are removed from the Cr(IV) by reduction, and two electrons are removed from the carbon of the alcohol by oxidation. Hence, this is known as a reduction-oxidation step.

Also, Cr(IV) participates in the further oxidation steps, and it is eventually reduced to Cr(III). Often, Cr(III) is present as a Hexa aqua chromium (III) ions — (Cr(H2O)6)3+ - and Cr(III) complexes, whereas the H2O molecules are replaced either by one or more sulphate ions — Cr(H2O)5(SO4)+. All these complexes provide Cr(III), which is the characteristic green color.

The colour of the solution is not maintained by chromium reactivity with tertiary alcohols. Consequently, the Jones test helps differentiate between primary alcohols and secondary alcohols.

To learn more about different types of alcohols in organic chemistry and their identification methods, log on to Vedantu and seek professional mentoring from the top subject matter experts. Develop your conceptual foundation in this topic and excel in this subject. 

FAQs (Frequently Asked Questions)

1. How to Distinguish Between Classifications Using Alcohol Reactivity?

The alcohol presence is determined using test reagents that react with the -OH group. The initial test in identifying alcohol is to take the free of water, neutral liquid, and add a solid phosphorus (V) chloride. A burst of acidic steamy hydrogen chloride fumes specifies an alcohol presence. Some subsequent tests are required to distinguish between alcohol classifications.


Determining the Tertiary Alcohol

A few drops of alcohol are added to a test tube that contains potassium dichromate(VI) solution acidified with the dilute sulfuric acid. The tube is then warmed in a hot water bath.


After heating, the colors are observed, as shown below.


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For either primary or secondary alcohols, the orange solution turns into green. The Schiff's test will be required to be performed to distinguish between the primary and secondary alcohols. There is no color change with tertiary alcohol.

2. Explain the Identification of Alcohol Using an Oxidation Test?

Identification tests for alcohol can also be achieved by the oxidation test. In the oxidation test, alcohol is oxidized with the sodium dichromate (Na₂Cr₂O₇). The oxidation of primary alcohol varies with the secondary, and tertiary alcohol too. The primary secondary and tertiary alcohols are distinguished by the oxidation rate. Based on their oxidation rates, alcohols are distinguished as follows.

  • Primary alcohol can be easily oxidized to an aldehyde and can be oxidized further to carboxylic acids also.

  • Secondary alcohol will easily be oxidized to a ketone, but there is no possibility of further oxidation.

  • Tertiary alcohol does not oxidize in the presence of sodium dichromate.

  • Therefore, the oxidation rate upon the oxidation with sodium dichromate helps us identify primary, secondary, and tertiary alcohol.

3. How to identify the alcohol

A variety of processes can be used to identify primary, secondary, and tertiary alcohols. The various properties of alcohol types can be used to identify the types of alcohol in organic chemistry. Other qualitative tests can be run, as well as different techniques for instrumentation analysis, such as nuclear magnetic resonance (NMR). As with ketones and aldehydes, alcohol can be identified by combining these tests together.


4. What are the Uses of Alcohols?

Alcohol is used for a variety of purposes. Here are a few.

  • A specific percentage of ethanol of 30 to 40% is found in alcohols that are consumed as beverages.

  • These are used as anti-freezing agents by mixing ethylene glycol with water to form a solution.

  • The alcohol ethanol acts as an antiseptic.

  • The internal combustion engines use alcohols as fuels, such as methanol.

  • Some of them are used as preservatives for laboratory specimens in medicine.

5. What are the physical properties of alcohol?

At room temperature, most of the common alcohols are colorless liquids. Alcohols with fruity odors include ethyl alcohol, methyl alcohol, and isopropyl alcohol. Most higher alcohols have an oily consistency and a strong fruity aroma. Alcohols containing between 4 and 10 carbon atoms are somewhat viscous, or oily.

6. What are the two chemical properties of alcohol?

Alcohol is acidic in nature. It reacts with metals such as sodium, potassium, calcium, and magnesium. Hydrogen atoms and oxygen atoms in a hydroxyl group have a polar bond. Alcohols in the primary state are more acidic than alcohols in the secondary and tertiary states.

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