Basic effects in Organic chemistry

BASIC CONCEPTS IN
ORGANIC CHEMISTRY
JEEVANANTHAM P
I MSc Chemistry,
SRMV college of arts and science,
Coimbatore.
jeevananthamche@gmail.com
1

OBJECTIVES :
 Find the route of mechanism.
 We can able to predict the product.
 We can able to tell the property of the molecule.
 We can able to predict the stability.
2

TYPES :
1. Inductive effect(I)
2. Electromeric effect(E)
3. Resonance effect(R)
4. Hyperconjugation
3

Electronegativity plays a important role in inductive effect
Loves electron and
pull towards myself
EN atoms
N,
O,
X(halogens)
4

What is INDUCTIVE EFFECT?
The process of electron shift along a chain of atoms due
to the presence of electron withdrawing group(-I group)
or electron donating group (+I group) is called inductive
effect.
(OR)
The polarization of sigma bond due to electron
withdrawing or electron donating effect of adjacents
groups is called inductive effect.
5

The direction of displacement is shown by placing
an Arrow head midway along the line presenting the
sigma bond.
This effect decrease as the distance from the
electronegativity atom inreases.
This is a permanent effect and is almost negligible
beyond two carbon atom from the active group.
6

+I EFFECT
INDUCTIVE
EFFECT
-I EFFECT
PUSH PULL
8

-I effect :
electron withdrawing groups
-R3N+ > -NO2 > -SO2R > -CN > -COOH > -F
> -Cl >- Br > -I > -OR > -COR > -OH > -C6H5
> -CH=CH2 > -H
+I effect :
electron donating groups
(CH3)3C- > (CH3)2CH- > CH3CH2- > -CH3 > -D
> -H
9

Stability of carbocation by +I effect of methyl group
Acidity increased by presence of –I (group) effect
APPLICATION
10

SUMMARY
 The inductive effect plays a vital role in deciding the acidity and
basicity of a molecule
 Acidity increases with increasing the +I group
 Basicity increases with increasing the –I group
> >
14

• ELECTRON LOVING SPECISESELECTROPHILE
• NUTRON LOVING SPECISESNULEOPHILE
17

ELECTROMERIC EFFECT
 Electrons of pi bond are loosely held (due to sideways overlap)
and easily polarisable.
 therefore., when a compound having pi bond approached by a
charged reagent (electrophile or nucleophile) the electrons of the
bond are completely polaraised or displaced towards the one of
the constituent atoms.
18

 This is a temporary effect operating in unsaturated
compounds only.
 It involves complete transfer of pi electrons of multiple
bond.
19

+E effect :
When the transfer of pi electrons takes place towards the attaking reagent
(electrophile).this effect is called +E effet.
-E effect :
When the transfer of pi electrons occurs away from the attacking reagent
(nuleophile).this effect is called –E effect.
20

APPLICATION
1) Electrophilic addition reaction of unsaturation compounds
 the polarisation of the carbon – carbon double bond in the presence of attacking
electrophile like H+ etc.,
 electrophilic addition involving +E effect
2) Nucleophilic addition reation of carbonyl compounds
 the polarisation of the carbon – oxygen double bond in the presence of attacking
nucleophile
 nucleophilic addition involving –E effect
3) Ring polarisation
polarisation and the attack by electrophile may actually take plae concurrently in a
concentrated mechanism.
21

INDUCTIVE EFFECT ELECTROMERIC EFFECT
It is a permanent effect involving only the
displacement of electrons
It is a temporary effect involving a complete
transfer of pi electrons.
This has no such requirements It is operate only under the influence of a
suitable attacking reagent
This requires only a polar covalent bond This effect is shown only by compounds
containing one or more multiple bonds
Electron transfer shown by arrow head midway Electron transfer shown by curved arrow
INDUCTIVE EFFECT vs ELECTROMERIC EFFECT
22

DELOCALIZATION :
In conjugated system pi electrons are distributed over whole of the molecule
embracing all the carbon atom and do not remain localized. this phenomenon
is called delocalization.
Deloalization of electrons does not only take place in alternate single bond
and double bond in carbon atoms(1,3 buta diene and benzene) but in may
also occur in a conjugated system involving carbon and other than carbon.
Delocalization by pi – pi overlap
Delocalization by pi – p orbital overlap
24

RESONANCE OR MESOMERIC EFFECT
Resaonance effect describes the polarity produced in a molecule by the
interation between the lone pair and a pi bond or the interaction of two pi
bonds in adjustant atoms.
The resonance(canonical) effect is a chemical phenomenon observed in
compounds characteristic of double bonds of organic compounds.
25

 It is usually found in molecules with conjugated double
bonds or in molecules having at least one lone pair and one
double bond.
 Understanding resonance is important in understanding
stability of the compound and its energy state
 Aromatic compounds are especially stable due to this bond
delocalization and resonance effect.
26

-M or -R effect : It is shown by substituents or groups that withdraw
electrons by delocalization mechanism from rest of the molecule and are
denoted by -M or -R. The electron density on rest of the molecular entity
is decreased due to this effect.
E.g. -NO2, Carbony group (C=O), -C≡N, -COOH, -SO3H etc.
+M or +R effect : The groups show positive mesomeric effect when
they release electrons to the rest of the molecule by delocalization. These
groups are denoted by +M or +R. Due to this effect, the electron density
on rest of the molecular entity is increased.
E.g. -OH, -OR, -SH, -SR, -NH2, -NR2 etc.
TYPES
27

RESONANCE THEORY
1. Whenever a molecule can be represented by two or more structure that differ from one another
only in the arrangement of electrons and not In atomi nuclei.the molecule is said to be a
RESONANCE HYBRID of all these structure.
1. Each contributing structure must have the same number of unpaired electrons.
2. The greater the stability of a contributing structure, greater is its contribution to the hybrid.
3. Resonance is most important when the contributing structures are equivalent.
4. The resonance hybrid is more stable than any of the contributing structures.
5. The smaller the difference in the energy contents (or stabilities) of the contributing structures, the
greater the resonance energy of that hybrid.
6. The polar contributing structure involving distinct charges are less stable than those which do not
involve any charges.
8. The larger number of bonds in contributing structure, the greater the stability of that structure.
29

RESONANCE ENERGY
Resonance hybrid is more stable than any single discrete structure.this can be explaind in
terms of RESONANCE ENERGY,
This can be quantised from the heat of hydrogenation released on addition of hydrogen to
double bond.
the difference between the observed and calculated value (energy released) is called
resonance energy.
Minimum energy content is more stable than high energy content.
30

APPLICATION
1.Aromatic compound and conjugated diene stability due to resonance effect
2.Triphenyl methyl cation :
This is the only one of the most stable carbocations known.its stability is due to extensive
deloalization of +ve charge by three benzene ring.
3.Active methylene compound:
Malonic ester and acetoacetic ester have more acidic than alpha-hydrogens of carbonyl
compounds
Due to stability of forming carbanion.
31

QUESTN…?
Why carboxylic acid do not show the reaction of the
carbonyl group?
ANS :
The double bond character of the carbon –oxygen linkage in
carboxylic acis is considerably dereased due to resonace.
32

 The delocalization of σ-electrons or lone pair of electrons into
adjacent π-orbital or p-orbital is called hyperconjugation.
 It occurs due to overlapping of σ-bonding orbital or the orbital
containing a lone pair with adjacent π-orbital or p-orbital.
 It is also known as "no bond resonance" or "Baker-Nathan effect“
 There must be an α-CH group or a lone pair on atom adjacent to
sp2 hybrid (double bond)carbon or other atoms like nitrogen, oxygen
etc.
 E.g., Alkenes, alkyl carbocations, nitro compounds with α- hydrogen
34

The displacement of σ-electrons towards the
multiple bond occurs when there are hydrogens
on the α-carbon (which is adjacent to the multiple
bond). This results in the polarization of the
multiple bond
.
E.g. In propene, the σ-electrons of C-H bond of
methyl group can be delocalized into the π-orbital
of doubly bonded carbon as represented below.
In the same way, the other hydrogens on the
methyl group also participate in the
hyperconjugation. This is possible due to free
rotation of C-C bond so that the other C-H bonds
can also participate in the hyperconjugation. Thus
the propene molecule can show following
resonance structures, which confer stability to it.
35

In the contributing structures: (II), (III) & (IV) of propene, there is NO bond between
an α-carbon and one of the hydrogen atom. Hence the hyperconjugation is also known
as "no bond resonance".
These equivalent contributing structures i.e., (II), (III) & (IV) are also polar in nature
and hence are responsible for the dipole moment of propene (0.36 D).
The C-C bond lengths in propene are equal to 1.48. Its value is in between 1.54 (of C-
C) and 1.34 (of C=C). It is because the bond order of C-C bonds is approximately 1.5
due to hyperconjugation.
This type of hyperconjugation is also referred to as sacrificial hyperconjugation since
one bond is missing.
36

APPLICATION :
1) Stability of alkenes:
A general rule is that, the stability of alkenes increases with increase in the number of alkyl groups
(containing hydrogens) on the double bond. It is due to increase in the number of contributing no bond
resonance structures.
For example, 2-butene is more stable than 1-butane
2)Stability of carbocation :
This is because, the σ-electrons of the α-C-H bond in ethyl group are delocalized into the empty p-orbital
of the positive carbon . This also gives no bond resonance.
37

3) Reactivity & orientation of electrophilic substitution on benzene ring :
In Toluene, the methyl group releases electrons towards the benzene ring partly due to inductive
effect and mainly due to hyperconjugation. Thus the reactivity of the ring towards electrophilic
substitution increases and the substitution is directed at ortho and para postions to the methyl
group.
The no bond resonance forms of toluene due to hyperconjugation are shown below.
4)Dipole moments :
The large increase in dipole moment of 1,1,1-trichloroethane as compared with chloroform can
be attributed to hyperconjugated structures.
38

SUMMARY
• Acidity and basicity can be predictable by INDUCTIVE EFFECT…
• RESONANCE EFFECT – stability can be studied by delocalaization(pi –
bond)….
• HYPERCONJUGATION – stability can be studied by delocalization(sigma
bond)….
• Relative hyperconjugation strength: Hydrogen has greater strength than its
isotope Deuterium and Tritium has least ability to show hyperconjugation among
the three. This is because energy required to break C-T bond > C-D bond > C-H
bond, which makes it easier for H to hyperconjugate
39

REFERRANCE :
https://en.wikipedia.org/wiki/Inductive_effect
http://www.chem.ucla.edu/~harding/IGOC/I/inductive_effect.html
https://www.google.co.in/search?q=inductive+effect&source=lnms&tbm=isch&sa=X&ved=0ah
UKEwio5cyd8OzcAhWLL48KHej9Da4Q_AUIDCgD&biw=1242&bih=529#imgrc=qC9KjJZt0zqm9
M:
https://www.adichemistry.com/organic/basics/hyperconjugation/hyperconjugation-1.html
 Modern Organic Chemistry
-Jain and Sharma
 Organic Chemistry
-Jagadhambal singh
BOOKS :
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Basic effects in Organic chemistry

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