Alkenes - Properties and Trends

What's an Alkene?

An alkene is an organic compound that contains at least one carbons double bonded to another carbon. The general formula for an alkene is CnH2n. Historically, alkenes have been referred to as olefins.

Markovnikov's Rule

Vladimir Markovnikov, a Russian chemist, described an observation based on the reaction of halogen acids (HX, or more specifically HF, HCl, HBr, and HI) or water with alkenes. When such a reaction occurs, the double bond of the alkene will be broken. The hydrogen bonds to the carbon with the greater number of hydrogens (fewer substituents) and "X" (which represents the halogen, or the hydroxide ion if the reaction is with water) bonds to the carbon with fewer hydrogens (more substituents). Although the reverse can happen, it is considered the minor product. The major product follows what is now known as Markovnikov's rule.

Reactions - Halogenation

The halogens, which are diatomic, attack the double bond of an alkene. In breaking one of the bonds, a bonds is opened on adjacent carbons. An atom of the diatomic halogen attaches to the open spot on each carbon. Consider the example below, the reaction of propene with chlorine:

Compounds of the formula HX, where X is a halogen, will also react with alkenes. However, there is selectivity as to which carbon the hydrogen and the halogen will attach. According to Markovnikov's rule, the halogen will attach to the carbon that has more carbons already attached to it. It could also be said that the hydrogen will attach to the carbon that has the fewer carbons attached to it. Shown below is the reaction between propene and HBr:

According to Markovnikov's rule, the hydrogen in HBr would prefer the first position carbon since there are already two hydrogens bonded to it. The middle carbon (second position) only has one hydrogen bonded to it and will feature the bromine.

Reactions - Hydration

Hydration is the process of reacting water with an alkene, usually utilizing a catalyst such as sulfuric acid. The reaction follow's Markovnikov's rule, as the water molecule attacks the double bond so that a hydroxyl ion (OH-) bonds to one carbon and a proton (H+ to the other carbon. ΔHreaction values are very negative (exothermic) for reactions of this type.

Cis/Trans Isomerism and Properties

The table below can help investigate the effect of cis/trans isomerism in alkenes.

Name Melting Point (°C) Boiling Point (°C) ΔHf°
cis-2-butene -138.88 3.71 -29.8
trans-2-butene -105.52 0.88 -33.3
cis-2-pentene -151.36 36.93 -53.7
trans-2-pentene -140.21 36.34 -58.2
cis-2-hexene -141.11 68.8 -83.9
trans-2-hexene -133 67.9 -85.5

Firstly, note the differences in each property's values for each pair. The cis- isomer always has a higher melting point than the trans- isomer, although this is less pronounced as carbon chain length increases. The boiling point values are much closer, and the cis- isomer has a higher boiling point. Lastly, for each compound the heat of formation value is more negative for the trans- isomer. So, what does all of this mean?

The melting and boiling point trends come down to polarity and symmetry. In the compounds above, there are no functional groups present, so the effect of symmetry is the only factor to consider. Trans- isomers tend to be more symmetrical and more likely to resemble a straight chain. For this reason, trans- isomers can pack together more closely in the solid state, and require more energy to separate them. Thus, they have higher melting points. The trend is reversed for boiling points, where the value is higher for the cis- isomers. Since there must be a significant amount of separation between molecules to change it from liquid to gas, making polarity a significant factor for looking at boiling points. The greater chance for cis- isomers to intertwine with each other causes their boiling points to be slightly higher. Recall that in order to boil a substance, a great deal of separation must be created between molecules, and this is somewhat more difficult when molecules can interlock with each other. The addition of a functional group will certainly raise the boiling point of both isomers, but because of a greater dipole moment (due to the lack of symmetry), the cis- isomer will be raised more than the trans- isomer.

The heat of formation values can be used to make a statement about stability. The more negative a heat of formation value, the more thermodynamically stable it is. The trans- isomer is more stable (not surprising in light of the symmetry and polarity discussed earlier) and has a more negative heat of formation value.