Ch. 2 - Molecular RepresentationsWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins
Johnny Betancourt

There are two types of Van der Waals forces: London dispersion and dipole-dipole interactions. Van der Waals and other intermolecular forces like hydrogen bonding have observable effects like boiling and melting points.


London dispersion:

London dispersion is most easily observed in nonpolar molecules, and it’s sometimes called dipole-induced dipole. Electrons aren’t exactly point particles; they’re closer to clouds of probability. Electron density in a neutral compound can fluctuate, and that fluctuation can result in temporary polarization.  The images below show what happens:

Electron-Density fluctuations

Stage 1Stage 1

In stage 1, the electron density is pretty evenly distributed in both uncharged molecules.

Stage 2Stage 2

In stage 2, the electron density of the molecule on the left fluctuated, resulting in partial charges developing on opposite sides of the molecule. We know electrons repel each other, and the extra electron density is repulsive to the neighboring molecule’s electrons. This creates a dipole. 

Stage 3Stage 3

Stage 3 shows the attraction between the partial negative charge and the induced partial positive. This attraction (between two nonpolar molecules) is sometimes referred to as the Debye force.

Bonus Biology knowledge: geckos are actually able to walk on featureless surfaces like glass thanks to London dispersion forces. Look at that! Just another example of nature being super cool. Here’s a poorly drawn gecko for your viewing pleasure:

Poorly drawn geckoPoorly drawn gecko

Dipole-Dipole interactions:

Dipole dipole interactions are found in polar molecules, i.e. those with permanent dipole moments. These interactions are generally stronger than London dispersion because of the permanence of the dipole. The attractive force between two permanent dipoles is sometimes referred to as the Keesom force. Let’s look at acetone:

Acetone dipole-dipoleAcetone dipole-dipole

Acetone has an electronegative oxygen attached to a relatively electropositive carbon, and this bond results in a polar bond. The oxygen develops a partial negative while the carbon develops a partial positive. Since there’s more than one acetone molecule in solution, they can actually stack like in the image above. The oxygens will match up with the carbons according to polarity.


Johnny Betancourt

Johnny got his start tutoring Organic in 2006 when he was a Teaching Assistant. He graduated in Chemistry from FIU and finished up his UF Doctor of Pharmacy last year. He now enjoys helping thousands of students crush mechanisms, while moonlighting as a clinical pharmacist on weekends.