Organic Chemistry

38 CHAPTER 1 Electronic Structure and Bonding • Acids and Bases

You may wonder how an electron “knows”what orbital it should go into. In fact,

electrons know nothing about orbitals. They simply arrange themselves around atoms

in the most stable manner possible. It is chemists who use the concept of orbitals to ex-

plain this arrangement.

PROBLEM 20

Which of the bonds of a carbon–carbon double bond has more effective orbital–orbital

overlap, the bond or the bond?

PROBLEM 21

Why would you expect a bond formed by overlap to be stronger than a

bond formed by overlap?

PROBLEM 22

a. What is the hybridization of each of the carbon atoms in the following compound?

b. What is the hybridization of each of the carbon, oxygen, and nitrogen atoms in the fol-

lowing compounds?

PROBLEM 23

Describe the orbitals used in bonding and the bond angles in the following compounds.

(Hint:see Table 1.7).

a. b. c. d. e. HCOOH f.

1.15 Dipole Moments of Molecules

In Section 1.3, we saw that for molecules with one covalent bond, the dipole moment

of the bond is identical to the dipole moment of the molecule. For molecules that have

more than one covalent bond, the geometry of the molecule must be taken into account

because both the magnitudeand the directionof the individual bond dipole moments

(the vector sum) determine the overall dipole moment of the molecule. Symmetrical

molecules, therefore, have no dipole moment. For example, let’s look at the dipole

moment of carbon dioxide Because the carbon atom is bonded to two atoms, it

uses sporbitals to form the bonds. The remaining two porbitals on carbon

form the two bonds. The individual carbon–oxygen bond dipole moments

cancel each other—because sporbitals form a bond angle of 180°—giving carbon

dioxide a dipole moment of zero D. Another symmetrical molecule is carbon tetra-

chloride The four atoms bonded to the hybridized carbon atom are identi-

cal and project symmetrically out from the carbon atom. Thus, as with the

symmetry of the molecule causes the bond dipole moments to cancel. Methane also

has no dipole moment.

CO 2 ,

(CCl 4 ). sp^3

C¬Op

C¬Os

(CO 2 ).

BeH 2 BH 3 CCl 4 CO 2 N 2

N

N

NC

C

N

C

O

C

O CH

3

CH 2 OH

CH

HC C

O

CC

HO

HO OH

H 3 C

CH 3

CH

caffeine

O

vitamin C

CH 3

CH 3 CHCH CHCH 2 C CCH 3

s sp^3 – sp^3

C¬Cs sp^2 – sp^2

s p