IN THIS CHAPTER ALL IMPORTANT CONCEPT COVERED REGARDING HALOGEN DERIVATIVES.THIS CHAPTER HAS MORE WEIGHTAGE IN IIT JEE EXAM.IT COVERS ALL IMPORTANT QUESTIONS.I THINK IT IS USEFUL FOR THOSE WHO ARE PREPARING COMPETITIVE EXAMS
, 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
-------------------------------------------------------------------------------------------------------
, 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
, 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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