Chemistry - A Molecular Science

Chapter 13 Organic Chemistry

13.1

HYDROCARBONS Carbon atoms always have four bonds: f

our single bonds, two single and one double

bond, two double bonds, or a single bond and a triple bond. Although carbon is the basic building block, organic compounds usually

have hydrogen atoms as well. Oxygen and

nitrogen are also common, but almost any other element can be found in organic compounds. In this section, we deal with the class of compounds known as hydrocarbons

, which are compounds that contain only carbon and hydrogen.

Alkanes

are the simplest organic compounds. All of the carbon atoms in an alkane are

sp

3 hybridized and tetrahedral, and all bonds are sigma bonds, which makes alkanes

relatively unreactive. They have the general formula C

Hn

2n+2

, where n is an integer. Figure

13.1 shows CH

, C 4

H 2

and C 6

H 3

, the three simplest alkanes. The line-wedge-dash 8

representations attempt to give a three-dimensional view of the organic molecules. However, groups connected by a single bond can rotate relative to one another, so the structures of alkanes are very dynamic, and th

ere is not much to be gained by attempting

to show a three-dimensional structure. Instead, several shorthand notations are used for drawing organic molecules. These shorthand

notations are based on the following facts:

(^) •
carbon atoms form the backbone of organic molecules;
(^) •
carbon atoms always have four bonds in their molecules; and
(^) •
organic compounds usually contain many C-H bonds. Figure 13.2 shows four ways of representing a C
H 4
molecule. Figure 13.2a is the 10
line-wedge-dash representation. Figure 13.2b is
the Lewis structure, which shows all
atoms in a plane and all angles at 90
o. Structures like those shown in Figure 13.2c, which
give the number of hydrogen atoms but do not show the C-H bonds explicitly, are called condensed structures

. The number of carbon atoms and the number of C-H bonds in

organic molecules can be quite large, so organic molecules are often represented by skeletal structures,

Figure 13.2d, that show neither the carbon atoms nor the hydrogen

atoms attached to them explicitly. Rather, they show only the bonds connecting the carbon atoms. All other atoms, including hydroge

n atoms attached to non-carbon atoms, are

included explicitly. The fact that carbon alwa

ys has four bonds is used to determine

whether an intersection or end of a line segment represents a C, a CH, a CH

, or a CH 2

. 3

For example, Figure 13.2d shows two ends and

two intersections, so the molecule contains

four carbon atoms. Only one bond is drawn to the carbon atom at each end, so there must

H

HH C H

C

HHC H

H HH

HC

C HH

H

C HH

HH

CH

4

CH^2

6

CH^3

8

Figure 13.1 The simplest alkanes HC

C HH

C HH

HH

H C

HH

HC

H H

H CCCHHH

H H

H

HC^3

H^2 C

CH^2

CH

3

CH

CH 3

CH 2

CH 2

3

or

CH

(CH 3

)CH 22

3

(a)

(b)

(c)

(d)

Figure 13.2 Four representations of C

H 4

(^10)
a) Line-wedge-dash representat
ion; b) Lewis structure;
c) condensed structures; and d) skeletal structure
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