Today we explored the structures of the alkenes and the alkanes and naming these two other groups of hydrocarbons.

Today we explored the structures of the alkenes and the alkanes and naming these two other groups of hydrocarbons. We began by considering the general formulas for these three groups. We have learned from the Survival Organic chemistry exercise that there are three groups of hydrocarbons; alkanes, alkenes and alkynes. The general formulas are;

Hydrocarbon	Formula
alkanes	C_nH_2n+2
alkenes	C_nH_2n
alkynes	C_nH_2n-2

When given a formula it is critical that we know which group the compound belongs. Lets consider some of the members of the alkenes;

Formula	Name	Structure	Condensed-Formula	Comments
C₂H₄	ethene (ethylene)		H₂C=CH₂	Note the planar structure of this molecule. The bond angles are all 120 degrees around the 'central' carbon atoms.
C₃H₆	propene		H₂C=CHCH₃
C₄H₈	1 - butene		H₂C=CHCH₂CH₃	This is the first alkene where the double bond can be located between a different pair of carbon atoms.
C₄H₈	cis-2 - butene		H₃CHC=CHCH₃	In 2-butene there are two possible 'geometric' isomers, the cis- and the trans- forms. The one to the left is the cis-form. The cis forms always have the hydrogens on the same side of the carbon atoms sharing the double bond.
C₄H₈	trans-2 - butene		H₃CHC=CHCH₃	This is the trans- form of 2-butene. Notice the hydrogen atoms are on opposite sides of the carbon atoms sharing the double bond..
C₅H₁₀	1-pentene		H₂C=CHCH₂CH₂CH₃	This has the double bond located between the first and second carbon atom. This compound can not have cis- and trans- isomers because one of the carbon atoms sharing the double bond has two hydrogen atoms.
C₅H₁₀	cis-2-pentene		H₃CCH=CHCH₂CH₃	When we move the double bond to between the 2nd and 3rd carbon atoms we generate the cis- and trans-isomeric forms again. Note the 'cis' arrangement of the hydrogen atoms.
C₅H₁₀	trans-2-pentene		H₃CCH=CHCH₂CH₃	This is the trans isomer of 2-pentene. Notice the hydrogen atoms are on opposite sides of the carbon atoms sharing the double bond.

How do we name alkenes?

Find the longest continueous chain of carbon atoms which contains the double bond. Called the parent chain this chain establishes the base alkene name for the compound.
For alkenes the parent name is the same as the name for the alkane, only the -ane is replaced with -ene.
Number the parent chain, beginning at the end closest to the double bond, the location of the double bond is given by the lowest-numbered carbon sharing the double bond.

Lets try an example to see how we understand the alkenes.

Sample 1;

Name and draw the structures for all of the straight chain alkenes with the formula C₆H₁₂.

Answer.

Lets consider some of the members of the alkynes;

Formula	Name	Structure	Condensed-Formula	Comments
C₂H₂	ethyne (acetylene)		HCCH	Note the linear structure of this molecule. The bond angles are 180 degrees around the 'central' carbon atoms. The bond between the two carbon atoms is a triple bond.
C₃H₄	propyne		HCCCH₃	We could also call this compound 1-propyne. But since the triple bond can only go in one place (between the first and second carbon atom), we can call this compound propyne and not worry about specifying the location of the triple bond.
C₄H₆	1 - butyne		HCCCH₂CH₃	This is the first alkyne where the triple bond can be located between a different pair of carbon atoms.
C₄H₆	2 - butyne		H₃CCCCH₃	There are no geometric isomers in the alkynes as we observed in the alkenes. So there is only one possible isomer of 2-butyne.
C₅H₈	1 - pentyne		HCCCH₂CH₂CH₃
C₅H₈	2-pentyne		H₃CCCCH₂CH₃

How do we name alkynes?

Find the longest continueous chain of carbon atoms which contains the triple bond. Called the parent chain this chain establishes the base alkyne name for the compound.
For alkynes the parent name is the same as the name for the alkane, only the -ane is replaced with -yne.
Number the parent chain, beginning at the end closest to the triple bond, the location of the triple bond is given by the lowest-numbered carbon sharing the triple bond.

Lets try an example to see how we understand the alkynes.

Sample 1;

Name and draw the structures for all of the straight chain alkynes with the formula C₆H₁₀.

Answer.