Organic Chemistry

sp

sp

p

p

Figure 1.17

An sphybridized carbon. The two

sporbitals are oriented 180°away

from each other, perpendicular to

the two unhybridized porbitals.

(The smaller lobes of the sporbitals

are not shown.)

Section 1.9 Bonding in Ethyne: A Triple Bond 31

DIAMOND, GRAPHITE, AND

BUCKMINSTERFULLERENE:

SUBSTANCES CONTAINING ONLY

CARBON ATOMS

Diamond is the hardest of all substances. Graphite, in contrast,

is a slippery, soft solid most familiar to us as the “lead”in pen-

cils. Both materials, in spite of their very different physical

properties, contain only carbon atoms. The two substances dif-

fer solely in the nature of the carbon–carbon bonds holding

them together. Diamond consists of a rigid three-dimensional

network of atoms, with each carbon bonded to four other car-

bons via orbitals. The carbon atoms in graphite, on the other

hand, are hybridized, so each bonds to only three other car-

bon atoms. This trigonal planar arrangement causes the atoms in

graphite to lie in flat, layered sheets that can shear off of neigh-

boring sheets. You experience this when you write with a pencil:

Sheets of carbon atoms shear off, leaving a thin trail of graphite.

There is a third substance found in nature that contains only car-

bon atoms: buckminsterfullerene. Like graphite, buckminster-

fullerene contains only hybridized carbons, but instead of

forming planar sheets, the carbons in buckminsterfullerene

form spherical structures. (Buckminsterfullerene is discussed in

more detail in Section 15.2.)

sp^2

sp^2

sp^2

sp^3

C

C

H

H

c.

a.

C

C

H

H

HHCC

σ bond formed by

sp–sp overlap

σ bond formed by

sp–s overlap

b.

180 °

>Figure 1.18

(a) The bond in ethyne is

formed by sp–spoverlap, and the

bonds are formed by sp–s

overlap. The carbon atoms and the

atoms bonded to them are in a

straight line. (b) The two

carbon–carbon bonds are formed

by side-to-side overlap of the p

orbitals of one carbon with the p

orbitals of the other carbon.

(c) Electron density accumulates in

a cylinder that wraps around the

egg-shaped molecule.

p

C¬H

C¬Cs

Four electrons hold the carbons together in a carbon–carbon double bond; only two
electrons bind the atoms in a carbon–carbon single bond. This means that a carbon–carbon
double bond is stronger (152 kcal mol or 636 kJ mol) and shorter (1.33 ) than a
carbon–carbon single bond (88 kcal mol or > 368 kJ>mol,and 1.54 ).Å

> > Å

1.9 Bonding in Ethyne: A Triple Bond

The carbon atoms in ethyne (also called acetylene) are each bonded to only two
atoms—a hydrogen and another carbon:

Because each carbon forms covalent bonds with two atoms, only two orbitals (an s
and a p) are hybridized. Two degenerate sporbitals result. Each carbon atom in ethyne,
therefore, has two sporbitals and two unhybridized porbitals (Figure 1.17).

One of the sporbitals of one carbon in ethyne overlaps an sporbital of the other car-
bon to form a carbon–carbon bond. The other sporbital of each carbon overlaps the
sorbital of a hydrogen to form a C¬Hsbond (Figure 1.18a). To minimize electron

s

sp

ppp

sp

s

hybridization pp

two orbitals are hybridized

hybrid orbitals

H CCH

ethyne

(acetylene)