In the presence of sunlight, chlorine is added to hydrocarbons in a very fast reaction. Chlorine can replace the hydrogen atoms one by one. It is called a/an ______ reaction.

Appeared in: RRB Nsg. Superintendent-2026 (Shift -3rd)

Explanation

The reaction described involves one atom (hydrogen) being replaced by another atom (chlorine) within a molecule.
In organic chemistry, this process of swapping one atom or functional group for another is specifically termed a substitution reaction.
The presence of sunlight indicates a free-radical mechanism, which is characteristic of substitution reactions in saturated hydrocarbons (alkanes).
This reaction proceeds step-by-step, replacing hydrogen atoms one at a time, which is a hallmark of substitution.

Option A: An elimination reaction involves the removal of atoms or groups from a molecule, typically to form a double or triple bond. This question describes the replacement of an atom, not its removal to create a multiple bond.
Option C: An addition reaction involves adding atoms to a molecule, specifically across a double or triple bond. The hydrocarbons described (alkanes) are saturated and do not have multiple bonds to which atoms can be added.
Option D: While 'replacement' is a correct English synonym for the process, 'substitution' is the precise, standard scientific terminology used in chemistry to describe this specific type of reaction mechanism.

Visual 1: Diagram: Free-radical substitution of methane. The visual should illustrate the three key stages: 1. Initiation (UV light splits Cl₂ into 2 Cl• radicals), 2. Propagation (a Cl• radical abstracts a hydrogen from CH₄ to form HCl and a •CH₃ radical, which then reacts with Cl₂), and 3. Termination (radicals combine).

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Types of Organic Chemical Reactions as background academic context rather than a clinical decision trigger.
Understanding fundamental reaction types like substitution is crucial for pharmacology. Many drugs are metabolized (biotransformed) in the liver via Phase I reactions, which include oxidation, reduction, and hydrolysis, often involving substitution-like mechanisms.
This knowledge helps nurses understand how certain drugs are activated or inactivated in the body and provides a basis for anticipating potential drug interactions.
What if? If the hydrocarbon were unsaturated (e.g., ethene, an alkene) instead of saturated, the primary reaction with chlorine would be an addition reaction, not substitution. This reaction would not require sunlight and would break the double bond to add a chlorine atom to each carbon.

How to Approach the Question

First, analyze the process described in the question stem: 'Chlorine can replace the hydrogen atoms'.
Identify the key reactants and conditions: a hydrocarbon, chlorine, and sunlight.
Recall the definitions of the main types of organic reactions: addition (adding across a multiple bond), elimination (removing atoms to form a multiple bond), and substitution (replacing an atom or group).
Recognize that 'replacing' an atom directly corresponds to the definition of a substitution reaction.
Differentiate between the general term 'replacement' and the specific scientific term 'substitution'. In a science context, always choose the most precise term.
Conclude that 'substitution' is the correct classification for this reaction.

Concept Tested & Keywords

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Practise the full RRB Nsg. Superintendent-2026 (Shift -3rd)

Attempt every question from this paper in a timed mock, then review the full solution for each one.