Relative Acidities Of Alcohols - In Aqueous Solution

Alcohols have acidic character as the react with active metals like sodium or potassium liberating hydrogen. For example,

C2H5OH + Na → C2H5ONa + ½H2

However, alcohols are weak acids. This is because they have an electron-releasing alkyl group (+I effect) which increases electron density around oxygen so that the release of a proton is rendered difficult.

Acidic character of alcohols shows the following order:

primary alcohol > secondary alcohol > tertiary alcohol

The acidic character of alcohols depends on the release of H+ from O–H. The +I effect increases  from primary alcohols (having one alkyl group) to secondary alcohols (having two alkyl groups) to tertiary alcohols (having three alkyl groups).

As a result, in tertiary alcohols, the release of a proton is most hindered making them the weakest acids of the three classes of alcohols.

In the gas phase, order of acidity is the exact opposite of that given above. Currently, this ought to be beyond the scope of the CAPE syllabus.

Phenyl Radical - Structure And Reason For Reactivity

The group derived by loss of an H from benzene is a phenyl group abbreviated Ph.

The phenyl radical (C6H5·) is the prototypical σ-type aryl radical and one of the most common aromatic building blocks for larger ring molecules. Using a combination of rotational spectroscopy of singly substituted isotopic species and vibrational corrections calculated theoretically, an extremely accurate molecular structure has been determined.

Figure  1. Side-by-side comparison of the structures of benzene and the phenyl radical.

The phenyl radical (C6H5·) is a highly reactive species formed by the homolytic cleavage of a C–H bond in benzene (Figure 1), the prototypical aromatic  compound. It is one of the most common aromatic radicals, and plays a central role in many reactions, ranging from astronomy to combustion and biochemistry.

As the simplest aryl radical, it also serves as the benchmark for computational investigations of larger, open-shell ring molecules.

The reactivity of aryl radicals is due to the localization of the unpaired electron in a σ-type orbital at the C–H cleavage site, as indicated by the very high C–H bond dissociation energy of benzene (465 ± 3 kJ mol-1).