3.14 Aldehydes

Aldehydes are analogous to carboxylic acids except they end in -CHO instead of -CO₂H. Remember the O in -CHO is double-bonded to its carbon atom. Aldehydes are named in a few different ways. Some are named by analogy with the carboxylic acids; formaldehyde HCHO is the aldehyde equivalent of formic acid, and there is also acetaldehyde CH₃CHO, responsible for hangovers, and higher order aldehydes like propionaldehyde, butyraldehyde, etc. Most aldehydes are oil-soluble liquids.

Isobutyraldehyde (CH₃)₂CHCHO occurs in grains and cereals and contributes to their characteristic malty flavor.

Benzaldehyde is C₆H₅CHO and is the aroma and flavor of bitter almond. It also contributes to fruit flavors, especially cherry and other members of the rose family, as well as to many flower aromas.

Cinnamaldehyde C₆H₅CH=CHCHO occurs naturally in cinnamon and, as an artificial flavor, is responsible for the taste, the spiciness, and some of the sweetness of cinnamon candy.

Vanillin, which we saw in lesson 2.4, is simultaneously an aldehyde, an alcohol, a phenol, and an ether.

Aldehydes often have the ending -al such as citral (CH₃)₂C=CHCH₂CH₂C(CH₃)=CHCHO, the main component of lemon oil.

Aldehydes used in perfumery are often indicated by their carbon number, e.g. aldehyde C₆ is hexanal CH₃CH₂CH₂CH₂CH₂CHO; aldehyde C₈ is octanal, C₁₀ is decanal, etc. There is more than one aldehyde C₁₂ thanks to methyl nonyl acetaldehyde or aldehyde MNA, CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH(CH₃)CHO, a very wonderful smelling ingredient reminiscent of fresh lemon-lime soda and candle wax. The other aldehyde C₁₂ is lauric aldehyde or dodecanal, a straight chain aldehyde with a soapy smell. Some perfumery ingredients are incorrectly called aldehydes such as "aldehyde C₁₄", "aldehyde C₁₆", and "aldehyde C₁₈", which aren't aldehydes at all and were only named that by their manufacturers in order to confuse competing businesses. It's really sad that corporate interests can contaminate the accuracy of naming of molecules in this way.

Aldehydes with more than a certain number of carbon atoms are sometimes called fatty aldehydes, by analogy with fatty acids. Fatty aldehydes tend to be oily liquids. There are also fatty alcohols, such as decanol and dodecanol, again by analogy with fatty acids/aldehydes and because they too have similar physical properties.

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