chemistry. session general organic chemistry - 5
TRANSCRIPT
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Chemistry
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Session
GENERAL ORGANIC CHEMISTRY - 5
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Session Objectives
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Session Objective
1. Fission of a covalent bond: homolysis and heterolysis
2. Reaction intermediates: carbocations, carbanions and free radicals
3. Different reagents: electrophiles, nucleophiles and carbene
4. Brief idea about type of organic reactions: substitution, addition, elimination,condensation, rearrangement, isomerisation
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Reaction Mechanism
• Detailed description of sequence of steps involved in group from reactantsto products.
• Reactant intermediate product
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Bond Cleavage
B:AB:A A : B A– : B+
+vely charged ion – carbocation
-vely charged ion – carbaanion
Heterolytic Cleavage
Homolytic Cleavage
BAB:A
Free radicals.
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Carbonium ion
• Planar – sp2 hybridised bond angle 120o
• Has six electrons
• Stabilized by resonance or inductive effort or hyperconjugation
C
Empty unhybridisedp-orbital
sp2 Hybridisation ofcarbon
Planar Strucutre of carbnion
+
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Examples of Carbonium ion
Benzyl cation
CH2 CH2 CH2
etc
+
+ +
CH 2 CH CH 2 CH 2 CH CH 2 + +
Stabilised through resonance
Allyl cation
H2C CH+
Vinyl cation
no resonance hence unstable.
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Stability of Cabocation
(i) By inductive effect
The resonance effect is always more predominant than the inductive effect in stabilizing an ion.
CH3 C
CH3
CH3
CH3 C
CH3
H
> > H C
CH3
H
> >
3° 2°
+ +
1°
+
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Stability of Cabocation
(ii) By hyperconjugation
H3C — C
CH3
CH2 — H
H3C — C
CH3
CH2H
H3C — C
CH2H
C
CH3
CH3CH3
CH2H
+
+
+
etc. +
Thus, tertiary carbocation is more stable than secondary and so on.
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Carbanion
• Pyramidal - sp3 hybridised bond angle 109.28
• Has eight electrons
• Stabilized by resonance or by inductive effect.
. . sp3 hybrid orbitalcontaining lone pair
Tetrahedral structure of carboanion
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Stability of Carbanion
(i) By resonance
H
-
H
-
H
Cyclopentadienyl carbanion
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Stability of Carbanion
(ii) By inductive
CH3C H
CH3
CH3
CH3 C
CH3
H
C
CH3
H
3° 2° 1°
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Stability of Carbanion
(iii) Electron-donating groups destabilize a carbanion while electron-withdrawing groups stabilize it.
N O 2 3OCH
>
2CH
2CH
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Free Radical
• Planer or Pyramidal
• Has seven electrons
• Stabilized by resonance or by inductive effect.
• Order of stability of free radical 3o >2o> 1o
C(
Unhybridised orbitalcontaining odd electron
120oC
sp2 hybridised carbon
Planar Sturcutre
+
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Classification of Reagents
Nucleophilic Reagents (Nucleophiles)
• Attacks the positive end of a polar bond or nucleus-loving is known as nucleophile.
• Generally, negatively charged or electron rich species are nucleophilic.
3 3 2 3e.g. OH , OCH , CN , I , CH COO , NH , CH
2 3 3 2H O, NH , NH — NH
N..
..N H 3,CH 3 — O — CH 3,
..
. ...
C 2H 5 — O H,. ...H 2 O,
• All nucleophiles are in general Lewis bases.
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Classification of Reagents
Electrophilic Reagents (Electrophiles)
• Attacks a region of high electron density or electron-loving is known as electrophile.
• All positively charged or electron deficient species are electrophilic.
3 2H , CH , NO , Cl , Br , Ag
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Classification of Reagents
• Neutral reagents which contain an electron-deficient atom are also electrophiles.
AlCl3, SO3, BF3, SOCl2, POCl3, FeCl3, ZnCl2
• All electrophiles are in general Lewis acids.
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Carbenes
• Divalent carbon compound.
• Carbon atom is linked to two adjacent groups by covalent bonding.
• A carbene is neutral and possesses two free electrons, i.e. a total of six electrons.
• Electron deficient.
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Carbenes
Carbene is of two types
(i) Singlet carbene:
(ii) Triplet carbene:
Triplet carbene is more stable than single carbene.
CH2 hybridisation sp2
it is v-shaped
CH2 hybridisation spit is linear shaped
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Types of Organic Reaction
Substitution Addition
Elimination Rearrangement
Condensation Isomerisation
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Types of Organic Reaction
Substitution Reaction
Replacement of an atom or group byother atom of group
Nucleophilic substitution:
R X OH R OH X
SN1 Reaction: Unimolecular nucleophilic substitution reaction.
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Types of Organic Reaction - SN1 Reaction
CH3 — C — CH2Cl
CH3
CH3
OH–
slow
CH3 — C — CH2
CH3
CH3SN1 +
CH3 — C — CH2
CH3
CH3
CH3 — C — CH2 – CH3
CH3
+
+
1, 2-Methyl anionshift
Fast OH–CH3 — C — CH2CH3
OH
CH3
(1)
(2)
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Types of Organic Reaction - SN2 Reaction
SN2 Reaction: This is called bimolecular nucleophilic substitution and it is one-step process.
H — C — Br + OH
CH2CH3
CH3
–OH C Br
H CH3
.
CH2CH3
FastHO — C — H
CH3
CH2CH3
d– d–
Transition state unstable
slow
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Addition
The reagent often adds to bond
and the bond is converted into bond.
Can be electrophilic addition or nucleophilic addition.
C C , C O or C N C C ,
Cl22 2 2 2
CCl4CH CH Cl CH CH Cl
OH2H
OH
+
(Hydration)
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Elimination Reaction
Two groups on adjacent atoms are lost as a double bond is formed.
CH3 – CH – CH – CH3
OH H
Conc. H2SO4
– H2OCH3 — CH CH – CH3
We divide elimination reactions into three classes.
(1) E1 (2) E1 CB (3) E2
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Rearrangement
• Migration of a group takes place within the same molecule.
C6H5
C6H5
C = N
OH
C6H5 — C — N — C6H5
O
H
H
ether
OHH+
(Beckmann rearrangement)
(Dehydration and rearrangement)
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Condensation
H3C — C — CH3 + H3C — C — CH3
O
H3C — C — CH — C — CH3
CH3O O
dil.
NaOH,
Two molecules of same or different reactants combine to give a new product with the elimination of simple byproducts like H2O, NH3, etc,
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Isomerisation
C
H
H3C
C
H
CH3
C
H
H3C
C
CH3
H
h
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Class Test
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Class Exercise – 1
Select the most stable carbocation among the following.
CH3
CH3HC
6 5 3(C H ) C
3 2 2CH CH CH
3 3(CH ) C
(a) (b)
(c) (d)
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Solution
C — C6H5
C6H5
C6H5
C+ +
This carbocation is highly stabilised through resonance with three benzene rings.
Hence answer is (b).
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Class Exercise - 2
Which of the following is an addition reaction?
3 3CH CH CH
|
Br
OH2 3
AlcoholCH CH CH
h3 3 2 3 2CH CH Cl CH CH Cl
(a)
(b)
3 2 3 2CH CH Br CN CH CH CN Br(c)
3 2 3 3CH CH CH H CH CH CH
|
Br
Br(d)
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Solution
H3C — CH — CH3
Br
AlcoholH2C CH — CH3
H3C — CH3 + Cl2 CH3CH2Cl + HCl
H3C — CH2Br + CN H3C — CH2CN + Br
H3C CH CH2 + HBr H3C CH CH3
Br
OH–
(Elimination)
h
–
(Substitution)
Substitution
–
AdditionHence answer is (d).
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Class Exercise - 3
Which of the following is the most effective group in stabilizing a free radical inductively?
(a) F (b) I
(c) Br (d) Cl
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Solution
Since free radical is electron deficient, any substituent with more electron releasing and less electron withdrawing ability will stabilize the radical inductively.
The decreasing order of electronegativity of halogens is: F > Cl > Br > I
Hence answer is (b).
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Class Exercise - 4
Which of the following is not a nucleophile?
(a) CN– (b) BF3
(c) RNH2 (d) OH–
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Solution
Among the following, BF3 is only electron deficient. Hence, it will not act as a nucleophile.
Hence answer is (b).
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Class Exercise - 5Which of the following is the correct order regarding –I effect of the substituents?
(a) –NR2 > –OR > –F
(b) –NR2 > –OR < –F
(c) –NR2 < –OR < –F
(d) –OR > –NR2 > –F
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Solution
–I effect increases with electronegativity of atom. The decreasing order of electronegativity is
F > O > N
The correct order for –I effect is
–NR2 < –OR < –F
Hence answer is (c).
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Class Exercise - 6The least stable carbonium ion is
(a) (b)
(c) (d)
3 2H CCH
6 5 2 2C H — CH — CH
6 5 2C H — CH
6 5 6 5C H — CH — C H
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Solution
Among the following, (a) is stabilised through +I effect and (b) is destabilized through –I effect of phenyl ring. Other two are stabilised through resonance.
Hence answer is (c).
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Class Exercise - 7Arrange the following ions in the decreasing order of stability.
2HC
CH3 CH3CH3
++
+
(a) (b) (c) (d)
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Solution
CH2
+
. It is a primary cation. Hence, minimum stability.
CH3CH3
+and
+(c) (b)
are secondary cations.
Hence, stabilised through +I effect of –CH3 group which decreases with distance. (c) is more stable as compared to (b).
(d) is most stable as it is tertiary cation and stabilised through +I effect of –CH3 group and hyperconjugation.
The order is (d) > (c) > (b) > (a)
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Class Exercise - 8
Arrange the following radicals in order of their decreasing stability
3 2 3 3 6 5 2 2 2CH CH , (CH ) C, C H CH , CH CH CH
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Solution
Radicals are stabilised through electron releasing resonance and inductive effect.
CH2CH2etc.
More resonating structure
H2C CH — CH2 H2C — CH CH2
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Solution
One resonating structure, although
both are primary radicals.
Among and , later is a tertiary radical. Hence, more stable.
The decreasing order of stability is
H3C — CH2 (CH3)3C
C6H5CH2 > H2C CH — CH2 > (CH3)3C > H3CCH2
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Thank you