Outline a synthesis of propene from each of the following:
a. Propyl chloride
b. Isopropyl chloride
c. Propyl alcohol
d. Isopropyl alcohol
The correct answer and explanation is:
To synthesize propene (CH₃–CH=CH₂) from various precursors, we must eliminate substituents (such as halides or hydroxyl groups) to form a double bond via elimination reactions. Here’s how it can be done from each compound:
a. Propyl chloride (CH₃–CH₂–CH₂Cl)
Type of reaction: Dehydrohalogenation (E2)
Reagents: Strong base like KOH or NaOH in ethanol, heat
Reaction:
CH₃–CH₂–CH₂Cl + KOH → CH₃–CH=CH₂ + KCl + H₂O
Explanation:
Propyl chloride is a primary alkyl halide. Under strong basic conditions (e.g., hot alcoholic KOH), an E2 elimination occurs, removing the β-hydrogen and the halide, forming propene.
b. Isopropyl chloride ((CH₃)₂CHCl)
Type of reaction: Dehydrohalogenation (E2 or E1)
Reagents: KOH or NaOH in ethanol, heat
Reaction:
(CH₃)₂CHCl + KOH → CH₃–CH=CH₂ + KCl + H₂O
Explanation:
Isopropyl chloride is a secondary alkyl halide. It can undergo E2 (or E1 in some cases) elimination with hot alcoholic base. The elimination of H and Cl leads to propene.
c. Propyl alcohol (CH₃–CH₂–CH₂OH)
Type of reaction: Dehydration (acid-catalyzed)
Reagents: Concentrated H₂SO₄ or H₃PO₄, heat (~170°C)
Reaction:
CH₃–CH₂–CH₂OH → CH₃–CH=CH₂ + H₂O
Explanation:
In the presence of a strong acid and heat, primary alcohols undergo E1 or E2 dehydration to form alkenes. For 1-propanol, elimination yields propene.
d. Isopropyl alcohol ((CH₃)₂CHOH)
Type of reaction: Acid-catalyzed dehydration
Reagents: Concentrated H₂SO₄, heat (~170°C)
Reaction:
(CH₃)₂CHOH → CH₃–CH=CH₂ + H₂O
Explanation:
Secondary alcohols dehydrate readily with acid and heat to form alkenes via E1 mechanism. The OH group is protonated, leaving as water, and a carbocation forms, followed by loss of a β-hydrogen, giving propene.
Conclusion
Each compound undergoes elimination reactions (either E1 or E2, depending on structure) to form propene. Halides are eliminated with a strong base, while alcohols require acid and heat for dehydration. These are classic organic transformations used in synthetic chemistry to construct alkenes.