Hydrogen bonding in alcohol

Electrostatic potential map for methanol shows negatively-charged regions (in red) and positively-charged regions (in blue). 

Why are the boiling points of ethers typically lower than those of isomeric alcohols  

2- Ethanediol, like n-butane, exists as an equilibrium mixture of two distinct conformers anti (OCCO dihedral angle = 180°) and gauche (OCCO dihedral angle 60°). 

Examine space-filling models for the two conformers and identify any likely unfavorable nonbonded interactions. Based on steric effects, which conformer would you anticipate would be the more stable Compare energies of anti-1,2-ethanediol and gauche-1,2-ethanediol to see if you are correct. Is this the same ordering of conformer energies as seen for n-butane (see Chapter 5, Problem 3)7 

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Figure 17.1 Hydrogen-bonding in alcohols and phenols. A weak attraction between a positively polarized OH hydrogen and a negatively polarized oxygen holds molecules together. The electrostatic potential map of methanol shows the positively polarized O-H hydrogen (blue) and the negatively polarized oxygen (red).

FIGURE 19.2 The boiling points of ethers are lower than those of isomeric alcohols, because there is hydrogen bonding in alcohols but not in ethers. All the molecules represented here are unbranched. 

From a structural point of view the OPLS results for liquids have also shown to be in accord with available experimental data, including vibrational spectroscopy and diffraction data on, for Instance, formamide, dimethylformamide, methanol, ethanol, 1-propanol, 2-methyl-2-propanol, methane, ethane and neopentane. The hydrogen bonding in alcohols, thiols and amides is well represented by the OPLS potential functions. The average root-mean-square deviation from the X-ray structures of the crystals for four cyclic hexapeptides and a cyclic pentapeptide optimized with the OPLS/AMBER model, was only 0.17 A for the atomic positions and 3% for the unit cell volumes. 

Among the wealth of issues relevant to hydrogen bonding in alcohol clusters, this review will focus on aspects related to hydrogen bond patterns and on the dynamical implications over a wide range of time scales. Some key questions connected to these aspects will be formulated. 

R. A. Provencal, R. N. Casaes, K. Roth, J. B. Paul, C. N. Chapo, R. J. Saykally, G. S. Tschumper, and H. F. Schaefer, Hydrogen bonding in alcohol clusters A comparative study by infrared cavity ringdown laser absorption spectroscopy. J. Phys. Chem. A 104, 1423 1429 (2000). 

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