Chlorination of Alkanes

1. Initiation

• Process: UV light causes the homolytic fission of a chlorine molecule ($Cl_2$).
• Equation: $Cl_2 \xrightarrow{UV} 2Cl\cdot$
• Key Outcome: One stable molecule produces two highly reactive radicals.

2. Propagation

• Step A: A chlorine radical attacks the alkane, removing a hydrogen atom to form $HCl$ and an alkyl radical.
• Step B: The alkyl radical then attacks a chlorine molecule to form the chloroalkane product and regenerates a chlorine radical.
• Key Outcome: The radical is 'recycled', allowing the reaction to continue in a loop.

3. Termination

• Process: Two radicals collide and react to form a stable covalent bond, effectively removing radicals from the system.
• Key Outcome: The chain reaction ends. Multiple different termination products can form depending on which radicals collide (e.g., two alkyl radicals forming a longer chain alkane).

• Homolytic vs. Heterolytic: In this mechanism, bonds break homolytically (equal electron sharing), whereas in nucleophilic substitution, bonds break heterolytically (one atom takes both electrons).

• The 'Dot' Placement: Always ensure the radical dot ($\cdot$) is clearly visible and placed on the atom that actually carries the unpaired electron (e.g., on the Carbon in an alkyl radical, not the Hydrogen).

• Identifying Steps: If a question asks to identify a step, look at the radical count. If radicals appear on both sides, it is propagation. If they only appear on the right, it is initiation. If they only appear on the left, it is termination.

• Predicting Products: Be prepared to show all possible termination products. For example, if you are reacting methane, a termination step between two methyl radicals ($CH_3\cdot$) will produce ethane ($C_2H_6$).

• Condition Check: Always mention UV light or High Temperature as the essential condition for the initiation of this specific mechanism.

• Forgetting the Second Propagation Step: Students often forget that propagation requires two distinct equations to complete the cycle and regenerate the original radical.

• Incorrect Arrows: Do not use standard 'curly arrows' (which represent electron pairs) for the movement of single electrons in radical mechanisms unless specifically asked for 'fish-hook' arrows.

• By-product Confusion: Students often assume only the main chloroalkane forms. In reality, termination can produce a variety of alkanes and further substitution can lead to di- or tri-chloroalkanes if chlorine is in excess.