Imagine a calm pond (an alkane) that suddenly gets a splash of sunlight (UV light). That splash gives the molecules the energy they need to break a bond and invite a new friend—chlorine—to join the party. This is called a photochemical substitution reaction.
Photochemical reactions are chemical changes that are driven by light. In our case, ultraviolet (UV) light supplies the activation energy (\$E_{\mathrm{a}}\$) needed to start the reaction. Think of UV light as a superhero that gives the alkane the power to break a C–H bond and form a new C–Cl bond.
The overall reaction for a simple alkane (RH) with chlorine is:
\$\mathrm{RH + Cl_2 \xrightarrow{h\nu} RCl + HCl}\$
Only one chlorine atom replaces one hydrogen atom—this is called monosubstitution.
Propane (CH₃–CH₂–CH₃) reacts with chlorine to give:
\$\mathrm{CH3–CH2–CH3 + Cl2 \xrightarrow{h\nu} CH3–CHCl–CH3 + HCl}\$
The new molecule, 1‑chloropropane, has a chlorine atom attached to the middle carbon. The structure can be drawn as:
CH₃–CHCl–CH₃
| Step | What Happens | Key Species |
|---|---|---|
| 1. Initiation | UV light splits a chlorine molecule into two chlorine radicals. | \$\mathrm{Cl_2 \xrightarrow{h\nu} 2Cl^\bullet}\$ |
| 2. Propagation 1 | A chlorine radical takes a hydrogen from the alkane, forming HCl and an alkyl radical. | \$\mathrm{Cl^\bullet + RH \rightarrow HCl + R^\bullet}\$ |
| 3. Propagation 2 | The alkyl radical reacts with another chlorine molecule, giving the chlorinated product and another chlorine radical. | \$\mathrm{R^\bullet + Cl_2 \rightarrow RCl + Cl^\bullet}\$ |
| 4. Termination | Two radicals combine to form a stable molecule, ending the chain. | \$\mathrm{R^\bullet + Cl^\bullet \rightarrow RCl}\$ or \$\mathrm{Cl^\bullet + Cl^\bullet \rightarrow Cl_2}\$ |
Remember:
Question: Write the product of the reaction between butane (CH₃–CH₂–CH₂–CH₃) and chlorine under UV light. Show the structural formula and the overall equation.
Answer:
\$\mathrm{CH3–CH2–CH2–CH3 + Cl2 \xrightarrow{h\nu} CH3–CH2–CHCl–CH3 + HCl}\$