Alcohol & Phenol

Hydroxyl-containing organic compounds: alcohols ($\text{R-OH}$) have the $-\text{OH}$ attached to an $\text{sp}^3$ hybridized carbon, while phenols have the $-\text{OH}$ attached directly to an aromatic ring.

[!important] Phenol is NOT an alcohol and NOT an aromatic alcohol.

SMILES Gallery

CO

Methanol

CCO

Ethanol

CC(C)O

Propan-2-ol (isopropyl alcohol)

CC(C)(C)O

tert-Butyl alcohol (2-methylpropan-2-ol)

c1ccccc1O

Phenol

c1ccccc1CO

Phenylmethanol (benzyl alcohol) — an aromatic alcohol, NOT a phenol

C1CCCCC1O

Cyclohexanol

O=[N+]([O-])c1ccccc1O

2-Nitrophenol

O=[N+]([O-])c1ccc(O)cc1

4-Nitrophenol

Nc1ccccc1O

2-Aminophenol

Nc1ccc(O)cc1

4-Aminophenol

Cc1ccc(O)cc1

4-Methylphenol (p-cresol)

Fc1ccc(O)cc1

4-Fluorophenol

Classification of Alcohols

Alcohols are classified by the carbon atom bearing the $-\text{OH}$ group (the alcoholic carbon), not by the number of R groups on oxygen.

Type	Structure	Oxidation Product
Primary (1°)	$\text{R-CH}_2\text{-OH}$	Aldehyde → Carboxylic acid
Secondary (2°)	$\text{R}_2\text{CH-OH}$	Ketone
Tertiary (3°)	$\text{R}_3\text{C-OH}$	Resistant to oxidation

[!warning] Contrast with amines Amine classification depends on R groups attached to N, not carbon.

Nomenclature

IUPAC Names

Determine longest chain containing $-\text{OH}$.
Number from end closer to $-\text{OH}$.
Replace -e of parent alkane with -ol.
Denote $-\text{OH}$ position with number.
Cyclic alcohols: numbering begins from carbon bearing $-\text{OH}$.
Multiple $-\text{OH}$: diol, triol, tetraol.

Common Names

Alcohol named as alkyl derivative of water.
Examples: methyl alcohol, ethyl alcohol, isopropyl alcohol.

Isomerism

Structural Isomers

Chain isomerism: different carbon skeleton
Positional isomerism: different $-\text{OH}$ position
Functional group isomerism: alcohol vs ether (e.g. ethanol vs dimethyl ether)

Optical Isomers

Require stereocenter(s) (chiral carbon with 4 different groups).
Enantiomers rotate plane-polarized light in equal but opposite directions.
Example: butan-2-ol exists as $(+)$/$d$ and $(-)$/$l$ enantiomers.

Physical Properties

Boiling Point Discussion Framework

When comparing b.p. across compounds, analyze in this priority:

Hydrogen bonding — strongest influence; requires H bonded to O/N/F
Dipole-dipole attractions — net dipole moment $\mu$
Exposed surface area — straight chain > branched
Relative molar mass (RMM) — more atoms → more vdW forces

General order: $\text{H-bond} > \text{dipole-dipole} > \text{vdW}$

Key Trends

Alcohols have much higher b.p. than hydrocarbons/ethers of similar RMM due to H-bonding.
Ethanol (b.p. 78 °C) vs propane (b.p. −42 °C): ~120 °C difference.
Ethanol vs dimethyl ether (b.p. −24 °C): H-bonding contributes >100 °C vs dipole-dipole alone.
Primary > Secondary > Tertiary for same RMM: exposed surface area decreases with branching.
Phenols > aliphatic analogues: electron delocalization strengthens intermolecular attractions.

Intramolecular vs Intermolecular H-Bonding

2-nitrophenol (intramolecular): b.p. 217 °C
3-nitrophenol (mixed): b.p. 230 °C
4-nitrophenol (intermolecular): b.p. 245 °C

Solubility

1–3 carbon alcohols: completely soluble in water.
4–10 carbon alcohols: oily liquids, decreasing solubility.
11 carbon alcohols: almost insoluble solids.
Solubility decreases with RMM (longer hydrophobic tail).
For same RMM: more $-\text{OH}$ groups → higher solubility.
Phenol: partially soluble below 66 °C, completely soluble above (compact shape + strong H-bonds with water).

Acidity of Alcohols

Alcohols are weak acids. The acid-dissociation constant:

$$K_a = \frac{[\text{H}_3\text{O}^+][\text{RO}^-]}{[\text{ROH}]}$$

Higher $K_a$ = stronger acid; higher $pK_a$ = weaker acid.

Factors Affecting Acidity

1. Inductive Effects

EWG in R (e.g. halogens): stabilize alkoxide, increase polarity of O–H → stronger acid.
EDG in R (alkyl groups): destabilize alkoxide, decrease polarity → weaker acid.

2. Solvation Effects

Bulky R groups inhibit solvation of alkoxide and hinder proton abstraction by water.
More R groups → weaker acidity (complementary to inductive effect).

pKa Values

Compound	$pK_a$	Notes
HCl	~−7	Strong acid (reference)
Acetic acid	4.8	Carboxylic acid (reference)
Phenol	10.0	Resonance-stabilized phenoxide
2,2,2-Trichloroethanol	12.2	EWG stabilizes alkoxide
Water	14.0	Reference
2-Chloroethanol	14.3	EWG, but farther from O
Methanol	15.5	Least substituted
Ethanol	15.9	—
Isopropyl alcohol	16.5	More EDG
tert-Butyl alcohol	18.0	Most EDG, most hindered
Cyclohexanol	18.0	Typical 2° alcohol

Acidity order: Carboxylic acids > Phenols > Water > Alcohols

[!note] Within alcohols: methanol > 1° > 2° > 3° Both inductive and solvation effects push in the same direction for alcohols.

Basicity of Alcohols

In strong acid, alcohol acts as a weak base (lone pairs on oxygen accept proton):

$$\text{ROH} + \text{H}^+ \rightleftharpoons \text{ROH}_2^+$$

Protonation is the first step in many alcohol reactions.

Acidity of Phenols

Phenols ($pK_a \approx 10$) are ~100 million times more acidic than typical alcohols ($pK_a \approx 18$).

Resonance Stabilization

The phenoxide ion is stabilized by delocalization of negative charge over oxygen and three ring carbons:

$$\text{C}_6\text{H}_5\text{OH} \rightleftharpoons \text{C}_6\text{H}_5\text{O}^- + \text{H}^+$$

Reactions with Bases

Reacts with Na and NaOH (more acidic than water).
Does NOT liberate $\text{CO}_2$ with $\text{Na}_2\text{CO}_3$ or $\text{NaHCO}_3$.
Diagnostic: dissolves in NaOH but no effervescence with carbonate.

Effect of Ring Substituents

Substituents alter acidity by changing electron density on the phenolic oxygen via resonance and inductive effects.

Electron-Withdrawing Groups (EWG)

Withdraw electron density from ring → weaken O electron density → stabilize phenoxide → increase acidity (lower $pK_a$).
Ortho/para EWG most effective: resonance places +ve charge on carbon adjacent to $-\text{OH}$.

Compound	$pK_a$	Explanation
Phenol	10.00	Reference
2-Nitrophenol	7.20	o-NO₂: resonance places +ve charge adjacent to O
4-Nitrophenol	7.20	p-NO₂: same resonance effect
3-Nitrophenol	8.40	m-NO₂: no resonance +ve charge adjacent to O; only −I

Electron-Donating Groups (EDG)

Donate electron density to ring → strengthen O electron density → destabilize phenoxide → decrease acidity (higher $pK_a$).
Ortho/para EDG most effective: resonance places −ve charge on carbon adjacent to $-\text{OH}$.

Compound	$pK_a$	Explanation
Phenol	10.00	Reference
4-Aminophenol	10.30	p-NH₂: resonance places −ve charge adjacent to O
2-Aminophenol	9.97	o-NH₂: similar EDG effect; slight enhancement from intramolecular H-abstraction
3-Aminophenol	9.82	m-NH₂: no resonance −ve charge adjacent to O; −NH₂ basicity aids proton removal

[!tip] Drawing resonance structures To explain o-/m-/p- effects, draw resonance forms showing charge on the carbon adjacent to $-\text{OH}$. EWG creating +ve charge there increases acidity; EDG creating −ve charge there decreases acidity.

Preparation of Alcohols

1. Fermentation

Yeast + sugars → ethanol + $\text{CO}_2$
Yield: 12–15%; distillation increases to 40–50%.

2. Hydration of Alkene

Alkene + dilute acid ($\text{H}_2\text{SO}_4$ or $\text{H}_3\text{PO}_4$).
Markovnikov addition; dilute acid favors alcohol (Le Châtelier).
Industrial: ethylene + steam, high T/P, catalyst ($\text{P}_2\text{O}_5$, etc.).

3. Nucleophilic Substitution of Haloalkane

$\text{R-X} + \text{NaOH}/\text{KOH} \rightarrow \text{R-OH} + \text{X}^-$
1° → 1° alcohol (SN2); 2° → 2° alcohol (SN2/E2 compete); 3° → alkene (E2 dominates).

4. Grignard Reagent

Formation: $\text{R-X} + \text{Mg} \xrightarrow{\text{dry ether}} \text{RMgX}$
Formaldehyde → 1° alcohol; Aldehydes → 2° alcohol; Ketones → 3° alcohol.
Anhydrous conditions essential — water destroys Grignard reagent.

5. Reduction of Carbonyl Compounds

Covered in later topics (Sem 2).

Preparation of Phenols

Cumene Process (Industrial)

Benzene + propene → cumene (alkylation)
Cumene → cumene hydroperoxide (oxidation)
Cumene hydroperoxide → phenol + acetone (rearrangement)

Laboratory Method

Aromatic amine + $\text{HNO}_2$ → diazonium salt → phenol + $\text{N}_2$ (with $\text{H}_2\text{O}$).

Reactions of Alcohols

1. Reaction with Active Metals

$$2,\text{ROH} + 2,\text{Na} \rightarrow 2,\text{RONa} + \text{H}_2 \uparrow$$

Reactivity: methanol > ethanol > 2° > 3°.
3° alcohols react very slowly (use K or NaH in THF).

2. Substitution to Haloalkane

Reagent	Conditions	Product	Key Notes
HX	—	R-X	Phenol < 1° < 2° < 3° < benzyl; HCl < HBr < HI
HCl + ZnCl₂	Lucas test	R-Cl	Differentiates 1°/2°/3° by cloudiness rate
PX₃	—	R-X	Good yields 1°/2°; no rearrangement; poor for 3°
SOCl₂	Pyridine	R-Cl	Gaseous by-products; retention of configuration

Lucas Test: conc. HCl + $\text{ZnCl}_2$. $\text{Zn}^{2+}$ complexes with O lone pairs, weakens C–O bond. Cloudiness indicates alkyl chloride formation. 3° fastest, 1° slowest. Limited to alcohols <6 C.

3. Dehydration (Elimination)

Reagents: conc. $\text{H}_2\text{SO}_4$ (180 °C), 85% $\text{H}_3\text{PO}_4$ (350 °C), $\text{Al}_2\text{O}_3$ (350 °C).
Lower temp (~140 °C) gives symmetrical ethers.
Follows Zaitsev's rule: most substituted alkene major product.
Ease: 3° > 2° > 1° (carbocation stability).
Usually E1 mechanism; rearrangements possible.

4. Oxidation

Class	Reagent	Product
1°	PCC, $\text{CH}_2\text{Cl}_2$	Aldehyde
1°	$\text{K}_2\text{Cr}_2\text{O}_7/\text{H}^+$ or $\text{KMnO}_4/\text{H}^+$	Carboxylic acid
2°	PCC, $\text{K}_2\text{Cr}_2\text{O}_7/\text{H}^+$, $\text{KMnO}_4/\text{H}^+$	Ketone
3°	—	Resistant (requires C–C cleavage)

Chromic acid test: Orange → green/blue indicates 1° or 2° alcohol. No change with 3° alcohol, ketone, or alkane.

5. Esterification

Alcohol + carboxylic acid $\xrightarrow{\text{H}^+}$ ester + $\text{H}_2\text{O}$ (equilibrium).
Alcohol + acyl chloride → ester + HCl (irreversible, exothermic).

6. Iodoform Test

Positive for structures with $\text{-CH}_3$ bonded to $\text{-C=O}$ (or oxidizable to it):

Ethanal, methyl ketones
Ethanol (only 1° alcohol)
2° alcohols oxidizable to methyl ketones

Negative: methanol, 3° alcohols. Product: yellow $\text{CHI}_3$ precipitate.

Reactions of Phenols

O–H Bond Reactions

Forms phenoxide with NaOH (more acidic than water).
Phenoxide reacts with acyl chlorides/anhydrides to form esters.
Phenol itself reacts poorly with carboxylic acids (weak nucleophile).

C–O Bond Limitations

Phenols do NOT undergo acid-catalyzed elimination or SN2.
Phenols are not easily oxidized (no H on carbinol carbon).

Electrophilic Aromatic Substitution

-$-\text{OH}$ is strongly activating and ortho-para directing.

Reactions occur without Lewis acid catalyst.

Halogenation:

Low T, non-polar: o- and p-monohalophenols.
Aqueous, higher T: 2,4,6-trihalophenol (white precipitate for bromine).

Nitration:

Dilute $\text{HNO}_3$, room temp: o- and p-nitrophenol.
Conc. $\text{HNO}_3$: 2,4,6-trinitrophenol (picric acid).

Identification Tests

Bromine water: rapid decolorization + white precipitate (2,4,6-tribromophenol).
Aqueous $\text{FeCl}_3$: light purple complex (positive for phenols and enols).

Important Alcohols and Phenols

Compound	Formula/SMILES	Uses
Methanol	`CO`	Solvent, fuel, formaldehyde synthesis
Ethanol	`CCO`	Beverages, solvent, fuel additive, antiseptic
Phenol	`c1ccccc1O`	Disinfectant, plastics (Bakelite), nylon precursor
Propan-2-ol	`CC(C)O`	Rubbing alcohol, topical antiseptic
Cyclohexanol	`C1CCCCC1O`	Nylon-6,6 intermediate

Industrial Applications

Ethanol

Fermentation → 12–15% → distillation → 95% azeotrope → absolute (100%) with CaO.
Fuel: Indianapolis 500 (since 2006), gasohol (10% in gasoline).
Solvent, antiseptic, mouthwash.

Methanol

Synthesis: CO + $\text{H}_2$ (high T/P).
Solvent, fuel (Indy 500, 1965–2006), precursor to methyl compounds.
Toxic: ~100 mL fatal.

Phenol → Nylon

Hydrogenation: $\text{C}_6\text{H}_5\text{OH} + 3,\text{H}_2 \rightarrow (\text{CH}_2)_5\text{CHOH}$ ($\Delta G = -55.31\ \text{kJ/mol}$, 100 °C).
Cyclohexanol/cyclohexanone → nylon-6 and nylon-6,6.
Uses: textiles, airbags, carpet fibres.