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Physical Properties of Organic

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One of the most revealing of all physical properties for a chemical substance is its boiling point. Boiling point reflects the strength of the intermolecular attractive forces that hold the molecules of a substance together in a condensed phase, and as such, it is useful to compare the boiling points for related compounds to see how structural differences account for the differences in intermolecular attractions. After briefly reviewing the nature of intermolecular attractive forces, this page will examine trends in boiling points for various groups of compounds to help the reader understand how size, shape, and functional group polarity affect boiling point.

Strongly Related Topics

Somewhat Related Topics

Glossary Terms
dipole moument electronegativity hydrogen bond
inductive effect lone pair electros normal alkane
polarity primary quarternary
secondary tertiary

Physical Properties of Organic Compounds

There are three types of intermolecular attractive forces: these are summarized in the table below:
Name Origin Present in...
1. London
Non-uniform motionof electrons
in atoms and molecules
...all substances
2. Dipole-dipole interactions Electrostatic attraction between (+) and (-) ends of molecules all molecules having polar covalent bonds
3. Hydrogen bonding An especially strong sort of dipole-dipole interaction ...molecules having H atom(s) covalently bound to N,O, or F
It is assumed that the reader is already somewhat familiar with the origins of these forces and the types of molecules in which they will appear. However, a brief review of these forces -- using organic molecules to illustrate the different types -- is probably useful.

1) London dispersion forces

In the present context, the bigger the molecule, the higher its boiling point. Thus, for a series of related compounds, the higher the molecular weight, the higher the boiling point. Note the trend for the first five straight-chain alkanes:
Compound Formula (mol. wt.) B.P.
methane CH4(16) -164C
ethane C2H6 (30) -88C
propane C3 H8 (44) -42C
n-butane C4 H10 (58) 0C
n-pentane C5 H12 (72) 36C
Self-test question #1 Based on the data in the table above, estimate the boiling point of hexane, the next member in this series.


Dipole-dipole interactions

Self-test question #2

Molecular nitrogen (N2) and carbon monoxide (CO) have identical molecular weights: 28g/mol. Which has the higher boiling point? Before looking at the answer, how do you know you are right?


Self-test question #3


Related reading in textbook (McMurry, Organic Chemistry, 4th ed.)

Related Computer Based Materials

Links to Related Internet Resources

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This page was prepared by Daniel Renaldi of the Penn State University, Schuylkill Campus, Spring 1996

Send questions, comments, or suggestions to:
Dr. Thomas H. Eberlein
Copyright © 1996 Thomas H. Eberlein

Version 1.1.7, 3/17/97