Nomenclature, Preparation and Physical Properties of Ethers

An ether's defining structural feature -- two carbon groups joined through a single oxygen, with no

hydrogen left on that oxygen -- shapes both how it is best prepared and why it behaves so

differently, physically, from an isomeric alcohol.

Williamson ether synthesis. The standard, general laboratory method for preparing an ether of

essentially any desired combination of alkyl groups is the Williamson synthesis: a sodium (or

potassium) alkoxide, $\text{R--O}^-\text{Na}^+$ (made by dissolving the corresponding alcohol in

sodium metal or reacting it with NaH), is treated with a primary alkyl halide,

$\text{R}'\text{--X}$, and the alkoxide's oxygen displaces the halide directly in a clean $S_N2$

substitution:

For example, sodium ethoxide with bromomethane gives methoxyethane,

$\text{CH}_3\text{CH}_2\text{ONa} + \text{CH}_3\text{Br} \rightarrow \text{CH}_3\text{CH}_2\text{OCH}_3 + \text{NaBr}$. The alkyl halide partner must be primary

(and, ideally, unhindered) for the reaction to work cleanly: the alkoxide ion is not just a

nucleophile but also a moderately strong, somewhat bulky base, so if the halide were secondary

or tertiary, the alkoxide would instead tend to remove a beta-hydrogen from the halide in a

competing E2 elimination, forming an alkene as the major product rather than the desired

ether -- for a tertiary halide, elimination dominates almost completely. The correct strategic

pairing, therefore, always puts the bulkier of the two target R groups into the alkoxide half (as

the base/nucleophile) and reserves the less hindered, primary group for the halide half, regardless

of which group is 'larger' in the final ether's name.

Preparation by dehydration of alcohols. A symmetrical ether can also be made directly from two

molecules of the same primary alcohol, heated with concentrated $\text{H}_2\text{SO}_4$ at a

moderate temperature (around $413\ \text{K}$/$140\,°\text{C}$ for ethanol, lower than the

temperature that favours alkene formation from the same alcohol):

This route is only practical for primary alcohols (secondary and tertiary alcohols dehydrate to

alkenes far too readily under these acidic conditions to give useful ether yields) and only for

symmetrical (same-alcohol) ethers, since mixing two different alcohols under these conditions gives

a statistical, hard-to-separate mixture of all three possible ethers.

Physical properties. Because an ether's oxygen has no hydrogen of its own, ether molecules

cannot hydrogen-bond to one another the way alcohol molecules can -- so an ether boils far

lower than the isomeric alcohol of identical molecular formula, despite the two compounds having

identical molar mass (Example 15 works through this exact comparison for ethanol versus …