Slide 1

SNi MECHANISMNucleophilic substitution of alcohol with HX gives haloalkane but there is possibilities for the formation of rearranged products especially when there is formation of carbocations.

However when sulphur or phosphorous halides are used , no rearrangement product obtained. It follows Sni mechanism (Substitution Nucleophilic Internal)Eg:

R-OH + SOCl2 R-Cl + so2 + HClMECHANISMFIRST STEP:

The lone pair of oxygen attacks electrophilic centre of SO2Cl2 (S)gives positive charge In oxygen and negative on other to form an unstable ion pair which eliminates a molecule of HCl to form Alkyl chloro sulphite intermediate (similar to SN1)SECOND STEPInternal attack of the nucleophile Chlorin to the electrophilic Carbon from the same side , breaking C-O bond with the elimination of SO2 molecule

Here Nucleophile attacks from the same side of the leaving group which leads to retention of configuration

EgNIGHBOURING GROUP PARTICIPATIONIn some cases the rate of SN2 reactions is found to be extremely high and the Product is obtained with the retention of configuration. These reactions follows Nighbouring group mechanismHere nighbouring group acts as nucleophile.. Ie one of the nucleophile is a part of the same molecule and is located at the C atom or farther from the leaving group

As it involves two steps with Sn2 mechanism two inversions leads to retention of configuration

In the first step attack of neighboring group at the carbon from the backside forming an unstable intermediate with inversion of configuration ( SN2 reaction )

In the second stage nucleophilic attack takes place to form the product with inversion of configuration ( Another SN2 reaction) ie two SN2 reactions with inversion of configuration . Final product with retention of configuration

Eg::

First stepSecond stepL-2- BROMOPROPIONATEL-2- METHOXY PROPIONATEFEATURES

High reaction rate compared to SN2

Followsfirst order keinetics (substrate only)

Retention as a result of two inversion

Also called Anchimeric assistanceE1CB MECHANISM(UNIMOLECULAR MECHANISM FROM A CONJUGATE BASE)In E1CB mechanism a proton is initially abstracted to generate conjugate base at equilibrium which then loses L- giving elimination products in therate determing step

As the proton has to be removed from a carbon from a neutral species ( a difficult job)A strong base is needed for E1CB reaction as in E2 reaction. More over increase in strength of base and concentration of base increase the rate of E1CB reaction

If more than one beta hydrogen more acidic hydrogen is removedBetter the leaving group, better will be the rate of E1CB reaction ( if E1CB is operating) Because rds involve the leaving group otherwise if the leaving group is so good It will leave before the anion is formed leading to E2 mechanismE1CB mechanism operates when the abstraction of proton is very easy ( removal from a highly electronegative atom)and the conjugate base is resonance stabilised)E1CB mechanism also operates when the expulsion of leaving group is difficult ( Leaving group is bad like F-,-OH-,RO- etc or if leaving group has partial double bond due to resonance)EgDehydrohaloganation of geminal halohydrin because H+ is removed from highly electronegative oxygen

2Dehydrohalogenation of carbonyl compounds because resultant carbanion is resonance stabilised

If the leaving group is a better leaving group, E1CB reaction will takes placeBut if the leaving group is very good E2 reaction will takesplace3- Dehydroflurination of because fluoride is a bad leaving group

Dehydrohalogenation when halogen has partial double bond due to resonance in vinyl halide and aryl halides

5 Dehydration in basic medium

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