Electronegativity

Electronegativity

Electronegativity is the tendency of a neutral atom in a small molecule to attract electrons.It is a property of the atom in a bonded state.Electronegativity is inherently fundamental property of the atom and is fundamentally different from electron affinity.Since,electron affinity represents the tendency of an isolated gaseous atom to attract the electron while electronegativity represents the tendency of a bonded atom(i.e. of an atom in a molecule) to attract the shared electron pair.In 1931,pauling defined the electronegativity of an atom as the tendency of the atom to attract electrons to itself when combined in a compound.

Electronegativity is not an absolute quantity,it is only a relative property of an atom in a molecule in the environment and under the influence of surrounding atoms.It is not a precise quantity rather it is a relative value.A number of scales have been devised to measure the electronegativity of the atoms.These scales are arbitary and are based on various types of experimental data like bond energy,dipole moment,ionization potential snd electron affinity.Pauling scale is the most important of these sacles.In this scale,Pauling assined an arbitary value of 4.0 for fluorine atom.with the help of his scale,he has arbitarily calculated electronegativity of other atoms.

i)Pauling's approach of electronegativity

In 1931,Linus Pauling proposed a scale of electronegativity.Pauling on the basis of electronegativity explain the fact that the covalent bond between two different atoms (A-B) is stronger than would be expected by taking the mean of the bond energy of A-A and B-B bonds.Pauling scale of electronegativity is based on the bond energy of molecule.

Pauling considered the reaction of the type:

$$\frac{1}{2}(A-A)+\frac{1}{2}(B-B) \rightarrow A-B$$

According to Pauling,for a covalent molecule(say A-B),its bond energy is equal to the geometric mean of the bond energy of A-A and B-B molecule.

∴$$E_{A-B}=(E_{A-A}×E_{B-B})^½$$

Where,

$$E_{A-B}$$=Bond energy of A-B bond

$$E_{A-A}$$=Bond energy of A-A bond

$$E_{B-B}$$=Bond energy of B-B bond

It has been found that the experimental(actual) bond energy is always greater than its theoritical bond energy.This excess bond energy is called ionic covalent resonance energy.This extra bond energy i.e. difference in bond energy is denoted as Δ(delta) given by the following empirical equation.

$$Δ=(E_{A-A}_expt.- \sqrt{ (E_{A-A})×(E_{B-B})}$$

where Δ=resonance energy

Resonance energy(Δ) of a molecule depends on the polarity of bond,which in turn depends on the electronegativity difference between two atoms A and B respectively,then,

$$χ_A-χ_B=0.182 \sqrt{Δ}$$

Here Δ is expressed in KCal mol^-1.However if Δ is expressed in eV per molecule then the above expression becomes

$$χ_A-χ_B=0.208 \sqrt{Δ}$$

The extrastabilty of the A-B bond originates from the polarity developed in it due to the difference in the electronegativities of A and B.

ii)Mulliken's scale of electronegativity

Mulliken approach of electronegativity depends on the electron affinity and the ionization energy of an atom.amulliken suggested that electronegativity is the arithmetic mean of ionization energy and electron affinity of an atom,both expressed in elecron volts(eV).

Electronegativity=$$\frac{I.E+E.A}{2}$$(eV unit)

If E.A and I.E are expressed in KJ/mole,the above relationship may be written as,

Electronegativity=$$\frac{I.E+E.A}{2×96.48}$$(1eV=96.48 KJmol^-1)

Mulliken values of electronegativity are about 2.8 times greater than as Pauling's value.Hence,to adjust Mulliken's values with Pauling's values,the expression will be,

Electronegativity=$$\frac{I.E+E.A}{2×96.48×2.8}$$

Electronegativity=$$\frac{I.E+E.A}{540}$$ kJmol^-1

iii)Allerd and Rochow scale of electronegativity

Allerd and Rochow scale of electronegativity depends on the effective nuclear charge.It is widely adopted empirical approach to electronegativity.They defined electronegativity as the attractive force between a nucleus and an electron.This force is electrostatic and given by,

$$F=Z_eff×\frac{e^2}{r}$$

where Z_eff=Effective nuclear charge

e=Charge on electron

r=Covalent radius

This force of attraction F,may be converted into electronegativity values of using empirical relationship.

Electronegativity=$$0.744+0.359\frac{Z_eff}{r^2}$$

Variation of electronegativity[Factors affecting electronegativity]

The overall tendency of a bonded atom to attract electron depends on its bonding environment.Electronegativity of an atom in a molecule is influenced by two factors.i)Valence state[Hybridization state] and ii)Atomic charge[Oxidation state]

i)Valence State[Hybridization State]

Hybridization affects the electronegativity because of the lower energy and hence greater electron-attracting power of s-orbitals.The electronegativity of an atom vary slightly with hybridization,with those orbitals having greater s-characters being more electronegative.Hence the electronegativity increases with the increase in 's' character of the hybrid orbitals.The electronegativity sequence run as:sp>sp²>sp³.This is because the percentage of s-character in these hybrid orbitals are 50%,33.33%, and 25% respectively.For C,the value in Pauling's scale are:

$$C(sp)=3.34,C(sp^2)=2.80,C(sp^3)=2.52$$

Examples

a)The acidity sequence (i.e.protonic character of hydrogen bound to carbon) runs as follows:

$$H-C≡C-H>H_2C=CH_2>CH_3-CH_3$$

The electronegativity of a hybrid orbital increases with increase in s-character.Thus the electronegativity sequence,$$C(sp)>C(sp^2)>C(sp^3)$$

runs paralleled to the acid strength sequence.

b)The basicity of amines is a function of the hybridization of the N-atom.The more electronegative the N-atom,the less readily it will share its lone pair electrons and act as a base.The electronegativity of the Nitrogen atom increases as the s-character of the hybridization increases and hence its basicity decreases.

ii)Atomic Charge(oxidation state)

With the increase in positive charge of atom,the electronegativity increases while it decreases with increase of negative charge.It is because the tendency to attract electron increases with the increase in positive charge and vice versa.

For example,

Fe(II)=1.83 Fe(III)=1.96

Sn(II)=1.80 Sn(IV)=1.96

Similarly,the electronegativity of an atom can vary in response to the partial charge generated by the substituent atoms and groups.The electron withdrawing(i.e electronegative) substituents produce an electron deficiency(i.e more positive charge) on the atoms to which they are bound.For example methyl iodide hydrolyses as expected for alkyl halides,but trifluoromethyl iodides give unusual products:

The nucleophilic attack occurs at the positive center.In the trifluoromethyl iodide,fluorines induce the positive charge at both C and I,but I is more polarisable than C because of large size of I.Thus the nucleophilic attack occurs on I,not on C with the formation of hypoiodous acid which then loses an H⁺ in the alkaline medium to form charge distribution.Similarly,the following reactions between carbonylate anion and alkyl iodides gives the different products.

i)$$Na^+[Mn(CO)_5]^- + CH_3I \rightarrow CH_3[Mn(CO)_5]+NaI$$

ii)$$Na^+[Mn(CO)_5]^- + CF_3I \rightarrow [Mn(CO)_5]I + CF_3Na$$

$$CF_I+CF_3Na \rightarrow F_3C-CF_3+NaI$$

In this reactions,the polarity of C-I bond depends upon the substituents on the carbon atom.The carbony;late anion can acts as a base as well as a nucleophile.It makes a nucleophile attack on 'C' of CH₃I while it makes a nucleophilic attack on I in CF₃I.