Premed · Premed · Organic Chemistry 1
Lecture 18: Ethers and Epoxides
Organic Chemistry I
Learning Objectives
By the end of this lecture, students will be able to:
- Name ethers using IUPAC and common nomenclature
- Describe the physical properties of ethers and their use as solvents
- Explain the synthesis of ethers by the Williamson ether synthesis and other methods
- Describe the cleavage of ethers with strong acids (HBr, HI)
- Describe the structure and strain of epoxides
- Explain the synthesis of epoxides from alkenes and from halohydrins
- Describe the ring opening of epoxides under acidic and basic conditions
- Predict the regiochemistry and stereochemistry of epoxide ring-opening reactions
Lecture Content
I. Structure and Nomenclature of Ethers
Ethers are compounds in which an oxygen atom is bonded to two carbon groups (R-O-R'), which may be identical (symmetric ethers) or different (unsymmetric ethers). The oxygen is sp3 hybridized with two lone pairs, and the C-O-C bond angle is approximately 112 degrees, slightly larger than tetrahedral due to lone pair repulsion.
In IUPAC nomenclature, the smaller alkyl group plus oxygen is named as an alkoxy substituent on the larger parent chain. Alkoxy groups include methoxy (CH3O-), ethoxy (C2H5O-), and propoxy (C3H7O-), giving names like methoxyethane and 2-ethoxypentane. Common nomenclature lists both alkyl groups alphabetically followed by "ether," as in diethyl ether, methyl tert-butyl ether (MTBE), and ethyl methyl ether. Cyclic ethers have their own names: oxirane (the three-membered ring epoxide), oxetane (four-membered), tetrahydrofuran or THF (five-membered), tetrahydropyran (six-membered), and 1,4-dioxane.
II. Physical Properties of Ethers
Ethers have boiling points much lower than alcohols of comparable molecular weight because they lack an O-H bond for self-hydrogen bonding. Diethyl ether (MW 74) boils at 35 degrees C, compared to 118 degrees C for 1-butanol (MW 74). Ether boiling points are similar to those of alkanes of comparable molecular weight. Although ethers have a small dipole moment from their polar C-O bonds, they cannot form hydrogen bonds with each other.
Ethers are slightly soluble in water because their oxygen lone pairs can accept hydrogen bonds from water molecules, though they are less soluble than comparable alcohols. They are excellent solvents for organic reactions, dissolving many organic compounds. Diethyl ether is a volatile, highly flammable extraction solvent. THF has a higher boiling point, is miscible with water, and is an outstanding solvent for organometallic reactions. Ethers coordinate to Lewis acids (such as BF3 and Grignard reagents) through their oxygen lone pairs.
Chemically, ethers are relatively unreactive. They lack acidic hydrogens, their C-O bond is a poor leaving group, and they are stable toward bases, nucleophiles, reducing agents, and most oxidizing agents. They react only with strong acids (HBr, HI) that can cleave the C-O bond. A safety concern is that ethers form explosive peroxides upon prolonged exposure to air and light.
III. Synthesis of Ethers: Williamson Ether Synthesis
The Williamson ether synthesis is the most important and reliable method for preparing ethers. It proceeds by an SN2 reaction between an alkoxide ion and a primary or methyl alkyl halide: R-O:- + R'-CH2-X yielding R-O-CH2-R' + X-. The procedure involves first deprotonating the alcohol with NaH, sodium metal, or NaNH2 to generate the alkoxide, then reacting it with the alkyl halide.
The critical requirement is that the alkyl halide must be primary or methyl to ensure SN2 reactivity. Secondary and tertiary halides undergo E2 elimination as the major pathway because the alkoxide is both a strong nucleophile and a strong base. When planning the synthesis of an unsymmetric ether, one must choose the disconnection that places the less hindered group as the alkyl halide component. For tert-butyl methyl ether, for example, the correct approach uses tert-butoxide plus methyl iodide (unhindered), not methoxide plus a tert-butyl halide (which would give mostly elimination).
<image>Panel A: Williamson ether synthesis planning example. Target: ethyl propyl ether (CH3CH2OCH2CH2CH3). Disconnection A: ethoxide + 1-bromopropane (both primary — good SN2, correct approach, circled with a checkmark). Disconnection B: propoxide + bromoethane (both primary — also works). Both synthetic routes are drawn with arrows. Panel B: A bad example: target is tert-butyl ethyl ether. Disconnection showing ethoxide + tert-butyl bromide is crossed out (tertiary halide → E2 elimination). The correct disconnection: tert-butoxide + ethyl bromide (primary halide → SN2) is shown with a checkmark.</image>
IV. Other Ether Synthesis Methods
Acid-catalyzed dehydration of primary alcohols at approximately 140 degrees C with H2SO4 can produce symmetric ethers through an SN2 mechanism in which one alcohol molecule attacks the protonated form of another. This method is limited to primary, symmetric ethers because higher temperatures favor elimination to alkenes. Alkoxymercuration-demercuration adds an alkoxy group across an alkene in Markovnikov fashion, analogous to oxymercuration but using an alcohol instead of water. Acid-catalyzed addition of alcohols to alkenes also follows Markovnikov regioselectivity and is important industrially, as in the synthesis of MTBE from isobutylene and methanol.
V. Cleavage of Ethers
Ethers can be cleaved only by strong hydrohalic acids, specifically HBr and HI. Treatment with one equivalent of HX produces an alkyl halide and an alcohol (R-O-R' + HX yielding R-X + R'-OH), while excess HX converts both groups to alkyl halides (R-O-R' + 2 HX yielding R-X + R'-X + H2O).
The mechanism begins with protonation of the ether oxygen, converting the -OR group into a good leaving group (the alcohol ROH). The halide ion then attacks either through SN2 (at a primary carbon) or the protonated ether undergoes SN1 loss of the better cation (at a tertiary carbon). HI is more effective than HBr because iodide is a better nucleophile, and HCl is insufficiently reactive to cleave ethers.
Silyl ethers (R-O-SiR3) serve as protecting groups for alcohols during multi-step syntheses. They are installed by treating the alcohol with TBSCl or TMSCl and removed with fluoride (TBAF) or mild acid, allowing the hydroxyl group to be temporarily shielded from other reactive conditions.
VI. Epoxides: Structure and Synthesis
Epoxides (oxiranes) are three-membered cyclic ethers with approximately 114 kJ/mol of ring strain due to their compressed 60-degree bond angles (far from the ideal 109.5 degrees for sp3 carbon). This strain energy makes epoxides far more reactive than ordinary ethers, particularly toward ring-opening by nucleophiles.
Epoxides are synthesized from alkenes by treatment with a peroxy acid, most commonly mCPBA. The mechanism is a concerted, syn addition of oxygen to the double bond, and it is stereospecific: cis-alkenes give cis-epoxides and trans-alkenes give trans-epoxides, preserving the substituent geometry. Alternatively, epoxides can be formed from halohydrins by treatment with base (NaOH), which deprotonates the hydroxyl group and triggers an intramolecular SN2 displacement of the halide in a 3-exo-tet cyclization. This pathway requires the OH and X to be anti (trans) for proper ring closure.
VII. Epoxide Ring Opening: Basic Conditions
Under basic or nucleophilic conditions, the nucleophile attacks the less substituted (less hindered) carbon of the epoxide through an SN2 mechanism. Backside attack produces inversion at the attacked carbon, and the epoxide oxygen departs as an alkoxide, which is protonated during acid workup to give the final product. The regiochemistry is governed by steric factors: the nucleophile preferentially attacks the more accessible, less crowded carbon. The stereochemistry is anti addition overall, with the nucleophile and the hydroxyl ending up on opposite faces.
A wide variety of nucleophiles participate in this reaction. Alkoxides (RO-) give beta-alkoxy alcohols. Hydroxide (HO-) gives anti-1,2-diols. Thiolates (RS-) give beta-hydroxy thioethers. Grignard reagents (RMgBr) extend the carbon chain by attacking at the less hindered carbon. Acetylide anions (RC-triple-bond-C:-) give propargylic alcohols. Amines yield amino alcohols. LiAlH4 delivers hydride to the less substituted carbon, effectively reducing the epoxide to an alcohol.
VIII. Epoxide Ring Opening: Acidic Conditions
Under acidic conditions, the epoxide oxygen is first protonated, which weakens the C-O bonds and increases the electrophilicity of the ring carbons. The nucleophile, often the solvent (water or an alcohol), then attacks the more substituted carbon, where the partial positive charge is greatest. This regioselectivity contrasts sharply with basic conditions and reflects the development of SN1-like carbocation character at the more substituted position in the protonated epoxide.
Despite this different regiochemistry, the stereochemistry remains anti addition, as the nucleophile still approaches from the opposite side of the breaking C-O bond. Acid-catalyzed hydrolysis (epoxide + H3O+) gives the anti-1,2-diol (trans-diol), and acid-catalyzed ring opening with alcohols gives beta-alkoxy alcohols.
The key distinction to remember is that basic/nucleophilic conditions favor attack at the less substituted carbon (steric control), while acidic conditions favor attack at the more substituted carbon (electronic control). Both pathways give anti addition products.
<image>Panel A: Ring opening of 2-methyloxirane (propylene oxide) under basic conditions: NaOCH3 in methanol attacks the less substituted carbon (CH2 end) via SN2, giving 1-methoxy-2-propanol. The arrow shows backside attack at the less hindered carbon. Panel B: Ring opening of the same epoxide under acidic conditions: H3O+ protonates the oxygen, then water attacks the more substituted carbon (bearing the methyl group), giving 2-methoxy-1-propanol. The partial positive charge on the more substituted carbon is indicated with delta+. Both products are drawn with anti stereochemistry highlighted. Panel C: A comparison table showing basic vs. acidic epoxide ring opening with columns for mechanism, regiochemistry, and stereochemistry.</image>
