9701 · 13.2
Characteristic organic reactions
Different families of organic molecules have predictable ways of reacting, known as characteristic reactions. By understanding these core patterns—substitution, addition, and oxidation—we can predict how molecules will transform.
Need to know
What you need to know
- **Initiation:** The reaction begins when UV light causes homolytic fission of a halogen molecule, creating two highly reactive halogen free radicals. e.g., $Cl_2 \xrightarrow{UV} 2Cl\cdot$
- **Propagation:** This is a two-step chain reaction. A chlorine radical attacks an alkane, forming HCl and an alkyl radical ($CH_4 + Cl\cdot \rightarrow \cdot CH_3 + HCl$). This alkyl radical then attacks another chlorine molecule, forming the product and regenerating a chlorine radical ($ \cdot CH_3 + Cl_2 \rightarrow CH_3Cl + Cl\cdot $).
- **Termination:** The reaction stops when two free radicals collide and combine, removing radicals from the system. e.g., $Cl\cdot + Cl\cdot \rightarrow Cl_2$ or $\cdot CH_3 + \cdot CH_3 \rightarrow C_2H_6$ or $\cdot CH_3 + Cl\cdot \rightarrow CH_3Cl$.
Explanation
Organic Reactions: The Basic Patterns
- Free-radical substitution: Alkanes are unreactive, but UV light can break a halogen molecule into highly reactive free radicals, starting a chain reaction that substitutes a hydrogen atom.
- Electrophilic addition: The electron-rich double bond in an alkene attracts electrophiles (electron-pair acceptors), breaking the pi bond to form new single bonds as the electrophile adds across the double bond.
- Nucleophilic substitution: The polar C-Halogen bond in a halogenoalkane is attacked by a nucleophile (electron-pair donor), which replaces the halogen atom. The mechanism (Sₙ1 or Sₙ2) depends on the halogenoalkane's structure.
- Oxidation: Alcohols can be oxidised using an oxidising agent like acidified potassium dichromate(VI). The product depends on whether the alcohol is primary, secondary, or tertiary, and the reaction conditions used.