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

London dispersion forces and dipole-dipole forces are subsets of Van der Walls forces, which themselves are the weakest intermolecular forces. Intermolecular forces directly determine properties like melting point, boiling point, and viscosity. London dispersion is most easily noticed in nonpolar molecules, where stronger IMFs don’t drown out the London dispersion. 

Overview

London dispersion forces are the weakest of the three types of intermolecular forces. London dispersion forces are actually what hold many substances together. Real-life examples include gasoline and petroleum jelly, which are mostly make of alkanes of varying lengths. Alkanes are non-polar and exhibit no hydrogen bonding, so Van der Waals London dispersion are the only IMFs they exhibit.

Temporary polarity resulting from unsymmetrical distribution of electrons creates an attractive force in otherwise nonpolar molecules like Br2. 

Br2Br2

Let’s say we’ve got a single dibromide molecule like the one above. The two atoms in the molecule are identical, so it might seem at first that the electrons will have a symmetrical distribution, but that’s an oversimplification. 

Br2 with partial chargesBr2 with partial charges

Electron density will actually oscillate—this happens because electrons aren’t point-objects; it might help to think about them as clouds of probability. Those partial charges will oscillate as electron density moves around; those partial positive charges result from lack of electron density. 

Of course, molecules don’t exist in isolation. Let’s look at what happens when there are more dibromide molecules in the area. Those temporary dipoles will actually induce dipoles in neighboring molecules. That makes sense if you think about it in terms of electron density. Electrons will repel each other, so a partial negative in one bromine will push electron density away in a neighboring dibromide. This results in a relatively stable attractive force between the partial negative and induced partial positive. 


Br2 conga lineBr2 conga line

The dibromides will actually line up with each other with opposite partial charges facing each other. This trend can continue indefinitely and isn’t limited to a single dimension like in the diagram above. 


Factors affecting strength

The strength of London dispersion forces is influenced by molecular size and molecular shape. Basically, the larger the molecule is, the more surface area there generally is for partial charges to develop. Based on that idea, which of the following molecules would you expect to have the higher boiling point? Remember that boiling point is directly determined by intermolecular forces.

propane vs pentanepropane vs pentane

Ding ding! The answer is pentane! The boiling point of propane is -42ºC and the boiling point of n-pentane is 36.1ºC. 

So that tackles the size… but what about shape? Let’s look at three molecules with 5 carbons: cyclopentane, n-pentane, and 2-methylbutane. Try to order them in order of increasing boiling point. Think about the ways that the molecules can arrange themselves to maximize contact. 

5-carbon molecules5-carbon molecules

Cyclic compounds can stack pretty easily since their sigma bonds don’t really rotate too much, but chain compounds do rotate around a lot. Branched chain hydrocarbons also rotate, but they have the extra substituents that prevent overlap.

 5-carbon molecules-stacking5-carbon molecules-stacking

The general order of increasing London dispersion forces, and therefore boiling points, is ring > straight chain > branched chain. The boiling points of the above compounds are as follows:

Molecule

Boiling Point (ºC)

Cyclopentane

49

n-pentane

36.1

2-methylbutane

27.8



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.