Acid-Catalyzed Reactions of Alcohols: SN2 and SN1 Mechanisms
The acid-catalyzed reactions of alcohols involve SN2 and SN1 mechanisms where protonated alcohols react with RX. In the SN2 mechanism, a nucleophile attacks a protonated 1o alcohol, leading to inversion. On the other hand, in the SN1 mechanism, a carbocation intermediate is formed, and a nucleophile attacks to give a racemic mix of products.
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The SN2 and SN1 Acid-Catalyzed Reactions of ROH RX in (crude) animated form The crucial first step: Equilibrium formation of protonated alcohol ROH2+ 3) To help H+ keep the electrons down we heat, driving equilbrium to right(reflux) .. ROH OH .. H+ OH2+ H+ from H2SO4 2) but can t quite keep it down so endlessly releases and repeats attempts to consume OH electrons ( equilibrium) 1) H+ sees yummy lone pairs on O and tries to eat (bind to) them..
SN2 reaction of protonated 1o alcohol starts by attack of Br(-) nucleophile Initial stage Br: - OH2+ Br begins to approach protonated alcohol Incipient bond forms from C----Br as lone pair of Br starts to donate Three attached groups move away from Br The C-OH2 bond lengthens
Activated complex stage 5-coordinate activated complex for SN2 formed after attack of nucleophilic Br- At this point C-O bond about to break away completely but is still attached. 2a) (-) from Br contributes to Br-C bond forming Complex now deforms away from its unstable peak 2c) Br makes bond while OH2 leaves + - OH2 Br: 1)two electrons in breaking bond goes to OH2(+) and neutralizes (+) 2a)Lone pair from Br contributes to C-Br bond forming on other side Br-C nearly formed 2b) while Br continues to approach, OH2 and other groups continue to move away to reach final inverted state
Inversion for SN2 Start Un-inverted reactant (Left leaning) OH2+ Br - Br H2O Leaving group has left End inverted product (Right leaning)
Non-chemical version of an `inversion Another chemical example of inversion: ammonia- the first laser source (called a maser) https://www.youtube.com/watch?v=6tL_cWvycTk
Start of SN1 reaction on 2o or 3o, protonated alcohol (step 2 after step 1 protonation) 2a) H2O leaves as H2O + OH2 and leaves (+) on central carbon 1b) At same time all the other attached groups move towards central carbon to form a flattened, sp2 like structure
Step 3 of SN1 : attack of Bromine on intermediate 2o or 3ocarbocation made in step 2 - - Br Br + + 3a) Br- attacks carbocation from either side equally, causing deformation back to sp3 shape 3b) + and charges neutralize 3c) C bonds to Br and molecules formed are opposite in inversion effect (racemic mix)
Example of rearrangement during SN1 Initial (primary) carbocation formation CH3 + CH3 OH2 reflux OH + H+ CH CH CH CH Unprotonated alcohol protonated alcohol CH3 Br CH+ Br - + Rate limiting step HC CH + H2O Unrearranged product Initial =primary carbocation
Rearrangements of primary carbocation 1,2 `hydride shift H moves to (+) CH3 CH3 (+) moves to site H vacates CH+ CH2 C+ CH 2o 3o Primary carbocation CH3 Secondary carbocation from 1,2 hydride shift rearrangement CH2 Br Br - Rearrangement product from 1,2 hydride shift
Rearrangements of primary carbocation 1,2 methyl shift CH3 moves to (+) (+) moves to site CH3 vacates 3o CH3 CH3 C+ CH+ HC CH 2o Secondary carbocation from 1,2 methyl shift rearrangement Br CH3 Br - HC Rearrangement product from 1,2 methyl shift
Rearrangements only occur if degree of secondary carbocation increases vs primary YES CH+ C+ CH CH2 CH CH 3o 2o CH+ CH2 H3C + H2 C NO 2o 1o
FACTS ABOUT . R-OH + HBr Acid + base R-Br +H2O salt + water 0.) need reflux and extra acid (H2SO4) to make ROH2+else no go 1) 3o ROH > 2o ROH > 1o ROH in rate 1o ROH 2o & 3o ROH Both ROH and HX concentrations affect rate Only ROH concentration affect rate No rearrangements Rearrangements occur Reaction rate follows the modest trend > Br > Cl > F No affect of halogen identity on rate Modestly polar aprotic solvents produce better yields Strongly Polar protic solvents produce better yields Product is inverted vs starting alcohol Product is racemized vs starting alcohol SN1 MECHANISM SN2 MECHANISM
