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