The document discusses the Sni (substitution nucleophilic internal) mechanism. In the Sni mechanism, part of the leaving group attacks the substrate as it detaches from the rest of the leaving group. This allows the reaction to occur with retention of configuration and no neighboring group participation. The document also discusses factors that affect the Sni mechanism like solvent, substituents, and chloride ion concentration. Finally, it examines nucleophilic substitution mechanisms at vinyl carbons including tetrahedral, addition-elimination, and elimination-addition pathways.

2. Sni mechanism :
It occurs with retention of
configuration and there is no possibility of
a neighboring group effect. In the Sni
mechanism (substitution nucleophilic
internal) part of the leaving group must be
able to attack the substrate, detaching itself
from the rest of the leaving group in the
process.

3. First step:
The reaction of alcohols with thionylcholoride to
give alkyl halides the chlorosulphinate is formed with
retention. Since R-O bond is not broken during this
reaction.
These alkyl cholorosulphinate can be isolated.

4. Second step:
(part of the leaving group attacks)

5. Evidence for this mechanism is as follows:
The addition of pyridine to the mixture of
alcohol and thionyl chloride results in the formation of
alkyl halide with inversion of configuration.
The pyridine coordinates with the Hcl produced
during the formation of intermediate chlorosulphite from
ROH and SOcl2 to form pyridine hydro chloride and the cl-
is an effective nucleophile.

6. The displacement of the chlorosulphinate ester by cl via SN2
mechanism gives product with complete inversion of
configuration.

7. Factor affecting Sni mechanism:
1. Effect of changing the solvent:
At 750C various by a factor of 10,000 in changing from dioxin to nitro
benzene. Which is in consistent with unpaired, because a covalent
process would show only moderate response to solvent polar.
However, nucleophilic solvent has to be avoided to get Sni
mechanism or retention, last there should be any attack by solvent.

8. 2. Effect of substituent:
Eg: α-phenyl ethyl chloroformate show a large negative
value -3.86. Which is explains a tight ion pair with positive
charge development in the transition state. So this reaction
is facilitated by substituents those increasing electron
density at the Centre carbon.

9. 3. Effect of chloride ion:
When excess chloride is added or pyridine is added
the external nucleophile or the cl- formed during the
reaction and ionization
Attacks from the rear side leading to inversion.
(Generally energies are added to remove Hcl formed.)

10. Nucleophilic substitution at a vinyl carbon:
While allyl carbocation is stabilized by resonance for vinyl
substrates the molecule is stabilized.
Hence vinylic substrates are somewhat non-reactive towards
nucleophilic reagents just as phenyl.

11. There are 3 different mechanism:
1. Tetra hedral mechanism
2. Addition – elimination mechanism
3. Elimination- addition mechanism

12. 1.Tetrahedral mechanism:
It takes place with much less facility than with carbonyl
groups, since negative charge of the intermediate must be
born by the less electronegative carbon. (Then O, S, and N)
hence also the addition is followed by elimination.
Intermediate can be stabilized by combining with a positive species
and that is addition to C=C bond here the addition and substitution
often complexes.

13. Evidence for tetrahedral intermediate mechanism:
When the leaving group is changed from Br or cl to F. the rate
increase although F is a poor leaving group, because of superior
electron with drawing character of F makes C-F bonds carbon more
positive and more susceptible to nucleophilic attack. This atoms that
C-X bond does not break in rate determinate step (as it would in SN1
and SN2) however in more cases second step is rate determining step

14. 2.Addition – elimination mechanism:
Nucleophilic addition of ArSH to 1, 1-dichloro ethylene
catalyzed by Eto-.
The product was not 1, 1-di thiophenoxy compound
but the rearranged 1, 2-dithiophenoxy compound.

15. 3. Elimination – addition mechanism:
The reaction does not proceed without Eto- . No simple
substitution Rate α [Eto] but not on [ArSH]. Both lead to
retention. Since both are anti addition and elimination.

16. Characteristics of nucleophilic substitution at vinyl
carbon:
1. Groups:
ZCH=CHX; when Z is electron withdrawing group. Such as
HCO, RCO, -COOEt, ArSO2, CN and F carbanion stability
increased by spreading of negative charge.
Rentetion observed here is attributed to hyper conjugation involving
the carbanion electron pair and the substitution on adjacent carbon
stereo conversance 1:1 mixture of E or Z observed, when the
carbanionic carbon has two electron with drawing group

17. 2. α - Statidisation
α - Aryl vinyl halide (ArCBr=CR2) or α - cyclo propyl, α - vinyl
and an adjacent double bond make vinyl substrate to undergo
sn1 reaction.
3. Good leaving group:
Even without α - stabilization good leaving groups such as (-O
SO2 CF3) triflurate make the vinyl substrate to undergo
nucleophilic substitution.

18. 4. Stereo chemical outcome:
Stereo chemical outcome of SN1 mechanism of
vinyl substrate is often randomization.
Cis or trans substrate gives 1:1 mixture of cis and trans
products indicating vinyl cation is linear (sp). But when
R1 and R2 are present the entry of the nucleophile is
influenced by the relative size of R1 and R2 as the
empty p orbital lies in the plane of the double bond.