Phenols

Chapter 24: Phenols

Nomenclature:
Functional group suffix = -common - phenol, systematic - benzenol (review)
Functional group prefix = hydroxy
Numbering of the ring begins at the hydroxyl-substituted carbon and proceeds in the direction of the next substituted carbon that possesses the lower number.

Ortho, meta or para ? Mono-substituted phenols are characterized using the prefix ortho (o-), meta (m-) or para (p-) depending on the placement of the substituent from the hydroxyl group or the hydroxyl group from a higher priority functional group, 1,2-, 1,3- or 1,4- respectively.


1,2-chlorophenol or o-chlorophenol	1,3-chlorophenol or m-chlorophenol	1,4-chlorophenol or p-chlorophenol

There are several other functional group suffixes, for various substituted phenols, that one needs to know, check here.

Physical Properties:

The polar nature of the O-H bond (due to the electronegativity difference of the atoms ) results in the formation of hydrogen bonds with other phenol molecules or other H-bonding systems (e.g. water). The implications of this are:

high melting and boiling points compared to analogous arenes
high solubility in aqueous media

The presence of intramolecular hydrogen bonding is believed responsible for the significantly lower boiling points of certain ortho-substituted phenols vs the meta- and para- analogs.

Structure:

The alcohol functional group consists of an O atom bonded to an sp²-hybridized aromatic C atom and a H atom via s bonds.
Both the C-O and the O-H bonds are polar due to the high electronegativity of the O atom.
Conjugation exists between an unshared electron pair on the oxygen and the aromatic ring.
This results in, compared to simple alcohols:

a shorter carbon-oxygen bond distance
a more basic hydroxyl oxygen
a more acidic hydroxyl proton (-OH)

phenol resonance

Acidity:

Phenols are more acidic (pK_a»10) than alcohols (pK_a»16 - 20), but less acidic than carboxylic acids (pK_a»5)
The negative charge of the phenolate ion is stabilized by resonance due to electron delocalization onto the ring as shown below:

The acidity difference means that it is possible to separate phenols from alcohols and/or carboxylic acids.

Mixing an ether solution, of either phenol and alcohol or phenol and carboxylic acid, with dilute base (sodium hydroxide and sodium bicarbonate, respectively), results in the stronger acid being converted to its alkali salt, which is then extracted to the aqueous phase and can be separated from the organic phase.

Nucleophilic substitution reactions of phenols are generally carried out under basic conditions as the phenolate ion is a better nucleophile.

Substituent Effects on Acidity
Substituents, particularly those located ortho or para to the -OH group, can dramatically influence the acidity of the phenol due to resonance and / or inductive effects. Electron withdrawing groups enhance the acidity, electron donating substituents decrease the acidity. The resonance stabilization of o-nitrophenol is shown below: resonance stabilisation of ortho-nitrophenolate

resonance stabilisation of ortho-nitrophenolate

Compound	pKa	Compound	pKa
Phenol	10.0
o-Methoxyphenol	10.0	p-Methoxyphenol	10.2
o-Methylphenol	10.3	p-Methylphenol	10.3
o-Chlorophenol	8.6	p-Chlorophenol	9.4
o-Nitrophenol	7.2	p-Nitrophenol	7.2
m-Nitrophenol	8.4

Questions:
Draw resonance structures to show the stabilization of p-nitrophenol.
How about m-nitrophenol ?

Study Tip:
Did you answer question about the m-nitrophenol correctly ? If you did, you probably have a good understanding of resonance and curly arrows. If not, you may want to review resonance and curly arrows again

Reactivity:

The image to the left shows the electrostatic potential for phenol.
The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.

The hydroxyl O atom is a region of high electron density (red) due to the lone pairs.
The hydroxyl O atom can function as a nucleophile or Lewis base.
There is low electron density (blue) on H atom of the hydroxyl group, i.e. H⁺character, therefore phenols are acidic (pK_a ~ 10)
Due to conjugation with the ring, phenols are more acidic than alcohols (pK_a ~ 16).
Removal of the proton generates a phenolate ion.

The image to the left shows the electrostatic potential for a phenolate ion.
The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.

Note the increased electron density on the oxygen compared to the phenol