Aldehydes are the next family of hydrocarbon derivatives. In the aldehyde family, most compounds are liquids, except for the one-carbon aldehyde, formaldehyde, which is a gas. Aldehydes have a wide flammable range from 3 to 55% in air; they are toxic and may polymerize. They have moderate boiling and flash points, and high ignition temperatures. Fires involving aldehydes should be fought with polar-solvent type foams because water may be ineffective. Aldehydes may also form explosive peroxides as they age much the same way ethers do.
Aldehydes are composed of a carbon double-bonded to oxygen with a hydrogen atom on the other carbon connection. Aldehydes are carbonyls and, therefore, polar. The degree of polarity is much the same as ketone and ester, and much less than alcohol and organic acid. They are miscible in water and require the use of polar-solvent foams to extinguish fires. Aldehydes have the general formula of R-CHO. There is one radical attached to the carbon of the aldehyde functional group. Aldehydes are one of the three derivatives in which the carbon in the functional group is counted when naming the compound.
The alternate terms for one- and two-carbon radicals are also used with the aldehydes. A one-carbon aldehyde uses "form" and a two-carbon uses "acet" as a prefix. The aldehydes are named by identifying the radical, naming it, and ending with the word aldehyde. Aldehydes may also be named in the same manner as the alternate names for alcohols; however, with the aldehydes, the ending is "al" instead of "ol". For example, a one-carbon aldehyde is called formaldehyde with the alternate name of methanal. In the following examples, the structures, molecular formulas, physical and chemical characteristics, and names are shown for one-, two-, three-, and four-carbon aldehydes.
Formaldehyde, HCHO, is an aldehyde hydrocarbon derivative. Formaldehyde is a gas with a strong, pungent odor. It readily polymerizes. Commercially it is offered as a 37 to 50% solution, which may contain up to 15% methanol to inhibit polymerization. Boiling points for solutions range from 206
