Phenol Flashcards
What makes phenol a moderately polar molecule with a high dipole moment?
Inductive Effect - electronegative oxygen atom draws electrons towards itself in the C-O bond.
Mesomeric Effect - a lone pair on the oxygen atom delocalises into the ring.
Mesomeric Effect is greater than Inductive Effect; electrons are drawn into the ring.
What is meant by the Mesomeric Effect in phenols?
It is when a lone pair on the oxygen atom delocalises into the ring.
Describe phenol.
White crystalline solid at room temperature with a smell of disinfectant.
Describe the solubility of phenol in water.
Unlike lower alcohols, phenol is only moderately soluble in water but does give a more acidic solution.
Explain the low solubility of phenols.
The presence of the bulky, non-polar benzene ring and delocalisation of one of the oxygen lone pairs reduce its availability for H bond formation.
Why is phenol a stronger acid than alcohols?
1) Mesomeric effect; reduces electron density in the O-H bond - weakens bond; proton lost more easily.
2) Phenate ion is stabilised by delocalisation of negative charge into the ring.
- Stabilisation discourages recombination with a proton; promotes acidity.
Acidic character of Phenol
1) How does phenol behave with sodium metal?
Phenol behave in a similar way to ethanol.
Releases a proton which then reacts with sodium in a redox reaction to form hydrogen gas.
Acidic character of Phenol
2) How does phenol behave with sodium hydroxide?
Unlike alcohols, phenol is a strong enough acid to react with sodium hydroxide solution to form a salt.
Acidic character of Phenol
3) What is the evidence for this acidic character?
Phenol is very slightly soluble in water.
However, if NaOH (aq) is added, phenol forms the ionic soluble salt.
This solution is the evidence for the acidic character possessed by phenol.
Is phenol a stronger acid than mineral acids?
No, if a strong mineral acid is added to the salt solution produced with NaOH(aq), the phenate ion is forced to accept the proton and the weaker acid is displaces from solution.
Although phenol is a stronger acid than alcohols and water, it is still very weak and much weaker than mineral acids.
Acidic character of Phenol
4) How does phenol behave with sodium carbonate or sodium hydrogen carbonate solution?
Phenol is not a strong enough acid to liberate CO2 gas from either solution.
Carbonic acid is a strong enough acid to displace phenol from its salt, sodium phenate.
Carbonic acid is a stronger acid than phenol.
Why doesn’t phenol react directly with carboxylic acids to form esters, like alcohol?
1) Conjugation of the lone pair on the O of the OH group with the C=O group reduces the electropositive character of the carboxyl C atom.
2) Delocalisation of the lone pair on the O of the phenol into the ring reduced its ability for nucleophilic attack.
Therefore, nucleophilic condensation does not occur.
How can esterification of phenol be achieved?
1) Carboxylic acid is first converted to the more reactive acid chloride.
2) Dissolve phenol in sodium hydroxide solution, creating the phenate ion.
3) With its full negative charge, the ion is a much more powerful nucleophile.
4) Reaction will then occur at room temperature forming the ester at high yield as a white solid suspension.
Compare benzene with phenol.
- Both possess a highly exposed delocalised π cloud.
- This is attacked by electron seeking electrophiles.
- Both undergo substitution reactions in which the stabilising π system is retained.
- Electrophilic substitution reactions are expected.
What does it mean by the ring in phenol is “activated” towards electrophilic attack?
- Delocalisation of the lone pair on O into the ring of phenol increases electron density.
- Attack by electrophiles is promoted.
