Chemistry - A Molecular Science

Chapter 13 Organic Chemistry

while the highly branched alkane C

H 8

causes little knocking and is assigned an octane 18

rating of 100. A gasoline with an octane

rating of 87 causes the same knocking as a

mixture that is 87% in the branched alkane and 13% of the straight chain alkane.

Alkenes

are organic compounds that contain carbon-carbon double bonds. The two

simplest alkenes, C

H 2

and C 4

H 3

, are shown in Figure 13.6. It might appear that the C 6

H 3

(^6)
molecules shown in Figures 13.6b and 13.6c are
different, but they can be interchanged by
a simple rotation, so they are representations of the same molecule and not isomers. Now consider the representations of C
H 4
shown in Figure 13.7. The double bond can be placed 8
after either the first (Figure 13.7a) or second carbon (Figure 13.7c). However, placing the double bond after the third carbon (Figure 13.7b) is the same as placing it after the first carbon atom because the two molecules can be
interchanged by a simple rotation. We
conclude that Figure 13.7 shows only
two
isomers of C
H 4
(Figures 13.7a and 13.7c). 8
Alkenes with several double bonds are called
polyenes
, and polyenes with alternating
single and double bonds have delocalized
systems. As the number of delocalized bonds π
in a polyene increases, the energy spacing between their
MOs decreases (Section 8.6) as π
does their HOMO-LUMO gap (the energy separ
ation between the highest occupied and
lowest unoccupied MOs). In large
systems, the HOMO-LUMO gap drops into the π
visible region of the spectrum (
E = hΔ
) and the polyene absorbs visible light. Indeed, ν
most dyes are polyenes of this type. Figure 13.8 shows
β−
carotene, which contains 21
alternating single and double bonds, so its
system is delocalized. The HOMO-LUMO π
energy gap is such that
β−
carotene absorbs blue light and reflects orange light.
β−
carotene
is the compound responsible for the orange color of carrots.
Alkenes are far more reactive than alkanes.
Most of the chemical reactivity of alkenes
comes from the fact that the pi bond can act as an electron pair source (it is a Lewis base) and can initiate the formation of new chem
ical bonds. As an example, hydrogen can add
across the double bond of an alkene to produce an alkane in a process called hydrogenation

. The hydrogenation of ethene is

CC

H H

H H

+ H

2

HC

H H

H C

H

H

Alkenes are said to be

unsaturated

because hydrogen atoms can be added to them.

Similarly,

poly

enes are said to be

poly

unsaturated. Alkanes, on the other hand, have no

multiple bonds, so no more hydrogen atoms can be

added to them. Therefore, alkanes are

said to be

saturated.

CC

H H

H H

CC

H H

H CH

H

H

CC

H CH

H

H H

H

(a)

(b)

(c)

Figure 13.6 Ethene and propene (a) C

H 2

, (b) and (c) are identical structures of C 4

H 3

. The skeletal 6

structures are shown beneat

h the Lewis structures.

(a)

(b)

(c)

Figure 13.7 Isomers of C

H 4

(^8)
(a) and (b) are identical molecules. (c) is the second other isomer.
Figure 13.8
β-carotene
The
π electrons are delocalized over the 21 alternating single and
double bonds.
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