Drawing the four sp³ orbitals as slightly elongated single lobes (emphasizing the major portion of the orbital drawn above) for illustration purposes, a carbon atom would "look" something like the tetrahedral sketch below.

A carbon-hydrogen covalent bond forms when the 1s orbital of a hydrogen atom overlaps one of the hybrid orbitals (in the case of an alkane, an sp³ orbital). Note that we represent electrons as arrows for the purposes of this illustration: these arrows are oriented either up or down to indicate their relative spins (or spin quantum numbers). Recall that for two electrons to exist in the same orbital, they must have different spin quantum numbers.

Another view of bonding is that free atoms have atomic orbitals, but atoms in molecules have molecular orbitals. The total number of molecular orbitals is the sum of the atomic orbitals, and these molecular orbitals are divided into lower-energy (and thus more stable) bonding orbitals and higher-energy (and thus less stable) antibonding orbitals. Consider, again, the simple case of two hydrogen atoms. Each atom has one atom in its atomic orbital, but when combined to form H₂, these atoms form two molecular orbitals. Following the sigma bond notation, the bonding orbital is labeled ?, whereas the antibonding orbital is labeled ?*.

This molecular orbital model is complementary-rather than contradictory-to the hybrid orbital model ("valence bond model") discussed previously. Each can be helpful in its own way in the study of organic chemistry.

Intermolecular Forces

At first glance, alkanes-and other organic molecules with a balance of positive and negative charges-would seem to tend to "ignore" each other, since they are net neutral molecules. But electrons are not always evenly distributed around nuclei, and as such, molecules may actually be electric dipoles (that is, they may have a dipole moment). Dipoles, even though they may, as a whole, be charge neutral, nonetheless exhibit a weak electric field that can influence other charges, dipoles, or even neutral molecules with no inherent dipole moment. Furthermore, the mobility of electrons can actually cause two neutral molecules with no inherent dipole moments to induce such dipole moments in each other (this electron mobility also explains why two neutral hydrogen atoms can attract each other). These induced dipoles may then weakly attract one another. The forces of attraction between neutral molecules owing to dipole moments or induced dipole moments are called van der Waals forces. To these forces, some of the characteristics of organic molecules, such as alkanes, can be attributed.

Consider, for instance, the boiling point of simple alkanes like methane, ethane, and propane. Because these molecules do not exhibit a significant dipole moment, the only van der Waals force that exists between them is due to induced dipoles. Thus, two methane molecules might form mutually induced dipoles, causing a weak attraction between them.

At sufficiently a low temperature (i.e., the boiling point), the van der Waals force between the molecules is sufficient to cause them to "stick" together in a somewhat orderly manner-in other words, the substance goes from a gas (virtually no order owing to high energy of motion) to a liquid (slight order owing to lower energy of motion). Larger alkanes-in particular, those that are longer, less branched chains-have more possible locations in which dipole moments can be induced, meaning that the van der Waals forces between these larger molecules can accumulate. The result is that more energy of motion is required to break these forces, and therefore, the boiling point is higher.

For example, the boiling point of methane is -160°C, ethane is -89°C, and propane is -42°C.

Combustion

You've probably heard of propane: a gas that is burned for heat in homes and cooking appliances, for example. Alkanes (and other hydrocarbons) burn (combust) rather easily, producing carbon dioxide, water, and heat when burned in the presence of oxygen. (Combustion is a type of oxidation-reduction reaction.) For instance, the following is a balanced chemical equation for the combustion of n-hexane:

2C₆H₁₄ + 19O₂ ? 12CO₂ + 14H₂O + Heat

In the case of n-hexane, combustion yields 4,163kJ/mol of heat.

Practice Problem: Write a balanced chemical equation for the combustion of ethylcyclohexane in pure oxygen.

Solution: First, we must determine a molecular formula for ethylcyclohexane. By following the IUPAC rules "in reverse," we obtain the molecule shown below. We can then count hydrogen and carbon atoms to get the molecular formula.

Now, write the chemical equation, making sure that the numbers of each element are the same on both sides.

C₈H₁₆ + 12O₂ ? 8CO₂ + 8H₂O

Functional Groups

We have reviewed alkanes, the simplest hydrocarbons. But what about carbon compounds that have multiple bonds between carbon atoms or that incorporate atoms other than carbon and hydrogen? These multiple bonds and different atoms are largely responsible for the varying properties (reactivity) of different organic molecules. Specific groups of atoms that give organic molecules their reactive characteristics are called functional groups.

Alkanes are generally viewed as fundamental molecules that do not have functional groups. But when a double or triple bond between carbon atoms is added, or some atom or group of atoms, then functional groups are present. The following molecules each include a functional group (shown in red).

Of course, a molecule may incorporate a number of functional groups, and the above are just a few examples. As with alkanes, organic molecules have specific rules for nomenclature that help to identify a molecule accurately. In some cases, these rules can yield long and highly complicated names; sometimes, it's easier just to use the common name for a substance, as with the molecule below.