Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
The group in phenol is ortho and para directing because it increases the electron density at these positions due to the (resonance) effect.
Phenols are highly reactive towards electrophilic substitution compared to benzene because the lone pair on the oxygen atom is delocalized into the ring.
Nitration with at low temperature () yields a mixture of -nitrophenol and -nitrophenol, which can be separated by steam distillation.
Nitration with in the presence of results in the formation of -trinitrophenol, also known as Picric acid.
Halogenation of phenol in low-polarity solvents like or at low temperature gives mono-substituted products (- and -bromophenols).
Reaction with bromine water () results in the formation of a white precipitate of -tribromophenol due to the high activation of the ring in aqueous medium.
Kolbe's Reaction: Phenoxide ion reacts with followed by acidification to produce -hydroxybenzoic acid (Salicylic acid).
Reimer-Tiemann Reaction: Phenol reacts with in the presence of to introduce an group at the ortho position, forming Salicylaldehyde. The intermediate electrophile is dichlorocarbene ().
📐Formulae
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💡Examples
Problem 1:
Explain why -nitrophenol is steam volatile while -nitrophenol is not.
Solution:
-nitrophenol has intramolecular hydrogen bonding, while -nitrophenol has intermolecular hydrogen bonding.
Explanation:
In -nitrophenol, the and groups are close, forming a ring via hydrogen bonding within the same molecule. This prevents association between different molecules, lowering the boiling point. In -nitrophenol, molecules are linked to each other via hydrogen bonds, requiring more energy (steam) to break, thus making it non-volatile.
Problem 2:
What happens when Phenol is treated with in at ?
Solution:
A mixture of -bromophenol and -bromophenol is formed, with -bromophenol being the major product.
Explanation:
In non-polar solvents like , the ionization of phenol is suppressed. The ring is less activated than in water, leading only to mono-substitution. The para isomer is major due to less steric hindrance.
Problem 3:
Identify the electrophile in the Reimer-Tiemann reaction.
Solution:
Dichlorocarbene ()
Explanation:
The reaction of with produces through alpha-elimination. This neutral but electron-deficient species acts as the electrophile that attacks the phenoxide ring.