2. Organic reactions
These are chemical reactions involving organic
compounds, in which carbon is linked with other atoms
through covalent bonds and found in cells (usually
hydrocarbons)
Organic reaction types include:
Addition reactions
Elimination reactions
Substitution reactions
Pericyclic reactions
Rearrangement reactions
Photochemical reactions
Redox reactions
3. Organic reactions require the breaking of
strong covalent bonds, which takes a
considerable input of energy
For relatively stable organic molecules to
react at a reasonable rate, they often need to
be modified with the use of highly reactive
materials or should be carried out in the
presence of catalyst
A reaction shows reactants and products
4. Mechanism in organic chemistry
A reaction mechanism shows how electrons
move and bonds form and break in a
reaction
The mechanism includes all intermediates
and how these intermediates are formed
5. Reaction intermediate
A reaction intermediate is a transient entity
within a multi-step reaction mechanism. It is
produced in the preceding step and consumed
in a subsequent step to ultimately generate the
final reaction product
It may be an atom, a molecule, or an ion
7. Most chemical reactions take more than one elementary
step to complete, and a reactive intermediate exists only
in one of the intermediate steps
Usually, cannot be isolated but sometimes observable
only through fast spectroscopic methods
Only in exceptional cases, these can be isolated and
stored
Their existence helps to explain how a chemical
reaction takes place
The series of steps together make a reaction mechanism
8. It is stable in the sense that an elementary
reaction forms the reactive intermediate and the
elementary reaction in the next step is needed to
destroy it
When a reactive intermediate is not observable,
its existence can be inferred through
experimentation
This usually involves changing reaction
conditions such as temperature or concentration
and applying the techniques of chemical kinetics,
chemical thermodynamics, or spectroscopy
11. Common Features
• Low concentration with respect to reactants and products
• Often generated on the chemical decomposition of a
chemical compound
• Often possible to prove their existence by spectroscopic
means or chemical trapping
• Cage effects - the cage effect illustrates how molecular
properties are affected by their surroundings- have to be
taken into account
• Often stabilized by conjugation or resonance
• Often difficult to distinguish from a transition state (along the
reaction coordinate, reactive intermediates are present not much lower in
energy from a transition state making it difficult to distinguish)
12. Covalent bond
Equal sharing of electrons between the bonded atoms
Energy is released when a covalent bond is formed-
exothermic process
13. Energy is needed to break a covalent bond-
endothermic process
14. Fission
Breakdown of a covalent bond, which is of 2
types;
Homolytic fission
Heterolytic fission
15. Homolytic Fission
This is the bond fission in which each of the atoms or
fragments. It occurs when the electronegativity
difference between the bonded atoms is zero or nearly
zero. For example in Cl–Cl and RO–X. acquires one of
the bonding electrons. The fragments produced as a
result of homolytic fission carry unpaired electrons
and are electrically neutral. They are called as Free
Radicals. Free radicals are extremely reactive because
of the tendency of unpaired electrons to become
paired.
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16. Heterolytic fission
This is the bond fission in which one of the atoms or
fragments acquires both of the bonding electrons.
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Heterolytic fission reactions take place at measurable
rates (Non-spontaneous Reactions). Heterolytic fission
occurs most readily with polar compounds in polar
solvents. These reactions usually occur in solution phase.
17. Heterolytic fission is favored when the
electronegativity difference between two bonded
atoms is significant. The more electronegative atom
acquires both of the bonding electrons and becomes
negatively charged. The less electronegative atom loses
both of the bonding electrons and becomes positively
charged. For example in H–Cl, R–X. The fragments
produced as a result of heterolytic fission are called as
‘ions’. The positively charged ions are called cations
and the negatively charged ions are called anions.
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27. Preparation
Carbocation may be formed by several ways:
By the heterolysis of the C–X bond in alkyl
halide:
Carbocations are formed by heterolytic
cleavage of covalent bonds in which the
leaving group takes away the shared pair of
electrons of a covalent bond with it
28. Kinds of carbocations
Primary carbocations
In a primary (1°) carbocation, the carbon that carries the
positive charge is only attached to one other alkyl group
29. Secondary carbocations
In a secondary (2°) carbocation, the carbon with the
positive charge is attached to two other alkyl groups,
which may be the same or different
Examples:
30. Tertiary carbocation
In a tertiary (3°) carbocation, the positive carbon atom is
attached to three alkyl groups, which may be any
combination of the same or different
32. Electron pushing effect of alkyl groups
Bromine is more electronegative than hydrogen
so that in a H-Br bond the electrons are held
closer to the bromine than the hydrogen
A bromine atom attached to a carbon atom
would have precisely the same effect - the
electrons being pulled towards the bromine end
of the bond
The bromine has a negative inductive effect
33. Alkyl groups do precisely the opposite and, rather than
drawing electrons towards themselves, tend to "push"
electrons away
This means that the alkyl group becomes slightly positive
(+) and the carbon they are attached to becomes slightly
negative (-)
The alkyl group has a positive inductive effect
This is sometimes shown as:
34. The arrow shows the electrons being "pushed" away
from the CH3 group. The plus sign on the left-hand
end of it shows that the CH3 group is becoming
positive
The spreading charge around makes ions stable
The general rule of thumb is that if a charge is very
localized (all concentrated on one atom) the ion is much
less stable than if the charge is spread out over several
atoms
36. The electron-pushing effect of the CH3 group is placing
more and more negative charges on the positive carbon
as we go from primary to secondary to tertiary
carbocations. The effect of this, of course, is to cut down
that positive charge
At the same time, the region around the various CH3
groups is becoming somewhat positive. The net effect,
then, is that the positive charge is being spread out over
more and more atoms as you go from primary to
secondary to tertiary ions
The more spread the charge around, the more stable the
ion becomes
38. The stability of the carbocations in terms
of energetics
When we talk about secondary carbocations being more
stable than primary ones, what exactly do we mean? We
are actually talking about energetic stability - secondary
carbocations are lower down an energy "ladder" than
primary ones
This means that it is going to take more energy to make
a primary carbocation than a secondary one
If there is a choice between making a secondary ion or a
primary one, it will be much easier to make the
secondary one
39. Similarly, if there is a
choice between making a
tertiary ion or a secondary
one, it will be easier to
make the tertiary one
40. 3.Carbanion
A carbon atom having a negative charge, electron-rich, is
called a carbanion
41. Structure of carbanion
When the negatively charged carbon atom in a
carbanion is bonded to hydrogen or alkyl groups (e.g.,
methyl or ethyl groups etc.), it is sp3 -hybridized and
forms three σ-bonds. The unshared electron pair
resides in the fourth sp3 -hybridized orbital.
42. When the negatively charged carbon atom in a
carbanion is bonded to an unsaturated group (e.g.,
in benzyl carbanion), it is sp2 -hybridized and
forms three σ-bonds. The unshared electron pair
resides in the unhybridized p-orbital. The
completely filled non-bonding sp3 or p-orbital
makes the carbon atom electron-rich and gives it a
formal negative charge. A carbanion will combine
with an electrophile in a reaction to donate its
electron pair thus making it a nucleophile.
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44. Inductive Effect
The inductive effect, via which highly electronegative
substituent groups attached to the carbanion help subdue
the negative charge on it and make the molecule more
stable
On the other hand, highly electropositive substituent
groups can increase the negative charge on the carbanion
and, therefore, decrease the overall stability of the
molecule
45. The +I groups decrease the stability of carbanions while
the -I groups increase their stability
For example, the alkyl groups (+I) donate electron density
to the negatively charged carbon resulting in the
destabilization of carbanion
Thus the order of stability is as:
46. Size (Steric hindrance)
The stability of a carbanion is also inversely
proportional to the size of the atom attached to
the carbanion
In other words, the smaller the atom, the more
stable the carbanion
47. The stability of a carbanion is directly
proportional to the number of resonance
structures that can be drawn for the carbanion
The more resonance structures that can be
drawn, the more stable the carbanion
Resonance
48. The resonance effect, via which the
delocalization of the electrons distributes the
negative charge all over the carbanion, adds
stability to the process
Aromatic systems add a great deal of stability
to carbanions when they are present as a
substituent group as a result of the resonance
effect (and the greater extent of delocalization
of electrons over the aromatic system)
49. Hybridization of the Charge-bearing Atom
The stability of a carbanion is directly proportional to
the s-character of the orbitals involved in the bond
between the carbon and the atom bearing the negative
charge. In other words, the more s-character in the
orbitals, the more stable the carbanion