You are viewing a free preview of this lesson.
Subscribe to unlock all 12 lessons in this course and every other course on LearningBro.
By the end of this lesson you should be able to:
Phenol is C6H5OH - a benzene ring with a hydroxyl (-OH) group directly attached. The -OH is attached to the sp2 carbon of the aromatic ring, not to an sp3 carbon as in aliphatic alcohols (e.g. ethanol CH3CH2OH).
Despite having an -OH like an alcohol, phenol is significantly more acidic than ethanol:
| Compound | Formula | pKa | Behaviour |
|---|---|---|---|
| Water | H2O | 15.7 | Neutral |
| Ethanol | CH3CH2OH | 15.9 | Very weakly acidic |
| Phenol | C6H5OH | 9.9 | Weakly acidic (~6 orders of magnitude more acidic than water) |
| Ethanoic acid | CH3COOH | 4.76 | Weak carboxylic acid |
Phenol is about 10,000 times more acidic than ethanol but is still 10,000 times less acidic than a carboxylic acid. In water, phenol partially dissociates:
C6H5OH(aq) + H2O(l) <=> C6H5O-(aq) + H3O+(aq)
The phenoxide ion (C6H5O-) is stabilised relative to ethoxide (CH3CH2O-) because the negative charge on oxygen can be delocalised into the benzene ring:
C6H5O- <-> o-C6H5=O- <-> m-C6H5=O- etc.
The pi-system of the benzene ring accepts some of the negative charge through the p-orbital of the oxygen, spreading the charge over the ortho and para positions of the ring. This delocalisation stabilises the anion and makes it easier to form - hence phenol is more acidic than a simple alcohol.
In ethanol, CH3CH2O- has no ring to delocalise into, so it is higher in energy and harder to form. Hence ethanol is barely acidic (pKa ~16).
Like any alcohol, phenol reacts with sodium to produce hydrogen gas and the sodium salt:
2 C6H5OH + 2 Na -> 2 C6H5O-Na+ + H2
Observation: effervescence (H2 gas), sodium dissolves, white solid (sodium phenoxide) forms.
This reaction proves phenol has an -OH group - the same test works for ethanol.
Phenol reacts with NaOH to form a water-soluble sodium phenoxide salt and water:
C6H5OH + NaOH -> C6H5O-Na+ + H2O
This is possible because phenol is acidic enough to neutralise a strong base (NaOH). Ethanol, being much less acidic, does not react with NaOH - a key distinction.
This reaction is often used in separation procedures: adding NaOH(aq) to a mixture of phenol and an alkane (or phenol and ether) dissolves the phenol into the aqueous layer as the sodium salt, leaving the other compound in the organic layer.
Perhaps the most striking reaction of phenol is with bromine water (Br2 dissolved in water). Unlike benzene, phenol reacts instantly and at room temperature with bromine water - without a catalyst:
C6H5OH + 3 Br2 -> C6H2Br3OH + 3 HBr
Specifically, three bromines substitute at the 2, 4, and 6 positions of the ring, giving 2,4,6-tribromophenol. This is a white crystalline solid that precipitates out of solution. The bromine water, which was orange-brown, is decolourised.
Observation: orange bromine water decolourised; white precipitate (2,4,6-tribromophenol) forms. Smell of TCP (a mild antiseptic) may be noticed.
Why does phenol react so readily with Br2 without a catalyst when benzene needs FeBr3 to react slowly? Because the -OH group activates the ring towards electrophilic substitution. The lone pair on the oxygen is delocalised into the ring, increasing the electron density at the ortho (2,6) and para (4) positions. This extra electron density polarises the Br-Br bond directly, generating a sufficient electrophile without needing a halogen carrier.
The resulting product has three bromines at the 2, 4, and 6 positions because these are the activated sites.
This bromination reaction is a standard test for phenol (white precipitate + decolourisation of bromine water).
Phenol reacts easily with dilute HNO3 at room temperature to give a mixture of 2-nitrophenol and 4-nitrophenol (ortho and para products):
C6H5OH + HNO3 -> 2-O2N-C6H4-OH + 4-O2N-C6H4-OH + H2O
No sulfuric acid is needed - phenol is so activated that dilute HNO3 alone is enough. Compare this with benzene, which needs concentrated HNO3 + concentrated H2SO4 at 50 degrees C. The difference demonstrates the activating effect of the -OH group.
When a benzene ring already has a substituent (X) and you add a new group (E) via electrophilic substitution, the new group does not land randomly. The original substituent X directs it to either the 2,4 (ortho, para) positions or the 3 (meta) position. OCR A-Level specifies two categories:
These groups have a lone pair (OH, NH2) or are electron-donating (alkyl). They push electron density into the ring, especially at the 2, 4, and 6 positions (ortho and para). Any new electrophile will preferentially attack at these positions.
Activation also makes the ring more reactive than benzene - as shown by phenol's instant reaction with Br2 water.
Example: Phenol + HNO3 (dilute) -> 2-nitrophenol + 4-nitrophenol (both at 2,4,6 positions)
The NO2 group is strongly electron-withdrawing (both inductively and through pi-delocalisation into the N=O bonds). It pulls electron density OUT of the ring, especially at the 2, 4, and 6 positions. The electron density at the 3 and 5 positions (meta) is relatively higher than at 2,4,6, so new electrophiles attack there.
Deactivation also makes the ring less reactive than benzene - nitrobenzene requires harsher conditions for further nitration than benzene itself.
Example: Nitrobenzene + HNO3 -> 1,3-dinitrobenzene (3-directing)
Nitrobenzene + Br2 (FeBr3) -> 3-bromonitrobenzene (3-directing)
| Group | Type | Direction | Activity compared to benzene |
|---|---|---|---|
| -OH | Activating, lone pair donor | 2,4- (ortho, para) | Much more reactive |
| -NH2 | Activating, lone pair donor | 2,4- (ortho, para) | Much more reactive |
| -NR2 | Activating | 2,4- | Much more reactive |
| -OR | Activating | 2,4- | More reactive |
| -CH3 | Weakly activating (induction) | 2,4- | Slightly more reactive |
| -NO2 | Strongly deactivating | 3- (meta) | Much less reactive |
| -COOH | Deactivating | 3- (meta) | Less reactive |
| -CN | Deactivating | 3- (meta) | Less reactive |
At OCR A-Level you need to know:
and the reasoning (lone pair donation for OH/NH2, electron withdrawal for NO2).
Two extra electrophilic substitutions on benzene are the Friedel-Crafts alkylation and Friedel-Crafts acylation. Both use a Lewis acid catalyst (usually AlCl3) to generate a carbocation (or acylium) electrophile.
Subscribe to continue reading
Get full access to this lesson and all 12 lessons in this course.