Basic Chemistry: Unsaturation – Alkenes and Alkynes

Even those scientifically un-inclined recognize the words saturated, unsaturated, and polyunsaturated - terms closely associated with dietary health. Most know these are chemical terms, though such terms go generally unexplained. This is a shame, since saturation and un-saturation can be explained at a level easily understood by practically any bright student.

Saturated Hydrocarbon Linkage

Living matter, whether plant or animal, consists of organic compounds, among which are fats, carbohydrates, and proteins. All these chemicals contain a kind of backbone or chain, consisting of atoms of carbon (C) bonded to hydrogen (H). Each atom of carbon is capable of bonding with four atoms of hydrogen. For purposes of illustration, we'll assume we have before us a chain of four carbon atoms, with every available bond possibility occupied by hydrogen atoms - in other words, n-butane, the "n" standing for a normal, or straight backbone. The structure can be drawn: CH₃‐CH₂‐CH₂‐CH₃. The possible unsaturated modifications of this four-carbon chain will well illustrate much of what we will discuss here.

Geometry of Saturation

The above drawing of the butane molecule is a little misleading. Saturated carbon forms tetrahedral angles of approximately 109.5° not the 180° angles required to form a backbone that lays flat like a straight line. It is thus more accurate to picture a chain of carbon in zigzag formation, as is illustrated in the image associated with this article, Figure 1a, with the teeth having a slightly obtuse angle. Since molecules are rarely stretched out, figure 1b suggests a more accurate representation.

Unsaturated Alkenes

If two hydrogen atoms are removed from adjacent carbon atoms of a saturated carbon-hydrogen backbone, with a second bond forming between the two affected carbons, the result is an alkene. In fact, there are two possible locations for the double bond to form. One is between carbon atoms 1 and 2 (fig. 1c); the other is between carbon atoms 2 and 3 (fig. 1d). There is nothing new if the double bond is drawn between carbon atoms 3 and 4; a mere flip of the molecule shows this is identical to the molecule in fig. 1c. Unlike the flexibility of the single bond, a double-bond is fairly rigid - it locks the two carbon atoms and their immediately attached four atoms in a plane.

Cis‐ and Trans‐ Alkenes

If the double bonded carbon atoms of an alkene is each bonded to another carbon atom, two classes of compound can form: cis-alkenes and trans-alkenes. If, of the four branches attached to the double bonded carbon, two are hydrogen atoms and the other two branches contain carbon, there can exist what are called cis- and trans-isomers of the very same compounds. The difference can be simply illustrated by imagining two carbon-based branches, Branch A and branch B. If such is the case, the compound 1e is cis, and the compound in 1f is trans. The cis-isomer has both branches on the same side of the rigid double bond, whereas the trans-isomer is trans or across the double bond. Generally speaking, cis- chemistry is the chemistry of life.

Alkynes

Remove another two hydrogen atoms from the two carbon atoms of a double bond, and an alkyne is the result (fig. 1g). The carbon atoms of a triple bond, as well as the immediately attached two atoms to those carbons, are locked into a straight-line configuration. Atoms further away are free to rotate. There are no additional isomers such as a cis- or a trans- introduced by the presence of alkyne linkage. If one of the atoms attached to an alkyne carbon is a hydrogen atom, that atom is said to be acidic - it can often be replaced by a metal atom. One thus needs to be careful what kind of container he puts a liquid alkyne such as acetylene (HC≡CH) into, if he is to avoid chemical attack. The compound resulting from attack of acetylene on metal is an acetylide, some of which are quite explosive.

Polyunsaturated

Monounsaturated - as used in the popular media - means a molecule (for example, peanut oil) that contains only one double bond. Polyunsaturated means each molecule contains a multiple number of double bonds.¹ One example of a polyunsaturated molecule, found in many common vegetable oils, is α-linolenic acid, which contains three conjugated (alternating) double bonds. All of its double bonds take the cis-format. It is categorized as a polyunsaturated, omega-3 fatty acid.

¹ Strictly speaking, the term polyunsaturated could refer to the presence of multiple triple bonds, but this is never the intended meaning in commercial advertisements.