Problem 3 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{C}_{3} \mathrm{H}_{8}+\mathrm{Cl}_{2} \stackrel{\text { Light }}{\longrightarrow} \mathrm{C}_{3} \mathrm{H}_{7} \mathrm{Cl}+\mathrm{HCl}\) is an example of (a) elimination (b) substitution (c) addition (d) rearrangement reaction

Step-by-Step Solution

Answer

The reaction is a substitution reaction.

Step 1: Identify the Reaction Type

In the given reaction, \( \mathrm{C}_{3} \mathrm{H}_{8} + \mathrm{Cl}_{2} \rightarrow \mathrm{C}_{3} \mathrm{H}_{7} \mathrm{Cl} + \mathrm{HCl} \), observe that \( \mathrm{Cl}_{2} \) is adding one chlorine atom to \( \mathrm{C}_{3} \mathrm{H}_{8} \), and simultaneously one hydrogen is replaced. This suggests that a substitution is occurring as one atom is substituted by another.

Step 2: Elimination or Substitution?

An elimination reaction involves removing a small molecule like \( \mathrm{H}_2 \) or \( \mathrm{H}_2\mathrm{O} \), which does not happen here. Instead, a hydrogen atom is replaced by a chlorine atom, confirming a substitution.

Step 3: Why Not Addition or Rearrangement?

Addition reactions involve adding atoms or groups without losing any part of the reactants. Rearrangement involves changing the structure of the molecule without addition or removal of atoms. Neither of these processes is occurring, as a chlorine atom replaces a hydrogen in \( \mathrm{C}_{3} \mathrm{H}_{8} \), confirming a substitution.

Step 4: Final Conclusion

The reaction \( \mathrm{C}_{3} \mathrm{H}_{8} + \mathrm{Cl}_{2} \rightarrow \mathrm{C}_{3} \mathrm{H}_{7} \mathrm{Cl} + \mathrm{HCl} \) is indeed a substitution reaction, as the chlorine substitutes for a hydrogen atom.

Key Concepts

Elimination Reaction

An elimination reaction is a type of chemical reaction where two atoms or groups are removed from a molecule, typically resulting in the formation of a double bond or a ring structure. This process often involves splitting off a small molecule like water (\(\mathrm{H}_2\mathrm{O}\)) or hydrogen gas (\(\mathrm{H}_2\)).

For a better understanding, consider a saturated compound like alcohol undergoing dehydration to form an alkene. During this process, a molecule of water is eliminated, thus fulfilling the criteria for an elimination reaction.

• Elimination often creates multiple bonds in the structure.
• Commonly involves the removal of 0two substituents from a single molecule.
• Results in an increase in unsaturation of the molecule.

A common example is the dehydration of ethanol to ethene. Although the original problem does not involve an elimination reaction, being able to differentiate between reaction types is crucial in organic chemistry.

Addition Reaction

An addition reaction involves two or more molecules combining to form a single product with more atoms than the original molecules. It typically occurs in molecules with double or triple bonds where the bond saturation increases.

Consider alkenes and alkynes; these are often the focus of addition reactions because they have unsaturated bonds, which easily accept additional atoms. A simple example is the reaction of ethene with hydrogen to form ethane over a nickel catalyst.

• Addition reactions are characterized by the direct combination of atoms or groups without the removal of other atoms.
• Usually increase the saturation of the molecule.
• Involve breaking of existing multiple bonds in favor of forming single bonds.

In the context of the original exercise, since no atoms are added across a bond without substitution, it doesn't qualify as an addition reaction.

Rearrangement Reaction

Rearrangement reactions are transformations where the molecular structure of a compound changes but the overall number of atoms remains the same. No atoms are added or removed, instead, the existing atoms are rearranged.

This type of reaction is quite common in the transition of one isomer into another. Isomerization is a key example, where a change in structure does not involve breaking or forming of bonds with external atoms or groups.

• Involves the reorganization of the existing molecular structure.
• The molecular formula remains unchanged.
• May significantly change the physical and chemical properties of the compound.

An everyday example is the conversion of glucose to fructose. As seen in the original problem statement, the reaction does not show rearrangement, as the molecular structure remains unchanged beyond the substitution event.