Identification of Alcohols

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What are Alcohols?

Alcohols are the most recognizable and familiar organic compounds, because they have a wide range of applications and uses in our daily life. Alcohols are organic molecules that contain a hydroxyl functional group connected to either an aryl or alkyl group (ROH). If only the hydroxyl carbon has a single R group, it is referred to as primary alcohol. However, if it holds two R groups, then it is a secondary alcohol, and if it is holding three R groups, it is clearly tertiary alcohol. As a number of other organic compounds, alcohols can be aromatic by holding a benzene ring. The simplest aromatic alcohol is called phenol.

Identification of Alcohols

Identification of primary secondary and tertiary alcohols can be done by using different processes. Identifying alcohol in organic chemistry can be achieved by exploiting various properties of the alcohol types. Different instrumentation analysis methods, like nuclear magnetic resonance (NMR), are used, and other qualitative tests can be utilized. By combining these tests together, the identification of alcohol can be determined in the same fashion as the way ketones and aldehydes can be identified.

Identification Methods of Alcohol

Ferric Chloride Test

One way to differentiate between aromatic alcohol and aliphatic alcohol is by using the iron (III) chloride. The iron chloride compound provides the solution with a red-orange appearance. In an aromatic alcohol presence, such as phenol, the chloride atoms are replaced by the aromatic alcohol by changing the central iron atom's coordination property. Then, this changes the solution color to purple. Aliphatic alcohols will not react with the iron (III) chloride and, therefore, the solution remains red-orange.

Jones Test

The Jones test is another method to identify alcohol, which utilizes chromium trioxide in the presence of a sulfuric acid to act as a powerful oxidizing agent. In the presence of Jones' reagent, a primary alcohol is first converted into an aldehyde. After that, into a carboxylic acid, whereas, the secondary alcohol will then be oxidized into a ketone. The chromium oxidation state is the key to this test. Chromium is in the oxidation state of +6 in the Jones' reagent. The Cr(VI) complexes present in the reagent give it a bright reddish and orange color.

In the reaction process, chromium is reduced from Cr (VI) to the oxidation state of +3 — Cr(III). First, the chromic and alcohol acid form a chromate ester. After that, a base, H2O, cleaves the C-H alcohol bond by forming the carbonyl group while reducing the Cr(VI) to Cr(IV). The Cr(IV) undergoes 2-electron reduction, and the carbon of the alcohol undergoes 2-electron oxidation. So, this is called 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.

Tertiary alcohols do not react with chromium, and therefore, no precipitate is made to keep the solution orange. Thus, the Jones test can help differentiate the primary and secondary alcohols from tertiary alcohols.

Differentiating Between Primary, Secondary and Tertiary Alcohols

  • Primary Alcohol

Alcohols, which are the Carbon(C) atom attached to the hydroxyl (OH) group, should attach directly either to a single or one carbon atom. These alcohol types have a - CH2OH  group, where the OH group is usually located at the end of a carbon chain.

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  • Secondary Alcohol

Alcohols, where the Carbon(C) atom attached to the hydroxyl (OH) group, should attach directly to two carbon atoms. These alcohols have a -CHOH group, where the OH group, in general, is located in the middle of a carbon chain.

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  • Tertiary Alcohol

Alcohols, where the Carbon(C) atom attached to the hydroxyl (OH) group, should attach directly to the three carbon atoms. These alcohols hold a -COH group, where the OH, in general, is located at the branched carbon chain junction.

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To determine the degree of alcohol, we should count the number of carbon atoms that are attached directly to the carbon bonded to the OH group.

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Schiff's Reagent 

To distinguish between the primary and secondary alcohols, the three classes of alcohol may be distinguished by Schiff's reagent, which is well known as a fuchsia dye decolorized by passing the sulfur dioxide through it. In the presence of aldehyde, even in even small amounts, it turns to bright magenta. It must, and however, be used absolutely cold, due to ketones react with it very slowly to provide the same color. Also, heat obviously causes a faster color change, but it is potentially confusing due to the competing ketone reaction. When warming this reaction mixture in a bath of hot water, pass some vapors produced through the Schiff's reagent.

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  • If the Schiff's reagent becomes magenta quickly, then aldehyde was formed from primary alcohol.

  • If there occurs no color change in the Schiff's reagent or only a simple trace of pink color within either a minute or so, no aldehyde was formed, and primary alcohol is not present.

  • The color change can identify secondary alcohol with an acidified potassium dichromate (VI) solution color change absence with the Schiff's reagent might.

FAQ (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.

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