Chemistry Hydrocarbons ETEA 2016
PMDC Verified Question 50 of 65
The correct order of the reactivity of hydrocarbon given below is
A
\( \text{C}_2\text{H}_4 > \text{C}_2\text{H}_2 > \text{C}_6\text{H}_6 \)
B
\( \text{C}_6\text{H}_6 > \text{C}_2\text{H}_4 > \text{C}_2\text{H}_2 \)
C
\( \text{C}_2\text{H}_2 > \text{C}_2\text{H}_4 > \text{C}_6\text{H}_6 \)
D
\( \text{C}_2\text{H}_2 > \text{C}_6\text{H}_6 > \text{C}_2\text{H}_4 \)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option A: \( \text{C}_2\text{H}_4 > \text{C}_2\text{H}_2 > \text{C}_6\text{H}_6 \)
Concept:

Reactivity of hydrocarbons towards electrophilic attack is based on the availability and energy of their pi-electrons.

Solution:

  • Ethene (\( \text{C}_2\text{H}_4 \), an alkene): Has one loosely held pi bond whose electron density sits above and below the plane. It is highly accessible to electrophiles, making it the most reactive.


  • Ethyne (\( \text{C}_2\text{H}_2 \), an alkyne): Possesses two pi bonds, but the sp-hybridized carbons pull the electron density closer and tighter, making the pi electrons less available than in alkenes. It is less reactive than ethene toward electrophiles.


  • Benzene (\( \text{C}_6\text{H}_6 \)): Contains a cyclic, delocalized pi electron system. This resonance creates extraordinary thermodynamic stability, making it highly unreactive toward addition reactions compared to both alkenes and alkynes.


  • Therefore, the correct reactivity sequence is \( \text{C}_2\text{H}_4 > \text{C}_2\text{H}_2 > \text{C}_6\text{H}_6 \).


Why other options are incorrect:

  • Any order placing benzene or ethyne before ethene misrepresents the resonance stability of benzene and the tight sp-orbital electronegativity of alkynes.

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