Keen on Chemistry

Alkanes and alkyl groups are great and all, but all their bonds are single. Are there hydrocarbons with double or triple bonds? Yes! Yes, there are. An alkene (pronounced al-keen) is a hydrocarbon with at least one double bond in it, and an alkyne (the last syllable rhymes with "fine") has at least one triple bond. We've already seen ethylene CH₂=CH₂ and acetylene CH≡CH, but another name for acetylene is ethyne.

Longer carbon chains can have a double or triple bond anywhere along their length. There is only one kind of propene (or propylene), because CH₂=CH-CH₃ is the same thing as CH₃-CH=CH₂. But butene comes in isomers: 1-butene or CH₂=CHCH₃CH₃ and 2-butene or CH₃CH=CHCH₃. Similarly, hexene has isomers 1-hexene, 2-hexene, and 3-hexene.

Butene and hexene have even more isomers than that because of the shape of the double bond. It can be thought of as a flat plane that doesn't rotate. Single bonds can (ordinarily) freely rotate, so is the same as , but is not the same as .

When the two substituents are on opposite sides, as in , the double bond is called trans, from the Latin word for "across". Under IUPAC naming conventions, this is written (E), which derives from some hard to remember German word. It's easiest to think of (E) with the mnemonic "they are like enemies."

When the substituents are on the same side, as in , the double bond is called cis, from the Latin word for "on this side". IUPAC calls this (Z), also from some German word, but it's easy to remember as "zey are on ze same side". One common compound that includes a cis double bond is leaf alcohol, also known as hexenol, cis-3-hexenol, or (Z)-3-hexen-1-ol. It occurs naturally in pretty much every vascular plant, and is the quintessential green aroma note. Leaf alcohol smells strongly of fresh mown grass mixed with fluorescent marker, and its skeletal formula looks like this: .

Alkenes also form groups that can act as substituents. The two that are recognized by IUPAC are vinyl, CH₂=CH-, and allyl, CH₂=CH-CH₂-. The material commonly known as "vinyl" is actually a family of similar polymers made from vinyl compounds. When exposed to a catalyst, the CH₂=CH-R molecule, where R is some group of atoms, breaks up its double bond and forms a series of repeating -CH₂-CH(R)- units. The most common type of vinyl is polyvinyl chloride or PVC. It is formed when CH₂CHCl polymerizes to -(-CH₂CH(Cl)-)-_n, where n is an arbitrarily large number. Allyl compounds occur naturally in such foods as onions, garlic, and mustard, usually in combination with sulfur, in fact allyl itself is named for garlic and onions (genus Allium).

Alkanes and their derivatives are called saturated because they have all the hydrogen they can hold. Alkenes and derivatives with one double bond are called mono-unsaturated, and with more than one double bond are poly-unsaturated. The healthiest dietary fats are cis-polyunsaturated fats because they have the lowest melting point and are the least likely to clog arteries. Trans fats are less healthy than cis fats, and saturated fats have the highest melting points and are the least healthy.

Alkynes and their derivatives are not very common.

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