Organic reactions and their mechanisms are described. Key topics covered include nucleophiles and electrophiles, reaction types (addition, elimination, substitution), and organic intermediates. Electron displacement effects such as inductive, mesomeric, electromeric and inductometric effects are also discussed. Common organic reactions like nitration, halogenation and nucleophilic aromatic substitution are summarized.

2. Organic Reactions and Mechanisms
 Organic chemistry –deals with organic compounds(which
contains C,H,O)
 Organic reactions are chemical reactions involving organic
compounds.
 A reaction mechanism is the step by step sequence of
elementary reactions by which overall chemical change
occurs.

3. Nucleophile
 A reagent which can donate an electron pair in a reaction is
called a nucleophile.
 The name nucleophile means nucleus loving and indicates
that it attacks regions of low electron density (positive
centres) in the substrate molecule.
 Nucleophiles are electron rich.

4.  They may be negative ions including carbanions or neutral
molecules with free electron pair.
 A nucleophile can be represented by a by general symbol Nu:-
 Examples
 Cl-,
 Br-,
 I-,
 CN -,
 OH-, RCH2
-, NH3, RNH2, H2O, ROH

5. Electrophiles
 A reagent which can accept an electron pair in a reaction
called an electrophile.
 The name electrophile means electron-loving and
indicates that it attacks regions of high electron density
(negative centers) in the substrates molecule.
 Electrophiles are electron deficient.

6.  They may be positive ions including carbonium ions or neutral
molecules with electron deficient centres.
 An electrophile can represented by E+.
 Examples
 H+,
 Cl+,
 Br+,
 I+,
 NO2
+, R3C+, +SO3H, AlCl3, BF3

7. Organic Reaction Mechanism
 A reaction mechanism is the step by step sequence of
elementary reactions by which overall chemical change
occurs.
 Although only the net chemical change is directly
observable for most chemical reactions, experiments can
often be designed that suggest the possible sequence of
steps in a reaction mechanism.

8. Electron displacement effects
Electron
displacement
effects
Permanent
effects
Inductive
effect
Mesomeric
effect
Temporary
effects
Inductometric
effect
Electromeric
effect

9. Inductive effect
 Inductive effect is defined as permanent displacement of
shared electron pair forming a covalent bond towards
more electronegative atom or group.
3

10. Types of Inductive effect :
 1. Negative Inductive Effect : (—I effect, Electron
withdrawing effect)
when an electronegative atom or group (more
electro negative than hydrogen)is attached to the terminal
of the carbon chain in a compound, the electrons are
displaced in the direction of the attached atom or group.
-NO2 > -CN > -COOH > F > Cl > Br > I > OH > C6H5
> H

11. 2.Positive Inductive effect : (+I effect, Electron
releasing effect)
 When an electro positive atom or group (more electro
positive than hydrogen)is attached to the terminal of the
carbon chain in a compound, the electrons are displaced
away from the attached atom or group.
(CH3)3C- > (CH3)2CH- > -C2H5 > - CH3

12. Mesomeric/ Resonance Effect
The flow of electrons from one part of a conjugated π
system to the other caused by phenomenon of resonance is
called resonance effect or mesomeric effect.
 The re-distribution of electrons which takes place in
unsaturated and especially in conjugated systems via their
π-orbitals.

13.  -M or -R effect : When the electron displacement
is towards the group.
 e.g :-NO2 , -CHO
 +M or +R effect : When the electron displacement is away
from the group.
 e.g :-OH , -OR,-Cl

14. Electromeric Effect
 Electromeric effect is defined as the complete transfer of
electrons of a multiple bond towards one of the bonded
atoms at the demand of an attacking reagent.
 Note :
 a) It is shown by those compounds containing multiple
bond
 b) It is a purely temporary effect & is brought into play
only at the requirement of attacking agent.

15. Types of Electromeric Effect
 +E effect : When displacement of electrons is away from
the atom or group.
 e.g : addition of H+ to alkene.
 -E effect : When displacement of electrons is towards the
atom or group.
 e.g : addition of cyanide ion(CN-) to the carbonyl
group.

17. Inductomeric effect
 Inductomeric effect is the temporary effect which
enhances the inductive effect and it accounts only in
the presence of an attacking reagent.
 Example
In methyl chloride the -I effect of set is further
increased temporarily by the approach of hydroxyl
ion.
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18. Organic intermediates
Carcocations
Carbenes
Carcoanions
Free radicals

19. Free radicals
 A Free radical is a species which has one or more
unpaired electrons.
 It is paramagnetic .
 It can be detected by electron spin resonance
spectroscopy.

20. Carbocations
 Carbocations are the intermediates in which the
positive charge is carried by the carbon atom with six
electrons in the valence shell.
 Carbocations are carbon atoms in an organic molecule
bearing a positive formal charge. Therefore they are
carbon cations.

21. Carboanions
 Carbanions arer thosev intermediates in which central
carbon atom carries negative charge and they posses
unshared pair of electrons.
 Carbanions are units that contain a negative charge on a
carbon atom.
CH3 >1 0>20>30

22. Carbenes
 The carbenes are neutral carbon intermediates in which
the central carbon has six electrons,two of which are free.
 In chemistry, a carbene is a molecule containing a neutral
carbon atom with a valence of two and two unshared
valence electrons.
 Ex,
 :CH2 ,Methylene

23. Types of reactions
Addition reactions
Elimination Reactions
Substitution reactions
Organic Redox reactions
Rearrangement reactions

24. Types of Reactions
Reaction Type Sub-type Examples
Addition reactions Electrophilic
Nucloephilic
radical
halognenation,
hydrohalogenation and
hydration
Elimination reaction Dehydration
Substitution reactions nucleophilic aliphatic
Substitution
nucleophilic aromatic substitution
nucleophilic acyl substitution
electrophilic substitution
electrophilic aromatic substitution
radical substitution
with SN1, SN2 and
SN reaction
mechanisms

25. Addition Reactions-Electrophilic addition
 An electrophilic addition reaction is an addition
reaction where, in a chemical compound, a π bond is
broken and two new σ bonds are formed. The
substrate of an electrophilic addition reaction must
have a double bond or triple bond.
 The driving force for this reaction is the formation of
an electrophile X+ that forms a covalent bond with
an electron-rich unsaturated C=C bond. The positive
charge on X is transferred to the carbon-carbon bond,
forming a carbocation.

26. Addition Reactions-Electrophilic addition

27. Addition Reactions-Electrophilic addition
 In step 1, the positively charged intermediate combines with
(Y) that is electron-rich and usually an anion to form the
second covalent bond.
 Step 2 is the same nucleophilic attack process found in an
SN1 reaction. The exact nature of the electrophile and the
nature of the positively charged intermediate are not always
clear and depend on reactants and reaction conditions.

28. Nucleophiic addition
 A nucleophilic addition reaction is an addition
reaction where in a chemical compound a π bond is
removed by the creation of two new covalent bonds
by the addition of a nucleophile.

29.  Addition reactions are limited to chemical compounds
that have multiple-bonded atoms
 molecules with carbon - hetero multiple bonds like
carbonyls, imines or nitriles

30. Nucleophiic addition
 An example of a nucleophilic addition reaction that occurs at
the carbonyl group of a ketone by substitution with hydroxide-
based compounds, denoted shorthand. In this example, an
unstable hemiketal is formed.

31. Substitution Reactions
The reactions in which an atom or group of atoms in a molecule is replaced or
substituted by different atoms or group of atoms are called substitution
reaction.
For example,

32. Nucleophilic Substitution
 Nucleophilic substitution is a fundamental class of
substitution reaction in which an "electron rich"
nucleophile selectively bonds with or attacks the
positive or partially positive charge of an atom
attached to a group or atom called the leaving group;
the positive or partially positive atom is referred to as
an electrophile.

33.  Nucleophilic substitution reactions can be broadly
classified as
 Nucleophilic substitution at saturated carbon
centres
 Nucleophilic substitution at unsaturated carbon
centres

34. Nucleophilic substitution at saturated carbon
centres
 In 1935, Edward D. Hughes and Sir Christopher
Ingold studied nucleophilic substitution reactions of
alkyl halides and related compounds.
 They proposed that there were two main mechanisms
at work, both of them competing with each other.

35.  The two main mechanisms are
 the SN1 reaction and
 the SN2 reaction.
 S stands for chemical substitution,
 N stands for nucleophilic,
 the number represents the kinetic order of the
reaction.

36.  In the SN2 reaction, the addition of the nucleophile
and the elimination of leaving group take place
simultaneously.
 SN2 occurs where the central carbon atom is easily
accessible to the nucleophile. By contrast the SN1
reaction involves two steps.
 SN1 reactions tend to be important when the central
carbon atom of the substrate is surrounded by bulky
groups, both because such groups interfere sterically
with the SN2 reaction (discussed above) and because
a highly substituted carbon forms a stable carbocation.

37. Nucleophilic substitution at carbon atom
SN1 Mechanism

38. Nucleophilic substitution at carbon atom
SN2 Mechanism

39. Nucleophilic substitution at unsaturated
carbon centers
 Nucleophilic substitution via the SN1 or SN2
mechanism does not generally occur with vinyl or aryl
halides or related compounds.
 When the substitution occurs at the carbonyl group,
the acyl group may undergo nucleophilic acyl
substitution. This is the normal mode of substitution
with carboxylic acid derivatives such as acyl
chlorides, esters and amides.

40. Nucleophilic Aromatic substitution
 A nucleophilic aromatic substitution is a
substitution reaction in organic chemistry in which the
nucleophile displaces a good leaving group, such as a
halide, on an aromatic ring.
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41. Nitration
 Nitration is a general chemical process for the introduction of
a nitro group into a chemical compound. Examples of
nitrations are the conversion of glycerin to nitroglycerin and
the conversion of toluene to trinitrotoluene. Both of these
conversions use nitric acid and sulfuric acid.
 In aromatic nitration, aromatic organic compounds are
nitrated via an electrophilic aromatic substitution mechanism
involving the attack of the electron-rich benzene ring by the
nitronium ion.

42. References
1.Organic chemistry by Morrison and Boyd
2. Engineering Chemistry by Jain and Jain,Dhanpat rai
Publication
3. http://chem-guide.blogspot.in/2010/04/inductive-
effect.html
4.http://www.expertsmind.com/learning/inductomeric-
effect-assignment-help-7342872160.aspx
5. http://nptel.ac.in/courses/Webcourse-contents/IIT-
Delhi/chemistry/chap4/chp4.htm