Alkynes contain a triple bond, so they add up, two molecules of dihydrogen, halogen, hydrogen halides etc.
-- NCERT Class 11 Chemistry, Ch. 9, p. 316Alkyne Addition Reactions
8 MCQs9-step worked example
Source: NCERT HydrocarbonsOfficial key: NTA-verifiedLast updated: 8 Oct 2026
Try this first
A gas decolourises the reddish orange solution of bromine in carbon tetrachloride. What does this test show?
- A.The gas is saturated
- B.The gas is unsaturated
- C.The gas is acidic
- D.The gas is soluble in water
Tap to see the answer
Answer: B. The reddish orange colour of bromine in carbon tetrachloride is decolourised when it adds to an alkyne, and this is used as a test for unsaturation (NCERT Class 11 Chemistry, Chapter 9, page 317).
A is wrong: A is wrong because a saturated compound does not add bromine, so the colour would stay; decolourisation is the test for unsaturation (NCERT Class 11 Chemistry, Chapter 9, page 317).
C is wrong: C is wrong because the test depends on addition of bromine across a multiple bond, not on acidity (NCERT Class 11 Chemistry, Chapter 9, page 317).
D is wrong: D is wrong because the test is carried out in carbon tetrachloride, not water, and tells about unsaturation, not solubility (NCERT Class 11 Chemistry, Chapter 9, page 317).
Alkyne Addition Reactions, explained for NEET
The question to answer first: an alkyne has a triple bond, so how many molecules of a reagent can it take up, and where do they go?
Alkynes contain a triple bond, so they add up two molecules of dihydrogen, halogen, hydrogen halides and so on (NCERT Class 11 Chemistry, Chapter 9, page 316). The product depends on the stability of the vinylic cation. Addition to unsymmetrical alkynes follows the Markovnikov rule, and the majority of alkyne reactions are electrophilic additions (NCERT Class 11 Chemistry, Chapter 9, page 317).
The four additions to know, all on page 317 of NCERT Class 11 Chemistry, Chapter 9:
- Dihydrogen over Pt, Pd or Ni: ethyne gives ethene and then ethane; propyne gives propene and then propane.
- Halogens: the reddish orange colour of bromine in carbon tetrachloride is decolourised, and this is used as a test for unsaturation.
- Hydrogen halides: two molecules of HCl, HBr or HI add to give gem dihalides, in which the two halogens are attached to the same carbon atom. Ethyne with HBr gives bromoethene and then 1,1-dibromoethane.
- Water: alkynes are immiscible with water and do not react with it. One molecule of water adds on warming with mercuric sulphate and dilute sulphuric acid at 333 K, giving carbonyl compounds.
Ethyne also polymerises. Under suitable conditions, linear polymerisation gives polyacetylene, a high molecular weight polyene that conducts electricity under special conditions (NCERT Class 11 Chemistry, Chapter 9, page 317). In cyclic polymerisation, ethyne passed through a red hot iron tube at 873 K gives benzene, three molecules joining into one (NCERT Class 11 Chemistry, Chapter 9, page 318).
Watch out: with a hydrogen halide the second molecule goes onto the same carbon as the first, giving a gem dihalide. A vicinal product, with halogens on neighbouring carbons, is not what NCERT shows.
How do you solve a Alkyne Addition Reactions question? A worked example
- 1
Given
5.2 × 10¹ g of ethyne (C₂H₂, molar mass 26 g mol⁻¹) is passed through a red hot iron tube at 873 K and converted completely into benzene (C₆H₆, molar mass 78 g mol⁻¹).
- 2
Required
The mass of benzene formed.
- 3
Concept
On cyclic polymerisation, three molecules of ethyne polymerise to form one molecule of benzene (NCERT Class 11 Chemistry, Chapter 9, page 318).
- 4
Formula
3 C₂H₂ gives C₆H₆. Moles of benzene = moles of ethyne ÷ 3; mass = moles × molar mass.
- 5
Substitution
Moles of ethyne = (5.2 × 10¹ g) ÷ (26 g mol⁻¹); moles of benzene = that value ÷ 3; mass of benzene = moles of benzene × 78 g mol⁻¹.
- 6
Calculation
Moles of ethyne = 52 ÷ 26 = 2.0 mol. Moles of benzene = 2.0 ÷ 3 = 0.667 mol. Mass = 0.667 × 78 = 52 g. The coefficients 3 and 1 are exact counts and do not limit the significant figures.
- 7
Final answer
The mass of benzene is 5.2 × 10¹ g. It equals the mass of ethyne because all the atoms of the three C₂H₂ units are in the one C₆H₆ molecule.
- 8
Common trap
Dividing the mass of ethyne by 3 gives 17 g. The factor 3 applies to the number of molecules, and benzene is three times as heavy per molecule as ethyne.
- 9
Similar NEET-style question
How many moles of ethyne are needed to form 0.50 mol of benzene by cyclic polymerisation? (Answer: 1.5 mol, since 3 mol of ethyne give 1 mol of benzene.)
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Can you answer these Alkyne Addition Reactions MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
A gas decolourises the reddish orange solution of bromine in carbon tetrachloride. What does this test show?
Show answer and why every option is right or wrong
Answer: B. The reddish orange colour of bromine in carbon tetrachloride is decolourised when it adds to an alkyne, and this is used as a test for unsaturation (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why A is wrong: A is wrong because a saturated compound does not add bromine, so the colour would stay; decolourisation is the test for unsaturation (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why C is wrong: C is wrong because the test depends on addition of bromine across a multiple bond, not on acidity (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why D is wrong: D is wrong because the test is carried out in carbon tetrachloride, not water, and tells about unsaturation, not solubility (NCERT Class 11 Chemistry, Chapter 9, page 317).
MCQ 2Easy RecallPractice
Two molecules of hydrogen halide add to an alkyne to form a gem dihalide. In a gem dihalide the two halogen atoms are
Show answer and why every option is right or wrong
Answer: A. Two molecules of hydrogen halides add to alkynes to form gem dihalides, in which two halogens are attached to the same carbon atom (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why B is wrong: B is wrong because halogens on neighbouring carbons make a vicinal dihalide, and NCERT describes the product of HX addition as a gem dihalide (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why C is wrong: C is wrong because in a gem dihalide both halogens sit on one carbon atom, not on carbons at opposite ends (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why D is wrong: D is wrong because the halogens are bonded to carbon in the product; they are not attached to hydrogen atoms (NCERT Class 11 Chemistry, Chapter 9, page 317).
MCQ 3Easy RecallPractice
Under which conditions does one molecule of water add to an alkyne to give a carbonyl compound?
Show answer and why every option is right or wrong
Answer: B. Alkynes are immiscible with water and do not react with it, but one molecule of water adds on warming with mercuric sulphate and dilute sulphuric acid at 333 K to form carbonyl compounds (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why A is wrong: A is wrong because alkynes are immiscible with water and do not react with it without the catalyst (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why C is wrong: C is wrong because a red hot iron tube at 873 K is the condition for the cyclic polymerisation of ethyne to benzene (NCERT Class 11 Chemistry, Chapter 9, page 318).
Why D is wrong: D is wrong because cold, dilute, aqueous potassium permanganate (Baeyer's reagent) is the reagent that oxidises alkenes to vicinal glycols; it is not the condition for adding water to an alkyne (NCERT Class 11 Chemistry, Chapter 9, page 313).
MCQ 4Direct ApplicationPractice
Propyne reacts with excess dihydrogen over a Pt catalyst. What is the final product?
Show answer and why every option is right or wrong
Answer: A. Propyne adds one molecule of dihydrogen to give propene, which adds a second molecule to give propane (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why B is wrong: B is wrong because propene is the intermediate after the first molecule of dihydrogen; with excess dihydrogen the reaction goes on to propane (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why C is wrong: C is wrong because no oxygen-containing reagent is present; the addition of dihydrogen gives a hydrocarbon (NCERT Class 11 Chemistry, Chapter 9, page 316).
Why D is wrong: D is wrong because addition of dihydrogen does not close a ring; it saturates the triple bond and gives propane (NCERT Class 11 Chemistry, Chapter 9, page 317).
MCQ 5Direct ApplicationPractice
Propyne reacts with an excess of HBr. What is the main product?
Show answer and why every option is right or wrong
Answer: C. Propyne is unsymmetrical, so addition follows the Markovnikov rule and the first bromine goes to the middle carbon, which carries the vinylic cation. The second HBr adds to give a gem dihalide, with both bromine atoms on that carbon: 2,2-dibromopropane (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why A is wrong: A is wrong because the bromine atoms are on neighbouring carbons, which is a vicinal dihalide; NCERT shows gem dihalides from hydrogen halides (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why B is wrong: B is wrong because it is a gem dihalide but both bromine atoms are on the end carbon, which is the opposite of Markovnikov addition to an unsymmetrical alkyne (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why D is wrong: D is wrong because the bromine atoms are on the two end carbons, so the product is neither a gem dihalide nor the Markovnikov product (NCERT Class 11 Chemistry, Chapter 9, page 317).
MCQ 6Direct ApplicationPractice
Which statement about the polymerisation of ethyne is correct?
Show answer and why every option is right or wrong
Answer: C. Linear polymerisation of ethyne produces polyacetylene, a high molecular weight polyene, which under special conditions conducts electricity (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why A is wrong: A is wrong because benzene is the product of cyclic polymerisation, in which three ethyne molecules join (NCERT Class 11 Chemistry, Chapter 9, page 318).
Why B is wrong: B is wrong because it swaps the two: cyclic polymerisation gives benzene and linear polymerisation gives polyacetylene (NCERT Class 11 Chemistry, Chapter 9, pages 317 and 318).
Why D is wrong: D is wrong because NCERT describes both linear and cyclic polymerisation of ethyne (NCERT Class 11 Chemistry, Chapter 9, pages 317 and 318).
MCQ 7CalculationPractice
Ethyne is treated with one mole of HBr and the product is then treated with a second mole of HBr. What are the first product and the final product?
Show answer and why every option is right or wrong
Answer: C. Ethyne adds one HBr to give bromoethene, CH₂=CHBr. A second HBr adds so that both bromine atoms end up on the same carbon, giving the gem dihalide 1,1-dibromoethane, CH₃CHBr₂ (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why A is wrong: A is wrong because the second bromine goes onto the same carbon as the first, giving 1,1-dibromoethane, not the vicinal 1,2-dibromoethane (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why B is wrong: B is wrong because the first product is bromoethene, which still has a double bond; bromoethane would need a molecule of dihydrogen as well (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why D is wrong: D is wrong because it reverses the order; the product with one bromine forms first and the dibromide forms from it (NCERT Class 11 Chemistry, Chapter 9, page 317).
MCQ 8CalculationPractice
Propyne is treated with two moles of dihydrogen over Pt and the product is shaken with bromine in carbon tetrachloride. What is observed?
Show answer and why every option is right or wrong
Answer: B. Two moles of dihydrogen take propyne through propene to propane (NCERT Class 11 Chemistry, Chapter 9, page 317). Propane is saturated, so bromine does not add and the colour stays; decolourisation is the test for unsaturation (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why A is wrong: A is wrong because after two moles of dihydrogen no multiple bond is left; the decolourisation test needs unsaturation (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why C is wrong: C is wrong because propyne does add dihydrogen over Pt, Pd or Ni, first to propene and then to propane (NCERT Class 11 Chemistry, Chapter 9, page 317).
Why D is wrong: D is wrong because a saturated hydrocarbon does not add water, and no precipitate is described in this test (NCERT Class 11 Chemistry, Chapter 9, page 317).
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What to remember before solving Alkyne Addition Reactions questions
8 NCERT lines
The addition product formed depends upon stability of vinylic cation. Addition in unsymmetrical alkynes takes place according to Markovnikov rule. Majority of the reactions of alkynes are the examples of electrophilic addition reactions.
-- NCERT Class 11 Chemistry, Ch. 9, p. 317Reddish orange colour of the solution of bromine in carbon tetrachloride is decolourised. This is used as a test for unsaturation.
-- NCERT Class 11 Chemistry, Ch. 9, p. 317Two molecules of hydrogen halides (HCl, HBr, HI) add to alkynes to form gem dihalides (in which two halogens are attached to the same carbon atom)
-- NCERT Class 11 Chemistry, Ch. 9, p. 317Like alkanes and alkenes, alkynes are also immiscible and do not react with water. However, one molecule of water adds to alkynes on warming with mercuric sulphate and dilute sulphuric acid at 333 K to form carbonyl compounds.
-- NCERT Class 11 Chemistry, Ch. 9, p. 317Under suitable conditions, linear polymerisation of ethyne takes place to produce polyacetylene or polyethyne which is a high molecular weight polyene
-- NCERT Class 11 Chemistry, Ch. 9, p. 317Under special conditions, this polymer conducts electricity.
-- NCERT Class 11 Chemistry, Ch. 9, p. 317Key Fact
Cyclic polymerisation: three ethyne molecules form benzene through a red hot iron tube at 873 K
Ethyne on passing through red hot iron tube at 873K undergoes cyclic polymerization. Three molecules polymerise to form benzene, which is the starting molecule for the preparation of derivatives of benzene, dyes, drugs and large number of other organic compounds.
-- NCERT Class 11 Chemistry, Ch. 9, p. 318Alkyne Addition Reactions: NEET previous year questions (PYQs) with answers
11 questions in Hydrocarbons
No question in our NEET 2020–2025 set targets this topic directly.
More in Hydrocarbons: 4 exam traps and mistakes · 1 formula · 2 question patterns from its other lessons.
Sources
NCERT refs: Class 11 Chemistry Chapter 9, p.316 | Class 11 Chemistry Chapter 9, p.317 | Class 11 Chemistry Chapter 9, p.318
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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