Section One

Chapter 12---Alkenes and Alkynes

Section One (corresponds to 12.1 - 12.4)

Objectives:

To learn the structure and shape of alkenes.
To learn IUPAC nomenclature for alkenes.
To learn the physical properties of alkenes.

12.1

Sometimes people categorize hydrocarbons as either saturated or unsaturated. (You may have heard of saturated and unsaturated fats.) What they are referring to is really whether or not the molecule has a C==C (or triple bond) in its structure. Unsaturated hydrocarbons contain one or more carbon-carbon double or triple bonds. Saturated hydrocarbons refer to the alkanes. The C==C is the functional group for the alkenes and the carbon-carbon triple bond is the functional group for the alkynes.

12.2 Structure/Shapes

ALKENES

The bond angles around alkene carbons are 120^o according to VSEPR (CHEM 1151). These structures are trigonal planar around the carbons of the double bond. Furthermore, there is no rotation around this bond. Build a model of ethene (C₂H₄) and check this out for yourself.

Because of this restriction of rotation, there exist cis/trans isomers for some alkenes. (Remember from cycloalkanes? Cis means "on the same side" and trans means "on opposite sides.") Go to these web sites for visualizations (must have "Quick Time" installed):

cis example

trans example

How do we tell whether or not an alkene can have cis/trans isomers? Look at both the carbons of the double bond. Both of the carbons should have 2 different "things" attached to them. For instance, in the structures above, each carbon has an -H and a -CH₃ attached. Thus, each carbon of the double bond has 2 different "things" attached. Now look at the structure below: one of the carbons with the double bond has 2 H's attached - two identical "things." Thus, this compound will not have cis/trans isomers.

1-butene

ALKYNES

Alkynes have a linear shape (180^o angle) around the carbon atoms of the triple bond.

2butyne

12.3 Nomenclature

For the IUPAC nomenclature of alkenes and alkynes, we have just a few modifications from the rules for alkanes.

The longest continuous chain of carbon atoms (parent chain) must include both carbons of the C==C (or triple bond).
The numbering system is established by the position of the double (or triple) bond, numbering from the direction that gives the lowest possible number to the position of the double bond. The number gets stated if there are 4 or more carbons in the parent chain.
The ending is "-ene" for alkenes and "-yne" for alkynes.
For alkenes: If the structure is shown as either the cis or trans isomer, then that term gets added as a prefix to the entire name.
For cycloalkenes, you must always consider the double bond to be between carbons 1 & 2, then go either clockwise or counterclockwise to get the lowest possible numbers for the substituents.
For compounds containing more than one double bond, the ending becomes "diene, triene, etc." You must use a number to indicate the position of each double bond.

alkenenomen

12.4 Physical Properties

Alkenes and alkynes are nonpolar compounds, so they are not soluble in water. They would however be soluble in each other and in alkanes and other nonpolar solvents.

The only type of intermolecular attraction that exists for nonpolar molecules is London dispersion forces. Therefore, their boiling points and physical states are similar to those of alkanes. Those that are liquid at room temperature have densities less than water and would float on water.

Assignment

Look at the examples in your book and then try to work the following questions: 12.1 - 12.3 and the homework problems related to Sections 12.1 - 12.4.