Ethers and Epoxides 340 Chem 1st 1439

Outline Structure of Ethers Nomenclature of Ethers Physical Properties of Ethers Preparation of Ethers Reactions of Ethers Epoxides: Structure and Nomenclature Synthesis of Epoxides Reactions of Epoxides 340 Chem 1st 1439

Ethers Ethers are also organic relatives of water, where alkyl groups replace both hydrogen atoms. Thus, ethers have two hydrocarbons bonded to an oxygen atom. Ethers have the general formula R-O-R or R-O-R`, where R may be an alkyl (or phenyl) group different from R`. In a symmetrical ether , the alkyl substituents are identical. In an unsymmetrical ether , the alkyl substituents are different. The simplest and most common ethers are diethyl ether and tetrahydrofuran (THF), which is a cyclic ether. 340 Chem 1st 1439

Nomenclature of ethers The common name of an ether consists of the names of the two alkyl substituents (in alphabetical order), followed by the word “ether.” The smallest ethers are almost always named by their common names. 340 Chem 1st 1439

alkoxy group rootname CH3—O—CH2—CH2—CH3 The IUPAC system names an ether as an alkane with an RO substituent. The substituents are named by replacing the “yl” ending in the name of the alkyl substituent with “oxy.” alkoxy group rootname CH3—O—CH2—CH2—CH3 340 Chem 1st 1439

3-Methoxyhexane 5-Ethoxy-2-heptene Propenoxy-benzene Common : Methyl Phenyl ether (anisole) IUPAC : Methoxy benzene m-Dimethoxybenzene 1,3-Dimethoxybenzene Diphenyl ether Phenoxybenzene 340 Chem 1st 1439

Physical properties of ethers Ethers are polar molecules in which oxygen bears a partial negative charge and each attached carbon bears a partial positive charge. However, only weak dipole-dipole interactions exist between ether molecules in the liquid state. The boiling points of ethers are much lower than those of alcohols of comparable molecular weight and are close to those of hydrocarbons of comparable molecular weight 340 Chem 1st 1439

Ethers can form hydrogen bonds with water molecules Because ethers cannot act as hydrogen bond donors, they are much less soluble in water than alcohols. However, they can act as hydrogen bond acceptors, which makes them more water-soluble than hydrocarbons of comparable molecular weight and shape. For example, diethyl ether and THF are common solvents used in the Grignard reaction. Ethers often form complexes with molecules that have vacant orbitals, e.g. THF complexes with borane (BH3.THF), which is used in the hydroboration–oxidation reaction . Ethers can form hydrogen bonds with water molecules 340 Chem 1st 1439

340 Chem 1st 1439

PREPARATION OF ETHERS Dehydration of Alcohols In general, this method is limited to the preparation of symmetrical ethers in which both alkyl groups are primary alcohols and H2SO4. 340 Chem 1st 1439

340 Chem 1st 1439

The Williamson Synthesis of Ethers Nucleophilic substitution of an alkyl halide by an alkoxide or phenoxide gives the carbon–oxygen bond of an ether. Methyl halides and primary alkyl halides are the best substrates. 340 Chem 1st 1439

340 Chem 1st 1439

Alkoxymercuration-Demercuration alkoxymercuration– reduction The reaction of an alkene with an alcohol in the presence of a mercury salt such as mercuric acetate Hg(OAc)2 or mercuric trifluoroacetate Hg(O2CCF3)2, in alcohol leads to an alkoxymercury intermediate, which on reaction with sodium borohydride NaBH4 yields an ether. When the alcohol reactant is also the solvent, the method is called solvomercuration–demercuration. 340 Chem 1st 1439

340 Chem 1st 1439

Alkylation of Alcohols A tert-butyl ether can be used to “protect” the hydroxyl group of a primary alcohol while another reaction is carried out on some other part of the molecule. A tert-butyl protecting group can be removed easily by treating the ether with dilute aqueous acid. 340 Chem 1st 1439

Reactions of Ethers Cleavage of Ethers Ethers are cleaved by heating with HBr or HI to give alkyl bromides or alkyl iodides. Ethers are unreactive toward most bases, but they can react under acidic conditions. 340 Chem 1st 1439

340 Chem 1st 1439

Epoxides Three-membered cyclic ethers are known as epoxides. Cyclic ethers have their oxygen as part of a ring—they are heterocyclic compounds Cyclic ethers can be named in several ways. One simple way is to use replacement nomenclature, in which we relate the cyclic ether to the corresponding hydrocarbon ring system and use the prefix oxa- to indicate that an oxygen atom replaces a CH2 group (common name). The IUPAC rules also permit oxirane (without substituents) to be called ethylene oxide. Tetrahydrofuran and tetrahydropyran are acceptable synonyms for oxolane and oxane, respectively. In each case the ring is numbered starting at the oxygen. IUPAC name Common name THF Oxacyclopropane Oxacyclobutane Oxacyclopentane 340 Chem 1st 1439

18-Crown-6 [C2H4O]6 IUPAC name 1,4,7,10,13,16-hexaoxa- Dioxane cyclooctadecane Dioxane Furan 1,4-Dioxacyclohexane One systematic method for naming epoxides is to name the rest of the molecule and use the term “epoxy” as a substituent, giving the numbers of the two carbon atoms bonded to the epoxide oxygen. 340 Chem 1st 1439

Synthesis of Epoxides Oxidation of Alkenes with Peroxycarboxylic Acids The most common laboratory method for the synthesis of epoxides from alkenes is oxidation with a peroxycarboxylic acid (a peracid). Three of the most widely used peroxyacids are meta-chloroperoxybenzoic acid (MCPBA), the magnesium salt of monoperoxyphthalic acid (MMPP), and peroxyacetic acid. 340 Chem 1st 1439

Internal Nucleophilic Substitution in Halohydrins treating the alkene with chlorine or bromine in water to form a chlorohydrin (or bromohydrin) followed by treating the halohydrin with a base (NaOH, NaOEt,..) to bring about intramolecular displacement of Cl. 340 Chem 1st 1439

Reactions of Epoxides Epoxides react rapidly with nucleophiles under conditions in which other ethers are inert. This enhanced reactivity results from the ring strain of epoxides. Reactions that lead to ring opening relieve this strain. 340 Chem 1st 1439

1- Acid –Catalyzed ring opening of epoxides in water to form glycols. 2- Acid –Catalyzed ring opening of epoxides in alcohol to form alkoxy alcohols 3- Acid –Catalyzed ring opening of epoxides with a hydrohalic acid (HCl, HBr, or HI), a halide ion attacks the protonated epoxide to give halo alcohol . 4- Ring opening of epoxides with amines 340 Chem 1st 1439

5- Ring opening of epoxides with Grignard and Organolithium Reagents to give longer alcohols 6- Epoxides are reduced to alcohols on treatment with lithium aluminum hydride. Hydride is transferred to the less crowded carbon. 340 Chem 1st 1439

Orientation of Epoxide Ring Open Symmetrically substituted epoxides (such as cyclopentene oxide, above) give the same product in both the acid-catalyzed and base-catalyzed ring openings. An unsymmetrical epoxide may produce different products under acid-catalyzed and base-catalyzed conditions. 340 Chem 1st 1439

340 Chem 1st 1439