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Name: Kelsey
Status: student
Grade: 9-12
Location: NE
Country: N/A
Date: April 2006

Question:
Which is more polar water or methanol? How do you know? How can you find out?



Replies:
Kelsey,

Water is more polar than methanol, and I believe is THE most polar solvent. Polarity of a solvent is proportional to the overal dipole of the molecule and dielectric constant, which can be found in the CRC Handbook, or charts like the one listed on this website:

http://www.wcrl.ars.usda.gov/cec/java/solvents.htm

Matt Voss


Unfortunately, "polarity" is one of those chemical terms that is often used carelessly and/or ambiguously. For example, liquid 'A' has a higher boiling (melting) point than liquid 'B'. Therefore, 'A' is more polar than 'B'. In reality boiling (melting) points are a function of many factors. To confuse matters more, "polarity" is sometimes confused with the term "polarizability" which resembles the term "polarity" but is entirely different. First, I will describe "polarizability" although that is not what you asked about, but we need to get it "out of the way".

Polarizability describes the response a molecule experiences when subjected to an external electric field. It pops up in the description of many physical phenomena -- index or refraction, dielectric constant, long range electronic interactions such as van der Waals constants, Raleigh scattering -- is a short list. Qualitatively, it measures "how squishy" the electrons in an atom or molecule are. So for example SCS (carbon disulfide) and OCO (carbon dioxide) are both linear molecules so neither have an electric dipole moment, but the polarizability of SCS and OCO is 8.74 and 2.911, respectively. The units of polarizability have the curious units of 10^-24 cm^3 -- the same units as volume. But that is a result of some fortuitous cancellation of units. Carbon disulfide is a lot more "squishy" than carbon dioxide. The polarizability of water and methanol is 1.45 and 3.29 in the same curious units, respectively. So methanol is more electronically "squishy" than water. Even atoms have "polarizabilities" even though they are spherically symmetric. For example, He and Xe have polarizabilities of 0.2 and 4.0 respectively (again in units of 10^-24 cm^3).

Now on to your question. More commonly, the term "polarity" refers to the permanent asymmetrical distribution of electric charge in a molecule. Linear and/or spherically symmetric molecules have zero dipole moment, as a result of their high symmetry. So atoms and linear molecules have zero permanent electric dipole moments. In the case of water and methanol the dipole moments are 1.855 and 1.70 respectively. The units here are in Debye units. In SI units 1 D = 3.336x10^-30 Coulomb*meters (qualitatively "charge" x "distance). The conversion is so one sided because the Coulomb is a large amount of charge relatively speaking and the meter is a large distance on the molecular scale. So water has the larger permanent electric dipole moment but the average electron cloud is more rigid than methanol as reflected by the smaller polarizability of water vs. methanol.

The CRC Handbook of Chemistry and Physics has rather large tables of both electric dipole moments and polarizabilities. In addition, a "Google" search on those terms will lead you to discussions from the very basic to the very complicated, depending upon how deeply you wish to probe the topics.

Vince Calder



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