GOC Theory
GOC Theory
GOC Theory
Introduction
The effect which appears due to electronic distribution is called electronic effect.
Classification :
> – > – > – NO2 > –SO2R > –CN > – SO2 Ar > – COOH > – F > – Cl >
– Br > – I > – OAr > –COOR > –OR > –COR > –SH > –SR > –OH > –CCR > Ar > –CH=CR2
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GOC-I # 1
D3: (b) + I effect : The group which release electron cloud is known as + I group and effect is + I effect.
> > – C(CH3)3 > – CH (CH3)2 > – CH2 – CH3 > – CH3 > – D > – H
The hydrogen atom is reference for + I and – I series. The inductive effect of hydrogen is assumed to be zero.
Ex. Since – NO2 is – I group it pulls or withdraws electron from cyclohexane ring making it electron deficient
(a) (b)
Due to e¯ donating nature of carbon chain has become partially negative but – COOH is – I group
therefore carbon chain has become partially positive.
Ex. Direction of electron displacements
1. 2. HOOC CH2 CH3 3.
CN
4. 5. 6.
+ + –
7. 8. 9. CH 3 CH CH NO2
Th2: Resonance
When two or more structures that differ only in the distribution of electrons can be written for a molecule, no
single Lewis structure is sufficient to describe it's true electron distribution. The true structure is said to be a
resonance hybride of the various Lewis formulas.
The various Lewis formulas called resonating structure/contributing structure/canonical structure, that can
be written for a molecule.
D4: Resonating structures are hypothetical but contribute to the real structure, which is called resonance hybrid.
The resonance hybrid is more stable than any other resonating structures.
Ex.
CH2 CH CH2 CH CH2 CH
CH CH2
CH CH2 CH CH2
Resonance hybrid
+
Resonance hybrid
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GOC-I # 2
Ex.
The most stable resonating structure contribute maximum to the resonance hybrid and less stable resonating
structure contribute minimum to resonance hybrid.
2.1 Conjugation:
A given atom or group is said to be in conjugation with an unsaturated system if:-
(i) It is directly linked to one of the atoms of the multiple bond through a single bond.
(ii) It has π bond, positive charge, negative charge, odd electron or lone pair electrons.
(b) The positions of the nuclei of the atoms must remain the same in all of the structures. Structures 3 is not
a resonance structure of 1 or 2, for example, because in order to form it we would have to move a
hydrogen atoms and this is not permitted :
(c) All atoms taking part in the delocalisation must lie in a plane so that orbitals overlaping become parallel
to each other.
i.e.
z
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GOC-I # 3
(d) All canonical forms must have the same number of unpaired electron.
(e) The energy of actual molecule is lower than of any form. Therefore delocalisation is a stabilizing
phenomenon.
(f) All canonical forms do not contribute equally to the true molecule. The more stable structure is the greater
contributor to its resonance hybrid.
Ex.
– O > –NH2 > –NHR > –NR2 > –OH > –OR > –NHCOR > –OCOR > –Ph > –F > –Cl > –Br > –I
Ex. (I)
(II) .. - +
H2C = CH – CH = CH – NH2 H2C – CH = CH – CH = NH2
(III)
(IV)
+ –
Ex. (I) (II) H 2C = CH – C N: H 2C – CH = C = N:
..
– + – + . .– – + ..
O – N = .O: O – N – .O:
. O – N = O:
.
(III)
+
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GOC-I # 4
Note :
1. When a +M group and –M group are at meta-positions with respect to each other then they are not in
conjugation with each other, but conjugation with benzene ring exists.
etc.
2. +M group increases electron density in benzene ring, called activating group while –M group decreases
electron density in the benzene ring, called deactivating group.
Ex. Write electron density order in the following compound.
(a)
(b)
Ans. (a) I > II > III > IV (b ) III > I > II > IV
(b) Nonpolar (uncharged) structure are most stable. Charge separation decreases stability. Separating opposite
charges requires energy. Therefore, structures in which opposite charges are separated have greater energy
(lower stability) than those that have no charge separation.
: :
CH2 – CH = Cl +
: :
: :
CH2 = CH – Cl
(I) (II)
(c) Structures with more covalent bonds are more stable than other structures
(d) Structure that carry negative charge on a more electronegative atom and positive charge on less
electronegative atom are comparatively more stable.
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GOC-I # 5
Ex. is more stable than )
Fig.2
Ex. Arange the following compounds in their decreasing SIR effect order.
(a)
(b)
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Th5: Hyperconjugation
D9: It is delocalisation of sigma electron with p-orbital. Also known as – -conjugation or no bond resonance. It
may take place in alkenes, alkynes, carbocations, free radicals and benzene nucleus.
Necessary Condition : Presence of at least one hydrogen at saturated carbon which is with respect to
alkenes, alkynes, carbocations, free radicals and benzene nucleus.
(i) Hyperconjugation in alkenes
Delocalisation of bond electrons in orbital by hyperconjugation in the case of alkene can be depicted as
in figure.
Hypercongugation
H
H
H H
H
H
(a) The effect of electron displacement due to this type of resonance is called hyperconjugative effect.
(b) Since canonical forms of this resonance may not contain any bond between H and C so hyperconjugation
is also known as no bond resonance.
(c) These resonating structures only suggest that
* There is some ionic character between C–H bond.
* Carbon-carbon double bond acquires some single bond character.
(d) Number of no bond resonating structures due to hyperconjugation = Number of -hydrogens (In aliphatic
systems)
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GOC-I # 7
Number of hydrogen in the following molecules/ion.
CH3 CH C 2H 5
CH
(i) CH3 CH C2H5 (ii) (iii)
CH3 CH3
H = 5 H=6
H = 6
CH3 CH3
(iv) C C (v)
CH3 CH3 H=6
H = 9
.N.
Nitrogen has bond as well as lone pair, but only bond of nitrogen will take part in delocalisation.
(d) Electrons of negative charge or lone pair behave as 2 electrons if it is in conjugation to bond.
; ;
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Th8 Applications of Electronic effects
(a) Stability of Alkenes : More is the number of hyperconjugative structures more stable is the alkene.
"More alkylated alkenes are more stable".
Stability of alkenes delocalisation of electrons
no. of hyperconjugative structures
Ex. <
(b) Heat of hydrogenation : Greater the number of hydrogen results greater stability of alkene.
Thus greater extent of hyperconjugation results lower value of heat of hydrogenation
1
Stability of alkenes no. of hyperconjugative structures
HHydrogenat ion
Ex. CH2 = CH2 > CH3 – CH = CH2 > CH3 – CH = CH – CH3 (HHydrogenation)
Ex.
(i) Bond length of C(II) – C(III) bond is less than normal C–C bond.
(ii) Bond length of C(II) – C(I) bond is more than normal C=C bond.
(iii) C–H bond is longer than normal C–H bond.
Cl Cl
C
H O
(I) (II)
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GOC-I # 9
Note: Resonance effect or delocalisation of electrons increases the stability of over all system.
For example carbocation, carbon free radical, carbanion & Alkenes.
(e) Dipole moment : Since hyperconjugation causes the development of charge, it also affects the dipole
moment of the molecule.
Ex. (i) CH2 = CH2 < CH3 – CH = CH2 (Dipole moment)
(ii) H–CH=O < CH3–CH=O < CH3–CH=CH–CH=O (Dipole moment)
–
COO COOH CH3 CH3
–
COO COOH NO2 NO2 OH OH
=3.93D =4.93D =1.54D =1.57D
– CH2 – E
D11: (ii) – E effect : In this effect the -electron of the multiple bond are transferred to that atom to which the
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GOC-I # 10
Comparision between aromatic, anti aromatic and non-aromatic compounds.
CHECK LIST
Definitions (D) Theories (Th)
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GOC-I # 11