Distinguishing Test of Terminal and Non-Terminal Alkynes- Tollen’s Reagent, Baeyer’s Test, Bromine Water Test, Ammoniacal Cuprous Chloride Test, Practice Problems and FAQs

Have you ever met twins or at least in a cinema? Identification of the individual is very difficult for those not in contact with both of them.

There are many twins in chemistry. Organic compounds exhibiting positional isomerism are examples. Alkynes are one such compound, where the triple bond can be at either end of the molecule or in the middle.

Assume you are in the final practical of your 12th grade. The teacher hands you two test tubes labeled A and B and asks you a simple question: "See, my friend, there are two alkynes present in each test tube; how can you predict which compound is terminal alkyne?"

Come and find out one of the most important reactions of alkyne.

Table of Contents

Distinguishing Test of Terminal and Non-Terminal Alkynes
Tollen’s Test
Baeyer’s Test
Bromine Water Test
Ammoniacal cuprous chloride test
Practice Problems
Frequently Asked Questions

Distinguishing Test of Terminal and Non-Terminal Alkynes

In the laboratory, you will use simple chemical tests to determine whether an unknown sample belongs to one of these classes of hydrocarbons, terminal or nonterminal alkyne, or both. You will not test for the presence of terminal alkynes in the laboratory alone.

You will run the tests for terminal and non-terminal alkynes on an unknown compound as well as some known representative compounds to see how different reagents affect these compounds to get appropriate results.

Let us discuss some important tests used to identify unsaturated compounds and then tests for distinguishing terminal and non-terminal alkenes.

Baeyer’s Test

In qualitative organic analysis is used to test for the presence of unsaturation. It is also known as Baeyer's reagent after the German organic chemist Adolf von Baeyer.

The reagent is an alkaline potassium permanganate solution.

When alkaline potassium permanganate is added to an unsaturated hydrocarbon, the pink colour of potassium permanganate disappears. The colour fades from purplish-pink to brown when it reacts with double or triple bonds (-C=C- or -C≡C-).

Bayer’s reagent will give brown precipitates with both terminal and Non- terminal alkynes. Hence, this test can be used for the detection of alkynes but does not specify whether they are terminal alkynes or non-terminal alkynes.

Baeyer’s Reagent Brown precipitates

Bromine Water Test

Bromine water is an oxidizing, intense yellow-to-red solution of diatomic bromine () in water

(). It is frequently used as a reactive in chemical assays of recognition for substances that, in an aqueous environment, react with bromine via the halogenation mechanism, primarily unsaturated carbon compounds (with a double or triple bond(s)).

The solution of bromine is brown. When a bromine solution is added to an unsaturated hydrocarbon in this test, the brown colour disappears if the hydrocarbon is unsaturated. Bromine reacts with the unsaturated hydrocarbon to form an addition product. It can be used for alkenes and alkynes both.

Let us discuss some important tests used to distinguish Terminal and non-terminal alkenes.

To differentiate or distinguish terminal and non-terminal alkynes, two tests are used: Tollen's test (which produces silver mirror precipitates) and the Ammoniacal Cuprous Chloride test (which produces red precipitates).

Tollen’s Test

Tollens' reagent is a chemical reagent used to distinguish between aldehydes and ketones, as well as terminal and non-terminal alkynes.

The reagent is a mixture of silver nitrate, ammonia, and sodium hydroxide (to maintain a basic pH of the reagent solution). It was named after the German chemist Bernhard Tollens, who discovered it. A positive test with Tollens' reagent results in the precipitation of elemental silver, which frequently produces a distinctive "silver mirror" on the inner surface of the reaction vessel.

Tollen’s reagent gives a white precipitate (silver alkynide) with terminal alkynes whereas non-terminal alkynes will show no results.

Tollen’s Reagent Silver Alkynide

Ammoniacal cuprous chloride test

Ammoniacal cuprous chloride solution is a blue solution made by dissolving cuprous chloride in ammonia and water.

is the formula for ammoniacal cuprous chloride solution.

It produces a red precipitate containing terminal alkynes. When terminal alkynes are passed through an ammoniacal cuprous chloride solution, a red precipitate of copper alkynide forms, whereas no precipitates form when non-terminal alkynes are present.

Ammoniacal cuprous Red chloride Precipitates

Practice Problems

Q 1. Which test can be used to distinguish between But-1-yne and But-2-yne?

a. Tollen’s Test
b. Baeyer’s Test
c. Bromine Water Test
d. None of these

Answer: But-1-yne and But-2-yne are terminal and non-terminal alkynes respectively. So, In order to distinguish between the terminal and non-terminal alkene, we can use two methods -The ammoniacal cuprous chloride test and Tollenn’s test. Tollen’s test is given in the option. Hence, it is the correct one.

Hence, the correct option is (A).

Q 2. Ammoniacal cuprous chloride gives red colour precipitates with?

a. Cycloheptyne
b. 3-methyl butyne
c. 1-methoxy propyne
d. Pent-2-yne

Answer: A red precipitate of copper alkynide forms when terminal alkynes are passed through an ammoniacal cuprous chloride solution, whereas no precipitates form when non-terminal alkynes are present.

In options (A), (C) ( )and (D)(), the alkynes are non-terminal and hence exclude the possibility of formation of red precipitates.

In option (B), the given alkyne is terminal and gives red colour precipitates with ammoniacal cuprous chloride solution.

Hence, the correct answer is option (B).

Q 3. Which test can be used to distinguish between Cyclohexyne and Cyclohexane?

a. Tollen’s Test
b. Ammoniacal cuprous chloride
c. Bromine Water Test
d. All of these

Answer: Cyclohexane being non-terminal alkyne and cyclohexane being alkane can be distinguished with bromine water tests as in option (A) Tolen’s test and option (B) Ammoniacal cuprous chloride give precipitates with terminal alkynes only.

Hence, when a bromine solution is added to an unsaturated hydrocarbon in this test, the brown colour disappears and forms an addition product. It can be used for alkenes and alkynes both.

Hence, the correct option is (C).

Q 4. Which test can be used to distinguish between Acetylene and Ethylene?

a. Tollen’s Test
b. Bromine Water test
c. Baeyer’s Test
d. All of these

Answer: Acetylene being terminal alkyne and Ethylene being alkene can be distinguished with Tollen’s tests as in option (C) Baeyer’s test and option (B) Bromine water test give positive result both with alkene and alkyne. Hence, they can not be used to distinguish between Acetylene and Ethylene.

Hence, Tollen’s reagent gives a white precipitate (silver alkynide) with terminal alkynes whereas alkenes will show no results.

Tollen’s Reagent Silver Alkynide

Hence, the correct option is (A).

Frequently Asked Questions

Q 1. Are alkynes electrophilic or nucleophilic?
Answer: An alkyne is an electron-rich molecule due to the clouds of electrons surrounding the sigma bond. As a result, they are nucleophiles that react with electrophiles. Because of their weak pi bonds, alkynes, like alkenes, undergo electrophilic addition reactions.

Q 2. Are alkynes used as fuel?
Answer: Acetylene is the most basic member of the alkyne series. Because it produces a large amount of heat when burned, it is frequently used as a fuel for welding torches.

Q 3. What are the applications of alkynes in everyday life?
Answer: Alkynes and compounds with alkynes in their chemical structures have applications in a variety of industries. For example, in the fuel and plastics industries, alkynes such as propyne and acetylene are used as starting materials in the manufacture of plastic products.

Q 4. Are alkynes combustible?
Answer: The primary danger of alkanes, alkenes, and alkynes is their flammability. Among the gases, the vapours of these compounds may be lighter or heavier than air, and among the liquids, they may be heavier than air.

Related topics

Physical and Chemical Properties of Alkynes	Reductive and Oxidative Ozonolysis of Alkynes
Reaction of Alkynes	Oxidation of Alkynes
Combustion of Alkynes	Halogenation of Alkyne