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7/31/2019 Lecture4 Characteristics of Covalent Bond (1)
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Uneven sharing of electrons
produces partial negative charges onthe atom having greater than half
share, leaving partial positive charge
on the atom having less than halfshare. There will be surplus of
negative charge around one nucleus
and deficiency around the other.
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Bond polarity depends on electronegativity
difference. The greater the electronegativity
difference, the greater the unevenness ofsharing, the greater share is acquired by the
initially more electronegative atom, thus
electron density will be displaced in the
direction of the more electron attracting atom.
The greater the difference between the electron
attracting power of the two atoms, the greater
is the ionic character of the covalent bonds.
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Electronegativity:F > O > Cl, N > Br > C, H
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Question: The electronegativity difference of
C - I bond is zero but the bond is polarizable .
Why?
In polar environment this covalent bond canacquire an appreciable degree of ionic character.
This is due to the polarizability of the outermostelectrons of iodine as a consequence of the bigsize of iodine atom. The distance between thenucleus and the outermost shell is great so that
the outermost electrons are not as stronglydrawn towards the nucleus as are the electronsof small atoms.
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Formal positive charge effectively increases
the electronegativity, thus increases polarity.
Calculation of Formal charge:
FC= group # - ( # of shared electrons) # of
unshared electrons
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Example: 1. N in ammonia
FC= 5- 3- 2 = 0
Example: 2. N in ammonium
FC = 5
4 = +1 more polar compared toammonia
N
H
HH
HH
H
N
H
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The formal positive charge effectively
increases the electronegativity of nitrogen.
There will be a tendency to pull the electron
closer to nitrogen resulting in increased partial
positive charges on each of the four hydrogen
atoms
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a. Determine which C O is more polar and explain.
or( FC = 0) (FC = +1)
The excess positive charge increases its electronattracting power so that there will be a greater
displacement of electron density towards oxygen.
H
H
H
C O H
H
H
H
C O H
H
+
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b. Which C H is more polar?
or
The C-H bond of chloroform is more polar than the C-H bond in methane because in chloroform adjacent
atoms are chlorine atoms which possess greaterelectron-attracting capacity than hydrogen atoms inmethane. The 3 chlorine will reduce the electron densityaround C, increasing the partial positive charge on Cwhich in turn will attract electron from hydrogen
increasing the partial positive charge on this atom.
Cl
Cl
Cl
C H
H
H
H
C H
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c. Which C H is more polar?
or
The C-H bond in acetylene is more polar than C-H bondin ethylene because C in acetylene is an sp ( s and pcharacter) while C in ethylene is an sp2 ( 1/3 s and 2/3 p).
The s character of hybrid orbital is a measure of theelectron-attracting capacity of the atom. The electron-attracting capacity of C in acetylene is greater than inthat of ethylene. The electron attracting power as a resultof s character of the hybrid orbital, responsible for the
polarity of C-H bond in acetylene is also known asORBITAL ELECTRONEGATIVITY.
CH CH CH2
CH2
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d. Which C H is more polar?
or
The C-H bond in ethylene is more polar than the C-H bondin ethane because oforbital electronegativity.
Importance of bond Polarity:1. It contributes to the characteristic of bond that is itinfluenced bond length and energy.2. It influences molecular polarity, thus in the end
determines the physical properties like melting point, boilingpoint, solubility.
3. It influences chemical reactivity.
C C
H
H
H
H
CH
H
H
C H
H
H
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2. BOND LENGTH
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The bond length is the distance
between those two atoms. Thegreater the number of electrons
between two atoms, the closer the
atoms can be brought towards oneanother, and the shorter the bond.This is measured in Angstrom units
which 1A is equal to 10-8 cm. It hasbeen shown that bond length is
correlated with bond polarity,
hybridization of orbital and
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When two atoms approach each
other, their interaction isinfluenced by:
a. repulsion between twoelectron, clouds
b. repulsion between the two
nuclei
c. attraction between the nucleus
of each, and
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Factors influencing bond length
a. Bond Polarity
Increasing polarity decreasing bond lengthType of Single Bonds Bond Length
C-C 1.538
C-N 1.471
CO 1.430
CF 1.380
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b. Hybridization
Increasing s ( near the nucleus) character of hybrid orbital
decreasing bond length or orbital electronegativity leads to
reduction of bond distance.
Ethane CH3-CH3 sp3 sp3 1.538
Propylene CH3-CH=CH2 sp3 sp2 1.501
Methylacetylene sp3-sp 1.459Ethylene CH2=CH2 sp
2-sp2; p-p 1.339
Acetylene sp-sp; p-p; p-p 1.207
C CHCH3
CH CH
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c.atomic size
small atoms will form shorter bond.
Example: H-H < CH4-CH4
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d. bond order
decreasing bond order increasing bond
length.
Triple bond< double bond< single bond
sp sp2 sp3
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e. and electron delocalization
illustrated by system containing atoms in the trigonal stateof hybridization.
Example:
a. CH2 = CH - Cl
b. O = C
c. CH3 CH = CH2
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3. BOND ENERGY orBOND STRENGTH
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energy required to break the bond and at the
same time the energy release when the bondis formed.
This is express in terms of kilo calories permole
Bond energy is variable depending on length,the shorter the bond the stronger the bond.
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Bond strengthening are attributed to:
a. orbital hybridization
sp3 sp3sp3- sp2sp3 spsp2 sp2 Bond energy increases bond length decreases
sp2 spsp - sp
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b. bond polarity - when bond polarity increases
bond energy also increases
- C C sp3sp3 0
= C
C = sp2
sp2
0- C = C - sp2 sp2 1 increasing pi bond increasing= C C= sp sp 0 bond energy- C = C- sp-sp 2
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c. pi bonding
- C C sp3sp3 0
= C C = sp2 sp2 0
- C = C - sp2 sp2 1 increasing pi bond
= C C= sp sp 0 increasing bond
-C = C- sp-sp 2 energy
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d.Reduction of bond strength is observed
when there is loss of overlap of hybrid
orbitals. The loss of overlap is a consequence
of forced bending of orbitals. A vivid exampleof loss of overlap as a result of forced bending
of sp3 orbitals is seen in cyclopropane. Forced
bending results in angular strain due tocompression of the tetrahedral bond angle.
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4. BOND order
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The bond order is equal to the number of
bonds between two atoms.
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The BO is an indication of the bond length,the greater the bond order, the shorter thebond and the greater the bond strength.
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5. BOND ANGLE
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a. governed by hybridization of central atom
sp3 - 109.5o
sp2 120 osp - 180 o
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b. influenced by the presence of lone pairs
CH4 sp3 109o28
NH3
sp3 106o47
H2O sp3 104o31
As the number of lone pair increases, the bond angle
decreases.