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Alcohol - Physical and chemical properties

Alcohol - Physical and chemical properties: Encyclopedia II - Alcohol - Physical and chemical properties

The hydroxyl group generally makes the alcohol molecule polar. Those groups can form hydrogen bonds to one another and to other compounds. Two opposing solubility trends in alcohols are: the tendency of the polar OH to promote solubility in water, and of the carbon chain to resist it. Thus, methanol, ethanol, and propanol are miscible in water because the hydroxyl group wins out over the short carbon chain. Butanol, with a four-carbon chain, is moderately soluble because of a balance between the two trends. Alcohols of five or more carbons (Pentanol and higher) are effecti ...

Alcohol: Encyclopedia II - Alcohol - Physical and chemical properties

Alcohol - Physical and chemical properties

Because of hydrogen bonding, alcohols tend to have higher boiling points than comparable hydrocarbons and ethers. All simple alcohols are miscible in organic solvents. This hydrogen bonding means that alcohols can be used as protic solvents.

The lone pairs of electrons on the oxygen of the hydroxyl group also makes alcohols nucleophiles.

Alcohols, like water, can show either acidic or basic properties at the O-H group. With a pK_a of around 16-19 they are generally slightly weaker acids than water, but they are still able to react with strong bases such as sodium hydride or reactive metals such as sodium. The salts that result are called alkoxides, with the general formula RO^- M⁺.

Alcohols conjugated to aromatic rings have a lower pKa (around 10). Electron-withdrawing groups also work to make alcohols more acidic. For example, Para-nitro phenol has a pKa of 7.15.

Meanwhile the oxygen atom has lone pairs of nonbonded electrons that render it weakly basic in the presence of strong acids such as sulfuric acid. For example, with methanol:

Alcohols can also undergo oxidation to give aldehydes, ketones or carboxylic acids, or they can be dehydrated to alkenes. They can react to form ester compounds, and they can (if activated first) undergo nucleophilic substitution reactions. For more details see the #Chemistry of alcohols section below.

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Adapted from the Wikipedia article "Physical and chemical properties", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki

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