Student Study Material

1. Reaction Mechanism of
SNi, SN1’, SN2’, SN1cA and SN2cA
ORGANIC CHEMISTRY – II (I-MSc)
Subject Code: 33CT22
By
Dr. G. Balakrishnan
Assistant Professor
Department of Chemistry
Vivekananda College
Madurai, Tamil Nadu

2. SNi –Mechanism (substitution nucleophilic internal)
In a few reactions, nucleophilic substitution proceeds with
retention of configuration, even where there is no possibility
of a neighboring-group effect.
In the SNi mechanism, part of the leaving group must be
able to attack the substrate, detaching itself from the rest of
the leaving group in the process.
The first step is the same as the very first step of the SN1
mechanism dissociation into an intimate ion pair.
But in the second step part of the leaving group attacks,
necessarily from the front since it is unable to get to the
rear, which results in retention of configuration.

3. The SNi mechanism is relatively rare.
Another example is the decomposition of (alkyl
chloroformates) ROCOCl → RCl + CO2

4. SN1’ Reaction
 When allylic substrates are treated with nucleophiles under
SN1 conditions, two products are usually obtained: the
normal one and a rearranged one.
 This reactant should give the same mixture of alcohol when
reacting with NaOH, Since carbocation (C+) form each should
be the same.
NaOH
40%Yield : 60%
CH3-CH=CH-CH2Cl → CH3-CH=CH-CH2-OH + CH3-CH(OH)-CH=CH3

5. Two products are formed because an allylic type of
carbocation is a resonance hybrid.
First step, the reactant give rise to two different
intimate ion pairs.
And than second, the field of the anion polarise the
allylic cation, making the nearly carbon atom more
electrophilic, so that it has a greater change of
attracting the Nucleophile.
SN1’ Reaction ….

6. SN2’ Reaction
 Nucleophilic substitution at an allylic carbon can also take
place by an SN2 mechanism, in which case no allylic
rearrangement usually takes place.
 However, allylic rearrangements can also take place under SN2
conditions, by the following mechanism, in which the
nucleophile attacks at the -carbon rather than the usual
position (called SN2’).

7.  The stereochemistry of SN2’ reactions has been investigated.
It has been found that both syn and anti reactions can take
place, depending on the nature of X and Y,
 Although the syn pathway predominates in most cases.
SN2’ Reaction ….

8. SN1cA (Nucleophilic Substitution Unimolecualr Conjugate Acid)
 Reactions in which the leaving group does not come off (labile) until
it has been protonated have been called SN1cA or SN2cA, depending
on whether after protonation the reaction is an SN1 or SN2 process.
Ex: -OH and -OR are not leaving groups from ordinary alcohols and
ethers, but can come off when the groups are protonated, that is,
converted to ROH2
+ Or RORH+.

9. Here alkoxyl group -OR is not a leaving group, so these
compounds must be converted to the conjugate acids before
they can be hydrolyzed.
Although 100% sulfuric acid and other concentrated strong
acids readily cleave simple ethers, the only acids used
preparatively for this purpose are HBr and HI.
These compounds (acetals/ketals/ester)are hydrolyzed with
greater facility because carbocations of the type RO-CH- are
greatly stabilized by resonance.
The above reactions therefore proceed by the
SN1 mechanism, as shown for acetals as a product.
SN1cA…..

10.  In general SN2 mecanism there is backside attack, the nucleophile
approaches the substrate from a position 180o away from the
leaving group. The reaction is a one-step process with no
intermediate.
 In SN2cA mechanisms, there is a preliminary step, the addition of a
proton, before the normal SN2 process occurs.
SN2cA (Nucleophilic Substitution Bimolecular Conjugate Acid)

11. It is obvious that the best nucleophiles (e.g., -NH2 , -OH)
cannot take part in SN1cA or SN2cA processes, because
they would be converted to their conjugate acids under
the acidic conditions necessary to protonate the leaving
groups.
 Because SN1 reactions do not require powerful
nucleophiles, but do require good leaving groups, most of
them take place under acidic conditions.
In contrast, SN2 reactions, which do require powerful
nucleophiles (which are generally strong bases), most
often take place under basic or neutral conditions.
SN2cA…..

12. Reference book: MARCH’S ADVANCED ORGANIC CHEMISTRY, 6th edition