Types of organic reactions

The organic reactions are broadly classified as

Substituted reactions, Addition reactions, Elimination reactions etc

of an aton or
Substituted reactions: involves replacement or substitution
of atoms
group of atoms in an organic molecule by another atom or group
molecule.
without change in the remaining part of the
For example: CH,CI + HCI
CH, + Cl,

Depending upon the nature of the attacking ispecies (nucleophiles,

electrophiles or a free radical) the substitution reactions are of three types.
Nucleophilic substitution reactions: Substitutioneaçtions, which are
substitution
brought about by nucleophiles, are called nucleophilic nucleophiles.
reactions. In this case stronger nucleophile displäces, weaker
These reactions are typical of alkyl halides. Fro eg,, hydrolysis of an alkyl
halide with an aqueous base to give an alcohöll

CH, + CI
CHs
Nu
CH-Nu

In this reaction halogen atom is replaced by a nucleophile (electron rich

species). Alkyl halide easily undergoes nucleophilic substitution reaction
becaus of th presence of polar covalent bond between carbon and halogen
atom. Due to large electronegativity difference between halogen and carbon,
the more, cletronegative halogen atom takes the bond towards itself hence
gets á hegative charge whereas carbon gets a positive charge. Therefore
"aucleophile easily attacks at positively charge carbon atom.
There are two types of nucleophilic substitution reactions at saturated
carbon namely,
a) unimolecular nucleophilic substitution reaction SN!
b) Bimolecular nucleophilic substitution reaction SN?

B. Sc. I| SEMESTER 1 YCM

 halide. The
in primary alkyl halide and least favoured in tertiary alkyl
reactivity order of alkyl halide in SN mechanism is 1 > 20 > 3
b) Nucleophilicity of the reagent- In SN² mechanism more powerful
nucleopile attacks the substrate faster and favours the reaction. In
SN mechanism the rate is independent of the nature of nucleophile
as nucleophile does not react in the slow step. It only reacts with the
carbocation

c) Solvent polarity: polar and protic solvents favor SN! mechänism

since these help ionization of alkyl halide by solvating R and X ions.
Protic solvents (those capable of furnishing hydrogen ions H for
hydrogen bonding) are very good at solvating X ions.
In the case of SN? mechanism the transition state is less
polar than the
reactants. Therefore polar solvent can solvate
thereactants more
effectively than the transition state. As, such polar
slow down the rate of the SN? reaction. solvents slightly
In short polar solvents favor SN mechanism whereas non polar
solvents favor SN? mechanism.gll
Difference between SNI and SN, reaction
SNI reaction
SN2 reaction
Unimolecular reaction Bimolecular reaction
Follows first order kinetics Follows second order kinetics
Occurs in two steps
Occurs in one steps
The rate of the reaction' depends on The
the concentration of the rate of the reaction depends on
substrate the concentration of both the
Formationl of intermçdiate occurs substrate and the nucleophile
No
As intermediate formation occurs
the h¡lo group
intermediatecarbocation leaves,Reaction occurs through a single
formed
followed by, the attack of nucleophile transition state
A racenic mixture of
Eormed products are Inversion of configuration
occurs
Pölar protic (donates
solvents are used hydrogen) Polar aprotic solvents are used
Follows dissociative
Occurs
mechanism Follows associative
Occurs mechanism
Favoured by tertiary alkyl halide
Favoured by primary alkyl halide
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 cither the front side or the back side, and each side gives one isomer. There
is an equal possibility for a reaction to occur from either side, so the two
isomers are formed with the same amnount, and the product is a racemic
(equal amount) mixture.
(ii) Sy2 reaction

Another feature of the Sy2 reaction mechanism is that the overall

configuration of the carbon in the product gets inverted compared to that of
the reactant. Such inversion of the
configuration is called Walden inversion.
H
OH
CH ACH,
-Br HO

CH;CH,
CH,CH
(R)-2-bromobutane (S)-2-butanol
Start with the (R-2-bromobutane. The SN2 reaction produces only one
enantiomer of the 2-butanol produCt, and it is
configuration of the product is supposed to bepredictable that the
S because of the
configuration inversion.

Factors effecting SN' and SN? reactions

Three factors effect SNl and SN? reactions

a) Nature of substrate b) nucleophilicity of the

reagent c) solvent polarity
a) Nature of substrate-primary alkyl halide
follows SN² mechanism
whereas tertiary alkyl halides react by SN!
alkyl halides sometimes undergo both but mechanism, secondary
solvents SE² predominates. Bulkier R groups favor except on very polar
it' is difficult for the SN! over SN² since
nucleophile to get to the reactive carbocation, the
alkyl halide ionizes before SN² attack can take
place to form planar
carbocation (less steric crowding than substrate). Also, the
Carbocation stabilizes easily by positive inductive effect of more alkyl
group and by hyper conjugation effect of more alkyl
alpha hydrogen atoms. Therefore SN! group containing
halide and least favoured in primary mechanism favoured in tertiary
alkyl halide in halide. The reactivity order of
SNI mechanism is 30 > 2o > 10
On the other hand the
transition state of SN2 mechanism is penta
coordinate and more crowded. Therefore SN²
mechanism is favoured
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 (iü) Sy2 Mechanism
represented in the energy
can be has
for this reaction reaction, so the diagram
The energy changes reaction IS a single-step in a lower energy than the
diagram shown. S2products CH3OH and Br are that the overall reaction
only one curve. The OH which indicates
reactants CHsBr and stable.
the products are more
is exothermic and

TS.

CH,Br + OH

CH,OH Br
Reaction coordinate

Energy Diagram for Sy2 reaction between CH,Br and OH

The top of the curve corresponds to the transition state, which is the
highest-energy structure involved in the reaction. A transition state always
involves partial bonds, partially formed bond and partially broken bonds,
and therefore it is very unstable with no
state therefore cannot isolated.
appreciable lifetime. The transition

Stereochemistry of (i) Sy1 reaction

The stereochemistry feature of the SNl reaction is different from that of Sx2

Step 1 CH, CH;

lonization
OH CH,
C;H-ç-CH, C;H-CH, + HO-C-C,H,
Br Slow
Tertiary carbocation C,H, C,Hs
Tertiaryalkyl bromide
Mirror images
+ Br

Starting with stereoisomer reactant, the SNl reaction

is the racemic mixture produces
mixture of two isomers which a 50:50
because the carbocation formed in the first product. This is
trigonal planar shape. When it reacts with step of an SNl reaction has a
B. Sc. Il SEMESTER nucleophiles, it may react from
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 The reactivity of alkyl halide towards SN reaction depends on the presence
of alkyl bulkiness of the molecule. Primary alky halide undergoes SN?
reaction very easily since in primary alkyl halide steric crowdedness is less
hence the nucleophile can easily attack the carbon atom from the back side.
Therefore the reactivity order towards SN² reaction is primary alkyl
halide>secondary alkyl halide>tertiary alkyl halide.
Energy profile diagrams of (i) S1 mechanism
SNl is a nultiple-step reaction so the diagram the
has three
multiple curves, and ach
steps, the activation
ep can be represented by one curve. Out of
energy for step 1 is the highest: therefore, step 1 is the slowest step, which
1s the rate--determining step.
Step 1
(TSI,
(CHJ,c B
Step Il
(TS:
energy
Potential (CHJ,c OH

Eas Ea,
Interrnediate
CH, CHH,

(CH),Brt:OH |+:Br
Reactants 2H H,
(CH).C OH +Br
Product
Reaction co-ordinates
Energy profile diagram of alkaline hydrolysis of t-butyl bromide by SN!
mechanism.

lut The connection between the first two curves represent the
carbocation
intermediate. Generally, the intermediate is the product of one step of a
reaction and the reactant for the next step. The intermediate is at a
relatively lower energy level compared to the transition state (which is at the
peak ofa curve), but the intermediate is also highly reactive and unstable.

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 carbonium ion (CH-CH-CH), which is more stable than primary
carbonium ion (CH3-CHa'). The reason is, in tertiary carbonium ion three
electron releasing alkyl groups are attached to tertiary carbon atom which
easily neutralizes the positive charge compare to secondary and primary
carbonium ion. The order of reactivity of different alkyl halide is

tertiary alkyl halide> secondary alkyl halide> primary alkyl halide.

b) Bimolecular nucleophilic substitution reaction SN?

Consider the hydrolysis of methyl bromide using aqueous potassium

hydroxide
CH3-Br CH3-OH + KB
Methyl bromide Methyl aljeohoi
Mechanism:

Slow Fast

H Attack of nuclephilet4 H
from back side ofi l Transition sate
bromine atom
The first step which is slow involves the attack of nucleophile OH at
carbon atom of methy] bromide from back side of bromine atom to from an
intermediate transition state, In transition state carbon atom is partially
bonded to both OH and Br. The bromine atom develops a partial negative
charge since it has partially taken the bond and OH group also carries a
partial negative charge since it starts sharing of electrons in transition state.
The carbon hydrogen bonds are in one plane while c-OH and C-Br bonds
are petpendiçular to the plane and lie on opposite sides. Therefore in
product OH group occupies opposite position to that of bromine atom. This
step is Fate determining step and it depends on the concentration of the
reactants, methyl bromide and-OH. According to law of mass action the rate
is proportional to concentrations of reactant, hence it is second order
outilréaction. Also this step involves two reactant molecules hence it is
bimolecular reaction. In second step, which is final step, the nucleophile
OH form a complete bond with carbon atom and simultaneously bromine
atom detaches to give methyl alcohol.
Since the attack of nucleophile at carbon atom from back side of
bromine atom inversion (Walden) in configuration takes place.
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 Toimolecular nucleophilic substitution reaction SN
CH; Aqueous KOH CH; A N U
CH;-C-CH,
Br
-- CH;-C-CH, + KBr
OH
Tertiary butyl bromide Tertiary butyl alcohol

Mechanism:

KOH
lonization K+OH

Step 1
CH, lonization
CH-C-CH, CH-CH+Br
Slow
Br
Tertiary butyl bromide Tertiary carbonium ion

CH; Step 2 CH,

CH-G-CH 4QH Nodleophilic CH-C-CH,
ättack
Fast OH
Tertiary butyl alcohol

First step is ionization of tertiary butyl bromide which is slow and rate
determining step. This step depends on the concentration of the reactant,
tertiaryibutyl bromide and according to law of mass action the rate is
proportional, to concentrations of reactant, hence it is first order reaction.
Also thís 'step involves only one reactant molecule hence it is unimolecular
h, reaction. In second step, which is final step, the nucleophile OH attacks the
tertiary carbonium ion to form tertiary butyl alcohol.
This reaction does not depend on the strength of the nucleopjhile.
Reactivity of alkyl halide in SNI reaction depends upon the stability of
carbonium ion formed as intermediate, Tertiary carboniumn ion (CH3-Q*-CH3)
is more stable than secondary H3

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