Step-by-step mechanisms for the 10 important organic conversions from the Haloalkanes & Haloarenes chapter.
Board Exam Important
NEET / JEE
(i) Ethanol to But-1-yne
Ascent of Chain
$$ \text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{SOCl}_2} \text{CH}_3\text{CH}_2\text{Cl} $$
$$ \text{HC}\equiv\text{CH} \xrightarrow{\text{NaNH}_2} \text{HC}\equiv\text{C}^-\text{Na}^+ $$
$$ \text{CH}_3\text{CH}_2\text{Cl} + \text{NaC}\equiv\text{CH} \to \text{CH}_3\text{CH}_2\text{C}\equiv\text{CH} $$
Concept: Convert alcohol to alkyl halide. React with Sodium Acetylide (from Ethyne) to add two carbons and a triple bond.
(ii) Ethane to Bromoethene
Double Elimination
$$ \text{CH}_3\text{-CH}_3 \xrightarrow[\text{h}\nu]{\text{Br}_2} \text{CH}_3\text{CH}_2\text{Br} \xrightarrow{\text{alc. KOH}} \text{CH}_2=\text{CH}_2 $$
$$ \text{CH}_2=\text{CH}_2 \xrightarrow{\text{Br}_2/\text{CCl}_4} \text{Br-CH}_2\text{-CH}_2\text{-Br} \xrightarrow{\text{alc. KOH}} \text{CH}_2=\text{CH-Br} $$
Concept: Convert alkane to alkene, halogenate to get vicinal dihalide, then partial dehydrohalogenation gives vinyl bromide.
(iii) Propene to 1-Nitropropane
Anti-Markovnikov
$$ \text{CH}_3\text{CH}=\text{CH}_2 \xrightarrow{\text{HBr/Peroxide}} \text{CH}_3\text{CH}_2\text{CH}_2\text{Br} $$
$$ \text{CH}_3\text{CH}_2\text{CH}_2\text{Br} \xrightarrow{\text{AgNO}_2} \text{CH}_3\text{CH}_2\text{CH}_2\text{NO}_2 $$
Concept: Peroxide effect adds Br to the terminal carbon. AgNO2 is used specifically to get the Nitro alkane (KNO2 would give alkyl nitrite).
(iv) Toluene to Benzyl Alcohol
Side Chain
$$ \text{C}_6\text{H}_5\text{CH}_3 \xrightarrow[\text{h}\nu]{\text{Cl}_2} \text{C}_6\text{H}_5\text{CH}_2\text{Cl} \xrightarrow{\text{aq. KOH}} \text{C}_6\text{H}_5\text{CH}_2\text{OH} $$
Concept: Free radical substitution attacks the benzylic position. Nucleophilic substitution ($S_N2$) with aqueous KOH yields the alcohol.
(v) Propene to Propyne
Unsaturation
$$ \text{CH}_3\text{CH}=\text{CH}_2 \xrightarrow{\text{Br}_2/\text{CCl}_4} \text{CH}_3\text{CH(Br)CH}_2\text{Br} $$
$$ \text{CH}_3\text{CH(Br)CH}_2\text{Br} \xrightarrow{\text{alc. KOH}} \xrightarrow{\text{NaNH}_2} \text{CH}_3\text{C}\equiv\text{CH} $$
Concept: Convert alkene to vicinal dihalide. Strong bases (KOH then NaNH2 ) perform double elimination to create the triple bond.
(vi) Ethanol to Ethyl Fluoride
Swarts Reaction
$$ \text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{SOCl}_2} \text{CH}_3\text{CH}_2\text{Cl} $$
$$ \text{CH}_3\text{CH}_2\text{Cl} \xrightarrow{\text{AgF or Hg}_2\text{F}_2} \text{CH}_3\text{CH}_2\text{F} $$
Concept: Direct fluorination is explosive. We use Swarts Reaction (Halide exchange with heavy metal fluorides) to synthesize alkyl fluorides.
(vii) Bromomethane to Propanone
Grignard
$$ \text{CH}_3\text{Br} \xrightarrow{\text{KCN}} \text{CH}_3\text{CN} $$
$$ \text{CH}_3\text{CN} + \text{CH}_3\text{MgBr} \to \text{Complex} \xrightarrow{\text{H}_3\text{O}^+} \text{CH}_3\text{COCH}_3 $$
Concept: Convert to nitrile (2 carbons). React with methyl Grignard reagent (provides 3rd carbon) to form a ketone upon hydrolysis.
(viii) But-1-ene to But-2-ene
Saytzeff Rule
$$ \text{CH}_3\text{CH}_2\text{CH}=\text{CH}_2 \xrightarrow{\text{HBr}} \text{CH}_3\text{CH}_2\text{CH(Br)CH}_3 $$
$$ \text{CH}_3\text{CH}_2\text{CH(Br)CH}_3 \xrightarrow{\text{alc. KOH}} \text{CH}_3\text{CH}=\text{CHCH}_3 $$
Concept: Add HBr (Markovnikov) to move the leaving group to the secondary carbon. Elimination yields the more substituted, stable alkene (But-2-ene) as the major product.
(ix) 1-Chlorobutane to n-Octane
Wurtz Reaction
$$ 2 \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{Cl} + 2\text{Na} \xrightarrow{\text{Dry Ether}} \text{n-Octane} + 2\text{NaCl} $$
Concept: Classic Wurtz reaction couples two alkyl halide molecules to double the carbon chain.
(x) Benzene to Biphenyl
Fittig Reaction
$$ \text{C}_6\text{H}_6 \xrightarrow{\text{Cl}_2/\text{FeCl}_3} \text{C}_6\text{H}_5\text{Cl} $$
$$ 2 \text{C}_6\text{H}_5\text{Cl} + 2\text{Na} \xrightarrow{\text{Dry Ether}} \text{C}_6\text{H}_5\text{-C}_6\text{H}_5 + 2\text{NaCl} $$
Concept: Halogenate benzene first. The Fittig Reaction couples two aryl halides using Sodium metal.
Essential Reagents Cheat Sheet
SOCl₂
Alcohol → Chloride
alc. KOH
Elimination (-HX)
HBr/Peroxide
Anti-Markovnikov
Na/Ether
Wurtz/Fittig Coupling
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