University:
California State University, San Marcos
Course:
CHEM 160 | General Chemistry II
Academic year:

2019
Views:

276

Pages:

4
Author:

Hallie Randolph

● Product: Pinacolone (3,3-dimethylbutan-2-one) ● Catalyst: Concentrated sulfuric acid (H₂SO₄) Reaction Summary: Mechanism of the Pinacol Rearrangement 1. Protonation of Hydroxyl Group ○ One -OH group gets protonated by H₂SO₄, making it a good leaving group (water). 2. Loss of Water ○ Water leaves, forming a carbocation at one of the carbon atoms. 3. Alkyl Shift (Rearrangement) ○ A methyl group migrates to stabilize the carbocation, forming a more stable tertiary carbocation. 4. Deprotonation ○ Deprotonation leads to the carbonyl group formation (C=O), producing the ketone pinacolone. Qualitative Tests Used ● Periodic Acid Test: ○ Tests for 1,2-diols (like pinacol). ○ Positive result: Formation of a white precipitate (silver iodate) means a diol was present. ● 2,4-DNP Test: ○ Tests for aldehydes/ketones (like pinacolone). ○ Positive result: Formation of a yellow-orange precipitate indicates a carbonyl group (C=O) is present. Calculations: Theoretical Yield and % Yield ● Theoretical Yield Steps: 1. Calculate moles of pinacol used: (Molar mass of Pinacol = 118.17 g/mol) 2. 1 mole of pinacol yields 1 mole of pinacolone. 3. Calculate grams of pinacolone expected: (Molar mass of Pinacolone = 100.16 g/mol) ● % Yield: IR Analysis: How to Tell If Reaction Worked ● Key Peaks for Pinacolone: ○ Strong C=O stretch around 1715 cm⁻¹ → carbonyl group (indicates ketone formation). ○ No broad O-H stretch (~3300 cm⁻¹) → shows the diol is gone. ● If Reaction is Successful: ○ You should see a strong C=O peak and no O-H peak. ○ If both are present, incomplete reaction or impure product. ● Purity: ○ Pure pinacolone will have a clean, sharp C=O peak with minimal/no extra peaks for alcohol (O-H). ○ Multiple extra peaks might suggest impurities (leftover pinacol or water). Synthesis of Banana Oil Overall Reaction Equation Reactants: ● Isopentyl alcohol (3-methyl-1-butanol) ● Acetic acid (ethanoic acid) Catalyst: ● Concentrated sulfuric acid (H₂SO₄) Product: ● Isopentyl acetate (banana oil) ● Water (byproduct) Balanced Reaction: Isopentyl alcohol + Acetic acid → Isopentyl acetate+Water Mechanism: Fischer-Esterification Key steps (nucleophilic acyl substitution): ● Protonation of the carbonyl oxygen (by H₂SO₄). ● Nucleophilic attack by the alcohol on the carbonyl carbon. ● Proton transfers rearrange bonds. ● Water (H₂O) leaves. ● Deprotonation to regenerate the catalyst and form the ester. Running the Reaction + Purification Week 1 (Synthesis): ● Measure and mix 5.0 mL isopentyl alcohol + 7.0 mL acetic acid. ● Add 1.0 mL concentrated H₂SO₄ carefully. ● Reflux for 60 minutes. ● Cool to room temperature. Work-up Steps (washing in separatory funnel): ● Water Wash: Removes soluble impurities (mostly acetic acid). ● 5% Sodium Bicarbonate (NaHCO₃) Wash: Neutralizes and removes residual acid (CO₂ bubbling is expected). ● Saturated Sodium Chloride (NaCl) Wash: "Salting out" — preliminary drying of organic layer by reducing water solubility. Week 2 (Purification): ● Dry the organic layer with anhydrous sodium sulfate. ● Distill the crude product carefully (collecting fraction around 143°C). ● Collect purified banana oil. Purpose of Each Wash During Work-Up