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Spec Mapping — OCR H432 Module 6.1.1 — Aromatic compounds, covering the structure, acidity and reactions of phenol (C₆H₅OH), the activating effect of the –OH group on the aromatic ring (rationalising why phenol brominates with Br₂(aq) at room temperature without a halogen carrier), the directing effects of substituents on EAS regiochemistry (–OH and –NH₂ as 2,4-directors / activators; –NO₂ as 3-director / deactivator — the OCR-specific labelling), the Friedel-Crafts alkylation with alkyl halides (RCl + AlCl₃ → carbocation R⁺ electrophile) and Friedel-Crafts acylation with acyl chlorides (RCOCl + AlCl₃ → acylium cation R-C(=O)⁺ electrophile), the comparative acidity of phenol vs water and ethanol (with delocalisation of the phenoxide anion into the ring as the AO3 explanation), and the industrial significance of phenol-derived chemistry (refer to the official OCR H432 specification document for exact wording).
Lesson 11 established the universal three-step EAS template for benzene. Lesson 12 extends that template in three directions: (i) onto phenol, where the –OH group activates the ring strongly enough for Br₂(aq) to react at room temperature with no halogen carrier and to give 2,4,6-tribromophenol as a white precipitate — one of A-Level's most striking qualitative tests; (ii) onto the OCR-specific Friedel-Crafts chemistry, both alkylation (giving alkylbenzenes via R⁺) and acylation (giving phenyl ketones via the acylium cation RC(=O)⁺); and (iii) onto the directing effects that govern where the second substituent goes on a pre-substituted ring — OH and NH₂ direct to 2,4-positions and activate the ring, NO₂ directs to the 3-position and deactivates it. Phenol's enhanced acidity (pKa 9.9, vs ≈ 16 for water and ethanol) and the reasoning behind it (phenoxide anion stabilised by delocalisation into the ring) is the AO3 hook this lesson is built around. OCR is one of only two UK A-Level boards that requires both Friedel-Crafts alkylation and acylation; AQA requires only acylation, so this is an OCR-specific point that examiners reward heavily.
Key Reactions (memorise the products and conditions):
- Phenol + Na (s) → sodium phenoxide + H₂(g) — same as any alcohol.
- Phenol + NaOH(aq) → sodium phenoxide + H₂O — unlike ethanol, which does NOT react with NaOH(aq). Diagnostic of phenol's enhanced acidity.
- Phenol + 3 Br₂(aq) → 2,4,6-tribromophenol (white ppt) + 3 HBr — room temperature, no catalyst. Br₂(aq) decolourises orange.
- Phenol + dilute HNO₃ → 2-nitrophenol + 4-nitrophenol — room temperature, no concentrated H₂SO₄ required.
- Benzene + RCl/AlCl₃ → alkylbenzene + HCl — Friedel-Crafts alkylation; electrophile R⁺.
- Benzene + RCOCl/AlCl₃ → phenyl ketone + HCl — Friedel-Crafts acylation; electrophile RC(=O)⁺.
Phenol is C₆H₅OH — a benzene ring with a hydroxyl (–OH) group bonded directly to one ring carbon. The –OH is attached to an sp² aromatic carbon, not to an sp³ carbon as in aliphatic alcohols (e.g. ethanol CH₃CH₂OH). This single structural difference — sp² vs sp³ carbon — is the root of every chemical difference between phenol and an ordinary alcohol, including the dramatic enhancement in acidity, the activation of the ring towards electrophilic substitution, and the failure of phenol to undergo Friedel-Crafts reactions with AlCl₃ (because AlCl₃ complexes with the lone pair on the phenolic oxygen — see "Friedel-Crafts on phenol" below).
Despite having an –OH group 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 10⁶-fold more acidic than water (pKa 9.9 vs 15.7 — about six orders of magnitude) and 10⁶-fold more acidic than ethanol, but it is still about 10⁵-fold less acidic than a typical carboxylic acid (ethanoic acid pKa 4.76). Phenol is a weak acid — strong enough to liberate H₂ from sodium metal and to react with NaOH(aq), but not strong enough to liberate CO₂ from carbonates (a classic discriminator between phenols and carboxylic acids in a qualitative-analysis exercise). In aqueous solution, 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 aromatic ring. The oxygen lone pair overlaps with the ring π-system; the negative charge spreads over the ortho (2 and 6) and para (4) ring carbons:
C6H5O−↔−C6H4=O (charge on C2/4/6)
Four resonance forms contribute: the original phenoxide with the negative charge on O, plus three "ring quinoid" forms with the negative charge on C2, C4 and C6 respectively. This delocalisation lowers the energy of phenoxide by ≈ 30 kJ mol⁻¹ relative to a hypothetical "ethoxide-with-a-benzene-ring-but-no-conjugation" species, making the phenoxide easier to form and the dissociation step thermodynamically more favourable. In ethanol, CH₃CH₂O⁻ has no aromatic ring into which the charge can delocalise; the negative charge sits localised on a single oxygen atom; the ethoxide is therefore much higher in energy and ethanol is barely acidic (pKa ≈ 16). The contrast between pKa 9.9 (phenol) and pKa 15.9 (ethanol) is the experimental signature of this delocalisation effect.
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.
Note: OCR covers BOTH alkylation and acylation, whereas AQA only requires acylation. This is a key distinction for OCR students.
Reagents: an alkyl halide (e.g. CH3Cl, CH3CH2Cl) + AlCl3 catalyst.
Mechanism:
Step 1: Generate R+ electrophile RCl + AlCl3 -> R+ + AlCl4-
Step 2: Attack on benzene C6H6 + R+ -> sigma complex
Step 3: Loss of H+ sigma complex -> C6H5R + H+ (and H+ + AlCl4- -> HCl + AlCl3, regenerating catalyst)
Overall: C6H6 + RCl -> C6H5R + HCl (with AlCl3 catalyst)
Example: methylation C6H6 + CH3Cl -> C6H5CH3 + HCl (methylbenzene / toluene)
Problem with alkylation: The product (e.g. methylbenzene) is itself activated by the alkyl group and is MORE reactive than benzene towards further substitution. So you get multiple alkylations - it is hard to stop at mono-alkylation. This is why acylation (below) is often preferred.
Reagents: an acyl chloride (RCOCl, e.g. CH3COCl) + AlCl3 catalyst.
Mechanism:
Step 1: Generate RCO+ (acylium ion) RCOCl + AlCl3 -> RCO+ + AlCl4-
The acylium cation is stable because it is resonance-stabilised (the charge can be delocalised onto the carbonyl oxygen: R-C+=O <-> R-C=O+).
Step 2: Attack on benzene C6H6 + RCO+ -> sigma complex
Step 3: Loss of H+ sigma complex -> C6H5COR + H+
Overall: C6H6 + RCOCl -> C6H5COR + HCl (with AlCl3 catalyst)
Example: ethanoylation C6H6 + CH3COCl -> C6H5COCH3 + HCl (phenylethanone / acetophenone)
Advantage of acylation: The product is a ketone (C6H5COR), and the carbonyl group is electron-withdrawing (deactivating). This means the product is LESS reactive than benzene, so the reaction stops cleanly at monoacylation (no over-reaction).
After acylation, the ketone can be reduced (e.g. with Zn/Hg and HCl in the Clemmensen reduction, or with hydrazine and base in the Wolff-Kishner) to give the alkylbenzene:
C6H5COR -> C6H5CH2R (reduction, OCR does not require the mechanism)
This two-step "acylation + reduction" route is the preferred way to add a single alkyl chain to benzene, because it avoids the over-alkylation problem.
The OCR specification (6.1.1) requires both Friedel-Crafts alkylation AND acylation. AQA only requires acylation. If you are sitting OCR A-Level Chemistry A (H432), make sure you can write both mechanisms and give examples of both. The starting reagents (RCl vs RCOCl) and the intermediate electrophiles (R+ vs RCO+) are the key differences.
graph TD
A[Friedel-Crafts OCR only: both] --> B[Alkylation: RCl + AlCl3]
A --> C[Acylation: RCOCl + AlCl3]
B --> D["Electrophile R+<br/>carbocation"]
C --> E["Electrophile RCO+<br/>acylium ion"]
D --> F["Product: C6H5R<br/>alkylbenzene<br/>over-reacts to multi-alkyl"]
E --> G["Product: C6H5COR<br/>ketone<br/>stops at mono-acyl"]
Friedel-Crafts alkylation/acylation of phenol fails if you simply add RCl (or RCOCl) and AlCl₃. The –OH oxygen has two lone pairs; the second (not the one delocalised into the ring) is highly available for Lewis-base donation to AlCl₃. AlCl₃ forms a Lewis adduct with the phenolic oxygen (ArO–H·AlCl₃), tying up both the catalyst and the activating substituent. The catalyst is no longer available to make R⁺; the –OH activation is also removed. For phenol bromination, no Lewis-acid catalyst is needed (the –OH activation alone is sufficient). OCR may test recognition that Friedel-Crafts on phenol fails but does not require alternative synthetic routes.
Problem: Write the balanced equation for the reaction of phenol with aqueous bromine at room temperature. State two observations and explain the mechanistic reason why this reaction occurs without a halogen-carrier catalyst, whereas the analogous reaction of benzene requires FeBr₃.
Solution:
Equation: C6H5OH+3Br2⟶C6H2Br3OH+3HBr — the product is 2,4,6-tribromophenol.
Observations: (i) the orange-brown Br₂(aq) is decolourised as Br₂ is consumed; (ii) a white crystalline precipitate of 2,4,6-tribromophenol forms in the reaction mixture (the product is sparingly soluble in water).
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