'A' Level Chemistry Problem Analysis: Reaction and Solubility of Phenol

Thought process:

(a) With Br2 (in CCl4), and at room temperature, phenol is mono-brominated at either the ortho or para positions (since the OH substituent is electron donating by resonance). With Br2 (aq) and at room temperature however, phenol is tri-brominated at the ortho and para positions (2,4,6 carbon atoms).

The reason for the greater extent of electrophilic aromatic substitution, has to do with the polar water molecules inducing a stronger dipole in the Br2 molecule, hence generating a stronger electrophile (as compared to the weaker instantaneous dipole - induced dipole in Br2 with a non-polar solvent such as CCl4).

(b) Tribromophenol is less soluble than phenol for 2 reasons :

Reason #1 - The electron withdrawing by induction effect outweighs the electron donating by resonance effect of the Br substituents, resulting in a shift of electron density away from the OH substituent of the phenol; consequently weakening the strength of any hydrogen bonding of the phenol with solvent water molecules.

Reason #2 - The greater no. of electrons, as well as increased polarizability, of the 3 bromine substituents, results in a significantly increased induced dipole - induced dipole van der Waals non-polar hydrophobic interaction with other tribromophenol molecules.

For non-polar hydrophobic interactions, the non-polar groups are first 'pushed away' by the polar solvent water molecules, which would rather hydrogen bond with each other (macham elite-class snobs) than with the non-polar tribromophenol molecules. Subsequently, the now stronger (thanks to increased no. of electrons and increased polarizability, due to the bromine substituents) induced dipole - induced dipole van der Waals non-polar hydrophobic interactions between the tribromophenol molecules cause them to 'stick together', hence precipitating out of aqueous solution.


The above content is contributed by Mr Heng, owner and 'A' Level Chemistry tutor at Bedok Funland JC. He also goes by the handle UltimaOnline on various online popular homework forums.


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