IIT JEE , AIEEE Forum - Chemistry , Organic Chemistry

ayushaggarwal

give me clear characteristics of sni and E1CB reactions

t_c511

posted in the community shelf for SNi

do check it out

iitkgp_bipin

SNi or Substitution Nucleophilic internal stands for a specific but not often encountered nucleophilic aliphatic substitution reaction mechanism. A typical representative organic reaction displaying this mechanism is the chlorination of alcohols with thionyl chloride and the main feature is retention of stereochemical configuration. Thionyl chloride first reacts with the alcohol to form an alkyl chloro sulfite. In the second step the sulfite group is lost and just like in an SN1 reaction an alkyl carbocation is created. The actual nucleophile in the third reaction step is the chlorine atom attached to the sulfite group which recombines with this carbocation. The crucial difference with the standard SN1 mechanism is that since the nucleophile resides at the same side as the original leaving group i.d. the hydroxyl group, the stereochemistry is retention of configuration and not racemization.

The E1cB elimination reaction is a special type of elimination reaction in organic chemistry. This reaction mechanism explains the formation of alkenes from (mostly) alkyl halides through a carbanion intermediate given specified reaction conditions and specified substrates. The abbreviation stands for Elimination conjugate Base.The reaction takes place around a sp³ - sp³ carbon to carbon covalent bond with an ?-acidic hydrogen atom substituent and a ?-leaving group.This leaving group can be a halide or a sulfonic acid ester such as a tosyl group. A strong base abstracts the ? proton generating a carbanion.The electron pair then expels the leaving group and the double bond is formed. When the first step to the carbanion is slow and the second step fast the reaction is irreversible and named (E1cB)_i. When the first step is fast and the deprotonation reversible then the reaction mechanism is(E1cB)_r. In the (E1cB)_anion variation the carbanion is especially stable with a rapid first step and a slow second step.

madhusudan.chavan

7470

S_Nⁱ reaction ? If in a nucleophilic substitution reaction a part of leaving group attacks the carbon site of the substrate and detaches itself from the rest of the leaving group, it is known as substitution nucleophilic internal, since the attacking nucleophile has already been present on the substrate and its attack has occurred internally.

The best example is reaction of alcohol with thiony chloride in ether medium

ROH + SOCl₂ RCl +SO₂ +HCl

The reaction takes place in three steps :

1.Formation of alkylchlorosulphite as an intermediate compound

R-OH + Cl₂S=O R-O-S(Cl)=O + HCl

2.Alkylchlorosulphite then dissociates slowly into an intimate ion pair, a carbenium ion & ^-OSOCl

R-O-S(Cl)=O ^slow R⁺ + ^-OSOCl

3.Now the nucleophile Cl ^- , a part of the leaving group (^-OSOCl) attacks the carbenium ion from the side of leaving group, since it is unable to get to the rear.

R⁺ + ^-OSOCl RCl + SO₂

Rate=k[alcohol][thionylchloride]

Since a carbenium ion is produced in the r.d.s. of chlorosulphite decomposition reaction, the rate of the reaction depends on the stability of the carbenium ion. The greater is the stability , greater is the rate. For the same reason increasing power of the solvent increases the rate.

E1cB reaction : This is a two step base catalysed 1,2 elimination reaction which involves a carbanion, the conjugate base of the substrate as the reaction intermediate.

1.The base abstracts the most acidic b - H from the substrate & a carbanion forms. This is a reversible and a fast step.

H

| | B:^- - |

-C-C-G Û BH + -C-C-G

| | | |

2.The leaving group leaves the carbanion & C=C double bond forms.

C^-¾ C¾G C=C + G^?

(Tansition state f E1cB reaction C ^d^-_¾^??C ?.G ^d^-)

Since the transition state for this step involves the conjugate base (cB) of the substrate only, the rate of this step = k[cB]

This sort of 1,2 elimination reaction is popularly called E1cB ? elimination unimolecular w.r.t. conjugate base of the substrate.

E1cB reactions usually proceed in the presence of conc. Strong base in polar media at elevated temp. However, pure E1cB reactions are very rare.

As the most acidic b H is abstracted by the base before the leaving group departs the substrate, the Hoffmann?s product is expected to be formed predominantly.Since the carbanions are formed in this reaction and since a carbanion can adopt most stable conformation the E1cB reactions are non-stereospecific.

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