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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 requires the transformation of But-1-ene ($CH_3-CH_2-CH=CH_2$) to n-butyl iodide ($CH_3-CH_2-CH_2-CH_2-I$).

  • The reactant is an alkene with 4 carbon atoms.
  • The product is a primary alkyl halide with the iodine atom attached to the terminal carbon ($C_1$).
  • This is an anti-Markovnikov addition of hydrogen iodide ($HI$) to the alkene.

Step 2: Selecting the Reaction Mechanism

Direct addition of $HI$ to an unsymmetrical alkene follows Markovnikov's Rule [per the mechanism involving the formation of the most stable carbocation], which would place the iodine on $C_2$ (forming 2-iodobutane). To achieve anti-Markovnikov orientation, we must employ the Peroxide Effect (also known as the Kharasch Effect).

Step 3: Execution via Free Radical Addition

The reaction proceeds via a free radical mechanism in the presence of an organic peroxide (e.g., benzoyl peroxide). Note: While the peroxide effect is typically effective for $HBr$, $HI$ does not undergo this reaction effectively because the step involving the addition of the iodine radical to the double bond is endothermic [per Thermodynamic Analysis of Bond Dissociation Energies]. Therefore, the synthesis follows a two-step sequence: Hydroboration-Oxidation followed by substitution.

Step 4: The Two-Step Synthetic Pathway

Step A: Hydroboration-Oxidation to yield n-butanol

But-1-ene reacts with Diborane ($BH_3)_2$ followed by alkaline hydrogen peroxide ($H_2O_2/OH^-$) to yield the primary alcohol.

$CH_3-CH_2-CH=CH_2 + (BH_3)_2 \xrightarrow{THF} CH_3-CH_2-CH_2-CH_2-BH_2$

$CH_3-CH_2-CH_2-CH_2-BH_2 \xrightarrow{H_2O_2, OH^-} CH_3-CH_2-CH_2-CH_2-OH$ (n-butanol)

Step B: Nucleophilic Substitution (Finkelstein reaction or Halogenation)

Convert the alcohol to an alkyl halide, then substitute with iodide, or react directly using red Phosphorus and Iodine.

$CH_3-CH_2-CH_2-CH_2-OH + P + I_2 \rightarrow CH_3-CH_2-CH_2-CH_2-I + H_3PO_3$

Visual Representation of Transformation

But-1-ene 1. BH3, THF 2. H2O2, OH- Butan-1-ol P / I2 n-butyl iodide

Reaction Summary Table

Reagent/Catalyst Intermediate/Product Rationale
$(BH_3)_2$ / $H_2O_2$ n-Butanol Anti-Markovnikov hydration of alkene
$P + I_2$ n-Butyl iodide Alcohol to Alkyl Halide conversion

Final Solution: The conversion is achieved via a two-step process: (1) Hydroboration-Oxidation of But-1-ene to produce Butan-1-ol, followed by (2) reaction of the alcohol with Phosphorus and Iodine ($P/I_2$) to yield n-butyl iodide.


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