Organic Chemistry Exam Helper: Introduction to Organic Chemistry by Matthew Pasek

Author:Matthew Pasek [Pasek, Matthew]
Language: eng
Format: azw3
Published: 2017-09-17T04:00:00+00:00

KEY POINTS AND HINTS:

Stereoisomers are like conformation isomers, if you break down the molecule to a descriptive sentence, there’s no way of distinguishing between stereoisomers, compared to conformational isomers.

Stereoisomers for alkenes change physical properties, but for alkanes don’t change as much.

12. Molecular Attractive Forces

Recall again from general chemistry that there are several varieties of molecular attractive forces. Most molecular attractive forces are related to the polarity of molecules—how much one side of a molecule is charged compared to the other side. The polarity of a molecule is expressed in its dipole moment, which is a vector that describes the direction and strength of charge in a molecule. The dipole moment is the sum of individual bond dipole moments. In general, the stronger the dipole moment of a molecule, the stronger its intermolecular forces of attraction.

Intermolecular forces are those forces between separate molecules. Think of an interstate (in the US). These roads nominally take you across at least two states, sometimes more (and occasionally less- there are interstates in Hawaii! Let’s just ignore those). In contrast, intra- means within something, so intrastate is a road stuck in one state, and intramolecular forces are those that occur only within the molecule.

The dipole moment of a molecule is dependent on both its shape and on its functional groups. C-N single bonds and C-H bonds have the weakest dipoles and can generally be considered nonpolar. C-O and C=O bonds are moderately and very strong, respectively. C-X bonds can be quite strong, where X is a halogen, though C-I bonds are weaker than C-F, C-Cl, and C-Br. N-H and O-H bonds have a strong dipole. One of the strongest commonly encountered organic dipole moments is a C-N triple bond.

A special variety of molecular forces are hydrogen bonds. These are discussed in detail in section 13.

A variety of molecular forces important to organic chemistry are the induced dipoles. Induced dipoles happen as the electrons move around a molecule randomly, and another molecule happens to approach. These molecules then induce a dipole in each other and stick together better than they would if the electrons didn’t move around. This effect is also known as the London Dispersion Force. It is more important for big molecules than for small molecules, as it’s easier to induce a dipole somewhere on a big (or long) molecule than on a smaller molecule. The effect of these forces is that many molecules stick together more than would be predicted by their weight alone.

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