Organic Chemistry · Alcohols and Phenols
Properties of Alcohols and Phenols
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In 30 seconds
Almost every physical property of an alcohol traces back to one feature: the –OH group can hydrogen-bond. The oxygen is electronegative and carries lone pairs, and the hydrogen is attached to that electronegative oxygen — exactly the donor–acceptor pair needed for hydrogen bonding Strong dipole–dipole attraction between O–H (or N–H/F–H) and a lone pair on O/N/F Full entry → between molecules. As a result, alcohols have strikingly high boiling points for their size, small ones dissolve in water in any proportion, and they are far less volatile than analogous hydrocarbons.
The second defining property is acidity. Alcohols are weak acids, with pKa Negative log of the acid dissociation constant; lower pKa = stronger acid Full entry → values near 16–18 — about as acidic as water (pKa 15.7). Phenols are dramatically more acidic: phenol itself has pKa ≈ 10, about a million times more acidic than a typical alcohol, because the phenoxide ion C6H5O⁻, the conjugate base of phenol Full entry → (C6H5O⁻) is stabilized by resonance Delocalizing charge/electrons over several atoms Full entry → delocalization of the negative charge around the aromatic ring. This acidity gap is exploited constantly in the lab — phenols dissolve in dilute aqueous NaOH as their salts while most alcohols do not — and it explains why phenol and its derivatives behave like mild antiseptics while ethanol does not.
Why this matters
Hydrogen bonding and acidity explain real-world behavior you can observe: ethanol (bp 78 °C) is a liquid at room temperature while its isomer dimethyl ether (bp −24 °C) is a gas; methanol, ethanol, and ethylene glycol mix with water completely, which is why antifreeze works; and the solubility of phenols in base is the basis of an entire class of separation experiments ("extract the phenol into the aqueous layer with NaOH"). In medicine, the acidity of phenols matters for drug absorption and for antiseptic use — phenol derivatives such as hexachlorophene and triclosan were used as antibacterial agents, and the acidity of aspirin's phenolic neighbor (salicylic acid) shapes its chemistry in the body. In industry, the acidity of phenols is exploited to make phenolic resins and to purify phenols from reaction mixtures. Understanding pKa and hydrogen bonding lets you predict boiling points, solubility, and acid–base behavior instead of memorizing data tables.
The college version
Core Concepts
Hydrogen bonding and physical state
An –OH group is simultaneously a hydrogen-bond donor (its H) and acceptor (its lone pairs). Alcohols therefore hydrogen-bond to each other, and the energy needed to break these intermolecular bonds raises boiling points far above those of comparable alkanes and ethers. Two molecules of ethanol (C2H6O, M = 46 g/mol) hydrogen-bond strongly; two molecules of dimethyl ether (also C2H6O, M = 46 g/mol) cannot donate a hydrogen bond to each other (no O–H), so they are held only by weak dipole and dispersion forces. Result: ethanol boils at 78 °C, dimethyl ether at −24 °C — a 102 °C gap from one structural difference. Boiling points rise with chain length (more dispersion forces) and fall with branching (less surface contact), but the –OH contribution dominates for small molecules.
Water solubility
The same hydrogen bonding makes small alcohols water-soluble: methanol, ethanol, and 1-propanol are miscible Mixes in all proportions without separating Full entry → with water in all proportions, because the –OH group forms hydrogen bonds with water that pay for breaking the alcohol–alcohol and water–water contacts. As the hydrocarbon chain grows, the nonpolar portion takes over: 1-butanol is only moderately soluble (~7 g per 100 g water), and 1-octanol is essentially immiscible. The rule of thumb: the –OH "wins" for about four carbons or fewer; beyond that, the chain dominates. Phenols are only slightly soluble in pure water (phenol ≈ 8 g per 100 mL) but dissolve readily in aqueous NaOH because the phenoxide salt is ionic and water-soluble.
Acidity of alcohols
Alcohols ionize by transferring the –OH proton to a base:
ROH + H2O ⇌ RO- + H3O+
with equilibrium constants that are small: pKa values are roughly 15.5–18 (methanol ≈ 15.5, ethanol ≈ 16, water 15.7, 2-methyl-2-propanol ≈ 17–18). Inductive effects explain the ordering: alkyl groups donate electron density and destabilize the alkoxide anion, so more alkyl substitution (tertiary > secondary > primary) makes the alcohol less acidic. Electron-withdrawing substituents make alcohols more acidic: 2,2,2-trifluoroethanol (CF3CH2OH) has pKa ≈ 12.4 because the fluorines pull electron density away from the –O⁻, stabilizing it.
Acidity of phenols and the resonance explanation
Phenol (C6H5OH, pKa ≈ 10) is about 10⁶ times more acidic than ethanol (pKa ≈ 16). The reason is in the conjugate base: the phenoxide ion distributes the negative charge by resonance across the ortho and para positions of the ring, so the charge is spread over several atoms rather than concentrated on one oxygen. An alkoxide ion RO⁻, the conjugate base of an alcohol Full entry → (RO⁻) has no such delocalization — the charge sits on a single oxygen. Substituents tune phenol acidity: electron-withdrawing groups (NO2, Cl) stabilize the phenoxide and raise acidity — p-nitrophenol pKa ≈ 7.2, picric acid (2,4,6-trinitrophenol) pKa ≈ 0.4 — while electron-donating groups (CH3, OCH3) lower it slightly.
The NaOH solubility test
Because phenol (pKa 10) is more acidic than bicarbonate but much more acidic than water, phenols react with dilute NaOH to form water-soluble sodium phenoxide salts (ArO⁻Na⁺), while most alcohols (pKa 16+) do not. This simple difference is used to separate phenols from alcohols and from neutral hydrocarbons in the lab: shake the mixture with aqueous NaOH; the phenol leaves the organic layer as its salt.
How It Works / Step-by-Step Process
To predict or explain a property of an alcohol or phenol:
- Count carbons and note the –OH position (1°, 2°, 3°) — the alkyl part sets dispersion forces and solubility limits.
- Ask: can these molecules hydrogen-bond to each other? (Yes if –OH is present; no for ethers.)
- Compare hydrogen bonding + dispersion to estimate relative boiling points.
- For acidity: identify the conjugate base and ask whether its negative charge is delocalized (phenoxide: yes → acidic) or localized (alkoxide: no → weak acid).
- Apply substituent effects: electron-withdrawing groups raise acidity; electron-donating groups lower it.
- For solubility in base: pKa < ~11 means the compound forms a water-soluble salt with NaOH (phenols yes, simple alcohols no).
Common Confusions
| Common Confusion | Correct Understanding |
|---|---|
| "Ethanol and dimethyl ether have the same boiling point because they have the same formula." | Same molar mass, different boiling points: ethanol 78 °C, ether −24 °C, because ethanol hydrogen-bonds and the ether cannot donate H-bonds. |
| "All alcohols are water-soluble." | Only small ones (roughly ≤ C4). 1-Octanol is essentially insoluble despite its –OH. |
| "Phenol is an alcohol, so it has an alcohol's pKa." | Phenol is a phenol (aromatic –OH); its pKa ≈ 10 vs ≈ 16 for simple alcohols — a million-fold difference. |
| "Alkoxide and phenoxide ions are equally stabilized." | Alkoxide localizes charge on one oxygen; phenoxide delocalizes it by resonance — that is the whole acidity story. |
| "More alkyl groups on the –OH carbon make the alcohol more acidic." | Alkyl groups donate electron density, destabilizing RO⁻, so tertiary alcohols are the least acidic (highest pKa). |
| "Electron-withdrawing groups always lower pKa of alcohols and phenols." | Yes — they stabilize the conjugate base; CF3CH2OH (pKa ~12.4) and p-nitrophenol (pKa ~7.2) are both more acidic than their parent compounds. |
| "Phenol dissolves in water because it is acidic." | Phenol is only slightly soluble in pure water; it dissolves well in basic water (NaOH) because the salt is ionic. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Alcohols are like magnets with two sticky ends: they stick to each other and to water, which is why small ones pour like water and boil hotter than you'd expect for their size. Phenols are the same family's older, grumpier cousins: the ring steals some of the negative charge when they lose a proton, so they give up that proton much more easily — which is why phenol dissolves in soapy, basic water but ethanol does not.
Worked example
Example 1: Boiling point comparison
Predict which has the higher boiling point and explain: ethanol (CH3CH2OH) vs dimethyl ether (CH3OCH3). Both have the formula C2H6O and molar mass 46 g/mol.
Ethanol molecules hydrogen-bond to each other: each –OH donates its hydrogen to a neighbor's oxygen lone pair, forming O–H···O bridges that must be broken for the liquid to vaporize. Dimethyl ether has no O–H, so it cannot donate hydrogen bonds; only dipole–dipole and dispersion forces hold its molecules together.
More intermolecular attraction → more energy to escape → higher boiling point:
Tb(ethanol) = 78 °C > Tb(dimethyl ether) = -24 °C
The identical molar masses prove the difference is hydrogen bonding, not size.
Example 2: Comparing acid strengths with pKa
Arrange ethanol (pKa ≈ 16), water (pKa ≈ 15.7), and phenol (pKa ≈ 10) in order of increasing acidity, and calculate how many times more acidic phenol is than ethanol.
Smaller pKa = stronger acid, so:
phenol (10) < water (15.7) < ethanol (16) ⇒ phenol > water > ethanol
Each unit of pKa is a factor of 10 in Ka, so the acidity ratio is:
Ka(phenol)Ka(ethanol) = 10(16 - 10) = 106
Phenol is about a million times more acidic than ethanol — entirely because the phenoxide ion spreads its negative charge by resonance while the ethoxide ion cannot.
Example 3: Dimensional analysis with molar mass
How many moles of ethanol are in 9.20 g of ethanol (M = 46.07 g/mol)? Show the formula, then substitute:
n = mM = 9.20 g46.07 g/mol = 0.200 mol
Units check: g ÷ (g/mol) = mol. If this ethanol is dissolved in enough water to make 0.500 L of solution, the molarity is:
c = nV = 0.200 mol0.500 L = 0.400 mol/L
Example 4: Predicting the NaOH test
A mixture contains phenol and 1-hexanol. Explain how to separate them using aqueous NaOH, and which layer each component ends up in.
Phenol (pKa ≈ 10) reacts with NaOH to form sodium phenoxide (C6H5O⁻Na⁺), an ionic salt that is soluble in the aqueous layer. 1-Hexanol (pKa ≈ 16) does not react measurably with NaOH, so it remains in the organic layer. Shake the mixture with aqueous NaOH: the phenol migrates to the aqueous layer as its salt; the 1-hexanol stays in the organic layer. Acidifying the aqueous layer (adding HCl) regenerates the free phenol, which can then be extracted into an organic solvent.
Key takeaways
- –OH can both donate and accept hydrogen bonds → high boiling points, water solubility, low volatility.
- Ethanol (bp 78 °C) vs dimethyl ether (bp −24 °C): same formula, different boiling points — the O–H bond makes the difference.
- Small alcohols (≤ C4) are water-soluble; longer chains are not; phenols are slightly soluble in water but dissolve in dilute NaOH.
- Alcohol pKa ≈ 15.5–18 (about water's); phenol pKa ≈ 10 — about 10⁶× more acidic.
- Phenoxide ion resonance (charge spread to ortho/para positions) is why phenols are so much more acidic than alcohols.
- Inductive effects: EWG raise acidity (trifluoroethanol, nitrophenols); alkyl groups lower it (tert > sec > prim).
- NaOH solubility test: phenols form water-soluble salts; simple alcohols do not.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
Explain the boiling-point difference between ethanol (78 °C) and dimethyl ether (−24 °C).
Show answer
Ethanol's –OH can donate and accept hydrogen bonds between molecules, so extra energy is needed to break those bonds to vaporize it; dimethyl ether has no O–H and only weak dipole–dipole and dispersion forces.
Why is phenol about 10⁶ times more acidic than ethanol?
Show answer
The phenoxide ion (C6H5O⁻) delocalizes its negative charge by resonance over the ring's ortho and para positions, stabilizing the conjugate base; ethoxide (CH3CH2O⁻) keeps the charge on one oxygen.
Arrange in order of increasing acidity: phenol, ethanol, water.
Show answer
Ethanol (pKa ≈ 16) < water (pKa ≈ 15.7) < phenol (pKa ≈ 10) — increasing acidity goes ethanol, water, phenol.
Would 1-octanol be more or less water-soluble than 1-butanol? Why?
Show answer
Less soluble: the longer eight-carbon chain is more nonpolar, so the nonpolar portion dominates over the single –OH's hydrogen bonding with water.
How many moles are in 4.61 g of ethanol (M = 46.07 g/mol)?
Show answer
n = m/M = 4.61 g ÷ 46.07 g/mol = 0.100 mol.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- hydrogen bonding
- Strong dipole–dipole attraction between O–H (or N–H/F–H) and a lone pair on O/N/F
- pKa
- Negative log of the acid dissociation constant; lower pKa = stronger acid
- phenoxide ion
- C6H5O⁻, the conjugate base of phenol
- alkoxide ion
- RO⁻, the conjugate base of an alcohol
- inductive effect
- Electron withdrawal or donation through σ bonds
- resonance
- Delocalizing charge/electrons over several atoms
- miscible
- Mixes in all proportions without separating
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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