HALOGEN DERIVATIVES
1
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, SUBSTITUTION & ELIMINATION
Solvents are most important for reactions since they provide medium for chemical reactions.
Molecular collisions are possible only in gaseous phase or in solvent phase.
Solvent
Cl
NonPolar C — Cl ; ; CS2
Solvent Cl Cl
Polar protic
Polar O O
H H/ R H/ NH 3
Polar aprotic
O O O
CH3 CH3
S C—N CH—
3 C—N
CH3 CH3 H CH3 CH3
DMSO DMF DMA
Polar protic Solvent:– ..
Can solvate both cations & anions (a) CH3 — .NH
. CH3NH2
H ..
H H O (b) CH3 — .O. : CH3OH
O H
H H O—H H
O
O O
O
H H H H —
O O (c) CH—
3 C—O CH—
3 C — OH
O—H
H H H (d) R — O— R — OH
Solvation of cation Solvation of anions
S
(e) H H H—S
Polar aprotic solvent ..
Can solvate only cations & not anions (f) R — SH R — .S. :
CH3 +
(g) H3O H2O
S CH3 Ans. (a) 1>2 (b) 1>2 (c) 1>2
CH3 O
O=S (d) 1>2 (e) 1<2 (f) 1<2
O CH3 (g) 1<2
S Rule 2:
CH3 CH3
For atoms of comparable size, better e— donor
—
Nucleophiles:- They are e pair donors . Particles is better nucleophile (lower electronegativity
which donate e— pair to generate covalent or better e— donor)
..
coordinate bonds are nucleophiles. 1. CH3 — NH2 > CH3 — O — H
..
2. R — NH2 > R — OH
Strength of Nucleophile
Rule-1 (—) charge .. ..
Conjugate bases are strong nucleophiles compared 3. NH2 < NH2
to acids:- O
4. R—O—H > R—C—O—H
2
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, O (2) L Leaving group
— (3) Solvent (4) Nucleophile.
5. R — O— > R—C—O
Types of nucleophilic substitution.
Rule 3:-
If e— donating atoms are of different size in polar 1. SN1 2. SN2 3. SNAR
NGP
protic solvent 4. SN 5. SNi 6. SN1’
Nucleophilicity size of atom 7. Benzyne.
(a) R — OH < R — SH
(b) (C6H5)3N < (C6H5)3P SN1 — Nucleophilic substitution first order:-
. .
(c) F Cl Br I 1. R — L R L
..
If e— donating atoms are of different size in polar 2. R Nu R — Nu
aprotic solvent R1
R1 . .
Nucleophilicity basicity 1. Slow
+ L
C—L rds. C
Leaving group (L):- Particles which are substituted R2 R2 R3
by stronger nucleophiles or depart from a molecule to R3
generate carbocation are leaving groups. R1
1 .. Θ
Leaving group ability
Base strength 2. C + Nu
Weak bases are good leaving groups. R2 R3
. . R1
R L R L R1
R2 Nu Nu R2
1. I > Br— > Cl— >> F—
O R3 R3
—
2. H H > OH
O
3. — Characteristics
R H > R— O
O (1) rate = k [R — L]1; rate is independent on
concentration of nuclophile.
—
4. R—C—O > R — O— (2) rate is directly proportional to stability of
O O carbocation
– (3) Rearrangement possible
5. O — S — CH3 < O — S — CF3 (4) Planar carbocation can be attacked from both
O O sides
Methylsulphonate Trifluoro methylsulphonate (5) Supported by polar protic solvent.
(Mesylate) (Triflate) (6) 2 step reaction
O O
O
6. –
–
< O—S— (7) Normally 3° – carbocation & resonance
O—S— —N
stabilized carbocation support this reaction
O O CH3 mechanism if attacking Nuclophile is neutral
polar protic solvent.
Nucleophilic substitution (SN – Reactions)
. .
Nu . . (8) Solvolysis (SN1)
R — L R — Nu + L O
H H
Ph — CH2 – Cl Ph – CH2 — OH + HCl
4 Components
(1) R Substrate
3
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, Q.1 Compare the rate of SN1 reaction:- EtOH
(3) Ph — CH2 — Br
(a) Ph — CH2 — Cl Ph — CH — Cl O
||
H—C—OH
Ph (4) Ph — CH2 — Br
(i) (ii)
Ph PhOH
(5) Ph — CH2 — Br
Ph — C — Cl Ans: (1) Ph — CH2 — OH
(2) Ph — CH2 — OCH3
Ph (3) Ph — CH2 — OEt
(iii) O
I I I ||
(4) Ph — CH2 — O — C — H
(b) (5) Ph — CH2 — OPh
(i) (ii) (iii) Q.3 Which SN1 reaction is expected to occur
I I I rapidly.
O
1. (a) (CH3)3 CCl + H H (CH3)3 C OH+ HCl
(c) O
(b) (CH3)3 CBr + H H (CH3)3 C OH + HBr
O
(i) (ii) (iii)
(d) CH3 — O — CH2 — Cl 2. (a) Cl + EtO— O — Et + Cl—
(i)
CH3 — O — CH2 — CH2 — Cl
(ii) M 1.0 M
CH—Cl
2
CH—Cl
2 1.0
(b) Cl + tO— O — Et + Cl—
(e)
2.0 M 1.0 M
(f) CH3 — O — CH = CH — CH2 — Cl & 3. (a) Cl + EtO— O — Et + Cl—
CH3 — O — CH2 — CH = CH — CH2 — Cl
100% 1.0 M 1.0 M
(g) (i) Ph — CH — Cl
H2O
CH3 (b) Cl + EtO— O — Et + Cl—
80%H2O
(ii) Ph — CH — Cl
20%EtOH 1.0 M 1.0 M 1.0 M 2.0 M
CH3 Ans. (1) b > a (2) a < b (3) a = b
Ans: (a) (iii) > (ii) > (i) (b) (ii) > (iii) > (i)
(c) (i) > (ii) > (iii) (d) (i) > (ii) Q.4 Compare rate of SN1 reaction
(e) (i) > (ii) (f) (i) > (ii) (g) (i) > (ii) O
1. (a) Cl + H H
1
Q.2 SN reactions are also known as solvolysis as O
.. Θ (b) Br + H H
solvent molecules behave as Nu . Write
the product in each case. O
2. (a) Cl + H H
H2O
(1) Ph — CH2 — Br
O
CH3OH Cl + CH3 H
(2) Ph — CH2 — Br (b)
4
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1
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, SUBSTITUTION & ELIMINATION
Solvents are most important for reactions since they provide medium for chemical reactions.
Molecular collisions are possible only in gaseous phase or in solvent phase.
Solvent
Cl
NonPolar C — Cl ; ; CS2
Solvent Cl Cl
Polar protic
Polar O O
H H/ R H/ NH 3
Polar aprotic
O O O
CH3 CH3
S C—N CH—
3 C—N
CH3 CH3 H CH3 CH3
DMSO DMF DMA
Polar protic Solvent:– ..
Can solvate both cations & anions (a) CH3 — .NH
. CH3NH2
H ..
H H O (b) CH3 — .O. : CH3OH
O H
H H O—H H
O
O O
O
H H H H —
O O (c) CH—
3 C—O CH—
3 C — OH
O—H
H H H (d) R — O— R — OH
Solvation of cation Solvation of anions
S
(e) H H H—S
Polar aprotic solvent ..
Can solvate only cations & not anions (f) R — SH R — .S. :
CH3 +
(g) H3O H2O
S CH3 Ans. (a) 1>2 (b) 1>2 (c) 1>2
CH3 O
O=S (d) 1>2 (e) 1<2 (f) 1<2
O CH3 (g) 1<2
S Rule 2:
CH3 CH3
For atoms of comparable size, better e— donor
—
Nucleophiles:- They are e pair donors . Particles is better nucleophile (lower electronegativity
which donate e— pair to generate covalent or better e— donor)
..
coordinate bonds are nucleophiles. 1. CH3 — NH2 > CH3 — O — H
..
2. R — NH2 > R — OH
Strength of Nucleophile
Rule-1 (—) charge .. ..
Conjugate bases are strong nucleophiles compared 3. NH2 < NH2
to acids:- O
4. R—O—H > R—C—O—H
2
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, O (2) L Leaving group
— (3) Solvent (4) Nucleophile.
5. R — O— > R—C—O
Types of nucleophilic substitution.
Rule 3:-
If e— donating atoms are of different size in polar 1. SN1 2. SN2 3. SNAR
NGP
protic solvent 4. SN 5. SNi 6. SN1’
Nucleophilicity size of atom 7. Benzyne.
(a) R — OH < R — SH
(b) (C6H5)3N < (C6H5)3P SN1 — Nucleophilic substitution first order:-
. .
(c) F Cl Br I 1. R — L R L
..
If e— donating atoms are of different size in polar 2. R Nu R — Nu
aprotic solvent R1
R1 . .
Nucleophilicity basicity 1. Slow
+ L
C—L rds. C
Leaving group (L):- Particles which are substituted R2 R2 R3
by stronger nucleophiles or depart from a molecule to R3
generate carbocation are leaving groups. R1
1 .. Θ
Leaving group ability
Base strength 2. C + Nu
Weak bases are good leaving groups. R2 R3
. . R1
R L R L R1
R2 Nu Nu R2
1. I > Br— > Cl— >> F—
O R3 R3
—
2. H H > OH
O
3. — Characteristics
R H > R— O
O (1) rate = k [R — L]1; rate is independent on
concentration of nuclophile.
—
4. R—C—O > R — O— (2) rate is directly proportional to stability of
O O carbocation
– (3) Rearrangement possible
5. O — S — CH3 < O — S — CF3 (4) Planar carbocation can be attacked from both
O O sides
Methylsulphonate Trifluoro methylsulphonate (5) Supported by polar protic solvent.
(Mesylate) (Triflate) (6) 2 step reaction
O O
O
6. –
–
< O—S— (7) Normally 3° – carbocation & resonance
O—S— —N
stabilized carbocation support this reaction
O O CH3 mechanism if attacking Nuclophile is neutral
polar protic solvent.
Nucleophilic substitution (SN – Reactions)
. .
Nu . . (8) Solvolysis (SN1)
R — L R — Nu + L O
H H
Ph — CH2 – Cl Ph – CH2 — OH + HCl
4 Components
(1) R Substrate
3
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, Q.1 Compare the rate of SN1 reaction:- EtOH
(3) Ph — CH2 — Br
(a) Ph — CH2 — Cl Ph — CH — Cl O
||
H—C—OH
Ph (4) Ph — CH2 — Br
(i) (ii)
Ph PhOH
(5) Ph — CH2 — Br
Ph — C — Cl Ans: (1) Ph — CH2 — OH
(2) Ph — CH2 — OCH3
Ph (3) Ph — CH2 — OEt
(iii) O
I I I ||
(4) Ph — CH2 — O — C — H
(b) (5) Ph — CH2 — OPh
(i) (ii) (iii) Q.3 Which SN1 reaction is expected to occur
I I I rapidly.
O
1. (a) (CH3)3 CCl + H H (CH3)3 C OH+ HCl
(c) O
(b) (CH3)3 CBr + H H (CH3)3 C OH + HBr
O
(i) (ii) (iii)
(d) CH3 — O — CH2 — Cl 2. (a) Cl + EtO— O — Et + Cl—
(i)
CH3 — O — CH2 — CH2 — Cl
(ii) M 1.0 M
CH—Cl
2
CH—Cl
2 1.0
(b) Cl + tO— O — Et + Cl—
(e)
2.0 M 1.0 M
(f) CH3 — O — CH = CH — CH2 — Cl & 3. (a) Cl + EtO— O — Et + Cl—
CH3 — O — CH2 — CH = CH — CH2 — Cl
100% 1.0 M 1.0 M
(g) (i) Ph — CH — Cl
H2O
CH3 (b) Cl + EtO— O — Et + Cl—
80%H2O
(ii) Ph — CH — Cl
20%EtOH 1.0 M 1.0 M 1.0 M 2.0 M
CH3 Ans. (1) b > a (2) a < b (3) a = b
Ans: (a) (iii) > (ii) > (i) (b) (ii) > (iii) > (i)
(c) (i) > (ii) > (iii) (d) (i) > (ii) Q.4 Compare rate of SN1 reaction
(e) (i) > (ii) (f) (i) > (ii) (g) (i) > (ii) O
1. (a) Cl + H H
1
Q.2 SN reactions are also known as solvolysis as O
.. Θ (b) Br + H H
solvent molecules behave as Nu . Write
the product in each case. O
2. (a) Cl + H H
H2O
(1) Ph — CH2 — Br
O
CH3OH Cl + CH3 H
(2) Ph — CH2 — Br (b)
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