Chemical Tests for Alcohols: Lucas Test & Oxidation Tests // HSC Chemistry

Summary

This video provides an in-depth overview of chemical tests for identifying different types of alcohols: primary, secondary, and tertiary. It explains the Lucas test, which differentiates alcohols based on their reactivity with hydrochloric acid and zinc chloride, producing halogenated alkanes. The video also covers oxidation tests, detailing how primary and secondary alcohols react with oxidizing agents like acidified dichromate and permanganate, turning from orange to green or colorless, while tertiary alcohols remain unaffected. The tests help distinguish alcohols by their molecular structure and reactivity.

Takeaways

• 😀 Primary alcohols have one carbon atom attached to the alcohol group and are bonded to one other carbon atom.
• 😀 Secondary alcohols have the alcohol group bonded to a carbon atom that is attached to two other carbon atoms.
• 😀 Tertiary alcohols have the alcohol group bonded to a carbon atom connected to three other carbon atoms.
• 😀 The Lucas test helps differentiate between primary, secondary, and tertiary alcohols using Lucas reagent (HCl + ZnCl2).
• 😀 Tertiary alcohols react the fastest in the Lucas test, turning the solution turbid quickly.
• 😀 Secondary alcohols react more slowly in the Lucas test, with turbidity forming in 3-5 minutes at room temperature.
• 😀 Primary alcohols do not react with the Lucas reagent at room temperature but can react when heated.
• 😀 The oxidation of primary alcohols produces aldehydes, which can be further oxidized to carboxylic acids.
• 😀 Secondary alcohols are oxidized to ketones, whereas tertiary alcohols cannot be oxidized.
• 😀 Acidified dichromate (orange) is used to identify primary and secondary alcohols, turning green upon oxidation. Tertiary alcohols do not react, so the solution stays orange.
• 😀 Acidified permanganate (purple) decolorizes when oxidizing primary or secondary alcohols, but remains purple when reacting with tertiary alcohols.

Q & A

What is a primary alcohol?

A primary alcohol is an alcohol where the carbon atom attached to the hydroxyl group (OH) is bonded to only one other carbon atom.

How does a secondary alcohol differ from a primary alcohol?

A secondary alcohol has a carbon atom attached to the hydroxyl group, which is bonded to two other carbon atoms, in contrast to a primary alcohol where the carbon is bonded to only one.

What defines a tertiary alcohol?

A tertiary alcohol is defined by having a carbon atom attached to the hydroxyl group that is bonded to three other carbon atoms.

What is the Lucas Test used for?

The Lucas Test is used to differentiate between primary, secondary, and tertiary alcohols based on their reactivity with Lucas reagent, which consists of concentrated hydrochloric acid and zinc chloride.

How does the reaction proceed in the Lucas Test?

In the Lucas Test, the hydroxyl group of the alcohol is replaced by a chlorine atom, forming a halogenated alkane. The reaction leads to the solution becoming turbid as the solubility of the final product is lower than the alcohol.

What is the observed result when a tertiary alcohol undergoes the Lucas Test?

Tertiary alcohols react the fastest in the Lucas Test, causing the solution to turn turbid very quickly due to the rapid formation of the halogenated alkane.

How do secondary alcohols behave in the Lucas Test?

Secondary alcohols react slower than tertiary alcohols in the Lucas Test, with the solution typically becoming turbid in about 3 to 5 minutes at room temperature.

What happens when a primary alcohol is tested with Lucas reagent?

Primary alcohols usually do not react with Lucas reagent at room temperature, leaving the solution clear. However, if heat is applied, the reaction can occur, and the solution will become turbid.

What is the chemical basis of the oxidation of alcohols?

The oxidation of alcohols involves the loss of electrons and the addition of oxygen. Primary alcohols are oxidized to aldehydes and then to carboxylic acids, while secondary alcohols are oxidized to ketones. Tertiary alcohols do not undergo oxidation.

What is the significance of the dichromate test for alcohols?

The dichromate test uses acidified dichromate as an oxidizing agent to differentiate between primary, secondary, and tertiary alcohols. The orange color of dichromate changes to green in the presence of a primary or secondary alcohol due to the reduction of dichromate to chromium ions.

How does acidified permanganate work in the oxidation of alcohols?

Acidified permanganate ions are purple, and when used to test alcohols, they are reduced to colorless manganese ions. Primary and secondary alcohols will decolorize the purple permanganate solution, while tertiary alcohols will not react, leaving the solution purple.