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+
.. H+ from H2SO4
1) H+ sees yummy lone pairs on O
and tries to eat (bind to) them..
OH2+
2)…but can’t quite keep it down so
endlessly releases and repeats
attempts to consume OH electrons
(…equilibrium)
Initial stage
Br:
SN2 reaction of protonated 1o alcohol
starts by attack of Br(-) nucleophile
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 Br2a) (-) from Br
contributes to Br-C
bond forming
Br:
At this point C-O bond about to
break away completely but is still
attached.
2c) Br makes bond while
OH2 leaves
-
Br-C nearly
formed
2b) while Br continues to approach,
OH2 and other groups continue to
move away to reach final inverted
state
Complex now
deforms away
from its unstable
peak
OH2 +
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
Inversion for SN2
Start
Un-inverted reactant (Left leaning)
Br
OH2
+
Br
H2O
End
inverted product
(Right leaning)
Leaving
group has
left
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
-
-
3a) Br- attacks
carbocation from
either side
equally, causing
deformation back
to sp3 shape
+
+
3b) + and
– charges
neutralize
Br
3c) C bonds to Br and
molecules formed are
opposite in inversion
effect (racemic mix)
Example of rearrangement during SN1
Initial (primary) carbocation formation
CH 3
CH 3
reflux
OH
+ H+
CH
+
OH2
CH
CH
CH
Unprotonated alcohol
Br
HC
protonated alcohol
CH 3
Br -
+
+
CH
Rate limiting step
CH
+ H2O
Unrearranged product
Initial =primary carbocation
Rearrangements of primary carbocation
1,2 `hydride’ shift
CH 3
CH 3
H moves to (+)
(+) moves to site H
vacates
+
CH
CH 2
+
CH
C
2o
3o
Primary carbocation
Secondary carbocation from 1,2 hydride
shift rearrangement
CH 3
CH 2
Br
Rearrangement product
from 1,2 hydride shift
Br -
Rearrangements of primary carbocation
1,2 methyl shift
CH 3
CH3 moves to (+)
(+) moves to site
CH3 vacates
+
CH
2o
3o
+
C
CH 3
HC
CH
Br
CH 3
HC
Rearrangement product
from 1,2 methyl shift
Br -
Secondary carbocation
from 1,2 methyl shift
rearrangement
Rearrangements only occur if degree of secondary
carbocation increases vs primary
YES
CH
+
+
CH
C
CH 2
CH
CH
3o
2o
+
CH
H3 C
+
2o
NO
CH2
H2C
1o
FACTS
ABOUT ….
R-OH + HBr  R-Br +H2O
Acid
+ base
“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
•No rearrangements
•Reaction rate follows the
modest trend > Br > Cl > F
•Modestly polar aprotic
solvents produce better yields
•Product is inverted vs
starting alcohol
SN2 MECHANISM
•Only ROH concentration
affect rate
•Rearrangements occur
•No affect of halogen identity on rate
•Strongly Polar protic solvents
produce better yields
•Product is racemized vs starting alcohol
SN1 MECHANISM