 Alkenes are the second group of hydrocarbons and differ from alkanes in that they have a double bond. The presence of a double bond brings up some important structural and functional changes. Let’s first mention that the C=C double bonds reduce the number of hydrogens per carbons and the general formula changes to C2H2n vs the C2H2n+n of the alkanes. This corresponds to one degree of unsaturation – hydrogen deficiency index (HDI). The root name is the same, however, the suffix changes from “ane” to “ene”: There is a separate, detailed post about all the rules for naming alkenes, so feel free to check that out as well. The Structure of AlkenesThe two carbons of a C=C double bond are sp2-hybridized. Remember, the sp2 hybridization occurs when the s orbital is mixed with two p orbitals and therefore, three orbitals are mixed, and the outcome is three hybrid orbitals which are called sp2 hybrid orbitals. The two sp2 hybridized carbon atoms then make a sigma (σ) bond by a linear overlap and one π bond by a side-by-side overlap of the two 2p orbitals on each carbon. Ethylene can be used as an example to illustrate the structure and geometry of a typical double bond: As a short summary for the key parameters about double bonds, remember that:
Restricted RotationYou may remember from alkanes and cycloalkanes that they were able to adopt different conformations via the free rotation about sigma bonds. For example, this flexibility allows for the ring flip, and in general, all the struggle with Newman projections that you may have gone through is due to the free rotation about C-C single bonds. Now, unlike the single bonds, the C=C double bond is locked as there is no rotation about the bond without breaking it. Because of this we have the cis and trans isomerism of alkenes. In short, if the two identical groups (alkyl or hydrogen) on both carbon atoms of the double bond are on the same side of the double bond, then it is cis and if they are on opposite sides of the double bond, then we have a trans isomer: The cis and trans alkenes are stereoisomers and because they are not mirror images, they are diastereomers: If there are no two identical groups, then the E and Z notation is used which is a more universal approach for classifying the configuration of alkenes. Yes, cis/trans and E/Z notations describe the configuration of and not conformation of alkenes since, remember, there is no rotation about a C=C double bond. Physical Properties of AlkenesAlkenes can have different states of matter from a gas to a solid depending mainly on their molecular mass: Like for any other class of compounds, the boiling point of alkenes typically increases with stronger intermolecular interactions which can be London forces or dipole-dipole interactions when halogens are present in the molecule. The melting point, on the other hand, depends also on the symmetry of the molecule. More symmetrical molecules pack better in the solid phase and therefore have higher melting points. Trans alkenes usually have a higher melting point then cis alkenes. An interesting example is the comparison of melting and boiling points of cis– and trans alkenes. Despite being nonpolar, the trans isomer of 1,2-dichloroethane has a higher melting point (−50 oC) than the cis isomer (−80 oC) because of higher symmetry which allows for compact packing in the solid phase. In contrast, the cis isomer is a polar molecule with a higher boiling point (60 oC vs 48 oC ) because of the net molecular dipole moment and intermolecular dipole-dipole interactions. These are all interesting and each concept here deserves a separate post, which are right here to go over! Stability of AlkenesThere are two factors to consider when deciding the more stable alkene. One is the sterics which is expressed when comparing cis and trans isomers – trans alkenes are more stable because of a less steric hindrance: Sterics is the fancy word for the space available to atoms/groups. High steric strain means less space, bulky groups, and therefore, less stability. In cis alkenes, the two (larger) alkyl groups are on the same side of the double bond fighting for space and yet cannot switch to a trans arrangement. The second factor is the number of alkyl groups bonded to the double bond; the more alkyl groups, the more stable the double bond: Another wording for this is the classification of alkenes as monosubstituted, disubstituted, trisubstituted and tetrasubstituted. In other words, the stability of alkenes increases with the substitution: An alkene with two substitutes on one sp2 carbon has about the same stability as the corresponding trans alkene: Now, the question is why having more alkyl groups connected to the C=C bond makes it more stable? One way, suitable for a typical undergraduate class, to look at it is the electronegativity of the sp2 and sp3-hybridized carbon atoms. Remember about the “s character” which is referred to the % of the s orbital initially involved in the hybridization process. In the sp3 hybridization, there is a total of four orbitals – one s and three p, and out of these only one is (was) an s. Therefore, the s character of an sp3 orbital is ¼ = 25%. The sp2 orbitals, on the other hand, have a 33% s character: Higher s character means a more electronegative atom because s orbitals are smaller and more electronegative than p orbitals. This has to do with the fact that in a smaller orbital, the attraction force is stronger, and the electrons are closer to the nucleus. Now, the sp2 hybridized carbon atoms in the C=C double bond, being more electronegative, readily accept the electron density from sp3 hybridized carbon atoms of the alkyl group(s). So, the more electron-donating alkyl groups on the C=C carbon atoms, the more stable the alkene. Therefore, we have this pattern of stability: Tetrasubstituted > Trisubstituted > Disubstituted > Monosubstituted. To summarize, remember that:
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