2.5.1 Dipole–dipole interactions
The interactions between the positive end of one dipole and the negative end
of another dipole are called dipole–dipole interactions. As a result of
dipole–dipole interactions, polar molecules are held together more strongly
than nonpolar molecules. Dipole–dipole interactions arise when electrons
are not equally shared in the covalent bonds because of the difference in
electronegativity. For example, hydrogen fluoride has a dipole moment of
1.98 D, which lies along the HÀ ÀF bond. As the fluorine atom has greater
electronegativity than the hydrogen atom, the electrons are pulled towards
fluorine, as shown below.
H F
δ +
δ −
µ = 1.98 D
The arrow indicates the electrons are towards the more electronegative atom
fluorine. The d
þ and d
À symbols indicate partial positive and negative
charges.
2.5.2 van der Waals forces
Relatively weak forces of attraction that exist between nonpolar molecules are called van der Waals forces or London dispersion forces.
Dispersion forces between molecules are much weaker than the covalent
bonds within molecules. Electrons move continuously within bonds and
molecules, so at any time one side of the molecule can have more electron
density than the other side, which gives rise to a temporary dipole.
Because the dipoles in the molecules are induced, the interactions
between the molecules are also called induced dipole–induced dipole
interactions.
van der Waals forces are the weakest of all the intermolecular interactions. Alkenes are nonpolar molecules, because the electronegativities of
carbon and hydrogen are similar. Consequently, there are no significant
partial charges on any of the atoms in an alkane. Therefore, the size of
the van der Waals forces that hold alkane molecules together depends on the
area of contact between the molecules. The greater the area of contact, the
stronger are the van der Waals forces, and the greater is the amount of
energy required to overcome these forces. For example, isobutane (b.p.
À10.2
C) and butane (b.p. À0.6
C), both with the molecular formula
C 4 H 10 , have different boiling points. Isobutane is a more compact molecule
than butane. Thus, butane molecules have a greater surface area for
interaction with each other than isobutane. The stronger interactions that
2.5 BOND POLARITY AND INTERMOLECULAR FORCES
29
The interactions between the positive end of one dipole and the negative end
of another dipole are called dipole–dipole interactions. As a result of
dipole–dipole interactions, polar molecules are held together more strongly
than nonpolar molecules. Dipole–dipole interactions arise when electrons
are not equally shared in the covalent bonds because of the difference in
electronegativity. For example, hydrogen fluoride has a dipole moment of
1.98 D, which lies along the HÀ ÀF bond. As the fluorine atom has greater
electronegativity than the hydrogen atom, the electrons are pulled towards
fluorine, as shown below.
H F
δ +
δ −
µ = 1.98 D
The arrow indicates the electrons are towards the more electronegative atom
fluorine. The d
þ and d
À symbols indicate partial positive and negative
charges.
2.5.2 van der Waals forces
Relatively weak forces of attraction that exist between nonpolar molecules are called van der Waals forces or London dispersion forces.
Dispersion forces between molecules are much weaker than the covalent
bonds within molecules. Electrons move continuously within bonds and
molecules, so at any time one side of the molecule can have more electron
density than the other side, which gives rise to a temporary dipole.
Because the dipoles in the molecules are induced, the interactions
between the molecules are also called induced dipole–induced dipole
interactions.
van der Waals forces are the weakest of all the intermolecular interactions. Alkenes are nonpolar molecules, because the electronegativities of
carbon and hydrogen are similar. Consequently, there are no significant
partial charges on any of the atoms in an alkane. Therefore, the size of
the van der Waals forces that hold alkane molecules together depends on the
area of contact between the molecules. The greater the area of contact, the
stronger are the van der Waals forces, and the greater is the amount of
energy required to overcome these forces. For example, isobutane (b.p.
À10.2
C) and butane (b.p. À0.6
C), both with the molecular formula
C 4 H 10 , have different boiling points. Isobutane is a more compact molecule
than butane. Thus, butane molecules have a greater surface area for
interaction with each other than isobutane. The stronger interactions that
2.5 BOND POLARITY AND INTERMOLECULAR FORCES
29