A cycloalkene (also sometimes called a cycloolefin) is a type of alkene hydrocarbon which contains a closed ring of carbon atoms. The prefix cyclo- comes from ancient Greek and in this case means round. Whenever the ends of a carbon chain are joined together, that molecule is said to be cyclic, and alkenes are no different than other carbon chains in that respect.
Cycloalkenes are perhaps among the most important organic substances for biological and industrial purposes because they are used in the production of molecules essential to a broad spectrum of applications. Understanding cycloalkenes is a critical part of understanding organic chemistry and its applications to biology, medicine, industry and every other relevant field.
What They Are
As stated above, a cycloalkene is merely a chain of carbon molecules containing at least one double bond between two carbon atoms that has joined two ends to form some type of ring structure. The number of carbons involved and the number of double bonds shared affect molecule stability and other properties, but those factors do not affect the definition of what a cycloalkene is.
If an organic chemist knows the empirical formula of a molecule and also knows that it is a cyclic molecule, she can derive the shape of the molecule from this information. She does so using the concept of sites of unsaturation. As you know from the chapter on alkanes, a saturated hydrocarbon chain is one that has only single bonds between carbon atoms and enough hydrogens attached to give every single carbon a total of four molecular bonds. If there are four carbon atoms, then using the saturation formula below we can calculate that there will be ten hydrogen atoms attached.
- nC * 2 + 2 = nH
Where nC equals the number of carbon atoms and nH equals the number of hydrogen atoms.
But what if the empirical formula we are given does not show ten atoms of hydrogen for every four atoms of carbon? What if it only shows eight? You might guess that there was an error, but it is more likely that the molecule features one site of unsaturation where either a double bond has formed (as in an alkene) or the molecule is cyclic (where the two ends have joined together to form a ring). The formula for calculating the number of double bonds in an alkene, and for accounting for cyclic structures, is below:
- ALKENES: (nC * 2 + 2) - (2 * nDBL) = nH
- CYCLOALKENES: (nC * 2 + 2) - (2 * nDBL) - 2 = nH
Where nC equals the number of carbon atoms, nH equals the number of hydrogen atoms, and nDBL equals the number of double bonds. For cycloalkenes, the - 2 just before the equal sign signifies the fact that two hydrogen atoms are displaced when a carbon chain joins from head to tail in order to form a ring.
Cycloalkenes have slightly fewer bonds than equivalent straight-chain alkenes, which means that they have less internal energy. The additional site of unsaturation due to the ring means that there are two fewer hydrogen atoms on the molecule, which is why there are fewer bonds in cycloalkenes than in straight chains.
On the macroscopic scale, cycloalkenes are most often found as liquids at standard temperature and pressure. This is because a certain number of carbons are needed to make ring structures, which means the molecules are generally too heavy to be gaseous at STP. Likewise, too many carbon atoms makes for too large a ring, so cycloalkenes are very rarely solid at STP as well.
Depending on which functional groups and side-chains a molecule may possess, some cycloalkenes can polymerize at STP thus forming a solid substance, but this should be considered a reasonable exception to the generalities listed above.
Alkenes contain double bonds by definition, and cycloalkenes benefit from enhanced stability if the double bonds are conjugated. Cyclohexene is one such case, where if it contains a total of three double bonds it is no longer called cyclohexene. A six-carbon ring with three alternating (and therefore conjugated) double bonds is called benzene, and benzene is such a stable molecule that it displays completely different chemistry than other non-aromatic cycloalkenes.
Star of the Show: Benzene
Benzene is critical molecule to the study of organic chemistry. It is a colorless and flammable carcinogenic liquid at STP, with a pleasant, sweet smell.
Benzene presents a special problem in that, to account for all the bonds, there must be alternating double carbon bonds. Using X-ray diffraction, researchers in the early 20th century discovered that all of the carbon-carbon bonds in benzene are of the same length, but it was well known at the time that a single bond should be longer than a double bond. In benzene, the distance between the two bonded atoms was greater than the length of a double bond, but shorter than the length of a single bond. There seemed to be, in effect, a bond and a half between each carbon!
Today this fact can be explained by the concept of electron delocalization. Essentially, the electrons are not held tightly by one carbon atom or another, but instead they are somewhat "smeared" out, like cream cheese around the edges of a bagel. The actual physics of the situation are far more complex than breakfast-food analogies could do justice, but the essential concept is the same. The electrons - and thus, the electronegativity - of a benzene ring are spread evenly around the molecule.
This electron delocalization gives benzene many interesting and useful properties, as you will see in later chapters. Of course benzene isn't the only molecule in organic chemistry with "smeared" electrons; the delocalization of electron density is also useful in discussing other molecules, which are called aromatics.
Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone electron pairs, or empty electron orbitals exhibit a stabilization stronger than would be expected by other factors alone. Aromaticity can also be considered a manifestation of resonance, in that multiple possible conformations of the aromatic molecule contribute to its stability.
An aromatic compound contains a set of covalently-bound atoms with very specific characteristics, namely: a delocalized conjugated pi system, a coplanar structure, atoms arranged in one or more rings, and a number of pi delocalized electrons that is even but not a multiple of four.
The permissible numbers of pi electrons in an aromatic structure are generally the even numbers not divisible by four, which would include 6, 10, 14, and so on. Aromaticity is most often estimated using Hückel's rule:
- A cyclic ring molecule should be aromatic when the number of π electrons equals 4n + 2, where n is any positive integer or zero.
But there have been exceptions found to this rule in more recent decades.
Aromatic molecules display special reactivity in organic reactions -- for example electrophilic aromatic substitution and nucleophilic aromatic substitution -- which can be used distinguish them from other cycloalkenes.
Uses of Benzene
Benzene is an important industrial solvent and a precursor in the industrial production of many types of drugs, plastics, synthetic rubbers, and dyes. It is used in most organic compounds of commercial value because it is so very versatile and has many desirable chemical properties. It is also used as a solvent in certain organic chemistry laboratory experiments, but great care should be taken when using it due to its carcinogenic properties.
Sources of Benzene
Benzene can be found naturally occurring in crude oil, but today it is also often synthesized from other compounds which can be found in unrefined petroleum. Another source of benzene is benzoic acid and certain other biological compounds, but the first industrial source of benzene was distillation from coal tar.