Page 79 - Physical chemistry understanding our chemical world

46 INTRODUCING INTERACTIONS AND BONDS

Table 2.2 The energies of hydrogen bonds

Atoms in H-bond Typical energy/kJ mol −1
Hand N 20
Hand O 25
Hand F 40

positive charge δ . Since the hydrogen atom is so small and so electropositive, its
+
excess charge leads to the formation of an unusually strong dipole, itself leading
to a strong van der Waals bond. The bond is usually permanent (unlike a typical
dipole–dipole interaction), thereby ‘locking’ the structure of DNA into its pair of
parallel helices, much like the interleaving teeth of a zip binding together two pieces
of cloth.
We call these extra-strong dipole–dipole bonds ‘hydrogen bonds’,
IUPAC is the Interna- and these are defined by the IUPAC as ‘a form of association between
tional Union of Pure an electronegative atom and a hydrogen atom attached to a second,
andAppliedChemistry. relatively electronegative atom’. All hydrogen bonds involve two
It defines terms, quan- dipoles: one always comprises a bond ending with hydrogen; the
tities and concepts in other terminates with an unusually electronegative atom. It is best
chemistry. considered as an electrostatic interaction, heightened by the small
size of hydrogen, which permits proximity of the interacting dipoles
or charges. Table 2.2 contains typical energies for a few hydrogen
bonds. Both electronegative atoms are usually (but not necessarily)
Strictly, the dipole–
dipole interactions dis- from the first row of the periodic table, i.e. N, O or F. Hydrogen
cussed on p. 42 are bonds may be intermolecular or intramolecular.
also hydrogen bonds, Finally, as a simple illustration of how weak these forces are,
since we discussed note how the energy required to break the hydrogen bonds in
the interactions arising liquid hydrogen chloride (i.e. the energy required to vaporize it)
−1
between H–O-bonded is 16 kJ mol , yet the energy needed break the chemical bond
species. between atoms of hydrogen and chlorine in H–Cl is almost 30
−1
times stronger, at 431 kJ mol .

Aside

The ancient Greeks recognized that organisms often pass on traits to their offspring, but
it was the experimental work of the Austrian monk Gregor Mendel (1822–1884) that
led to a modern hereditary ‘theory’. He entered the Augustinian monastery at Br¨ unn
(now Brno in the Czech Republic) and taught in its technical school.
He cultivated and tested the plants at the monastery garden for 7 years. Starting in
1856, painstakingly analysing seven pairs of seed and plant characteristics for at least
28 000 pea plants. These tedious experiments resulted in two generalizations, which
were later called Mendel’s laws of heredity. Mendel published his work in 1866, but
it remained almost unnoticed until 1900 when the Dutch botanist Hugo Marie de Vries
referred to it. The full significance of Mendel’s work was only realized in the 1930s.