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MCQEasyJEE 2025Alkenes (Electrophilic Addition, Polymerisation)

JEE Chemistry 2025 Question with Solution

Total number of sigma (σ\sigma) and pi (π\pi) bonds respectively present in hex-1-en-4-yne are:

  • A

    1313 and 33

  • B

    1111 and 33

  • C

    1313 and 1313

  • D

    1414 and 33

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: The compound is hex-1-en-4-yne.

Find: The total number of σ\sigma bonds and π\pi bonds present in the molecule.

Write the structure as:

CH2=CH−CH2−C≡C−CH3\text{CH}_2 = \text{CH} - \text{CH}_2 - \text{C} \equiv \text{C} - \text{CH}_3

Count the carbon-carbon σ\sigma bonds:

C1=C2→1σ\text{C}_1 = \text{C}_2 \rightarrow 1\sigma C2−C3→1σ\text{C}_2 - \text{C}_3 \rightarrow 1\sigma C3−C4→1σ\text{C}_3 - \text{C}_4 \rightarrow 1\sigma C4≡C5→1σ\text{C}_4 \equiv \text{C}_5 \rightarrow 1\sigma C5−C6→1σ\text{C}_5 - \text{C}_6 \rightarrow 1\sigma

So, total carbon-carbon σ\sigma bonds =5= 5.

Count the carbon-hydrogen σ\sigma bonds:

C1(CH2)→2\text{C}_1(\text{CH}_2) \rightarrow 2 C2(CH)→1\text{C}_2(\text{CH}) \rightarrow 1 C3(CH2)→2\text{C}_3(\text{CH}_2) \rightarrow 2 C6(CH3)→3\text{C}_6(\text{CH}_3) \rightarrow 3

So, total carbon-hydrogen σ\sigma bonds =8= 8.

Therefore, total σ\sigma bonds are:

5+8=135 + 8 = 13

Now count the π\pi bonds:

One C=C bond→1π\text{One } \text{C}=\text{C} \text{ bond} \rightarrow 1\pi One C≡C bond→2π\text{One } \text{C}\equiv\text{C} \text{ bond} \rightarrow 2\pi

Hence,

π bonds=1+2=3\pi \text{ bonds} = 1 + 2 = 3

Therefore, hex-1-en-4-yne has 1313 sigma bonds and 33 pi bonds. The correct option is A.

Bond Counting Breakdown

Given: In organic compounds, each single bond is a σ\sigma bond, each double bond contains one σ\sigma bond and one π\pi bond, and each triple bond contains one σ\sigma bond and two π\pi bonds.

Find: The number of σ\sigma and π\pi bonds in hex-1-en-4-yne.

Hex-1-en-4-yne consists of a C=C\text{C} = \text{C} double bond and a C≡C\text{C} \equiv \text{C} triple bond.

From the multiple bond contributions:

C=C→1σ+1π\text{C} = \text{C} \rightarrow 1\sigma + 1\pi C≡C→1σ+2π\text{C} \equiv \text{C} \rightarrow 1\sigma + 2\pi

The remaining bonds are single bonds. Counting all carbon-carbon and carbon-hydrogen single-bond contributions gives a total of 1313 σ\sigma bonds in the molecule.

The total number of π\pi bonds is:

1+2=31 + 2 = 3

Therefore, the correct answer is 1313 and 33, so the correct option is A.

Common mistakes

  • Counting a double bond as two σ\sigma bonds is incorrect because a double bond contains one σ\sigma bond and one π\pi bond. Count only one σ\sigma contribution from each double bond.

  • Counting a triple bond as three σ\sigma bonds is incorrect because a triple bond contains one σ\sigma bond and two π\pi bonds. Separate σ\sigma and π\pi contributions before adding totals.

  • Ignoring carbon-hydrogen bonds gives the wrong σ\sigma count. After writing the structure, count all attached hydrogens on each carbon and include every C-H bond as one σ\sigma bond.

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