Organic Chemistry

26 CHAPTER 1 Electronic Structure and Bonding • Acids and Bases

The potential map of methane shows that neither carbon nor hydrogen carries much

of a charge: There are neither red areas, representing partially negatively charged

atoms, nor blue areas, representing partially positively charged atoms. (Compare this

map with the potential map for water on p. 14). The absence of partially charged atoms

can be explained by the similar electronegativities of carbon and hydrogen, which

cause carbon and hydrogen to share their bonding electrons relatively equally.

Methane is a nonpolar molecule.

You may be surprised to learn that carbon forms four covalent bonds since you

know that carbon has only two unpaired electrons in its ground-state electronic con-

figuration (Table 1.2). But if carbon were to form only two covalent bonds, it would

not complete its octet. Now we need to come up with an explanation that accounts for

carbon’s forming four covalent bonds.

If one of the electrons in the 2sorbital were promoted into the empty 2patomic or-

bital, the new electronic configuration would have four unpaired electrons; thus, four

covalent bonds could be formed. Let’s now see whether this is feasible energetically.

Because a porbital is higher in energy than an sorbital, promotion of an electron

from an sorbital to a porbital requires energy. The amount of energy required is

The formation of four bonds releases of energy be-

cause the bond dissociation energy of a single bond is If the

electron were not promoted, carbon could form only two covalent bonds, which would

release only So, by spending (or 402 kJ mol) to promote

an electron, an extra (or 879 kJ mol) is released. In other words, promo-

tion is energetically advantageous (Figure 1.9).

210 kcal>mol >

210 kcal>mol. 96 kcal>mol >

C¬H 105 kcal>mol.

96 kcal>mol. C¬H 420 kcal>mol

s

pp p

s

promotion ppp

before promotion after promotion

Linus Carl Pauling (1901–1994)
was born in Portland, Oregon. A
friend’s home chemistry laboratory
sparked Pauling’s early interest in
science. He received a Ph.D. from the
California Institute of Technology
and remained there for most of his
academic career. He received the
Nobel Prize in chemistry in 1954 for
his work on molecular structure. Like
Einstein, Pauling was a pacifist, win-
ning the 1964 Nobel Peace Prize for
his work on behalf of nuclear
disarmament.

Potential energy

promotion

96 kcal/mol

420 kcal/mol

4 covalent

bonds

Figure 1.9N

As a result of electron promotion,

carbon forms four covalent bonds

and releases 420 kcal mol of

energy. Without promotion, carbon

would form two covalent bonds

and release 210 kcal mol of energy.

Because it requires 96 kcal mol to

promote an electron, the overall

energy advantage of promotion is

114 kcal>mol.

>

>

>

We have managed to account for the observation that carbon forms four covalent

bonds, but what accounts for the fact that the four bonds in methane are identi-

cal? Each has a bond length of 1.10 and breaking any one of the bonds requires the

same amount of energy (105 kcal mol, or 439 kJ mol). If carbon used an sorbital and

three porbitals to form these four bonds, the bond formed with the sorbital would be

different from the three bonds formed with porbitals. How can carbon form four iden-

tical bonds, using one sand three porbitals? The answer is that carbon uses hybrid

orbitals.

Hybrid orbitalsare mixed orbitals—they result from combining orbitals. The con-

cept of combining orbitals, called orbital hybridization, was first proposed by Linus

Pauling in 1931. If the one sand three porbitals of the second shell are combined and

then apportioned into four equal orbitals, each of the four resulting orbitals will be one

part sand three parts p. This type of mixed orbital is called an (stated “s-p-three”

not “s-p-cubed”) orbital. (The superscript 3 means that three porbitals were mixed

sp^3

> >

Å,

C¬H