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8.1 Alkene and Alkyne Overview

By definition, alkenes are hydrocarbons with one or more carbon-carbon double bonds (R2C=CR2), while alkynes are hydrocarbons with one or more carbon-carbon triple bonds (R-C≡C-R). Collectively, they are called unsaturated hydrocarbons, which are defined as hydrocarbons having one or more multiple (double or triple) bonds between carbon atoms. As a result of the double or triple bond nature, alkenes and alkynes have fewer hydrogen atoms than comparable alkanes with the same number of carbon atoms. Mathematically, this can be indicated by the following general formulas:In an alkene, the double bond is shared by the two carbon atoms and does not involve the hydrogen atoms, although the condensed formula does not make this point obvious, ie the condensed formula for ethene is CH2CH2. The double or triple bond nature of a molecule is even more difficult to discern from the molecular formulas. Note that the molecular formula for ethene is C2H4, whereas that for ethyne is C2H2. Thus, until you become more familiar the language of organic chemistry, it is often most useful to draw out line or partially-condensed structures, as shown below:

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8.5 Geometric Isomers

Within alkane structure there is free rotation about the carbon-to-carbon single bonds (C-C). In contrast, the structure of alkenes requires that the carbon atoms form a double bond. Double bonds between elements are created using p-orbital shells (a...

Figure 8.8 Alkene Double Bonds Can Form Geometric Isomers. (a) Shows the free rotation around a carbon-carbon single bond in the alkane structure. (b) Shows the fixed position of the carbon-carbon double bond that leads to geometic (spatial) isomers.

Click Here for a Kahn Academy Video Tutorial on Alkene Structure.

For this section, we are not concerned with the naming that is also included in this video tutorial.(Note: All Khan Academy content is available for free using CC-BY-NC-SA licensing at www.khanacademy.org )

Cis-Trans Nomenclature

The cis-trans naming system can be used to distinguish simple isomers, where each carbon of the double bond has a set of identical groups attached to it. For example, in Figure 8.8b, each carbon involved in the double bond, has a chlorine attached to it, and...

Click Here for a Kahn Academy Video Tutorial on Cis/Trans Isomerization

(Note: All Khan Academy content is available for free using CC-BY-NC-SA licensing at www.khanacademy.org )Cis-trans isomerism also occurs in cyclic compounds. In ring structures, groups are unable to rotate about any of the ring carbon-carbon bonds. Therefore, groups can be either on the same side of the ring (cis) or on opposite sides of the ring (trans). For our purposes here, we represent all cycloalkanes as planar structures, and we indicate the positions of the groups, either above or below the plane of the ring.

To Your Health

Possibly the most common place that you will hear reference to cis-trans conformations in everyday life is at the supermarket or your doctor’s office. It relates to our consumption of dietary fats. Inappropriate or excessive consumption of dietary fa...

Click Here for a Kahn Academy Video Tutorial on Saturated-, Unsaturated-, and Trans-Fats

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Key Factors for Determining Cis/Trans Isomerization

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E-Z Nomenclature

The situation becomes more complex when there are 4 different groups attached to the carbon atoms involved in the formation of the double bond. The cis-trans naming system cannot be used in this case, because there is no reference to which groups are be...

Click Here for a Kahn Academy Video Tutorial on E/Z Isomerization.

(Note: All Khan Academy content is available for free using CC-BY-NC-SA licensing at www.khanacademy.org )

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