Chem 226 / Dr. Rusay

Review: Ionic bonding, Covalent bonding and Electronegativity.

The forces that bind atoms can vary considerably and can be related to the differences in the electronegativities of the atoms. Very large differences relate to very strong bonds (higher in energy) between the atoms. These are ionic bonds. For cesium flouride, CsF, which has a bond that is among the highest in ionic character the difference is 3.2: 4.1 (e.n. F) minus 0.9 (e.n. Cs). In diatomic molecules such as chlorine, the difference is zero. The atoms are bound, but there is no ionic character. The bond is purely covalent, i.e. where the electrons are shared equally by both atoms. Ionic character is represented by the more electronegative atom having a stronger share of the bonding electron(s). The scale from 3.2 to 0 is one of relative bond strength with a spectrum of different shades of bonding character across it. Bonds typically will range somewhere along the scale and are not one of the extremes.

In organic molecules, the bonds will be for the most part covalent, but they will not usually be purely covalent like diatomic chlorine. They will have small differences. These differences polarize the bond and can be represented by a partial positive charge (⁺ ) for the less electronegative atom and a partial negative charge (^-) for the more electronegative. In general chemistry, you were asked to determine whether simple, covalently bonded molecules such as HCl were polar or non-polar. This question is very important in complicated organic molecules, but it is more difficult to answer since the overall polarity of the molecule will be determined by sum of all of the polar bond effects in 3-dimensions. Structure and bond polarity must both be considered.

At this point we will only consider relatively simple molecules. We will then look at individual bonds in various functions and determine their polarities in a qualitative sense. Finally as we consider each function in detail, we will look at the polarity of the entire molecular.

The following table provides the electronegativities of the key elements that will appear during the course of study.

H 2.2

Note: Using the experimental electronegativity values provides some anomalies, particularly for Iodine. Remember that the smaller the difference in e.n., the weaker the bond. The observed acid strengths HI > HBr > HCl > HF are consistent with this. However, the chemistry of organo-iodides suggest that Iodine is less electronegative than Carbon. Therefore, for calculation purposes use a value of 2.6 for Iodine.

Hydrocarbons tend to be non-polar molecules. Although there is an electronegativity difference of 0.3 for a C-H bond, the geometry of the hydrocarbon molecules generally produce a net effect of zero or near zero. For most all cases, the carbon backbone of a molecule does not need to be considered in determining the overall polarity of a molecule. Substituents and functionalities can then be focused on. For example, comparing methane CH₄ and the methyl halides. Methane is non-polar with a dipole moment equal to zero (=0) but the methyl halides are polar and since the electronegativities are greater than carbon the partial charge on the halide is drawn as ^- (the e.n. difference is semi-quantitative and is not expressed in drawings numerically) as in the following example where X- generically represents the halides.

Assignment:

On the back of the Worksheet: Organic Molecules: Functions / Polarity / Formal Charge, (Worksheet Not included On-line.) draw the 4 different methyl halide structures in 3-d with polarity notations included as above. List them in order of increasing chemical reactivity. Reactivity is based on breaking bonds and/or making bonds. In this case consider the ease of breaking bonds as the criteria for relative chemical reactivity.

Review: Formal Charge

Solomons, pp. 12-15. Table 1.3

Within a neutral molecule there may be atoms with formal charges. In such a case there must be an equal balance between the positive and negative charges. An understanding of the formal charges related to atoms is as important as polarity in determining the sites within a molecule that might serve as "reactive centers", where bonds can be made or broken. It is most important that you very quickly recognize polarity and formal charge in structures. The rules for calculating formal charge are cumbersome and time consuming. You are not expected to use the calculation, but you are expected to recognize polar and charged atoms in a molecular structure. Table 1.3 may be more useful than the calculation in helping pick out the important structural patterns.

In Worksheet #2: Organic Molecules: Functions / Polarity / Formal Charge, there are a variety of neutral compounds. After completing it you are expected to be expert in correlating polarity, formal charge and function. The Worksheet includes all free pairs of electrons drawn in which is not normally the case. They are most often omitted. Worksheet #3: Organic Molecules (II) : Charged Species is a continuation of Worksheet #2 but with non-neutral entities that can be cationic or anionic. (Worksheet Not included On-line.)

© Copyright 1997 R.J. Rusay