Chemistry - A Molecular Science

Figure 6.17 Bonding in allene Green identifies electron density in the two C-C

σ bonds, while

brown represents electron density in the two C-C

π bonds. Note

that the planes of the two

π bonds are perpendicular (

π and x

π) y

as are the planes of the two CH

groups. 2

C C

C

C C

C

H

H

H H

C

O

O

O

H

O C

H C

H

H

1

3 2

4

5

6

(a) (b)

Example 6.5 Aspirin (a) Lewis structure (b) ball-and-stick structure

is sp

2 hybridized. The sp

2 hybrid orbitals of C

lie in the xz plane bA

ecause that is the plane

defined by the C-H bonds, so the s, p

and px

orbitals are used to construct them. This z

leaves the p

orbital available to form a y

bond with the central carbon. If the central π

carbon uses its p

orbital to y

bond to Cπ

, then it must use its pA

orbital to x

bond with Cπ

. B

If C

uses its pB

orbital to form a x

bond, then it must use its s, pπ

, and py

orbitals to form z

the sp

2 hybrid orbitals, which lie in the yz plane and are perpendicular to the sp

2 plane of

CA

. Thus, the two CH

groups must be perpendicular 2

if the central atom is to

bond to π

both terminal carbon atoms because each

bond requires a different p orbital. Figure 6.17 π

shows the bonding geometry in allene.

We now apply VSEPR and valence bond theory to the Lewis structures of several

molecules and ions. The procedure gives us a very powerful tool to be used in the understanding molecular properties. With Lewis structures we can:

(^) •
determine shapes of molecules and ions, and

-^

understand the bonding in terms of or

bitals, hybridization, and bond type (

σ or

π).

Example 6.5 Describe the bonding at the labeled positions in the aspirin molecule, shown in the margin. Note that the lone pairs have be

en omitted, but C and O obey the octet rule.

Position 1. The C is in a benzene ring. It has three electron regions and is sp

2 hybridized.

The three sp

2 hybrid orbitals are used for two C-C

σ bonds and one C-H

σ bond. The p

orbital is involved in

π bonding.

Position 2. The C is in an acid group (COOH). It is sp

2 hybridized. It has a C-C

σ bond,

two C-O

σ bonds, and a C-O

π bond.

Positions 3 & 4. Both O atoms are sp

3 hybridized. Only four electrons are shown around

each in the figure, but oxygen obeys the octet rule, so there must be two lone pairs as well. Two orbitals are used for

σ bonds and two contain lone pairs.

Position 5. There are three electron groups around this C, so it is sp

2 hybridized. The

three sp

2 orbitals are involved in

σ bonds, and the remaining p orbital is used in the

π^

bond to the oxygen atom. Position 6. The C has four electron regions and is sp

3 hybridized. Three of the hybrids are

used in C-H

σ bonds and one is used in a C-C

σ bond.

Chapter 6 Molecular Structure & Bonding

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