Physical organic chemistry is that branch of organic chemistry which deals with mechanistic studies of chemical reactions for the elucidation of structure and reactivity of organic compounds and reaction pathways. For this purpose scientists in this area of research use spectroscopic methods and techniques such as nuclear magnetic resonance and infra red spectroscopy in addition to mass spectroscopy in order to identify the structure of unstable and stable compounds. These spectroscopic techniques are powerful in the that sense. They are relatively new and other methods were used in previous time.
Acidity of organic compounds is one parameter that is being investigated in the organic lab. It is a measure of reactivity of chemical compounds in general and for organic chemicals in particular. For example, the Hammett equation was originally designed to measure the acidity of benzoic acid as a function of substituents on the para and meta positions of the benzene ring that bears the carboxyl group.
Hammett found a linear correlation between the rate of dissociation of benzoic acid as a function of the substituent in the ring. Namely, changing the substituent on the ring will have an effect on the dissociation of benzoic acid. Electron donating substituents such as amines enhance the dissociation of the benzoic acid while electron withdrawing substituents will retard the dissociation of the same acid.
This equation was then developed for other systems than benzoic acid which included delocalization effects. acidity according to Hammett is measured according to resonance effect of the substituents. Inductive effects are taken care in other equations that were developed separately such as the Taft equation.
In addition to acidity, other parameters are studied in physical organic chemistry such as aromaticity. Benzene is an unusual organic compound. It has an extraordinary stability. This is due to its aromatic character as compared with three ethylene molecules. Aromaticity obeys the 4n+2 rule where n denotes the number of pi electrons.
I myself did research on aromaticity of heterocylic compounds that contain nitrogen and sulfur and oxygen in addition to phosphorus. These compounds include furan, thiophene and pyrrole and phosphole. We found in this research that the degree of aromaticity depends on the extent of sharing of the electron pair on the heteroatom by the heterocyclic ring. In addition, it was found also that the degree of planarity of the ring will matter if the ring will be aromatic or not.
Phosphole for example was found to be less aromatic than the other heterocycles investigated due to its nonplanar structure. We found also that furan is not as aromatic as the other compounds and it behaves more like butadiene. This is so due to the conservation of the electron lone pair on the oxygen. This was also confirmed by the zero p orbital on the oxygen of the HOMO of furan. Therefore, there is no participation of this lone pair on oxygen in the aromaticity of the ring.
The other concept that physical organic chemistry deals with is the concept of nucleophilicity versus basicity. This means that whether a carbon center which bears an electron pair has affinity for a proton or alternatively for a carbon center that bears a positive charge. For example, a hydride ion has an electron pair which is not bonded. This ion has high affinity for a proton and less so for a carbon center that is positively charged.
Iodine ion in contrast has a high affinity for a carbon center and less so for a proton. It is said to be a good nucleophile. The concept of acid and base belongs to the concept of soft and hard ions. The stability of the ion will determine how the ion will react. Methyl carbanion for example is a very good base and less so as a nucleophile. This is due to its exceptional instability. It has a much higher affinity for a proton than for a carbon center.
Another concept that is discussed in physical organic chemistry is the carbocation and carbanion concepts. Carbocation is an organic species which has an electron deficient carbon which bears a positive charge. An example is a methyl carbocation. A carbanion is a negatively charged carbon species. An example is methyl anion.
Other concepts that are investigated in mechanistic organic chemistry are reactions mechanisms such as SN2 and SN1 reactions, in addition to electrophilic aromatic substitution.