Chemistry

Problem

Three structural isomers with molecular formula

C_{4}H_{10}O

are listed below along with measured viscosities at

25\,^{\circ}\text{C}

Substance (condensed formula)	Dynamic viscosity (mPa·s)
1-Butanol ( $CH_{3}CH_{2}CH_{2}CH_{2}OH$ )	2.95
sec-Butanol ( $CH_{3}CH_{2}CH(OH)CH_{3}$ )	3.26
tert-Butanol ( $((CH_{3})_{3}C-OH)$ )	12.4

Answer the following questions. a. Part 1: (a) Rank the three isomers in order of increasing expected surface area available for dispersion interactions. Justify your ranking. b. Part 2: (b) Despite your answer in part (a), tert-butanol has the highest measured viscosity. Explain this apparent anomaly by discussing all relevant intermolecular forces. c. Part 3: (c) Predict which isomer would have the lowest vapor pressure at

25\,^{\circ}\text{C}

and justify your answer. d. Part 4: (d) The student replaces the

-OH

group in tert-butanol with

-OCH_{3}

, producing tert-butyl methyl ether. Predict qualitatively how the viscosity of the new compound will compare with that of tert-butanol and explain your reasoning.

Intuitive explanation

Viscosity measures how much a liquid resists flow, and it's exquisitely sensitive to hydrogen bonding and molecular shape. The three isomers have identical formulas but very different structures: 1-butanol is a long chain, sec-butanol is slightly branched, and tert-butanol is highly compact. The key insight is that tert-butanol's bulky, spherical shape creates a dense 3D hydrogen-bonding network that traps molecules together, while the linear isomers slip past each other more easily despite having similar $\text{OH}$ groups.

Intuitive explanation

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