Chemistry Hydrocarbons MDCAT 2018
PMDC Verified Question 39 of 65
Reaction mechanism of alkanes with halogens is known as
A
Addition
B
Elimination
C
Free radical substitution
D
Propagation
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: Free radical substitution
Concept:

Alkanes are saturated hydrocarbons containing only strong sigma bonds. They lack pi electrons and are unreactive towards electrophiles or nucleophiles under standard conditions.

Solution:

  • In the presence of UV light or high heat, halogen molecules (like \( \text{Cl}_2 \)) undergo homolytic cleavage to form highly reactive free radicals.


  • These radicals abstract a hydrogen atom from the alkane, and the resulting alkane radical reacts with another halogen molecule.


  • Because a hydrogen atom is completely replaced by a halogen atom via radical intermediates, the overall mechanism is definitively named free radical substitution.


Why other options are incorrect:

  • Addition occurs only in unsaturated compounds (alkenes/alkynes).


  • Elimination removes atoms to form double bonds; it does not replace them.


  • Propagation is merely one step within the free radical mechanism, not the name of the overall reaction itself.

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