Chemistry - A Molecular Science

Chapter 13 Organic Chemistry

be three additional bonds, which are assumed to be C-H bonds, which makes the end positions CH

groups. There are two bonds drawn to the two carbon atoms in the middle, 3

so there must also be two C-H bonds to each. These intersections represent CH

groups. 2

There is only one way to connect the atom

s in each of the alkanes shown in Figure

13.1, but there are multiple ways to connect th

e atoms in all other alkanes. Consider the

four-carbon alkane C

H 4

. Figure 13.2 and Figure 13.3a show C 10

H 4

as a 10

straight

or

continuous chain

hydrocarbon (no carbon bound to more

than two other carbons).

However, C

H 4

can also exist as a 10

branched

hydrocarbon (at least one carbon atom

bound to three or four other carbon atoms), as shown in Figure 13.3b. The two structures shown in Figure 13.3 have the same formula, C

H 4

, but they are different molecules. 10

Different molecules with the same formula are called

isomers

. The two molecules shown

in Figure 13.3 are the two isomers of C

H 4

. The number of isomers can be very large for 10

a formula containing a large number of carbon atoms. Isomers are prevalent in organic chemistry and are one of the reasons there is such a diversity of organic molecules. We consider isomers in more

detail in Section 13.3.

The two major sources of alkanes are natural gas (mostly CH

and C 4

H 2

) and 6

petroleum (a mixture of thousands of substa

nces, mainly hydrocarbons that were formed

from the decomposition of plants and animals). Although alkanes are not very reactive, they do burn. As discussed in Section 9.3, combustion of alkanes such as C

H 3

(propane, 8

home heating and cooking), C

H 4

(butane, lighter fluid), and C 10

H 8

(octane, a component 18

of gasoline) are an important source of heat and power. As an example, the combustion of propane is

C^3

H^8

(g) + 5O

(g) 2

→

3CO

(g) + 4H 2

O(l) 2

ΔH

o = -2220 kJ/mol

C-H and C-C bond cleavage during a combustion

reaction is homolytic,* so each atom

in the bond retains one of the bonding

electrons to produce two species, called

free

radicals

, that have unpaired electrons (Figure 13.4). Free radicals are highly reactive, but

they are stabilized when the unpaired electron

is on an atom that is attached to other

carbon-containing groups. Thus, free radicals formed from highly branched carbon atoms, such as (CH

) 33

.C

, are more stable and less reactive th

an those formed from straight chain

hydrocarbons. Indeed, straight chain hydrocarbons react so fast and violently that they can cause an engine to ‘knock’. The octane

rating of a gasoline indicates the extent of

knocking it causes. The reference molecules ar

e shown in Figure 13.5. The straight chain

hydrocarbon C

H 7

causes substantial knocking and is 16

assigned an octane rating of 0,

H^3

C

H^2 C

CH^2

CH

3

(a)

CH

3

CH

H^3

CCH

3

(b)

Figure 13.3 Straight or continuous chain (a) and branched (b) isomers of C

H 4

(^10)
CH
3
CH
C 3
CH
3
H^3
CC
CH
3
CH
H C 3
H
H

• H C
  H
  H
  Figure 13.4 Free radicals have unpaired electrons Homolytic bond cleavage of a C-C bond produces two free radicals. The radicals are stabilized when the carbon atom with the unpaired electron is bound to one or more carbon atoms, so (CH
  ) 33
  .C
  is more stable and less reactive than H
  C 3
  ..
  HC^3
  H^2 C
  CH^2
  H^2 C
  CH^2
  H^2 C
  CH
  3
  HCCC^3
  H^2
  CH
  CH
  3
  CH
  3
  CH
  3
  CH
  3
  octane rating = 0
  octane rating = 100
  Figure 13.5 Reference molecules used in establishing the octane rating of a gasoline

• Bond breaking in which one bonding electron resides on each atom
  after the bond is broken is called
  homolytic
  bond cleavage. If both
  bonding electrons remain on one atom after the bond breaks, the cleavage is said to be
  heterolytic
  .
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