Chapter 4: Chemical bonding
As a result: ■
■ ■ ■
■
the centre of positive charge does not coincide with the centre of negative charge we say that the electron distribution is asymmetric the two atoms are partially charged we show – the less electronegative atom with the partial charge δ+ (‘delta positive’) – the more electronegative atom with the partial charge δ– (‘delta negative’) we say that the bond is polar (or that it has a dipole).
Figure 4.32 shows the polar bond in a hydrogen chloride molecule. H δ+
Cl δ–
QUESTION
Figure 4.32 Hydrogen chloride is a polar molecule.
As the difference in electronegativity values of the atoms in a covalent bond increases, the bond becomes more polar. The degree of polarity of a molecule is measured as a dipole moment. The direction of the dipole is shown by the . The arrow points to the partially negatively sign charged end of the dipole. In molecules containing more than two atoms, we have to take into account: ■ ■
the polarity of each bond the arrangement of the bonds in the molecule.
Trichloromethane, CHCl3, is a polar molecule. The three C Cl dipoles point in a similar direction. Their combined effect is not cancelled out by the polarity of the C H bond. This is because the C H bond is virtually non-polar. The electron distribution is asymmetric. The molecule is polar, with the negative end towards the chlorine atoms. This is shown in Figure 4.33a. a
H C
Cl
δ+
b
δ–
Cl
Cl Cl
trichloromethane, a polar molecule
cancel each other out. An example is tetrachloromethane, CCl4 (Figure 4.33b). Tetrachloromethane has four polar C Cl bonds pointing towards the four corners of a tetrahedron. The dipoles in each bond cancel each other. So, tetrachloromethane is non-polar. The charge distribution in molecules and ions can be determined by a method called X-ray spectroscopy. One method involves firing X-rays at molecules and measuring the energy of the electrons given off. Using this method, scientists have found that in a sulfate ion, the sulfur atom has a charge of +1.12 units and the four oxygen atoms each have a charge of –0.78 units.
Cl C
Cl Cl
9 Are the following molecules polar or non-polar? In each case give a reason for your answer. (Electronegativity values: F = 4.0, Cl = 3.0, Br = 2.8, S = 2.5, C = 2.5, H = 2.1) a Chlorine, Cl2 b Hydrogen fluoride, HF c
The V-shaped molecule, sulfur dichloride, SCl2
d The tetrahedral molecule, chloromethane, CH3Cl e The tetrahedral molecule tetrabromomethane, CBr4.
Polarity and chemical reactivity
Bond polarity influences chemical reactivity. For example, both nitrogen, N N, and carbon monoxide, C O, have triple bonds requiring a similar amount of energy to break them. Nitrogen is a non-polar molecule and is fairly unreactive. But carbon monoxide is a polar molecule, and this explains its reactivity with oxygen and its use as a reducing agent. Many chemical reactions are started by a reagent attacking one of the electrically charged ends of a polar molecule. For example, chloroethane, C2H5Cl, is far more reactive than ethane, C2H6. This is because reagents such as OH– ions can attack the delta-positive carbon atom of the polarised C Cl bond (see also page 220).
tetrachloromethane, a non-polar molecule
Figure 4.33 The polarity of a trichloromethane and b tetrachloromethane.
Some molecules contain polar bonds but have no overall polarity. This is because the polar bonds in these molecules are arranged in such a fashion that the dipole moments
H
H
H
C
δ+ δ– C Cl
H
H
Such an attack is not possible with ethane because the C H bond is virtually non-polar. This helps to explain why alkanes, such as ethane, are not very reactive.
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