The pillars of chemistry

1st Pillar = We encounter atoms on the mole (macroscopic) scale

Chemistry is dictated by individual atoms interacting on the macroscopic scale

We must be able to deal with both scales

2nd Pillar = Systems in nature usually seek the lowest Energy possible

Almost all of chemistry is based on the interaction of charges seeking the lowest Energy

3rd Pillar = "Periodic Law"

arranged by atomic number, elements exhibit periodicity in their properties

elements grouped by properties

4th Pillar = electron motion in atoms is guarenteed

it occurs acoording to discrete functions, only at discrete energies

 “The Six Pillars of Organic Chemistry”

Electronegativity – in an example provided, going across a period  leads to increasing nucleophilicity with decreasing electronegativity. For example, nucleophilicity follows the following order:

Polar covalent bonding – when bonds form between atoms of unequal electronegativity, the resulting dipole has a positively charged terminus and a negatively charged terminus, which will provide insight into its preferred mode of chemical re-activity. So in the above example, the C-Cl bond is polarized whereas the C-H bonds (C and H having relatively similar electronegativities) are not, resulting in selective breakage of the carbon-halogen bond during the SN2 instead of the carbon-hydrogen bonds.

Steric effects – the rate of the SN2 is greatly affected by the presence of neighboring bulky groups. So the trend for the above reaction would be:

Inductive effects. The author gives the example of Markovnikoff’s rule as an example of inductive effects, where increasing substitution on carbon leads to increasing inductive stabilization of the carbocation [he also notes that hyperconjugation, a more fundamentally sound explanation, can be covered in the resonance section, below].

Resonance – resonance effects are widespread in organic chemistry. One example is that they explain the selective bromination of cyclohexene at the allylic position under free-radical conditions, versus competing bromination at the secondary (or vinylic positions). Another example is the planarity of peptide bonds due to the π donation of electrons into the carbonyl π* orbital.

Aromaticity – Aromaticity is an important driving force in chemical reactions and a powerful stabilizing influence on molecules. The decreased reactivity of benzene in bromination reactions versus, say, cyclohexene is an extension of resonance stabilization, which helps to explain why tryptophan is not considered a basic amino acid even though (like lysine) it contains a nitrogen – in tryptophan, the nitrogen lone pair is tied up in the π system.