Química Orgânica 1 · Reações radicalares e seletividade
1-Chloropropane and 2-chloropropane from propane
Condições: Cl₂, hν
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Initiation: split Cl₂ with light
Light breaks the Cl–Cl bond homolytically into two chlorine radicals. Two alkane molecules and two more Cl₂ molecules are drawn so that both product-forming branches can be followed.
As setas curvas
- One electron of the Cl–Cl bond stays on the first atom.
- The other electron of the Cl–Cl bond stays on the second atom, so the bond splits evenly into two radicals.
Propagation: abstract a primary hydrogen
One chlorine radical removes a hydrogen from a terminal CH3, giving the primary propyl radical. Chlorine abstraction is exothermic with an early transition state, so the primary C–H reacts almost as readily as a secondary one.
As setas curvas
- The unpaired electron on chlorine supplies one electron of the new bond to hydrogen.
- One electron of the primary C–H bond pairs with it to complete the new bond to hydrogen.
- The other primary C–H electron stays behind as the unpaired electron of the new radical.
Propagation: abstract a secondary hydrogen
The other chlorine radical removes a hydrogen from the central CH2 of a second propane, giving the more stable isopropyl radical. Per hydrogen this is only about four times faster than primary abstraction.
As setas curvas
- The unpaired electron on chlorine supplies one electron of the new bond to hydrogen.
- One electron of the secondary C–H bond pairs with it to complete the new bond to hydrogen.
- The other secondary C–H electron stays behind as the unpaired electron of the new radical.
Predict the ratio
Product ratio = (number of hydrogens) × (relative reactivity per hydrogen). For Cl•, secondary : primary ≈ 3.9 : 1, so 2 × 3.9 : 6 × 1 ≈ 57 : 43.
Propagation: the propyl radical takes chlorine
The first carbon radical attacks Cl₂, forming its C–Cl bond and releasing a chlorine radical that continues the chain.
As setas curvas
- The unpaired electron on the propyl carbon supplies one electron of the new bond to Cl.
- One electron of the Cl–Cl bond completes that new bond.
- The other Cl–Cl electron stays on the far Cl, regenerating the chain-carrying radical.
Propagation: the isopropyl radical takes chlorine
The second carbon radical reacts the same way with another Cl₂. Both chlorine-transfer steps are fast and exothermic; the product ratio was already fixed when the hydrogens were removed.
As setas curvas
- The unpaired electron on the isopropyl carbon supplies one electron of the new bond to Cl.
- One electron of the Cl–Cl bond completes that new bond.
- The other Cl–Cl electron stays on the far Cl, regenerating the chain-carrying radical.
1-Chloropropane, 2-chloropropane and HCl
Both monochlorides form. Because chlorine is unselective, the ratio is close to the statistical 6:2 corrected by only a modest preference for the secondary hydrogens.
O que você vai praticar
Draw both branches of propane chlorination, primary and secondary hydrogen abstraction, and explain why chlorine gives a mixture of 1-chloropropane and 2-chloropropane.
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