DISSOCIATION ENTHALPIES, BOND ANGLES AND VSEPR THEORY
1.2 ELECTRONEGATIVITY AND BOND POLARITY
Electronegativity is the tendency of an atom to pull the bonding electrons towards itself.
It increases across a row of the periodic table (excluding the noble gases) and it decreases down a column of the periodic table. The electronegativity values of some common elements are given in the following table.
A bond with the electrons shared equally between the two atoms, i.e., a bond in which each electron spends as much time in the vicinity of one atom as in the other, is called a nonpolar covalent bond. For example, the H—H covalent bond, the Br—Br covalent bond and the C—C covalent bond in ethane etc. are nonpolar covalent bonds. When two atoms of different electronegativities form a covalent bond, the electrons are not shared equally between them. The atom with greater electronegativity draws the electron pair closer to it. As a result of this unequal distribution of the bonding electrons, the bond acquires a slight positive charge (indicated by the symbol d+) on the end that has the less electronegative atom and a slight negative charge (indicated by the symbol d–) on the other end that has the more electronegative atom, i.e., a polarity developes within a bond. Such a bond is called a polar covalent bond. An example of a polar covalent bond is
the one in hydrogen fl uoride (H—F). The fl uorine atom, with its greater electronegativity, pulls the bonding electrons towards itself. As a consequence, the hydrogen atom becomes somewhat electron defi cient and acquires a partial positive charge (d+) while the fl uorine atom becomes somewhat electron rich and acquires a partial negative charge (d–). So, the hydrogen fl uoride molecule is a dipole (Hd+— Fd–). The direction of bond polarity is indicated by an arrow, the head of which indicates the negative end of the bond. A short perpendicular line is drawn near the tail of the arrow to indicate the positive end of the bond.
The extent of polarity in a covalent bond is expressed quantitatively by the physical property known as ‘ dipole moment’ m, which is, in fact, the product of the magnitude of charge e on any polar end and the distance d between the centres of positive and negative charges, i.e., m = e × d.
Dipole moment is expressed in Debye unit (D). The charge e and the distance d are of the order 10–10 esu and 10–8 cm, respectively. Therefore, m is of the order of 10–10 × 10–8 = 10–18 esu.cm (in C.G.S. system). This value of dipole moment is equal to one Debye unit, i.e., 1D = 10–18 esu. cm. The SI unit of dipole moment is coulomb-meter (C.m.) and 1 C.m. = 2.9962 × 1029 D.
The value of m for a nonpolar molecule is zero. For example, m = 0D for the molecules like H2, N2, O2, etc. For polar molecules, m has a defi nite value, e.g., for HF molecule m = 1.91 D. Polarities of molecules increase with increase in the value of m.
Dipole moment is a vector quantity. The dipole moment of a molecule is the result of the vector sum of the individual bond moments present in the molecule. When a molecule is formed by two atoms of different electronegativities (i.e., by two different atoms), the molecule must possess a dipole moment because there is no question to cancellation of the moment of this only polar bond. However, when a molecule contains two or more polar bonds, i.e., when a molecule contains more than two atoms, the molecule may or may not possess dipole moment and in that case the polarity of the molecule depends on the shape of the molecule. A symmetrical polyatomic molecule possesses no dipole moment because the individual dipole moments of the bonds cancel each other. It is to be noted that the symmetry in calculating dipole moment is not the molecular symmetry. H2O molecule, for example, has a symmetrical structure because it has two planes of symmetry (su planes).
But, it possesses dipole moment (1.85 D) because of its angular structure . The value of dipole moment of a molecule is equal to the product of the charge e and the distance, d, between the positive and negative charge centres, i.e., m = e × d, it can be well understood by the following examples:
(i) Although an H—F (0.92 Å) molecule is smaller than an H—Cl (1.27 Å molecule, it has a dipole moment larger than HCl (1.75 D compared to 1.03 D). Since fl uorine (electronegativity: 4.0) is more electronegative than chlorine (electronegativity:
3.0), therefore, the magnitude of e in H—F is much higher than that in H—Cl and for this reason although d in HF is smaller than in HCl (because of smaller size of F compared to Cl), the product e × d, i.e., m, is larger.
(ii) Chloromethane (CH3Cl) has a larger dipole moment (1.87 D) than fl uoromethane CH3F (1.81 D), even though fl uorine is more electronegative than chlorine. Due to higher electronegativity of fl uorine than chlorine, the separation of charge in the C—X bond, i.e., the magnitude of e in CH3F, is somewhat higher than that in CH3Cl.
However, the C—F bond is shorter than the C—Cl bond (1.42 Å compared to 1.77 Å) and because of this, the value of the product, i.e., m, is larger for chloromethane than for fl uoromethane.
That the polarity of a molecule depends upon the shape of the molecule can be well understood by the following example. NH3 possesses considerable dipole moment and although the N—F bonds are more polar than N—H bonds, NF3 (0.26 D) has a much smaller dipole moment than NH3 (1.46 D). Both NH3 and NF3 with sp3 hybridized N atom are pyramidal in shape. The unshared electron pair on nitrogen occupying an sp3 orbital contributes a large dipole moment (because an unshared pair has no other atom attached to it to partially neutralize its negative charge) in the direction opposite to the triangular base of the pyramid. In NH3, the net moment resulting from three N—H bond moments adds to the moment contributed by the unshared pair because they act in the same direction and for this reason, it has considerable dipole moment. In NF3, on the other hand, the vectorial sum of these N—F bond moments acts in the direction opposite to that of the moment caused by the unshared pair. Since these moments are of about the same size, therefore, NF3 has a much smaller dipole moment than NH3.
Resonance often plays an important role in determining the polarities of molecules. For example:
(i) The dipole moment of ethylchloride, CH3 CH2Cl (2.05 D), is larger than that of vinyl chloride, CH2 == CH—Cl (1.44 D). Chlorine is more electronegative than carbon and so, it attracts the C—Cl bonding electrons more towards itself. As a result,
polarity develops in the C—Cl bond in ethyl chloride and the compound shows considerable dipole moments. In vinyl chloride, on the other hand, the unshared electron pair on chlorine becomes involved in resonance interaction with the p-orbital of the double bond. This resonance interaction, therefore, tends to oppose the usual displacement of electrons towards chlorine. Also, the sp2 hybridized carbon being more electronegative than the sp3 hybridized carbon is less willing to release electrons to chlorine. So, although there is still a net displacement of electrons towards chlorine, it is less than in ethyl chloride. Hence, ethyl chloride is more polar than vinyl chloride, i.e., the dipole moment of ethyl chloride is greater than that of vinyl chloride.
(ii) The dipole moment of chlorobenzene is larger than that of fl uorobenzene. In chlorobenzene, the Cl atom withdraws electrons from the ring by its –I effect and donates electrons to the ring by its +R effect. Because of large size and high electronegativtiy of chlorine as compared to that of carbon, the resonance electron donation is much weaker than inductive electron withdrawal. So, the moment caused by the –I effect is much stronger than the moment caused by the +R effect and because of this, chlorobenzene possesses a net dipole moment which is relatively high. In fl uorobenzene, on the other hand, the +R effect fl uorine even being more electronegative than chlorine is much more important because of effective p orbital overlap between the orbitals of carbon and fl uorine which are of comparable size.
So, the moment due to inductive electron withdrawal is predominantly balanced by the moment due to resonance electron donation and the result is that the dipole moment of fl uorobenzene is relatively low.
Aromaticity often plays a role in determining the polarity of a molecule. For example:
(i) Azulene (a bicyclic hydrocarbon) has a much higher dipole moment. Azulene contains 10 p-electrons. Redistribution of these electrons between the two rings generates an aromatic system which consists of an aromatic cycloheptatrienyl cation and an aromatic cyclopentadienyl anion. Since this aromatic dipolar form is quite stable and more contributing, azulene has a much higher dipole moment.
(ii) Pyrrole and furan are polar molecules, but the dipole moment of furan is smaller than and opposite in direction from that of pyrrole. In pyrrole, the unshared electron pair on nitrogen is highly delocalized with the p-electrons of the ring for maintaining aromaticily, i.e., to form a delocalized cyclic system of (4n + 2)p electrons, where n = 1. Since the moment caused by the electron delocalization is much greater than that caused by the –I effect of nitrogen atom, pyrrole has a net dipole momemt of 1.81 D and the dipole points towards the ring. In furan, on the other hand, the unshared electron pair on oxygen is not well delocalized into the ring due to greater electronegativity of oxygen. Hence, the moment due to electron delocalization is small and in fact, it is somewhat smaller than that caused by the –I effect of oxygen. For this reason, the net dipole moment of furan is relatively small (0.70 D) and the dipole points towards oxygen.
Formation of charge-transfer complex is often responsible for exhibiting dipole moment by some molecules. 1,3,5-Trinitrobenzene, for example, shows signifi cant dipole moment in benzene but does not display dipole moment in carbon tetrachloride.
1,3,5-Trinitrobenzene being symmetrical possesses no net dipole moment, i.e., it is a nonpolar molecule. When it dissolves in benzene, a charge-transfer bonding involving some kind of donor–acceptor interaction (benzene is the electron donor and the electron defi cient 1,3,5-trinitrobenzene is the electron acceptor) is formed. Due to charge separation in the resulting complex, the compound shows signifi cant dipole moment in benzene. Since carbon tetrachloride cannot act as a donor molecule, the compound does not display dipole moment in carbon tetrachloride.
Conformation plays an important role in determining the polarities of some molecules. For example:
(i) 1, 2-Dichloroethane has a very little dipole moments, where as 1,2-ethanediol has considersable dipole moment. 1,2-Dichloroethane exists in anti and gauche staggered form. Since the two C—Cl dipoles in the anti form are antiparallel, its dipole moment is assumed to be zero. However, the value of dipole moment of the gauche form is approximately 3.2 D (calculated with reference to C2H5Cl). Because of dipolar and steric repulsion, the gauche form is less stable than the anti form by 1.2 kcal/mol and in the vapour phase the compound exists in 88% anti and 12%
gauche form. Because of such distribution of the conformers, the overall dipole moment of 1,2-dichloroethane is relatively low (1.2 D).
In the gauche conformer of 1, 2-ethanediol (ethylene glycol), the two —OH groups become involved in the formation of intramolecular hydrogen bond. However, in the anti conformer, no intramolecular hydrogen bond is formed because the two —OH groups are oppositely placed. The steric and polar repulsion of the hydroxyl groups in the gauche form is more than outweighed by the energy gained by the formation of hydrogen bond (5 kcal/mol). Because of this, the compound, particularly in the gas phase, exists almost exclusively in the gauche form having a fi nite dipole moment. For such conformational distribution, 1,2-ethanediol is found to possess considerable dipole moment.
(ii) The optically inactive (meso) form of 1,2-dichloro-1,2-di-p-tolylethane has a dipole moment lower than that of the active (d or l) form. The meso form of the compound exists in the following three conformations:
In the conformer I, bulky tolyl groups are anti to each other and also the two Cl atoms are anti to each other. So there is no steric interaction caused by the tolyl groups and repulsive interaction caused by the C—Cl dipoles. Therefore, this conformer is relatively more stable and it has practically no dipole moment. The conformers II and III are polar because the two Cl atoms are gauche to each other.
However, these are very unstable because the bulky p-tolyl groups which are gauch to each other are involved in steric interaction and there occurs repulsive interaction between the two C—Cl dipoles. Therefore, the most favoured conformer
of meso-1,2-dichloro-1,2-di-p-tolylethane is I and as a consequence, the overall dipole moment of the meso compound is relatively low.
The optically active form of this compound exists in the following three conformations:
The conformer IV in which C—Cl dipoles are antiparallel has no appreciable dipole moment. However, this conformer is unstable because the bulky p-tolyl groups which are gauche to each other are involved in steric interaction. The conformer VI has an appreciable dipole moment but it is unstable due to the same steric interaction and also due to repulsive interaction between the two C—Cl dipoles. In the conformer V, the two p-tolyl groups are anti to each other. So, it is relatively stable and the preferred conformer. Since it has an appreciable dipole moment, the overall dipole moment of active 1,2-dichloro-1,2-ditoylethane is relatively large.
(iii) The dipole moment of cis-1,2-dichlorocyclohexane is larger than that of its trans- isomer. Each of the conformational isomers of cis-1,2-dichlorcyclohexane has one axial and one equatorial chlorine atom. As both the forms are equally stable, 50% of the molecules exist in one and 50% in the other form. Since Cl atoms are gauche to each other in both the forms, they have appreciable dipole moment and consequently, the overall dipole moment of the compound is relatively large. The trans-isomer, on the other hand, exists in diequatorial and diaxial forms. The diequatorial form in which the two Cl atom are gauche to each other has appreciable dipole moment. Although favoured sterically, it is disfavoured by dipolar repulsion.
The diaxial form with a very small dipole moment, although disfavowred sterically, is free from dipolar repulsion and is relatively stable. This form, therefore, exists predominantly and so, the overall dipole moment of this isomer is much lower than that of the cis-isomer.
(iv) The dipole moment of hexane-3,4-dione is very small, whereas that of cyclohex- 3,5-dien-1,2-dione is very large. Hexane-3,4-dione may exist in two conformations such as s-cis (cisoid) and s-trans (transoid). In the s-trans conformer, the two C == O dipoles are antiparallel. Since the C == O bond moment cancels each other, therefore, this conformer has no net dipole moment. Again, as the negatively polarized oxygen atoms are as far as possible from each other, this conformer is free from any repulsive interaction between the two oxygen atoms. In the s-cis conformer, on the other hand, the C == O dipoles remain on the same side of the double bond make an angle of 60° with each other. Consequently, a large net moment of two strong C == O bond moments operates. Again, as the two C == O dipoles are close enough to repeal each other, this conformer is relatively less stable. For this reason, the compound exists almost exclusively, in the more stable s-trans conformation and the overall dipole moment of the compound is very small.
The planar and rigid cyclohex-3,5-dien-1,2-dione molecule exists exclusively in the highly polar s-cis conformation in which the two C == O bond moments make an angle of 60° with each other. It cannot exists in the s-trans conformation because it is very much unstable due to severe angle strain. For this reason, the dipole moment of the compound is quite large.
Induced moment often plays an important role in determining the dipole moments of molecules. The expected order of dipole moment of various fl uormethanes is CH2F2 > CH3F ê CHF3 > CF4 . However, the experimental order is: CF3F > CH2F2 > CHF3 > CF4. This anomaly can be explained in terms of opposing induced moment. In the symmetrical tetrahedral molecule of CF4, the resultant of three C — F bond moments cancels the fourth C — F bond moment and because of this, the molecule possesses no net dipole moment.
In CH3F, the moment due to the C—F bond is not cancelled and the molecule possesses a net dipole moment which is the resultant of C—F and C—H bond moments. The dipole moment of CH2F2 in which the two C—F bonds make an angle of nearly 109°5 is expected to be larger than that of CH3F because the resultant of two C—F bond moments must be greater than one C—F bond moment. Again, the dipole moment of CH3F is expected to be equal to that of CHF3 because the moment of a —CF3 group is equal to that of a C—F bond and the moment of a —CH3 group is equal to that of a C—H bond. Thus, the expected order of dipole moment is CH2F2 ề CH3F ê CH3F ề CF4. However, this order does not agree with the experimental dipole moment values. This can be explained by considering the moment acting in the opposite direction induced by each C—F dipole in the other. Since there is only one C—F bond in CH3F, the opposing induced moment is absent in it. So it has the largest dipole moment. In CH2F2, there are two C—F bond. Thus, opposing induced moment operates in this case and that partly cancels the resultant moment of two C—F bonds. Because of this, its dipole moment is smaller than CH3F. In CHF3, there are three C—F bonds and so, the magnitude of the opposing induced moment is relatively large in
this case. In fact, this reduces the resultant of three C—F bond moments considerably and because of this its dipole moment is smaller than that of CH2F2 and much smaller than that of CH3F. Hence, the order of decreasing dipole moments of these four fl uoromethanes in CH3F > CH2F2 > CHF3 > CF4.
Dipole moment of disubstituted benzenes:
The dipole moment of a disubstituted banzene C6H4 AB can be given as
2 2 2 cos
AB A B A B
m = m +m + m m a
where mA and mB are the two group moments and a is the angle between their direction.
When both the groups are electron donating or both are electron withdrawing, the values of a for ortho-, meta- and para-isomer are 60°, 120° and 180°, respectively. However, when one group is electron donating and the other is electron withdrawing, the value of a for ortho-, meta- and para-isomer are 120°, 60° and 0°, respectively. For example:
(i) Dichlorobenzenes: In each of the three isomeric dichlorobenzenes, the angle between the vectors of the two group moment may be shown as follows:
In this case, A == B == Cl and mA =mB=mC H Cl6 5 =1.55 D, The group moment of
—Cl is to be used with negative sign because it is directed away from the benzene ring.
For o-dichlorobenzene, where a = 60°, the net dipole moment is
6 4 2
2 2 2
C H Cl ( 1.55) ( 1.55) 2( 1.55) cos60
mO- = - + - + - ∞
2.40525 2.4025 2 2.4025 1
= + + ¥ ¥2
= 7.2075 2.68 D=
Similarly, for m-dichlorobenzene, where 120 cos120 1 ,
a = ∞ÊÁË ∞ =2ˆ˜¯ the net dipole moment is 1.55 D and for p-dichlorobenzene, where a = 180° (cos180° = –1), the net dipole moment is 0 D.
(ii) Nitrotoluenes: In this case,
6 5 3
3 2 A C H CH
A = CH , B = NO ,m =m =0.4 D and mB=mC H NO6 5 2 =3.95 D. The group moment of the electron–withdrawing —NO2 group is negative because it is directed away from the ring. The group moment of the electron-releasing —CH3 group is also negative because it is assumed to directed away from the ring.