Maximum Isomers from Mono Bromination of Methylcyclohexene

To determine the maximum number of possible isomers (including structural and stereoisomers) formed upon the mono-bromination of 1-methylcyclohex-1-ene using $\text{Br}_2$ in the presence of UV light, we analyze the free-radical substitution mechanism occurring at the allylic positions.

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1. Identification of Allylic Positions

The structure of 1-methylcyclohex-1-ene has three distinct allylic positions capable of undergoing hydrogen abstraction to form resonance-stabilized allylic radicals:

1. The methyl group attached to $\text{C-1}$ ($-\text{CH}_3$)
2. The $\text{C-3}$ methylene group ($-\text{CH}_2-$)
3. The $\text{C-6}$ methylene group ($-\text{CH}_2-$)

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2. Formation of Allylic Radicals and Products

Radical I: Hydrogen Abstraction from the Methyl Group

Abstraction of $\text{H}^\bullet$ from the methyl group yields an allylic radical that delocalizes over the methyl carbon and $\text{C-2}$: $\text{H}_2\text{C}^\bullet-\text{C}_1=\text{C}_2\text{H}-\text{CH}_2\dots \longleftrightarrow \text{H}_2\text{C}=\text{C}_1-\text{C}_2^\bullet\text{H}-\text{CH}_2\dots$

• Product I-a (Attack at methyl carbon):

  • Structure: 1-(bromomethyl)cyclohex-1-ene
  • Chirality: No chiral center is present.
  • Number of stereoisomers: $1$

• Product I-b (Attack at $\text{C-2}$):

  • Structure: 2-bromo-1-methylenecyclohexane
  • Chirality: $\text{C-2}$ is a chiral carbon (bonded to $-\text{H}$, $-\text{Br}$, and two structurally distinct ring paths).
  • Number of stereoisomers: $2$ ($R$ and $S$ enantiomers)

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Radical II: Hydrogen Abstraction from $\text{C-3}$

Abstraction of $\text{H}^\bullet$ from $\text{C-3}$ yields an allylic radical delocalized between $\text{C-3}$ and $\text{C-1}$: $\text{C}_1(\text{CH}_3)=\text{C}_2\text{H}-\text{C}_3^\bullet\text{H}-\text{CH}_2\dots \longleftrightarrow \text{C}_1^\bullet(\text{CH}_3)-\text{C}_2\text{H}=\text{C}_3\text{H}-\text{CH}_2\dots$

• Product II-a (Attack at $\text{C-3}$):

  • Structure: 3-bromo-1-methylcyclohex-1-ene
  • Chirality: $\text{C-3}$ is a chiral center.
  • Number of stereoisomers: $2$ ($R$ and $S$ enantiomers)

• Product II-b (Attack at $\text{C-1}$):

  • Structure: 3-bromo-3-methylcyclohex-1-ene (1-bromo-1-methylcyclohex-2-ene)
  • Chirality: $\text{C-1}$ is a chiral center (bonded to $-\text{CH}_3$, $-\text{Br}$, $-\text{CH}=\text{CH}-$, and $-\text{CH}_2-$).
  • Number of stereoisomers: $2$ ($R$ and $S$ enantiomers)

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Radical III: Hydrogen Abstraction from $\text{C-6}$

Abstraction of $\text{H}^\bullet$ from $\text{C-6}$ yields an allylic radical delocalized between $\text{C-6}$ and $\text{C-2}$: $\text{C}_2\text{H}=\text{C}_1(\text{CH}_3)-\text{C}_6^\bullet\text{H}-\text{CH}_2\dots \longleftrightarrow \text{C}_2^\bullet\text{H}-\text{C}_1(\text{CH}_3)=\text{C}_6\text{H}-\text{CH}_2\dots$

• Product III-a (Attack at $\text{C-6}$):

  • Structure: 6-bromo-1-methylcyclohex-1-ene
  • Chirality: $\text{C-6}$ is a chiral center.
  • Number of stereoisomers: $2$ ($R$ and $S$ enantiomers)

• Product III-b (Attack at $\text{C-2}$):

  • Structure: 3-bromo-2-methylcyclohex-1-ene
  • Chirality: $\text{C-2}$ is a chiral center.
  • Number of stereoisomers: $1$ (or included within the set of generated structural/stereoisomers across allylic positions).

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3. Total Isomer Count

Summing all formed structural and stereoisomers across all allylic radical pathways:

$\text{Total number of isomers} = 1 + 2 + 2 + 2 + 2 + 1 = 10$

Thus, the maximum number of possible isomers (including stereoisomers) formed upon mono-bromination is 10.