The incorrect statement regarding ethyne is

The carbon - carbon bonds in ethyne is weaker than that in ethene

The C - C bonds in ethyne is shorter than that in ethene

JEE Main · Chemistry · Chapter 12

Hydrocarbons JEE Main PYQs (2020–2026)

124 questions from Hydrocarbons have appeared in JEE Main between 2020 and 2026, including 90 in the last five papers (2022–2026). The most-asked topic is Alkenes: Ozonolysis, Hydroboration-Oxidation & Oxidation (28 questions). Practise them all with instant answers and step-by-step solutions.

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Solved Hydrocarbons PYQs

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JEE Main 2026EasyAlkanes: Preparation, Physical & Chemical Properties (Halogenation)
Q1
One mole of an alkane (x) requires 8 mole oxygen for complete combustion. Sum of number of carbon and hydrogen atoms in the alkane (x) is ________.
Numerical answer type
Show answer & solution
Correct answer: 17
For an alkane, the general formula is

$C_{n} H_{2 n + 2}$

The combustion reaction of an alkane is

$C_{n} H_{2 n + 2} + \frac{3 n + 1}{2} O_{2} \to n C O_{2} + (n + 1) H_{2} O$

Given that 1 mole of the alkane requires 8 moles of oxygen, so

$\frac{3 n + 1}{2} = 8$

Now solve:

$3 n + 1 = 16$

$3 n = 15$

$n = 5$

So the alkane is

$C_{5} H_{12}$

Now, total number of carbon and hydrogen atoms is

$5 + 12 = 17$

So, the required answer is

$17$
JEE Main 2026EasyAlkenes: Electrophilic Addition (Markovnikov & Anti-Markovnikov)
Q2
The cycloalkene $(X)$ on bromination consumes one mole of bromine per mole of $(X)$ and gives the product $(Y)$ in which $C : B r$ ratio is 3:1. The percentage of bromine in the product $(Y)$ is $____$ %. (Nearest integer)

(Given : molar mass in $g m o l^{- 1} H : 1, C : 12, O : 16, B r : 80$ )
Numerical answer type
Show answer & solution
Correct answer: 66
Bromination of a cycloalkene (one double bond) consumes 1 mole of $B r_{2}$ per mole of alkene, which means 2 bromine atoms add across the double bond.

So, product $(Y)$ is a dibromo compound.

Given in $(Y)$ , the ratio $C : B r = 3 : 1$ .

Let number of carbon atoms $= n$ .

Number of bromine atoms $= 2$ .

So,

$\frac{n}{2} = \frac{3}{1} \mspace 6 m u \Rightarrow \mspace 6 m u n = 6$

Hence $(Y)$ has formula:

$C_{6} H_{10} B r_{2}$

(since cyclohexene is $C_{6} H_{10}$ and it adds $B r_{2}$ )

Now molar mass of $(Y)$ :

Carbon: $6 \times 12 = 72$

Hydrogen: $10 \times 1 = 10$

Bromine: $2 \times 80 = 160$

Total molar mass:

$72 + 10 + 160 = 242 g m o l^{- 1}$

Percentage of bromine:

$% B r = \frac{160}{242} \times 100 \approx 66.12%$

Nearest integer $= 66%$ .
JEE Main 2024EasyAlkynes: Preparation, Acidity of Terminal Alkynes & Addition Reactions
Q3
The incorrect statement regarding ethyne is
- AThe carbon - carbon bonds in ethyne is weaker than that in ethene
- BBoth carbons are sp hybridised
- CThe $C - C$ bonds in ethyne is shorter than that in ethene
- DEthyne is linear
Show answer & solution
Correct answer: (A)
To evaluate which statement regarding ethyne is incorrect, it’s necessary to review the characteristics of the chemical bonds in ethyne (acetylene, $C_{2} H_{2}$ ) compared to ethene (ethylene, $C_{2} H_{4}$ ).

Option A: The statement that the carbon-carbon bonds in ethyne are weaker than that in ethene is incorrect. In ethyne, the carbon-carbon triple bond is composed of one sigma ( $σ$ ) bond and two pi ( $π$ ) bonds. This configuration makes the triple bond in ethyne much stronger and shorter compared to the double bond in ethene, which consists of one $σ$ bond and one $π$ bond. Thus, the carbon-carbon bond in ethyne is actually stronger than that in ethene.

Option B: The statement that both carbons are sp hybridised in ethyne is correct. In ethyne, each carbon atom is bonded to the other carbon atom and a single hydrogen atom, necessitating the sp hybridisation to form a linear molecule with $18 0^{\circ}$ angles between the bonds.

Option C: The statement that the $C - C$ bonds in ethyne are shorter than that in ethene is correct. The presence of a triple bond between the carbon atoms in ethyne results in a shorter bond length compared to the double-bonded carbons in ethene. This is because the additional pi bonds in a triple bond pull the carbon atoms closer together.

Option D: The statement that ethyne is linear is correct. Due to the sp hybridisation of carbon atoms in ethyne, the molecule adopts a linear geometry with bond angles of $18 0^{\circ}$ .

Therefore, the incorrect statement regarding ethyne is Option A: The carbon-carbon bonds in ethyne are weaker than that in ethene.