Alkenes

There are several types of strain in cycloalkanes. There is angle strain as a result of the smaller than desirable angle between carbon atoms. There is also torsional strain, which involves hydrogen atoms being closer together than they would like to be. The combination of angle strain and torsional strain is referred to as ring strain. The ring strain decreases as the size of the ring increases so that, with cyclohexane, there is no ring strain at all. Any rings higher than that will not be seen generally in organic chemistry.

Even with no ring strain, cyclohexane is not a straight planar molecule. It can be in two different shapes in nature: the chair conformation and the boat conformation. These are the most desirable shapes. The chair conformation is more desirable than the boat conformation because it has less interference between the hydrogen atoms, called “transannular strain”. Only the chair conformation has no angle strain because it has no “eclipsing strain” and a small amount of “steric strain” (which is crowding of hydrogen atoms). Steric strain occurs when two atoms are in close proximity to one another.

A summary of the different types of strain involves the following:

• Transannular strain—this is the crowding of two side groups in a ring.

• Eclipsing strain—this is also called torsional strain, which is strain between molecules because of the bond interaction between two eclipsed atoms or groups.

• Bond angle strain—this is when there is poor overlap between carbon atoms that allows overlapping of the atomic/hybrid orbitals.

ALKENES

As the molecular formula grows in size and number of carbon atoms, the number of possible structures increases. For example, with C5H8, what’s clear is that the hydrogen saturation is 4 less than is acceptable. This means that there may be a triple bond, a couple of double bonds, or a ring structure with a double bond, and two rings, among others. This means that there needs to be a consistency in the nomenclature that defines these types of molecules.

Things you need to know about determining alkene names include the following:

• Use the suffix “ene” to indicate an alkene or a cycloalkene.

• The longest chain listed in the root chain must include both carbon atoms of the double bond.

• The numbering of the carbon atom is done so that the double bond has the lowest number.

• If the double bond is in the middle, the numbering is so that the side chain has the lowest number.

• The double bond locator carbon atom is the smaller number of the two carbon

atoms.

• The term diene or triene is used to determine the number of double bonds if there is more than one, with each double bond assigned a locator number.

• There are side chains labeled with “common names”, including the vinyl group (which is CH2CH- side group) and the CH2CHCH2- or allyl side group. Figure 36 shows the vinyl and allyl side groups written out:

1.

Simple common alkenes include the following, which are very similar to the names of the alkanes:

• Ethene is C2H4

• Propene is C3H6

• Butene is C4H8

• Pentene is C5H10

• Hexene is C6H12

• Heptene is C4H14

• Octene is C8H16

• Nonene is C9H18

• Decene is C10H20

As you can see, the ratio of carbon atoms to hydrogen atoms in a single double-bonded molecule is 1:2. You need to number the double bond unless the double bond is at the far end of the chain structure, which would be something like 1-pentene. This is not necessary; however, 2-pentene is a possible type of alkene name. Another way of saying it is pent-2-ene. The only time it is necessary to say 1 before the molecule is if there is a side group, such as 2-ethyl, 1-pentene or 2-ethyl pent-1-ene.

If there is more than one double bond in an alkene, all of the bonds should be numbered in the name of the alkene, even if there is a terminal double bond. An example would be deca 1,5 diene. Conjugated double bonds are double bonds separated by a single bond, which adds stability to the molecule. This makes making a conjugated double bond more favorable than other types of double, double bonds. Double bonds that are separated by a single carbon atom are called “cumulated”. An isolated diene has two double bonds separated by a greater distance from each other.

Double bonds can have isomers using the cis-trans designation or the E-Z designation. The cis-form means that the side groups are on the same side of the double bond, while the trans-form means that the side groups are on the opposite side of the double bond. The E-Z system is based on the German words for “entgegen” or opposite side and “zusammen” or the same side. In this way, E means trans and Z means cis. The priority of the groups is based on the atomic mass of the attached atoms and not on the size of the group.

There can be two types of cyclic bonds in cyclic alkenes. The first is the endocyclic double bond, in which both carbons are within the ring structure. The second is the exocyclic double bond, in which only one carbon of the bond is within the ring structure.