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Q6.19(xii):
How the following conversions can be carried out? (xii) But-1-ene to n-butyliodide

Solution :

Step 1: Analyzing the Molecular Transformation

The conversion of But-1-ene ($CH_3CH_2CH=CH_2$) to $n$-butyl iodide ($CH_3CH_2CH_2CH_2I$) involves two critical chemical requirements:

  • Regioselectivity: The terminal carbon (C1) must be functionalized, while the double bond is reduced to a single bond.
  • Anti-Markovnikov Addition: Since Markovnikov's rule dictates the addition of a nucleophile to the more substituted carbon, we must employ a peroxide-effect mechanism (Kharasch effect) to direct the halogen to the terminal position.

Step 2: Step-by-Step Reaction Mechanism

Step A: Anti-Markovnikov Addition of Hydrogen Bromide ($HBr$)

Direct hydroiodination ($HI$) does not proceed via the peroxide effect due to the thermodynamics of the iodine radical. Therefore, we first add $HBr$ in the presence of organic peroxides ($R_2O_2$) [Per the Kharasch Effect].

$CH_3CH_2CH=CH_2 + HBr \xrightarrow{Peroxide} CH_3CH_2CH_2CH_2Br$

Justification: The peroxide initiates a free-radical chain mechanism, ensuring the bromine atom attaches to the less substituted carbon atom [Mechanism: Free Radical Addition].

Step B: Finkelstein Reaction (Halogen Exchange)

To convert the primary alkyl bromide ($1^\circ$ bromide) to an alkyl iodide, we utilize the Finkelstein reaction. This is an $S_N2$ reaction using sodium iodide ($NaI$) in acetone.

$CH_3CH_2CH_2CH_2Br + NaI \xrightarrow{Acetone} CH_3CH_2CH_2CH_2I + NaBr \downarrow$

Justification: Sodium bromide ($NaBr$) is insoluble in dry acetone and precipitates out of the solution, which drives the equilibrium to the right [Per Le Chatelier's Principle and the nature of $S_N2$ displacement].

Step 3: Visual Representation of the Conversion

But-1-ene HBr / Peroxide 1-Bromobutane NaI / Acetone n-Butyl iodide

Step 4: Summary of Chemical Transformations

Step Reagents/Conditions Intermediate Product
1 $HBr$ / Benzoyl Peroxide $1$-Bromobutane
2 $NaI$ / Acetone (reflux) $n$-Butyl iodide

Final Solution: The conversion of But-1-ene to $n$-butyl iodide is achieved via the anti-Markovnikov addition of $HBr$ to form $1$-bromobutane, followed by a Finkelstein substitution reaction using $NaI$ in acetone.


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