Difficulty: Intermediate | Prerequisites: Functional groups, SN1/SN2 mechanisms, alcohols (Ch. 10–11 typical), acid–base chemistry.
Ethers are one of the first functional groups where you see oxygen bridging two carbon groups rather than sitting on the end of a chain as in alcohols. This chapter sits squarely between your earlier work on alcohols and nucleophilic substitution (SN1/SN2) and the more reactive oxygen-containing rings (epoxides) you will meet in the same unit. You need ethers to understand common laboratory solvents (diethyl ether, THF) and the logic of the Williamson synthesis, which itself is an SN2 reaction in disguise. If you are comfortable with alcohols, alkyl halides, and the basics of nucleophilic substitution, you are ready for this material.
Ethers (R–O–R’) are generally inert, making them excellent solvents, but they can be made via Williamson synthesis (alkoxide + alkyl halide, SN2) or acid-catalysed addition of alcohols to alkenes. Cleaving an ether requires a strong acid with a good nucleophile (HI or HBr at high temperature), proceeding by either an SN2 or SN1 pathway depending on the substitution of the carbon centres.
Ether (R–O–R’)
A compound with an oxygen atom bonded to two carbon groups (neither of which is H). Think of it as an oxygen sandwiched between two organic fragments.
Alkoxy group
The smaller –OR substituent when naming an ether using IUPAC substitutive nomenclature (e.g. methoxy, ethoxy). In simple terms, it is the "ether piece" treated as a side chain on the larger parent.
Williamson ether synthesis
The classic laboratory preparation of ethers: a strong base deprotonates an alcohol to form an alkoxide, which then displaces a halide from an alkyl halide via SN2. Think of it as "make the nucleophile, then let it attack."
Peroxide formation (auto-oxidation)
The slow reaction of ethers with atmospheric O2 to form explosive alkyl peroxides. In simple terms, old ether bottles can blow up.
Acidic cleavage of ethers
Breaking the C–O bonds of an ether using a strong acid (HI or HBr) and heat, producing an alcohol and an alkyl halide. The mechanism is either SN2 (primary/methyl centres) or SN1 (tertiary/benzylic/allylic centres).
MTBE (methyl tert-butyl ether)
A widely known ether once used as a fuel additive. Its synthesis illustrates both acid-catalysed and Williamson approaches.
Oxymercuration (for ether formation)
Using Hg(OAc)2 with an alcohol (instead of water) followed by NaBH4 reduction to add an OR group across an alkene in a Markovnikov fashion. Think of it as "oxymercuration but with an alcohol instead of water."
Two systems coexist. Common names list both R groups alphabetically then add "ether" (e.g. diethyl ether, tert-butyl methyl ether).
IUPAC substitutive nomenclature treats the smaller group as an alkoxy substituent on the larger parent chain (e.g. methoxybenzene, para-bromomethoxybenzene).
Ethers have polar C–O bonds, giving them a net dipole moment and therefore higher boiling points than alkanes of similar molecular weight.
They are generally unreactive (inert) towards acids and bases, which makes them outstanding laboratory solvents (diethyl ether, THF).
Ethers do react slowly with atmospheric oxygen to form explosive peroxides (auto-oxidation). Old ether bottles must always be tested before distillation.
Acid-catalysed addition (industrial route)
An alkene is protonated to form a carbocation, which is then attacked by an alcohol nucleophile.
MTBE is made this way: isobutylene + methanol with H2SO4 catalyst.
Limitation: the strongly acidic conditions can be incompatible with sensitive functional groups elsewhere in the molecule.
Williamson ether synthesis (laboratory workhorse)
Step 1: Deprotonate the alcohol with a strong base (NaH, KH) to generate the alkoxide (R–O⁻).
Step 2: The alkoxide performs an SN2 attack on a primary or methyl alkyl halide (R’–X).
The alkyl halide must be unhindered (primary or methyl). Using a secondary or tertiary halide invites elimination (E2) instead.
Planning tip for unsymmetrical ethers: always put the bulkier group on the alkoxide side and the smaller, unhindered group on the halide side.
Ag2O with an alkyl halide can methylate alcohols without pre-forming the free alkoxide, a mild alternative for sensitive substrates.
Oxymercuration with an alcohol
Replacing water with an alcohol in the standard oxymercuration–demercuration sequence (Hg(OAc)2, ROH, then NaBH4) gives an ether directly, with Markovnikov regiochemistry.
Ethers are tough to break apart. You need a strong acid paired with a good nucleophile (typically HI or HBr) and heat (~100 °C).
SN2 pathway
The acid protonates the ether oxygen, turning it into a good leaving group.
The halide ion (I⁻) attacks the less hindered carbon via backside displacement.
Products: an alcohol + an alkyl halide.
Example: phenetole (PhOCH2CH3) + HI gives phenol + iodoethane.
SN1 pathway
When a tertiary, benzylic, or allylic carbon is present, protonation of the oxygen is followed by departure of the alcohol, generating a stabilised carbocation.
The carbocation then either captures a nucleophile or loses a proton to form an alkene.
Example: phenyl tert-butyl ether + CF3CO2H gives phenol + isobutylene.
Students often think any base and any alkyl halide will work in the Williamson synthesis. They will not. A secondary or tertiary halide undergoes elimination (E2) rather than substitution, giving an alkene instead of an ether.
Students sometimes assume ethers are completely unreactive. Ethers are unreactive towards most reagents, but they do react with atmospheric oxygen over time to form dangerously explosive peroxides.
Confusing the SN2 and SN1 pathways for ether cleavage is common. The pathway depends on the substitution of the carbon: primary and methyl go SN2, tertiary and benzylic/allylic go SN1. The nucleophile attacks the less hindered carbon in SN2, while in SN1 the more substituted carbon forms the carbocation.
Students frequently try to cleave ethers with weak acids or at room temperature. Ether cleavage requires strong hydrohalic acids (HI or HBr) and elevated temperature.
⚠️ The Williamson synthesis is one of the most commonly tested reactions in this chapter. You will almost certainly be asked to design a synthesis of an unsymmetrical ether and choose the correct alkoxide/halide pairing.
⚠️ Predicting whether ether cleavage goes SN2 or SN1 (and therefore predicting the products) is a standard exam question. Remember: primary/methyl = SN2, tertiary/benzylic/allylic = SN1.
⚠️ Peroxide formation is a safety topic that appears on exams as a conceptual question ("why must old ether bottles be tested?").
⚠️ Nomenclature (common name vs. alkoxy IUPAC name) is low-effort marks. Know both systems.
True or false: Ethers can hydrogen-bond with water but not with each other. True. Ethers lack an O–H, so they cannot donate a hydrogen bond to another ether, but their lone pairs can accept one from water.
Fill in the blank: In a Williamson synthesis the alkyl halide should be ______ (primary/secondary/tertiary). Primary (or methyl). Secondary and tertiary halides undergo elimination.
True or false: HCl at room temperature is sufficient to cleave diethyl ether. False. You need a strong acid with a good nucleophile (HI or HBr) at elevated temperature.
Fill in the blank: When a tertiary carbon is attached to the ether oxygen, acidic cleavage proceeds by an ______ mechanism. SN1 mechanism.
True or false: The Williamson synthesis is fundamentally an SN2 reaction. True.
Q: Propose a Williamson synthesis of methyl tert-butyl ether (MTBE). Which alcohol should supply the alkoxide and which should supply the halide?
A: Use tert-butanol with KH to form the tert-butoxide, then react it with methyl iodide (CH3–I). Putting the methyl group on the halide side ensures SN2 proceeds smoothly. The reverse pairing (methoxide + tert-butyl iodide) would give elimination.
Q: Predict the products when cyclohexyl phenyl ether is treated with HI at 100 °C.
A: The SN2-accessible carbon is the cyclohexyl side (secondary, but if forced; more likely the I⁻ attacks the less hindered position). With a phenyl group that cannot undergo SN2, iodide attacks the cyclohexyl carbon to give cyclohexyl iodide and phenol.
Q: Why are ethers commonly used as solvents in Grignard reactions?
A: Ethers are inert to the strongly basic Grignard reagent (they resist nucleophilic and basic attack) and their lone pairs coordinate to the Mg centre, stabilising the reagent in solution.
Q: Explain why treating an ether with HCl is much less effective than HI for ether cleavage.
A: Ether cleavage requires both protonation of the oxygen and nucleophilic displacement. Cl⁻ is a poorer nucleophile than I⁻, and HCl is a weaker acid than HI, so the reaction is far slower and often incomplete.
Q: Draw the mechanism for the acid-catalysed industrial synthesis of MTBE from isobutylene and methanol.
A: The alkene is protonated to form the tertiary carbocation [(CH3)3C+]. Methanol attacks the carbocation. Loss of a proton from the oxonium ion gives MTBE.
The Williamson synthesis is a direct application of SN2 from your substitution/elimination chapter, so revising that mechanism strengthens both topics at once. Ether cleavage connects forward to the next section on epoxides, where ring strain makes the same kind of C–O bond breaking far easier. Oxymercuration for ether formation ties back to alkene addition reactions and Markovnikov selectivity from your alkenes chapter.
Ether, diethyl ether, Et2O, THF, tetrahydrofuran, MTBE, methyl tert-butyl ether, alkoxy group, methoxy, ethoxy, Williamson ether synthesis, alkoxide, SN2 ether synthesis, acid-catalysed ether formation, ether cleavage, HI cleavage, HBr cleavage, SN1 ether cleavage, peroxide formation, auto-oxidation of ethers, oxymercuration ether, R–O–R’, nomenclature of ethers, organic solvents, Grignard solvent.