50
ATOMIC STRUCTURE AND BONDING
this allows the partially positive hydrogen atom to
associate with a centre in another molecule carrying a
partial negative charge. This is likely to be the oxygen
atom in another molecule.
O H
O H
d+
d−
d+
d−
hydrogen bond
If we consider the interaction between two water
molecules, then the partial positive charge on hydrogen induced by the electronegativity of oxygen is
attracted to the high electron density of the oxygen lone pair in another molecule. This hydrogen is
now linked to its original oxygen by a σ bond, and
to another oxygen by an electrostatic attraction. The
bond length of the H–O hydrogen bond is typically
about twice the length of the H–O covalent bond,
and the hydrogen bond is very much weaker than the
covalent bond, though stronger than other interactions
between molecules. In water, further hydrogen bonds
involving other molecules are formed, leading to a
network throughout the entire sample. The extensive
hydrogen bonding in water is responsible for some of
water’s unusual properties, its relatively high boiling
point, and its high polarity that makes it a particularly good solvent for ionic compounds. Alcohols
also exhibit hydrogen bonding; but, with only a single O–H, the network of bonds cannot be as extensive
as in water.
Hydrogen bonds connecting different molecules
are termed intermolecular, and when they connect
groups within the same molecule they are called
intramolecular. A simple example of an intramolecular hydrogen bond is seen in the enol form of
acetylacetone (see Section 10.1). Carboxylic acids
are known to form hydrogen-bonded ‘dimers’ in solution through two quite strong intermolecular hydrogen bonds. Hydrogen bonds involving N–H are also
common, and we can also meet hydrogen bonding
between alcohols and amines.
H 3 C
C
C
C
CH 3
O
O
H
H
intramolecular hydrogen bonding
in enol form of acetylacetone
R C
O
O
H
R
C
O
O
H
intermolecular hydrogen bonding
in carboxylic acids
Box 2.2
Hydrogen bonds and DNA
The nucleic acids known as deoxyribonucleic acid
(DNA) are the molecules that store genetic information. This information is carried as a sequence of
bases in the polymeric molecule. Remarkably, the
interpretation of this sequence depends upon simple
hydrogen bonding interactions between base pairs.
Hydrogen bonding is fundamental to the double helix
arrangement of the DNA molecule, and the translation and transcription via ribonucleic acid (RNA) of
the genetic information present in the DNA molecule.
O
H
H
O H
H
d+
d+
d+
d+
d−
d−
O H
H
O
H
H
O H
H
O
H
H
O
H
H
extensive hydrogen
bonding in water
O
H
CH 3
O
CH 3
H
O
H
CH 3
O
CH 3
H
hydrogen bonding in methanol
ATOMIC STRUCTURE AND BONDING
this allows the partially positive hydrogen atom to
associate with a centre in another molecule carrying a
partial negative charge. This is likely to be the oxygen
atom in another molecule.
O H
O H
d+
d−
d+
d−
hydrogen bond
If we consider the interaction between two water
molecules, then the partial positive charge on hydrogen induced by the electronegativity of oxygen is
attracted to the high electron density of the oxygen lone pair in another molecule. This hydrogen is
now linked to its original oxygen by a σ bond, and
to another oxygen by an electrostatic attraction. The
bond length of the H–O hydrogen bond is typically
about twice the length of the H–O covalent bond,
and the hydrogen bond is very much weaker than the
covalent bond, though stronger than other interactions
between molecules. In water, further hydrogen bonds
involving other molecules are formed, leading to a
network throughout the entire sample. The extensive
hydrogen bonding in water is responsible for some of
water’s unusual properties, its relatively high boiling
point, and its high polarity that makes it a particularly good solvent for ionic compounds. Alcohols
also exhibit hydrogen bonding; but, with only a single O–H, the network of bonds cannot be as extensive
as in water.
Hydrogen bonds connecting different molecules
are termed intermolecular, and when they connect
groups within the same molecule they are called
intramolecular. A simple example of an intramolecular hydrogen bond is seen in the enol form of
acetylacetone (see Section 10.1). Carboxylic acids
are known to form hydrogen-bonded ‘dimers’ in solution through two quite strong intermolecular hydrogen bonds. Hydrogen bonds involving N–H are also
common, and we can also meet hydrogen bonding
between alcohols and amines.
H 3 C
C
C
C
CH 3
O
O
H
H
intramolecular hydrogen bonding
in enol form of acetylacetone
R C
O
O
H
R
C
O
O
H
intermolecular hydrogen bonding
in carboxylic acids
Box 2.2
Hydrogen bonds and DNA
The nucleic acids known as deoxyribonucleic acid
(DNA) are the molecules that store genetic information. This information is carried as a sequence of
bases in the polymeric molecule. Remarkably, the
interpretation of this sequence depends upon simple
hydrogen bonding interactions between base pairs.
Hydrogen bonding is fundamental to the double helix
arrangement of the DNA molecule, and the translation and transcription via ribonucleic acid (RNA) of
the genetic information present in the DNA molecule.
O
H
H
O H
H
d+
d+
d+
d+
d−
d−
O H
H
O
H
H
O H
H
O
H
H
O
H
H
extensive hydrogen
bonding in water
O
H
CH 3
O
CH 3
H
O
H
CH 3
O
CH 3
H
hydrogen bonding in methanol