JEE Chemistry Alcohols Phenols and Ethers Notes Set 06

Ethers: Structure and Types

In an ether the oxygen atom is bonded to two carbon atoms. The groups may be alkyl, allyl, vinyl, alkynyl or aryl. Examples are dimethyl ether, ethyl methyl ether, methyl vinyl ether, methyl phenyl ether (anisole) and phenetole (ethyl phenyl ether).
Cyclic ethers with two carbons and one oxygen in a three-membered ring are oxiranes (epoxides), those with a four-membered ring are oxetanes, and those with a five-membered ring are tetrahydrofurans.

Preparation of Ethers

1. From alcohols: dehydration with concentrated H2SO4 gives an ether at about 140 °C (excess alcohol) and an alkene at 170 °C. The ether is distilled out as it forms. The mechanism is SN2: the alcohol is protonated, a second alcohol molecule attacks the carbon, and loss of a proton gives the ether. If the alkyl group is tertiary the path is SN1, with some alkene as a by-product. The method is limited to symmetrical ethers, because two different alcohols give a mixture of three ethers.

2. Williamson synthesis: an alkoxide ion attacks an alkyl halide, alkyl sulphate or alkyl sulphonate by SN2. The alkyl halide should be primary because tertiary halides give elimination, and a low temperature favours substitution. Phenoxides react with alkyl sulphates in the same way to give aryl alkyl ethers.

\[ \ce{RO-Na+ + R'-X -> R-O-R' + NaX} \]
\[ \ce{C6H5ONa + (CH3O)2SO2 -> C6H5OCH3 + CH3OSO3Na} \]

3. Gas-phase reaction: alcohol vapour passed over alumina at 260 to 280 °C gives the ether.
4. Alkoxymercuration-demercuration: an alkene with Hg(OAc)2 in an alcohol, followed by NaBH4, gives an ether (with water in place of alcohol the product is an alcohol).
5. With diazomethane: an alcohol and CH2N2 with HBF4 as catalyst give a methyl ether and N2.
6. From a Grignard reagent: an alpha-halo ether such as CH3OCH2Cl with CH3MgI gives CH3OCH2CH3.
7. From an alkyl halide and dry silver oxide: 2 (CH3)2CHCl + Ag2O gives diisopropyl ether and 2 AgCl.

Physical Properties of Ethers

1. Dimethyl ether and ethyl methyl ether are gases at room temperature; the other common ethers are pleasant-smelling colourless liquids that are lighter than water, volatile and highly flammable.
2. They are sparingly soluble in water (the oxygen can accept a hydrogen bond from water) and are good solvents for organic compounds.
3. Boiling points rise gradually with molar mass. Ethers boil much lower than isomeric alcohols because they have no intermolecular hydrogen bonding, and close to alkanes of similar mass.
4. The C-O-C angle is about 110°, so the two bond dipoles do not cancel and ethers have a small net dipole moment.

Reactions of Ethers

Ethers are fairly unreactive. They resist bases and nucleophiles and react mainly with acids, which together with their ability to solvate cations through the oxygen lone pair makes them good solvents for many reactions.
1. Halogenation: like alkanes, ethers undergo halogen substitution in the dark; the hydrogens on the carbon next to oxygen are replaced first.
2. Basic nature: ethers accept a proton from strong acids to form oxonium salts (diethyl ether with HBr gives an oxonium bromide).
3. Cleavage by acids. Hot concentrated HI or HBr breaks the C-O bond; the reactivity order is HI > HBr > HCl.

\[ \ce{R-O-R' + HX -> R-X + R'-OH} \]
\[ \ce{R-O-R ->[HX (excess, hot, conc.)] 2R-X} \]

(i) If both groups are primary or secondary, iodide attacks the smaller alkyl group by SN2, so the smaller group becomes the alkyl iodide and the larger group becomes the alcohol (CH3OC2H5 + HI gives CH3I and C2H5OH).
(ii) If one group is tertiary, the cleavage is SN1. The protonated ether loses the alcohol of the other group and the tertiary carbocation then combines with iodide, so the tertiary group ends up as the iodide.
(iii) Alkyl aryl ethers are cleaved at the alkyl-oxygen bond, because the aryl-oxygen bond is stronger and aromatic rings do not undergo nucleophilic substitution. The products are a phenol and an alkyl halide, and only HI (a good nucleophile) is effective.
(iv) Hot concentrated H2SO4 cleaves secondary and tertiary ethers into an alcohol and an alkene. The ease of alkene formation is 3° > 2° > 1°.

4. With PCl5: diethyl ether gives two molecules of ethyl chloride and POCl3.
5. With acid chlorides or anhydrides: heated with anhydrous ZnCl2, an ether and acetyl chloride give an alkyl chloride and an ester; with anhydrides and anhydrous AlCl3 only esters are formed.
6. Peroxide formation: on long exposure to air and light ethers form hydroperoxides (diethyl ether gives 1-ethoxyethyl hydroperoxide), and distilling an old sample can cause an explosion. Peroxides are detected by shaking the ether with freshly prepared FeSO4 solution followed by KCNS (a red colour shows peroxide), and they are removed by shaking the ether with aqueous KI or a ferrous salt.
7. Hydrolysis: dilute aqueous acid under pressure converts ethers to alcohols.

Epoxides (Oxiranes)

Epoxides are three-membered cyclic ethers, ethylene oxide being the commonest. The ring strain makes them reactive toward nucleophiles, unlike ordinary ethers.
Preparation: (1) partial oxidation of ethylene with air over silver; (2) epoxidation of an alkene with a peracid such as perbenzoic acid, peracetic acid or mCPBA. This is a stereospecific cis addition of oxygen, so a cis-alkene gives a cis-epoxide and a trans-alkene gives a trans-epoxide.

Ring-opening reactions:
Acid-catalysed opening (SN1-like): the epoxide is protonated and the nucleophile attacks the more substituted carbon, which carries more positive charge.
Base-catalysed opening (SN2): a strong nucleophile such as an alkoxide or hydroxide attacks the less hindered carbon from the back.
A symmetrical epoxide gives the same product under both conditions; an unsymmetrical epoxide gives different products.
Grignard reagents and LiAlH4 are strong nucleophiles and attack the less substituted carbon, so with RMgX the product is an alcohol with two more carbons than R, and with LiAlH4 an alcohol where the hydride is on the less substituted carbon.

Aromatic Ethers

The alkoxy group is activating and o-, p-directing, like OH in phenol. Anisole is brominated by Br2 in ethanoic acid without a catalyst, and undergoes Friedel-Crafts alkylation and acylation and nitration to give mainly the p- and o-substituted products.
Claisen rearrangement: allyl phenyl ether (made from sodium phenoxide and allyl bromide) on heating rearranges to o-allylphenol.

Worked Problem on Epoxides

Worked Problem 1. 2,2-Dimethyloxirane (isobutylene oxide) is treated with (a) methanol and acid and (b) sodium methoxide in methanol. What is the main product in each case?
Answer:
(a) Under acid catalysis the nucleophile (methanol) attacks the more substituted, tertiary carbon: product 2-methoxy-2-methylpropan-1-ol.
(b) Under basic conditions methoxide attacks the less hindered, primary carbon from the back: product 1-methoxy-2-methylpropan-2-ol.