Substitution or
Displacement Reactions
CH APTE R 15
15.1 Introd uction
Th. ·~ th ·· · · · Ives the
is Is e most unportant type of organic reactions. As the name implies, it mvo
replacement of an atom or group by another atom or group, e.g.,
A-B + C " A---C + B
It muS t be remembered that A-Band A---C both are covalent compounds. In the above exa~ple,
lhe atom or group B is replaced by another atom or group C to form a new compou~d, subStztuted
h
compou nd, A-C. During the formation of A-C from A-B two phenomenon occur (eil er separately
nd
or sunultaneously): (i) breaking of the bond between A and Band (ii)formation of a new bo between
nd 0th er
A an~ C; out of which the-first is very important and depends upon the nature of the reagent a
expe~e~t al conditions. This process, i.e., the rapturing of the covalent bond between A and B can take
place m either of the following three ways.
(i) Each atom or group receives one electron of the shared pair (homolyticfission) and thus, two
free radicals• are formed.
A:B--" A•+B·
Such type of fission generally occurs at high temperature and the substitution reactions which
proceed through free-radical formation are known as free-radical substitution , e.g ..
Cl 2
CH 3CH=CH 3 ---· CH 2CI.CH=CH 2 + HCl
400 - 500°
Propene
(ii) Atom A retains both the electrons (heterolytic fission or heterolysis) and thus an anion A and
a cation B are formed.
Such type of fission occurs when the reagent (C) is an electrophilic (electron seeking or cationoid
reagent) and thus, it will attack the atom or group having more electrons i.e., A, and hence, such type
of substitution reactions are kno~n as elect:op~ilic substitution reactions and designated as SE (S for
substitution and E for electroph1hc) e.g., mtrahon of benzene by the nitronium ion. ·
+
Ph: H + N02 - - - - " Ph- N02 + H+
(iii) Atom A loses the shared pair of electrons i. e., the shared pair of electrons remains 00 B (again
heterolysis). ;I
A : B - - --> A + : B I
• It must be remembered that the homolytic fission always produces free-ra,,1:cals
UJ
h b d fi
w h'JC can e e med as a j
species having odd number of electrons. .
, T
Substitution or
Displacement Reactions
[-·
,"'~
~
-- -- -- - ---·
C H A P T E R 15
15 .1 In tro du ct io n · ries. it involves the
s is th · .
lbi ~ . e mo st unponaut type of orga nic reactions. As the name imp
atom or group, e.g.,
replacement of an atom or group by another
A- B + C _ ____. A- C + B
~pl ~
both are covalent compounds. In the above exa
It muS t be remembered that A-B and A- C p C to form a new compou~d, sub slllu te
up B is repl aced by ano ther atom or grou
lhe atom or gro er separately
_P 0 nd , A-C ,. Dur ing the form ation of A- C from A- B two phenomenon occur (eith d
com " boo between
d between A and Band (ii)Jonnation of a new
or SlDlUltaneously): (i) breaking of the bon nd olher
of whi ch the first is very imp orta nt and depends upon the nature of the reagent a
A an~ C; out B can take
~ta l con ditions . Thi s proc ess, i.e., the rapturing of the covalent bond between A and
exp e~e
s.
place m either of the following three way
, two
of the shared pair (homolyticjissio n) and thus
(i) Each atom or group receives one electron
free radicals• are fonned.
'
A:B --- -1 A•+ B·
fissi on gen erally occu rs at high tem perature and the substitution reactions which
Such type of
known as free-radical substitution, e.g. .
proceed through free-radical formation are
CH 3CH =C H 3 CH 2Cl.CH= CH 2 + HCI
400 - 500°
Propene
(heterolytic fission or heternlysis) and thus
an anion A and
(ii) Atom A retains both the electrons
a cation B are fonned.
A: B - - A: + B+
onoid
h type of fission occ urs whe n the reagent (C) is an e!ectrophilic (electron seeking or cati
Suc ce, such type
gent) and thus, it will attack the atom or group having more electrons i.e., A, and hen
rea ted as SE (S for
t:op~1'/ic substitution reactions and designa
of substitution reactions are kno~n as e!ec
mtrahon of benzene by the nitronium ion .
substitution and E for electroph1hc) e.g.,
+
Ph: H + N07 --► Ph- N01 + J-t
of electrons remains on B (again
(iii) Atom A loses the shared pair of electrons i.e. , the shared pair
heterolysis).
A : B - --> A + : B
h can be defined as a
c fission always produces free-radicals whic
• It must be remembered that t~e homolyti .
species having odd number of electrons.
, 458 Reaction and Reagents
Such type of fission occurs when the reagent (C) is a n14c/eophilic ~nucleus-seeking) or ~nionoid
reagent or electron supplier and hence it wilt attack the atom or group havmg low electr~n density. Such
type of substitution reactions are known 11s 11ucleophi/ic substitution reactions an~ deSignated as SN (S
for substitution and N for uucleophilic), e.g., hydrolysis of alkyl halides by alkah. 'I
I
R: Cl + OH - - R-OH + Cl
So all the substituti on reactions may be cl11ssified under three headings.
(i) Free-radical substituti on : A• I •B + •C ---► A : C + B ·
I (ii) Heterolytic substitution :
+
(a) Electrophilic substitutiton (SE): A: I B + C - - A: C + B
+
These reactions are more common in aromatic compounds.
(b) Nucleophilic substitution (SN): Al :B +: C-- A: C + :B
These reactions are more common in aliphatic systems.
15.2 Nucle ophil ic Subst itutio n in Aliph atic Syste m
Every nucleophilic substitution reaction has three important components, viz. substrate, nucleophile
and solvent. The substrate in tum has two parts, alkyl group and leaving group. A typical nucleophilic
Alkyl group leaving group
, / Solvent
'---'R -l+ :Nu R-Nu + :L
Substrate Nucleophile Leaving group
reaction shows that it involves the replacement of a leaving group by a nucleophile. From our previous
knowledge we know that essential feature of a nucleophile is the presence of a pair ofnon-bonding electrons,
i.e., it may either be a negatively charged or a neutral species.
Likewise the substrate may be neutral or positively charged. TI1us four possibilities may be visualised.
(i) ~~
HO +CH 3- Br HO-CH 3 + Br-
t
(ii) -~
H 3N + CH 3·-~Br
+
[H 3N-CH 3}Br-
(iii)
-~
Cl +CH 3 -0H2
(;
Cl-CH 3 +H20
(iv) H,S~ Ji<cH,), - - (CH3)3N + H2;~R -- H+ HSR
The nucleophilic substitution reactions may take place mainly by two mechanisms: the unimolecular
and bimolecular.
Toe mechanism of a nuelophilic substitution reaction (whether unimolecular or bimolecular) can
be determined by the kinetic studies (Ingold) as follows. If the rate of reaction is proportional to the
concentration of the alkyl halide alone and independent of base, the reaction is said to proceed via
unimolec ular mechanism and is known as unimolecular nucleophilic substitution reaction (SN1). On the
other hand, if the rate ofreaction is proportional to the concentration of the alkyl halide as well as the base,
Displacement Reactions
CH APTE R 15
15.1 Introd uction
Th. ·~ th ·· · · · Ives the
is Is e most unportant type of organic reactions. As the name implies, it mvo
replacement of an atom or group by another atom or group, e.g.,
A-B + C " A---C + B
It muS t be remembered that A-Band A---C both are covalent compounds. In the above exa~ple,
lhe atom or group B is replaced by another atom or group C to form a new compou~d, subStztuted
h
compou nd, A-C. During the formation of A-C from A-B two phenomenon occur (eil er separately
nd
or sunultaneously): (i) breaking of the bond between A and Band (ii)formation of a new bo between
nd 0th er
A an~ C; out of which the-first is very important and depends upon the nature of the reagent a
expe~e~t al conditions. This process, i.e., the rapturing of the covalent bond between A and B can take
place m either of the following three ways.
(i) Each atom or group receives one electron of the shared pair (homolyticfission) and thus, two
free radicals• are formed.
A:B--" A•+B·
Such type of fission generally occurs at high temperature and the substitution reactions which
proceed through free-radical formation are known as free-radical substitution , e.g ..
Cl 2
CH 3CH=CH 3 ---· CH 2CI.CH=CH 2 + HCl
400 - 500°
Propene
(ii) Atom A retains both the electrons (heterolytic fission or heterolysis) and thus an anion A and
a cation B are formed.
Such type of fission occurs when the reagent (C) is an electrophilic (electron seeking or cationoid
reagent) and thus, it will attack the atom or group having more electrons i.e., A, and hence, such type
of substitution reactions are kno~n as elect:op~ilic substitution reactions and designated as SE (S for
substitution and E for electroph1hc) e.g., mtrahon of benzene by the nitronium ion. ·
+
Ph: H + N02 - - - - " Ph- N02 + H+
(iii) Atom A loses the shared pair of electrons i. e., the shared pair of electrons remains 00 B (again
heterolysis). ;I
A : B - - --> A + : B I
• It must be remembered that the homolytic fission always produces free-ra,,1:cals
UJ
h b d fi
w h'JC can e e med as a j
species having odd number of electrons. .
, T
Substitution or
Displacement Reactions
[-·
,"'~
~
-- -- -- - ---·
C H A P T E R 15
15 .1 In tro du ct io n · ries. it involves the
s is th · .
lbi ~ . e mo st unponaut type of orga nic reactions. As the name imp
atom or group, e.g.,
replacement of an atom or group by another
A- B + C _ ____. A- C + B
~pl ~
both are covalent compounds. In the above exa
It muS t be remembered that A-B and A- C p C to form a new compou~d, sub slllu te
up B is repl aced by ano ther atom or grou
lhe atom or gro er separately
_P 0 nd , A-C ,. Dur ing the form ation of A- C from A- B two phenomenon occur (eith d
com " boo between
d between A and Band (ii)Jonnation of a new
or SlDlUltaneously): (i) breaking of the bon nd olher
of whi ch the first is very imp orta nt and depends upon the nature of the reagent a
A an~ C; out B can take
~ta l con ditions . Thi s proc ess, i.e., the rapturing of the covalent bond between A and
exp e~e
s.
place m either of the following three way
, two
of the shared pair (homolyticjissio n) and thus
(i) Each atom or group receives one electron
free radicals• are fonned.
'
A:B --- -1 A•+ B·
fissi on gen erally occu rs at high tem perature and the substitution reactions which
Such type of
known as free-radical substitution, e.g. .
proceed through free-radical formation are
CH 3CH =C H 3 CH 2Cl.CH= CH 2 + HCI
400 - 500°
Propene
(heterolytic fission or heternlysis) and thus
an anion A and
(ii) Atom A retains both the electrons
a cation B are fonned.
A: B - - A: + B+
onoid
h type of fission occ urs whe n the reagent (C) is an e!ectrophilic (electron seeking or cati
Suc ce, such type
gent) and thus, it will attack the atom or group having more electrons i.e., A, and hen
rea ted as SE (S for
t:op~1'/ic substitution reactions and designa
of substitution reactions are kno~n as e!ec
mtrahon of benzene by the nitronium ion .
substitution and E for electroph1hc) e.g.,
+
Ph: H + N07 --► Ph- N01 + J-t
of electrons remains on B (again
(iii) Atom A loses the shared pair of electrons i.e. , the shared pair
heterolysis).
A : B - --> A + : B
h can be defined as a
c fission always produces free-radicals whic
• It must be remembered that t~e homolyti .
species having odd number of electrons.
, 458 Reaction and Reagents
Such type of fission occurs when the reagent (C) is a n14c/eophilic ~nucleus-seeking) or ~nionoid
reagent or electron supplier and hence it wilt attack the atom or group havmg low electr~n density. Such
type of substitution reactions are known 11s 11ucleophi/ic substitution reactions an~ deSignated as SN (S
for substitution and N for uucleophilic), e.g., hydrolysis of alkyl halides by alkah. 'I
I
R: Cl + OH - - R-OH + Cl
So all the substituti on reactions may be cl11ssified under three headings.
(i) Free-radical substituti on : A• I •B + •C ---► A : C + B ·
I (ii) Heterolytic substitution :
+
(a) Electrophilic substitutiton (SE): A: I B + C - - A: C + B
+
These reactions are more common in aromatic compounds.
(b) Nucleophilic substitution (SN): Al :B +: C-- A: C + :B
These reactions are more common in aliphatic systems.
15.2 Nucle ophil ic Subst itutio n in Aliph atic Syste m
Every nucleophilic substitution reaction has three important components, viz. substrate, nucleophile
and solvent. The substrate in tum has two parts, alkyl group and leaving group. A typical nucleophilic
Alkyl group leaving group
, / Solvent
'---'R -l+ :Nu R-Nu + :L
Substrate Nucleophile Leaving group
reaction shows that it involves the replacement of a leaving group by a nucleophile. From our previous
knowledge we know that essential feature of a nucleophile is the presence of a pair ofnon-bonding electrons,
i.e., it may either be a negatively charged or a neutral species.
Likewise the substrate may be neutral or positively charged. TI1us four possibilities may be visualised.
(i) ~~
HO +CH 3- Br HO-CH 3 + Br-
t
(ii) -~
H 3N + CH 3·-~Br
+
[H 3N-CH 3}Br-
(iii)
-~
Cl +CH 3 -0H2
(;
Cl-CH 3 +H20
(iv) H,S~ Ji<cH,), - - (CH3)3N + H2;~R -- H+ HSR
The nucleophilic substitution reactions may take place mainly by two mechanisms: the unimolecular
and bimolecular.
Toe mechanism of a nuelophilic substitution reaction (whether unimolecular or bimolecular) can
be determined by the kinetic studies (Ingold) as follows. If the rate of reaction is proportional to the
concentration of the alkyl halide alone and independent of base, the reaction is said to proceed via
unimolec ular mechanism and is known as unimolecular nucleophilic substitution reaction (SN1). On the
other hand, if the rate ofreaction is proportional to the concentration of the alkyl halide as well as the base,
