20.2 Alcohols and Ethers

Alcohols and Ethers

Alcohols and ethers are two families of organic compounds built around oxygen. Alcohols have a hydroxyl group ($-OH$) bonded to a carbon, while ethers have an oxygen atom sitting between two carbon groups. That single structural difference leads to big differences in boiling points, solubility, and reactivity, all of which trace back to intermolecular forces.

Alcohols

Structure and Properties of Alcohols

The defining feature of an alcohol is the hydroxyl group ($-OH$) covalently bonded to a carbon atom. Because oxygen is much more electronegative than carbon or hydrogen, the $-OH$ group makes the molecule polar.

That polarity has two major consequences:

• Hydrogen bonding between alcohol molecules. The partially positive hydrogen on one molecule's $-OH$ is attracted to the partially negative oxygen on a neighboring molecule. This extra intermolecular attraction is why alcohols have noticeably higher boiling points than alkanes of similar molecular weight. For example, methanol ($CH_3OH$, MW ≈ 32) boils at 64.7 °C, while methane ($CH_4$, MW ≈ 16) boils at −161 °C.
• Water solubility that depends on chain length. The $-OH$ group can hydrogen-bond with water, so short-chain alcohols like ethanol mix freely with water. As the nonpolar hydrocarbon chain gets longer, though, the molecule becomes more "oil-like" and less soluble. 1-Octanol, for instance, barely dissolves in water at all.

Alcohols are also classified by how many other carbon atoms are attached to the carbon bearing the $-OH$:

• Primary (1°): The $-OH$ carbon is bonded to one other carbon.
• Secondary (2°): The $-OH$ carbon is bonded to two other carbons.
• Tertiary (3°): The $-OH$ carbon is bonded to three other carbons.

This classification matters because it affects how the alcohol behaves in certain reactions, particularly oxidation. Primary alcohols can be oxidized to aldehydes and then to carboxylic acids, secondary alcohols can be oxidized to ketones, and tertiary alcohols resist oxidation because there's no hydrogen on the $-OH$ carbon to remove.

Nomenclature for Alcohols

Naming alcohols in the IUPAC system follows a clear set of steps:

1. Find the longest carbon chain that includes the carbon bonded to the $-OH$ group.
2. Replace the "-e" ending of the parent alkane name with "-ol".
3. Number the chain so the $-OH$ group gets the lowest possible number.
4. If there are multiple $-OH$ groups, use "-diol," "-triol," etc., and number each position (e.g., ethane-1,2-diol).

Some common names you should recognize:

• Methanol: $CH_3OH$
• Ethanol: $CH_3CH_2OH$
• Isopropyl alcohol (2-propanol): $CH_3CH(OH)CH_3$

Ethers

Characteristics and Uses of Ethers

Ethers have the general formula R-O-R', where R and R' are alkyl (or aryl) groups. The $C-O-C$ bond angle is about 110°, giving the oxygen a bent geometry similar to water.

The critical difference from alcohols: ethers have no $O-H$ bond, so they cannot hydrogen-bond with each other. This means ethers have much lower boiling points than alcohols of similar molecular weight. Diethyl ether (MW ≈ 74) boils at just 34.6 °C, while 1-butanol (MW ≈ 74) boils at 117.7 °C.

Ethers are also relatively unreactive because the $C-O$ bonds are strong and hard to break. That chemical stability, combined with their ability to dissolve many organic compounds, makes them popular solvents in the lab. A few important examples:

• Diethyl ether ($CH_3CH_2{-}O{-}CH_2CH_3$): historically used as an anesthetic; still a common lab solvent.
• Tetrahydrofuran (THF): a cyclic ether widely used as a solvent in reactions like Grignard reactions.
• 1,4-Dioxane: another cyclic ether used in organic synthesis.

Nomenclature for Ethers

For common naming, list the two alkyl groups attached to the oxygen in alphabetical order, then add the word "ether":

• $CH_3{-}O{-}CH_2CH_3$ → ethyl methyl ether
• $CH_3{-}O{-}CH_3$ → dimethyl ether
• $(CH_3)_3C{-}O{-}CH_3$ → methyl tert-butyl ether (MTBE)

If both alkyl groups are the same, use the "di-" prefix (diethyl ether).

Cyclic ethers can be named by replacing a $CH_2$ in the corresponding cycloalkane name with "oxa-" and assigning the oxygen the lowest possible number. THF, for example, can be called oxacyclopentane.

Intermolecular Forces and Physical Properties

The physical properties of alcohols and ethers come down to one key factor: the type of intermolecular forces each compound can form.