MORISSON   Organic Chemistry

MORISSON Organic Chemistry


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of course, 
the same electronegativity and share electrons equally; e is zero and hence p is 
zero, too. 
A molecule like hydrogen fluoride has the large dipole moment of 1.75 D. 
Although hydrogen fluoride is a small molecule, the very high electronegative 
4uorine pulls the electrons strongly; although d is small, e is large, and hence p is 
large, too. 
Methane and carbon tetrachloride, CCl,, have zero dipole moments. We 
certainly would expect the individual bonds-of carbon tetrachloride at least-to 
be polar; because of the very symmetrical tetrahedral arrangement, however, they 
exactly cancel each other out (Fig. 1.16). In methyl chloride, CH,Cl, the polarity 
++ H-F 
I 
tI ( I C-H 
! I 
Hydrogen Methdne C'a rbon Methyl chlorlde 
fluoride tetrdchlor~dc 
Figure 1.16 Dipole moments of some molecules. Polarity of bonds and of molecules. 
of the carbon-chlorine bond is not canceled, however, and methyl chloride has a 
dipole moment of 1.86 D. Thus the polarity of a molecule depends not only upon 
the polarity of its individual bonds but also upon the way the bonds are directed, 
that is, upon the shape of the molecule. 
Ammonia has a dipole moment of 1.46 D. This could be accounted for as a 
net dipole moment (a vector sum) resulting from the three individual bond moments, 
SEC. 1.16 POLARITY OF MOLECULES 25 
and would be in the direction shown in the diagram. In a similar way, we could 
account for water's dipole moment of 1.84 D. 
H Dipole moments 
H H expected from 
bond moments alone 
Ammonia Water 
Now, what kind of dipole moment would we expect for nitrogen trifluoride, 
NF,, which, like ammonia, is pyramidal? Fluorine is the most electronegative 
element of all and should certainly pull electrons strongly from nitrogen; the N-F 
bonds should be highly polar, and their vector sum should be large-far larger than 
for ammonia with its modestly polar N-H bonds. 
F 
Large dipole moment 
F expectedfrom 
bond moments alone 
Nitrogen trifluoride 
Z 
What are the facts? Nitrogen trifluoride has a dipole moment of only 0.24 D. It is 
not larger than the moment for ammonia, but rather is much smaller. 
How are we to account for this? We have forgotten the unshared pair of 
electrons. In NF3 (as in NH3) this pair occupies an sp3 orbital and must contribute 
a dipole moment in the direction opposite to that of the net moment of the N-F 
bonds (Fig. 1.17); these opposing moments are evidently of about the same size, 
p= 1.46 D 
I 
Ammonia Water Nitrogen 
trifluoride 
Figore 1.17 Dipole moments of some molecules. Contribution from un- 
I shared pairs. In NF,, the moment due to the unshared pair opposes the 
vector sum of the bond moments. 
26 STRUCTURE AND PROPERTIES CHAP. 1 
and the result is a small moment, in which direction we cannot say. In ammonia 
the observed moment is probably due chiefly to the unshared pair, augmented by 
the sum of the bond moments. In a similar way, unshared pairs of electrons must 
contribute to the dipole moment of water and, indeed, of any molecules in which 
they appear. 
Dipole moments can give valuable information about the structure of mole- 
cules. For example, any structure for carbon tetrachloride that would result in a 
polar molecule can be ruled out on the basis of dipole moment alone. The evidence 
of dipole moment thus supports the tetrahedral structure for carbon tetrachloride. 
(However, it does not prove this structure, since there are other conceivable 
structures that would also result in a non-polar molecule.) 
blem 1.5 
ro dipole I 
,. \ ,. 
the follow. 
(a) Carbo~ 
~.~ ~- ~ " - 
ing concei 
I at the ce 
.1 
vable struc 
nter of a s 
A . - L a . 
C1, would 
h a chlorir 
. . . 
also have 
le at each 
'- . . - . . - 
Which of 
moment? 1 
corner. (0) arbo on at the apex or a pyramla wlrn a cnlorlne ar eacn comer 01 a square 
base. 
'%Problem 1.6 
dipole momen 
Suggest a 
lt. 
shape for the CO, r nolecule tl hat would account fi Br its zero 
for 
trifl 
bonding, 1 
uoride to c 
In Sec. 1. 
If nitroger 
, ..,L_. :. .l 
12 we reje 
1 were sp2- 
- - >: - -a - - 
:onceivabb 
1, what d i ~ 
---_-:- l 
Pro1 cted two c e electronic configurations for 
amr hybridize( mle moment would you expect 
for ammonla ! w mar u me ulpoie moment of ammonia? (b) If nitrogen usedp orbitals 
low would you expect the dip i nitrogen 
:ompare? How do they compar 
lole mome 
,e? 
nts of am] 
.d 
monia anc 
The dipole moments of most compounds have never been measured. For these 
substances we must predict polarity from structure. From our knowledge of 
electronegativity, we can estimate the polarity of bonds; from our knowledge of 
bond angles, we can then estimate the polarity of molecules, taking into account 
any unshared pairs of electrons. 
1.17 Structure and physical properties 
We have just discussed one physical property of compounds : dipole moment. 
Other physical properties-like melting point, boiling point, or solubility in a 
particular solvent-are also of concern to us. The physical properties of a new 
compound give valuable clues about its structure. Conversely, the structure of a 
compound often tells us what physical properties to expect of it. 
In attempting to synthesize a new compound, for example, we must plan a 
series of reactions to convert a compound that we have into the compound that we 
want. In addition, we must work out a method of separating our product from all 
the other compounds making up the reaction mixture: unconsumed reactants, 
solvent, catalyst, by-products. Usually the isolation and purfication of a product 
take much more time and effort than theactual making of it. The feasibility of 
isolating the product by distillation depends upon its boiling point and the boiling 
points of the contaminants ; isolation by recrystallization depends upon its solubility 
in various solvents and the solubility of the contaminants. Success in the laboratory 
often depends upon making a good prediction of physical properties from structure. 
Organic compounds are real substances-not just collections of letters written on 
a piece of paper-and we must learn how to handle them. 
SEC. 1.18 MELTING POINT 27 
We have seen that there are two extreme kinds of chemical bonds: ionic 
bonds, formed by the transfer of electrons, and covalent. bonds, formed by the 
sharing of electrons. The physical properties of a compound depend largely upon 
which kind of bonds hold its atoms together in the molecule. 
1.18 Melting point 
In a crystalline solid the particles acting as structural units-ions or mole- 
cules-are arranged in some very regular, symmetrical way; there is a geometric 
pattern repeated over and over within a crystal. 
Melting is the change from the highly ordered arrangement of particles in the 
crystalline lattice to the more random arrangement that characterizes a liquid (see 
Figs. 1.1 8 and 1.19). Melting occurs when a temperature is reached at which the 
thermal energy bf the particles is great enough to overcome the intracrystalline 
forces that hold them in position. 
An ionic compound forms crystals in which the structural units are ions. Solid 
sodium chloride, for example, is made up of positive sodium ions and negative 
chloride ions alternating in a very regular way. Surrounding each positive ion and 
Figure 1.18 ,-Melting of an ionic crystal. The units are ions. 
equidistant from it are six negative ions: one on each side of it, one above and one 
below, one in front and one in back. Each negative ion is surrounded in a similar 
way by six positive ions. There is nothing that we can properly call a molecule of 
sodium chloride. A particular sodium ion does not "belong" to any one chloride 
ion; it is equally attracted to six chloride