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why lucas reagent follows sn1 mechanism?....what if,l lucas reagent test follows sn2 mechanism???.....why it can’t follow sn2 mechanism????

 
why lucas reagent follows sn1 mechanism?....what if,l lucas reagent test follows sn2 mechanism???.....why it can’t follow sn2 mechanism????
 

Grade:12

3 Answers

Bhavya
askIITians Faculty 1281 Points
8 years ago
Whether the displacement reaction is an SN1 or SN2 process depends on the structure of the alcohol. In general, the primary alcohols are considered to react by SN2 and the secondary and tertiary alcohols by SN1 mechanisms.

Hydrogen chloride is less reactive than hydrogen bromide toward primary alcohols, and a catalyst (zinc chloride) may be required. A solution of zinc chloride in concentrated hydrochloric acid (Lucas reagent) is a convenient reagent to differentiate between primary, secondary, and tertiary alcohols with less than eight or so carbons. Tertiary alcohols react very rapidly to give an insoluble layer of alkyl chloride at room temperature. Secondary alcohols react in several minutes, whereas primary alcohols form chlorides only on heating. The order of reactivity is typical of SN1 reactions.

SN1 and SN2 determination is also an experimental fact.Also, if Lucas reagent has followed SN2 then ,the order of reactivity would have been reversed i.e. primary > secondary > tertiary
Nikhil
13 Points
4 years ago
Lucas'reagent follows SN1 mechanism of reaction because in this test -OH group is protonated by reagent making -OH2+ group which further becomes better leaving group.
Thus when -OH2+ group leaves it forms cation and cation stability will be deciding factor of the primary, secondary or tertiary alcohol as tertiary alcohol stabilises  cation most it will give turbidity fastest then secondary and then primary alcohol
Arun
25750 Points
4 years ago
When primary alcohol reacts with Lucas reagent, ionization is not possible because primary carbocation is too unstable. So the reaction does not follow SN1 mechanism. Primary alcohol reacts by SN2 mechanism which is slower than SN1 mechanism

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