"H Cl C C Cl Cl Cl Cl Cl Cl Non-Symmetrical SymmetricalDue to the symmetrical structure of CCl the resultant of bond dipoles comes out4 to be zero. But in case of CHCl it is not possible as the presence of hydrogen3 introduces some dissymmetry.Example 25. Compare the dipole moment of H O and F O.2 2 Solution: Let‘s draw the structure of both the compounds and then analyse it.O O H F H F In both H O and F O the structure is quite the same. In H O as O is more2 2 2 electronegative than hydrogen so the resultant bond dipole is towards O, whichmeans both the lone pair and bond pair dipole are acting in the same directionand dipole moment of H O is high. In case of F O the bond dipole is acting2 2 towards fluorine, so in F O the lone pair and bond pair dipole are acting in2 opposition resulting in a low dipole.In C-H, carbon being more electronegative the dipole is projected towards C.Now the question comes whether hybridization has anything to do with the dipolemoment. The answer is obviously yes. If yes, why? Depending on thehybridization state the electronegativity of carbon changes and therefore thedipole moment of C-H bond will change. As the s character in the hybridizedstate increases, the electronegativity of C increases due to which C attracts theelectron pair of C-H bond more towards itself resulting in a high bond dipoles.Now as we have said about carbon hydrogen bonds, the question that is comingto your mind is whether we would be dealing with organic compounds or not. Yeswe would be dealing with the organic compounds. For instance but -2- ene. It exists in two forms cis and Trans.H CH H 3 CH3 C C C C H CH 3 CH 3 H Cis TransThe trans isomer is symmetrical with the 2 methyl groups in anti position. So thebond dipoles the two Me– C bonds acting in opposition, cancel each other resultsinto a zero dipole. Whereas in cis isomer the dipoles do not cancel each otherresulting in a net dipole.Example 26. The molecule having largest dipole moment among the following is (A) CH (B) CHCl4 3(C) CCl (D) CHI4 321 Solution:(B) Example 27. Compare the dipole moment of cis 1, 2 dichloroethylene and trans 1, 2dichloroethylene.Cl Cl Cl HSolution:C C C C Cl H H H Trans CisIn the trans compound the C-Cl bond dipoles are equal and at the same timeacting in opposition cancel each other while in cis compound the dipoles do notcancel each other resulting in a higher value.Generally all Trans compounds have a lower dipole moment corresponding toCis isomer, when both the substituents attached to carbon atom are eitherelectron releasing or electron withdrawing. PERCENTAGE OF IONIC CHARACTEREvery ionic compound having some percentage of covalent character according to Fajan‘s rule.The percentage of ionic character in a compound having some covalent character can becalculated by the following equation.Observed dipole moment ?100 The percent ionic character = Calculated dipole moment assuming 100% ionic bond –29 Example 28. Dipole moment of KCl is 3.336 ? 10 coulomb metre which indicates that it is+ – highly polar molecule. The interatomic distance between K and Cl is –102.6 ? 10 m. Calculate the dipole moment of KCl molecule if there wereopposite charges of one fundamental unit located at each nucleus. Calculate thepercentage ionic character of KCl.Solution:Dipole moment ? = e? d coulomb metre–10For KCl d = 2.6 ? 10 m For complete separation of unit charge–19e = 1.602 ? 10 C–19 –10 –29Hence ? = 1.602? 10? 2.6 ? 10 = 4.1652 ? 10 Cm–29? = 3.336 ? 10 CmKCl ?29 3.336 ?10? % ionic character of KCl = ?100 = 80.09%?29 4.165 ?10 Example 29. Calculate the % of ionic character of a bond having length = 0.83 Å and 1.82 Das it’s observed dipole moment.Solution: To calculate ? considering 100% ionic bond–10 –8=4.8 ?10 ? 0.83?10 esu cm–18= 4.8 ? 0.83? 10 esu cm = 3.984 D1.82? % ionic character = ?100 ?45.683.984 The example given above is of a very familiar compound called HF. The % ionic character isnearly 43.25%, so the % covalent character is (100 – 43.25) = 56.75%. But from the octet 22 rule HF should have been a purely covalent compound but actually it has some amount ofionic character in it, which is due to the electronegativity difference of H and F. Similarlyknowing the bond length and observed dipole moment of HCl, the % ionic character can beknown. It was found that HCl has 17% ionic character. Thus it can be clearly seen thatalthough we call HCl and HF as covalent compounds but it has got appreciable amount ofionic character. So from now onwards we should call a compound having more of ionic lessof covalent and vice versa rather than fully ionic or covalent.BOND CHARACTERISTICS1. Bond Length: The distance between the nuclei of two atoms bonded together is termed as? ?? bond length or bond distance. It is expressed in angstrom A units or picometer (pm). ?? ?? ? ?? ?? 8 12 1A?? 10 cm;1pm 10 m?? ?? ??Bond length in ionic compound = rr ? caSimilarly, in a covalent compound, bond length is obtained by adding up the covalent (atomic)radii of two bonded atoms.Bond length in covalent compound (AB) = rr ? ABThe factors such as resonance, electronegativity, hybridization, steric effects, etc., whichaffect the radii of atoms, also apply to bond lengths.Important features (i)The bond length of the homonuclear diatomic molecules are twice the covalent radii. (ii) The lengths of double bonds are less than the lengths of single bonds between the same two atoms, and triple bonds are even shorter than double bonds.Single bond > Double bond > Triple bond (decreasing bond length)(iii)Bond length decreases with increase in s-character since s-orbital is smaller than ap – orbital. 3 ? 2 ? ?sp C?? H 1.112A ; sp C?? H 1.103A ; spC?? H 1.08A ;(25% s-character as in alkanes) (33.3% s-character as in alkenes) (50% s-character as inalkynes)(iv)Bond length of polar bond is smaller than the theoretical non-polar bond length. 2.Bond Energy or Bond Strength: Bond energy or bond strength is defined as the amount ofenergy required to break a bond in molecule. Important features(i)The magnitude of the bond energy depends on the type of bonding. Most of the covalent?1 ?1 mol mol bonds have energy between 50 to 100 kcal (200-400 kJ ). Strength of sigmabond is more than that of a -bond. ? (ii)A double bond in a diatomic molecules has a higher bond energy than a single bond and atriple bond has a higher bond energy than a double bond between the same atoms.C ? C ? C ? C ? C ? C (decreasing bond length)(iii)The magnitude of the bond energy depends on the size of the atoms forming the bond, i.e.bond length. Shorter the bond length, higher is the bond energy. (iv)Resonance in the molecule affects the bond energy.23 (v)The bond energy decreases with increase in number of lone pairs on the bonded atom.This is due to electrostatic repulsion of lone pairs of electrons of the two bonded atoms. (vi)Homolytic and heterolytic fission involve different amounts of energies. Generally thevalues are low for homolytic fission of the bond in comparison to heterolytic fission. (vii)Bond energy decreases down the group in case of similar molecules. (viii)Bond energy increase in the following order: 23 s ? p ? sp ? sp ? spC ? C ? N ?N ? O ? O?No lone pair ? ?One lone pair ? ?Two lone pair ? 3.Bond angles: Angle between two adjacent bonds at an atom in a molecule made up of threeor more atoms is known as the bond angle. Bond angles mainly depend on the followingthree factors: (i) Hybridization: Bond angle depends on the state of hybridization of the central atom32 Hybridization sp sp sp ??? ?Bond angle 109 28 120 180Example CH BCl BeCl 4 3 2Generally s- character increase in the hybrid bond, the bond angle increases.(ii)Lone pair repulsion: Bond angle is affected by the presence of lone pair of electrons at thecentral atom. A lone pair of electrons at the central atom always tries to repel the shared pair(bonded pair) of electrons. Due to this, the bonds are displaced slightly inside resulting in adecrease of bond angle.(iii) Electronegativity: If the electronegativity of the central atom decreases, bond angledecreases. HYDROGEN BONDINGIntroduction:An atom of hydrogen linked covalently to a strongly electronegative atom can establish an extraweak attachment to another electronegative atom in the same or different molecules. Thisattachment is called a hydrogen bond. To distinguish from a normal covalent bond, a hydrogen… bond is represented by a broken line eg X – H Y where X & Y are two electronegative atoms.–1 –1 The strength of hydrogen bond is quite low about 2-10 kcal mol or 8.4–42 kJ mol as compared–1 –1 to a covalent bond strength 50–100 kcal mol or 209 –419 kJ molConditions for Hydrogen Bonding:? Hydrogen should be linked to a highly electronegative element.? The size of the electronegative element must be small.These two criteria are fulfilled by F, O, and N in the periodic table. Greater theelectronegativity and smaller the size, the stronger is the hydrogen bond which is evidentfrom the relative order of energies of hydrogen bonds.Types of Hydrogen Bonding:? Intermolecular hydrogen bonding: This type of bonding takes place between twomolecules of the same or different types. For example,H H H O—H----- O — H ------ O — H ------Intermolecular hydrogen bonding leads to molecular association in liquids like water etc. Thus inwater only a few percent of the water molecules appear not to be hydrogen bonded even at 90°C. 24 Breaking of those hydrogen bonds throughout the entire liquid requires appreciable heat energy.This is indicated in the relatively higher boiling points of hydrogen bonded liquids. Crystallinehydrogen fluoride consists of the polymer (HF) . This has a zig-zag chain structure involving n H-bond.F F H H H H H F F F ? Intramolecular hydrogen bonding: This type of bonding occurs between atoms of thesame molecule present on different sites. Intramolecular hydrogen bonding gives rise to aclosed ring structure for which the term chelation is sometimes used. Examples are o-nitrophenol, salicylaldehyde.H H O O O O C N H O Salicaldehyde o-nitrophenolEffect of Hydrogen BondingHydrogen bonding has got a very pronounced effects on certain properties of the molecules.They have got effects on? State of the substance? Solubility of the substance? Boiling point? Acidity of different isomersThese can be evident from the following examples.Example 30. H O is a liquid at ordinary temperature while H S is a gas although both O and S2 2 belong to the same group of the periodic table.Solution:H O is capable of forming intermolecular hydrogen bonds. This is possible due to2 high electronegativity and small size of oxygen. Due to intermolecular H-bonding,molecular association takes place. As a result the effective molecular weightincreases and hence the boiling point increases. So H O is a liquid. But in H S no2 2 hydrogen bonding is possible due to large size and less electronegativity of S. Soit‘s boiling point is equal to that of an isolated H S molecule and therefore it is a2 gas.Example 31. Ethyl alcohol (C H OH) has got a higher boiling point than dimethyl ether 2 5 (CH -O-CH ) although the molecular weight of both are same.3 3 Solution:Though ethyl alcohol and dimethyl ether have the same molecular weight but inethyl alcohol the hydrogen of the O-H groups forms intermolecular hydrogenbonding with the OH group in another molecule. But in case of ether the hydrogenis linked to C is not so electronegative to encourage the hydrogen to fromhydrogen bonding.H O H O H O C H C H C H 2 5 2 5 2 525 "