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Difficulty: HardAlkanols: Classification, Reactions, Industrial Preparation, and Fermentation

An organic compound XX with the molecular formula C4H10O\text{C}_4\text{H}_{10}\text{O} resists oxidation when treated with acidified potassium dichromate(VI) (K2Cr2O7/H+\text{K}_2\text{Cr}_2\text{O}_7/\text{H}^+). When compound XX is heated with concentrated tetraoxosulfate(VI) acid (H2SO4\text{H}_2\text{SO}_4) at 170∘C170^\circ\text{C}, it undergoes dehydration to produce a major organic product YY. What is the IUPAC name of compound YY?

  1. A
    2-methylpropanal
  2. B
    But-2-ene
  3. 2-methylpropeneAnswer
  4. D
    But-1-ene

Answer

2-methylpropene
The compound resisting oxidation must be a tertiary alkanol because the hydroxyl-bearing carbon lacks an alpha-hydrogen atom. The only tertiary alkanol with formula C4H10O\text{C}_4\text{H}_{10}\text{O} is 2-methylpropan-2-ol. Subjecting 2-methylpropan-2-ol to intra-molecular dehydration using concentrated tetraoxosulfate(VI) acid at 170∘C170^\circ\text{C} removes water to form 2-methylpropene as the major alkene product.

Step-by-Step Solution

1
Determine the structural class of compound X from its resistance to oxidation.
Compound X is a tertiary alkanol.
Primary and secondary alkanols are readily oxidized by acidified potassium dichromate(VI), whereas tertiary alkanols resist mild oxidation because the carbon atom bonded to the hydroxyl group (−OH-OH) carries no hydrogen atoms.
2
Identify the specific isomer of formula C4H10O\text{C}_4\text{H}_{10}\text{O} corresponding to a tertiary alkanol.
Compound X is 2-methylpropan-2-ol, (CH3)3C-OH(\text{CH}_3)_3\text{C-OH}.
Among the four structural isomers of C4H10O\text{C}_4\text{H}_{10}\text{O} alkanols, only 2-methylpropan-2-ol is tertiary.
3
Determine the elimination product when 2-methylpropan-2-ol undergoes acid-catalyzed dehydration.
Dehydration yields 2-methylpropene, (CH3)2C=CH2(\text{CH}_3)_2\text{C=CH}_2.
Heating with concentrated H2SO4\text{H}_2\text{SO}_4 at 170∘C170^\circ\text{C} removes a molecule of water (the −OH-OH group and a hydrogen atom from an adjacent methyl group), forming an alkene.

Key Concept

Classification of alkanols based on oxidation behavior and acid-catalyzed dehydration to alkenes
Estimated Time:2m 0s
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Topics

Alkanols: Classification, Reactions, Industrial Preparation, and Fermentation

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