Chapter 17: Alcohols and Phenols
What makes an alcohol, and how we classify and name it
Every alcohol shares one structural feature: hydroxyl attached to a carbon. That carbon — the one bonded to the –OH — is called the carbinol carbon, and it is the single most important carbon to locate in this chapter, because classification and most reactivity are decided by it alone, not by the size of the molecule. To classify, count the carbons bonded directly to the carbinol carbon: one other carbon makes the alcohol primary, two makes it secondary, three makes it tertiary. This label predicts reactivity throughout the chapter: tertiary alcohols dehydrate and ionize most easily, primary…
Hydrogen bonding sets the boiling points and solubility of alcohols
Two properties of alcohols follow directly from the polar O–H bond. First, boiling point: an alcohol can act as both a hydrogen-bond donor and acceptor, so its molecules are held together in a network that must be disrupted to vaporize the liquid. That is why ethanol boils near 78 °C while propane, of similar molar mass, boils near −42 °C, and why an isomeric ether — which can accept but not donate a hydrogen bond — boils lower than the alcohol. The same network is visible in the infrared: a neat alcohol shows its O–H stretch as a broad band centered near 3350 cm⁻¹, smeared out by hydrogen…
Why phenols are far more acidic than alcohols
Acidity is a statement about the conjugate base: the more stable the anion left after the proton leaves, the stronger the acid. For alcohols the conjugate base is an alkoxide, RO⁻, with the charge locked on a single oxygen; alcohols therefore have pKa values around 16, essentially the same as water, and even small structural changes move them only a little (added electron-withdrawing atoms lower the pKa modestly by induction). Phenol tells a different story. When phenol loses its proton, the resulting phenoxide can be drawn as several resonance forms that place the negative charge on the…
Substituents tune phenol acidity
The substituent effect on phenol acidity is one of the clearest demonstrations that acidity tracks conjugate-base stability. Start from phenol at pKa 10, and take the two ways a substituent can act, one at a time. Case 1 — pure induction. A group that only pulls electron density through the sigma framework stabilizes the phenoxide modestly, so the phenol is somewhat more acidic. A para-chloro group is the standard example: para-chlorophenol sits near pKa 9.4, a small step down from phenol, from inductive withdrawal alone. Case 2 — resonance plus induction. A group that can also accept the…
Making alcohols by reducing carbonyl compounds
Carbonyl reduction is the most common way to make an alcohol with a predictable structure. A hydride source adds H⁻ to the electrophilic carbonyl carbon; the pi electrons move onto oxygen to give an alkoxide, which is protonated on workup to the alcohol. Structure is set by the starting carbonyl: formaldehyde gives methanol, any other aldehyde gives a primary alcohol, and a ketone gives a secondary alcohol (a tertiary alcohol cannot be made this way because it would need a carbon, not a hydrogen, added to the carbonyl carbon). Reagent choice is about selectivity. Sodium borohydride is mild…
Grignard reagents build alcohols and new carbon–carbon bonds
The Grignard reaction is the chapter's most powerful carbon–carbon bond-forming route to alcohols. The reagent, formed from an alkyl or aryl halide and magnesium in dry ether, carries a strongly nucleophilic, carbanion-like carbon. It attacks a carbonyl carbon; the pi electrons shift onto oxygen to give a magnesium alkoxide, which aqueous acid converts to the alcohol in a separate step. The product's class is fully predictable: formaldehyde (HCHO) yields a primary alcohol, a higher aldehyde yields a secondary alcohol, and a ketone yields a tertiary alcohol. Esters are a special case worth…
Turning alcohols into alkenes and alkyl halides
Two classic ways alcohols are activated for further chemistry replace the poor hydroxide leaving group. Dehydration: heating an alcohol with concentrated sulfuric or phosphoric acid protonates the hydroxyl to water, which leaves to form the alkene. The most substituted alkene predominates (Zaitsev's rule). For tertiary and secondary substrates the reaction goes through a carbocation, so the reactivity order is tertiary > secondary > primary; a primary alcohol does not generate a free primary carbocation (too unstable) and reacts only under forcing conditions, by a concerted E2-like loss of…
Oxidation: how far an alcohol goes depends on its class
The single rule 'oxidation removes a carbinol hydrogen' organizes all of alcohol oxidation. A secondary alcohol has exactly one hydrogen on the carbon bearing the hydroxyl, so it can be oxidized once, to a ketone, and no further. A primary alcohol has two such hydrogens: removing one gives an aldehyde, and in the presence of water the aldehyde hydrates and loses the second hydrogen to become a carboxylic acid. That is why reagent and conditions matter for a primary alcohol — a strong aqueous chromium(VI) reagent (chromic acid, Jones reagent) drives all the way to the acid, whereas an…
Reactions distinctive to phenols
Because the hydroxyl oxygen can share a lone pair with the aromatic ring, a phenol's ring is electron-rich and highly reactive. In electrophilic aromatic substitution the –OH group is a powerful activator and an ortho–para director; phenol reacts with bromine so readily that, in water, all three available ortho and para positions are substituted without any Lewis-acid catalyst, and milder conditions give clean monosubstitution. This same electron richness makes phenols easy to oxidize. Removal of two electrons and two protons converts a 1,4-dihydroxybenzene (hydroquinone) into…
Silyl ethers protect an alcohol during incompatible reactions
Protecting groups let a chemist run a reaction on one part of a molecule while temporarily disabling an incompatible functional group elsewhere. The classic conflict in this chapter is a hydroxyl group in a substrate that must be treated with a Grignard reagent: the reagent is a strong base and would be destroyed by the O–H proton before it could add to the intended carbonyl. Capping the alcohol as a silyl ether solves this. Reaction with a silyl chloride — commonly tert-butyldimethylsilyl chloride (TBSCl), often with imidazole as base — converts R–OH into R–O–Si(t-Bu)(CH₃)₂, a silyl ether…
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