The major monochlorination product formed upon treatment of 1,1,2-trimethyl cyclopentane with Cl2/hv is 2-Chloro-1,1,2-trimethyl cyclopentane.
Understanding Monochlorination via Free Radical Halogenation
Monochlorination, specifically using Cl2 in the presence of light (hv) or heat, proceeds via a free radical mechanism. This reaction involves the selective replacement of a hydrogen atom in an alkane with a chlorine atom. The selectivity of this reaction is primarily governed by the stability of the transient carbon radical formed during the propagation step.
Formation of the Major Product
In the case of 1,1,2-trimethyl cyclopentane, the molecule contains various types of hydrogen atoms, each with differing reactivity towards radical abstraction:
- Tertiary (3°) Hydrogens: Attached to a carbon atom bonded to three other carbon atoms.
- Secondary (2°) Hydrogens: Attached to a carbon atom bonded to two other carbon atoms.
- Primary (1°) Hydrogens: Attached to a carbon atom bonded to one other carbon atom (typically found in methyl groups).
The general order of stability for carbon radicals is tertiary > secondary > primary. Consequently, the abstraction of a hydrogen atom that leads to the most stable radical intermediate is kinetically favored. For chlorination, while less selective than bromination, there is still a clear preference for abstracting tertiary hydrogens.
Let's analyze the 1,1,2-trimethyl cyclopentane structure:
- C1: This carbon is quaternary (bonded to four other carbons – two methyls and two ring carbons). It has no hydrogen atoms to abstract.
- C2: This carbon is tertiary (bonded to three other carbons – one methyl and two ring carbons). It has one tertiary hydrogen atom.
- C3, C4, C5: These are secondary carbons within the cyclopentane ring, each having two secondary hydrogen atoms.
- Methyl groups (on C1 and C2): These contain primary hydrogen atoms.
The tertiary hydrogen at the C2 position is the most reactive site for radical abstraction due to the formation of a more stable tertiary radical intermediate. Therefore, the chlorine radical (Cl•) preferentially removes this hydrogen atom, forming a tertiary carbon radical at C2. This radical then reacts with a molecule of Cl2 to yield the chlorinated product and regenerate another chlorine radical, continuing the chain reaction.
Reactivity Order for Chlorination
While chlorination is known for its lower selectivity compared to bromination, the relative reactivity of different C-H bonds is still observable:
| Type of C-H Bond | Relative Reactivity (Chlorination) |
|---|---|
| Primary (1°) | 1 |
| Secondary (2°) | ~3.8 |
| Tertiary (3°) | ~5 |
Given these relative reactivities, the tertiary hydrogen at C2 in 1,1,2-trimethyl cyclopentane is the most susceptible to abstraction, leading predominantly to 2-Chloro-1,1,2-trimethyl cyclopentane.
Summary of the Reaction
The major monochlorination of 1,1,2-trimethyl cyclopentane with Cl2/hv can be summarized as follows:
- Starting Material: 1,1,2-trimethyl cyclopentane
- Reagents: Cl2/hv (Chlorine gas and ultraviolet light)
- Reaction Type: Free radical halogenation
- Key Principle: Selective abstraction of the most reactive hydrogen atom (tertiary H) due to the formation of the most stable radical intermediate.
- Major Product: 2-Chloro-1,1,2-trimethyl cyclopentane
