Alkenes and alkynes differ from alkanes in shape because of their multiple bonds.
Methane is tetrahedral, but ethylene is flat (planar) and acetylene is linear (straight), as predicted by the VSEPR model discussed in Section 4.8.
C
Methane— a tetrahedral molecule with bond angles of 109.5°
H H H
H
109.5°
Ethylene— a flat molecule with bond angles of 120°
C
H H
H H
C 120°
Acetylene— a linear molecule with bond angles of 180°
H
180°
H C C
The two carbons and four attached atoms that make up the double-bond functional group lie in a plane. Unlike the situation in alkanes, where free rotation around the CiC single bond occurs, there is no rotation around a double bond, and the molecules are more rigid. However, their restricted freedom of rotation makes a new kind of isomerism pos- sible for alkenes. As a consequence of their rigid nature, alkenes possess ends and sides:
side
side
C
A E
B D
end C end
To see this new kind of isomerism, look at the four C4H8 compounds shown be- low. When written as condensed structures, there appear to be only three alkene iso- mers of formula C4H8 : 1-butene 1CH2“CHCH2CH32, 2-butene 1CH3CH“CHCH32, and 2-methylpropene 11CH322C“CH22. The compounds 1-butene and 2-butene are constitutional isomers of each other because their double bonds occur at different posi- tions along the chain, and 2-methylpropene is a constitutional isomer of both 1-butene and 2-butene because it has the same molecular formula but a different connection of carbon atoms (see Section 12.3). In fact, though, there are four isomers of C4H8. Because rotation cannot occur around carbon–carbon double bonds, there are two dif- ferent 2-butenes. In one isomer, the two iCH3 groups are on the same side of the double bond; in the other isomer, they are on opposite sides of the double bond.
C
1-Butene
H
H
CH2CH3
H C
In cis configurations, the groups are on the same side of the double bond.
C C
cis-2-Butene
H3C
H
CH3
H
In trans configurations, groups attach on the opposite sides of the double bond.
trans-2-Butene
C C H3C
H CH3
H
2-Methylpropene
C C H
H CH3
CH3
Recall from Section 12.5 that rotation around CiC single bonds allows a molecule to exist in multiple conformations.
S E C T I O N 1 3 . 3 The Structure of Alkenes: Cis–Trans Isomerism 401 The two 2-butenes are called cis–trans isomers. They have the same formula
and connections between atoms but have different three-dimensional structures because of the way that groups attach to different sides of the double bond. In this case, the isomer with its methyl groups on the same side of the double bond is named cis-2-butene, and the isomer with its methyl groups on opposite sides of the double bond is named trans-2-butene.
Cis–trans isomerism is possible whenever an alkene has two different substituent groups on each of its ends. (This means that in the earlier drawing illustrating the sides and ends of an alkene molecule, A ⬆ B and D ⬆ E.) If one of the carbons composing the double bond is attached to two identical groups, cis–trans isomerism cannot exist. In 2-methyl-1- butene, for example, cis–trans isomerism is not possible because C1 is bonded to two iden- tical groups (hydrogen atoms). To convince yourself of this, mentally flip either one of these two structures top to bottom; note that it becomes identical to the other structure:
These compounds are identical.
Because the carbon left of the double bond has two H atoms attached, cis – trans isomerism is impossible.
2-Methyl-1-butene
C and
H
H CH3
CH2CH3 CH3
CH2CH3
C C
H
H C
In 2-pentene, however, the structures do not become identical when one of them is flipped, so cis–trans isomerism does occur:
cis-2-Pentene trans-2-Pentene
These compounds are not identical. Neither carbon of the double bond has two identical groups attached to it.
C and
H H
H3C
H3C H
H CH2CH3 CH2CH3
C C C
It is important to note that the molecule must remain intact when you perform this analysis; you cannot break and reform any bonds when flipping and comparing the two structures.
The two substituents that are on the same side of the double bond in an alkene are said to be cis to each other, and those on opposite sides of the double bond are said to be trans to each other. In our generic molecule on the previous page showing ends and sides, for example, A and E are cis to each other, B and D are cis to each other, B and E are trans to each other, and A and D are trans to each other. Thus, in alkenes, the terms cis and trans are used in two ways: (i) as a relative term to indicate how various groups are attached to the double-bond carbons (for example, “groups A and E are cis”) and (ii) in nomenclature as a way to indicate how the longest chain in the molecule goes in, through, and out of the double bond (for example, cis-2-butene and trans-2-butene).
Worked Example 13.3 Molecular Structure: Cis and Trans Isomers Draw structures for both the cis and trans isomers of 2-hexene.
ANALYSIS First, draw a condensed structure of 2-hexene to see which groups are attached to the double-bond carbons:
C1 C2 C3 C4 C5 2-Hexene 6
C
Next, begin to draw the two isomers. Choose one end of the double bond, and attach its groups in the same way to generate two identical partial structures:
and H3C
H H3C
C H
C C C
Finally, attach groups to the other end in the two possible different ways.
Cis–trans isomer Alkenes that have the same connections between atoms but differ in their three-dimensional structures because of the way that groups attach to different sides of the double bond.