Difficulty: Intermediate | Prerequisites: Chapter 7 (alkyl halides, SN2/E2), Chapter 10 (alcohols)
Ethers are compounds with an oxygen atom bonded to two carbon groups (R–O–R'). They sit between alkanes and alcohols in polarity: they can accept hydrogen bonds but cannot donate them, so their boiling points are low but their water solubility is moderate. You can make ethers through Williamson ether synthesis (an SN2 reaction between an alkoxide and a primary alkyl halide) or by acid-catalysed addition of an alcohol across an alkene, which follows Markovnikov selectivity.
Ether
A functional group containing an oxygen atom bonded to two carbon atoms (R–O–R'). Think of it as the oxygen "bridge" that links two hydrocarbon pieces together.
Alkoxy group
The smaller –OR substituent named as a prefix in IUPAC nomenclature (methoxy, ethoxy, etc.). In simple terms, it is the chunk you name when labelling the ether as a substituent on the longer chain.
Williamson ether synthesis
An SN2 reaction between an alkoxide ion (RO⁻) and a primary alkyl halide (R'–X) that forms an ether (R–O–R'). Think of it as the go-to lab method for building ethers in a controlled way.
SN2 (bimolecular nucleophilic substitution)
A one-step mechanism in which a nucleophile attacks the electrophilic carbon at the same time the leaving group departs, with backside attack and inversion of configuration. In simple terms, the nucleophile shoves the leaving group out the back door.
E2 (bimolecular elimination)
A one-step mechanism in which a base removes a beta-hydrogen while the leaving group departs, forming an alkene. This is the competing side reaction you want to avoid during Williamson synthesis.
Markovnikov selectivity
In acid-catalysed addition, the electrophile (H⁺) adds to the less substituted carbon of the alkene and the nucleophile (the alcohol) ends up on the more substituted carbon. In simple terms, the oxygen lands on the carbon that already has more stuff attached to it.
Systematic (IUPAC) naming
Select the longest carbon chain as the parent
Name the –OR group as an alkoxy substituent (methoxy, ethoxy, propoxy, etc.)
Number from the end closest to the alkoxy group
Examples: 1-ethoxyethane, 1-ethoxypentane, 1-hexyloxycyclopentane
Common naming
Name both alkyl groups attached to the oxygen, then add "ether"
List alkyl groups alphabetically
Examples: diethyl ether, tert-butyl methyl ether
Cyclic ring is the parent when an alkoxy group is attached to a ring
Hydrogen bonding behaviour
Ethers can accept hydrogen bonds (the oxygen has lone pairs) but cannot donate them (no O–H)
They are H-bond acceptors only
Dipole-dipole interactions
The C–O bonds are polar, giving ethers moderate dipole-dipole interactions
Boiling points and solubility (comparison for similar MW compounds)
Ethanol: bp 78 °C, infinite water solubility
Dimethyl ether: bp −24 °C, 7.8 g / 100 g water
Propane: bp −42 °C, 0.004 g / 100 g water
Trend: alcohols > ethers > alkanes for both boiling point and water solubility, because alcohols can both donate and accept H-bonds, ethers can only accept, and alkanes cannot participate in H-bonding at all
Mechanism: SN2, one step, backside attack
Reagents: an alkoxide ion (RO⁻) + a primary alkyl halide (R'–X)
Critical rule: the alkyl halide must be primary (or methyl) to favour SN2. Secondary and tertiary halides undergo E2 elimination instead.
How to plan a Williamson synthesis
Look at the target ether and find the C–O bond to disconnect
Put the alkoxide on the more hindered side and the halide on the less hindered (primary) side
If both possible disconnections give a secondary or tertiary halide, the Williamson route will fail
Example: to make isopropyl propyl ether, use isopropoxide (from isopropanol + base) + 1-bromopropane (primary), not propoxide + 2-bromopropane (secondary, gives E2)
Overall: an alcohol adds across a C=C double bond in the presence of acid catalyst (H⁺)
Selectivity: Markovnikov, the –OR group ends up on the more substituted carbon
Mechanism (three steps)
Step 1: the alkene (base) is protonated by the acid to form a carbocation on the more substituted carbon
Step 2: the alcohol (nucleophile) attacks the carbocation, forming an oxonium ion
Step 3: another molecule of alcohol (or solvent) deprotonates the oxonium ion to give the ether product and regenerate the acid catalyst
Limitation: works best when one component is a simple alkene that forms a stable (tertiary) carbocation
Williamson ether synthesis: RO⁻ + R'–X → R–O–R' + X⁻ (SN2, primary halide required)
Competing E2: RO⁻ + R'–X (2° or 3°) → alkene + ROH + X⁻
Acid-catalysed addition: alkene + ROH + H⁺ → R–O–R' (Markovnikov product)
General ether structure: R–O–R' (the two R groups may be the same or different)
Diethyl ether was one of the first general anaesthetics used in surgery, and tert-butyl methyl ether (MTBE) has been used as a fuel additive to boost the octane rating of petrol. Ethers are also widely used as solvents in organic chemistry labs because they dissolve many organic compounds, are relatively inert, and have low boiling points that make them easy to remove.
Students often think ethers can hydrogen-bond to each other. They cannot. Ethers have no O–H bond, so they can only accept H-bonds from other molecules (water, alcohols) but never donate them.
Students sometimes pick the wrong disconnection in Williamson synthesis, placing the alkoxide on the primary side and the halide on the secondary side. That gives E2 elimination, not the ether.
Students confuse "SN1 favours tertiary" with "Williamson works with tertiary halides." It does not. Williamson is SN2, which requires primary (or methyl) halides.
Some students forget that acid-catalysed addition of alcohols follows Markovnikov selectivity and place the –OR on the less substituted carbon.
⚠️ Williamson ether synthesis retrosynthesis problems are a favourite exam question. You will be given a target ether and asked to choose the correct alkoxide + alkyl halide pair. Always put the halide on the primary carbon.
⚠️ Expect a question comparing boiling points and solubility of an alcohol, ether, and alkane of similar molecular weight. Know the trend and be able to explain it with hydrogen bonding.
⚠️ The competing E2 pathway is tested as a "what went wrong" question: you are shown a synthesis that gave an alkene instead of an ether and asked to explain why.
⚠️ IUPAC naming of unsymmetrical ethers comes up regularly. Remember: the smaller –OR group is the alkoxy substituent; the longer chain is the parent.
True or False: Ethers have higher boiling points than alcohols of the same molecular weight.
False. Ethers cannot donate hydrogen bonds, so their boiling points are lower than comparable alcohols.
Fill in the blank: In Williamson ether synthesis, the alkyl halide should be ______ to favour SN2.
Primary (or methyl).
True or False: In acid-catalysed addition of methanol to 2-methylpropene, the methoxy group ends up on the tertiary carbon.
True. Markovnikov selectivity places the nucleophile on the more substituted carbon.
Fill in the blank: The IUPAC name for CH₃–O–CH₂CH₃ is ______.
1-methoxyethane (or methoxyethane).
True or False: If you use a secondary alkyl halide in a Williamson synthesis with a strong alkoxide base, you will mainly get an ether.
False. You will mainly get an alkene from E2 elimination.
Q: Give the IUPAC name for the ether in which a hexyl group is bonded through oxygen to a cyclopentane ring.
A: 1-Hexyloxycyclopentane. The ring is the parent because it is named as a cycloalkane, and the hexyloxy group is the substituent.
Q: You want to synthesise ethyl isopropyl ether via Williamson synthesis. Which alkoxide and which alkyl halide should you use, and why?
A: Use sodium isopropoxide (the alkoxide from the more hindered alcohol) and bromoethane (a primary alkyl halide). This ensures SN2 proceeds cleanly. Using sodium ethoxide with 2-bromopropane would give mostly E2 elimination because the halide is secondary.
Q: Explain why dimethyl ether has a boiling point of −24 °C while ethanol (same molecular weight) boils at 78 °C.
A: Ethanol can both donate and accept hydrogen bonds (it has an O–H), so its intermolecular attractions are much stronger. Dimethyl ether can only accept hydrogen bonds, giving it weaker intermolecular forces and a much lower boiling point.
Q: Draw the product when 2-methylpropene reacts with methanol in the presence of H⁺.
A: 2-Methoxy-2-methylpropane (tert-butyl methyl ether). The methoxy group adds to the more substituted carbon of the alkene via a tertiary carbocation intermediate (Markovnikov selectivity).
Q: A student attempts a Williamson synthesis using sodium methoxide and 2-bromobutane. What is the major product, and why?
A: The major product is an alkene (but-1-ene or but-2-ene), not the ether. Sodium methoxide is a strong, small base, and the secondary halide favours E2 elimination over SN2.
This material connects directly to SN2 and E2 from Chapter 7 (alkyl halides). The substrate-choice rules you learnt there (primary favours SN2, secondary/tertiary favours E2 with strong bases) are exactly what drives the planning of a Williamson synthesis.
The acid-catalysed addition mechanism ties back to Markovnikov addition from Chapter 8 (alkene reactions). The same carbocation-stability reasoning applies: the proton adds to the less substituted carbon, leaving the cation on the more substituted one.
Alcohol chemistry from Chapter 10 is the immediate precursor. Converting an alcohol to an alkoxide (with NaH, Na, or NaOH) is the first step of Williamson synthesis, and knowing when alcohols act as nucleophiles vs. bases is essential.
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