Slide 1 : MANY PARAMETERS INFLUENCE
NUCLEOPHILIC SUBSTITUTION Sn1 & sn2
Slide 2 : NUCLEOPHILIC SUBSTITUTION MANY FACTORS INFLUENCE SN1 AND SN2 REACTIONS a) structure b) atom used c) concentration d) base strength a) structure of R,
stereochemistry a) nature of X b) concentration a) solvent b) temp. c) pH SOME PARAMETERS : b) atom used c) base strength c) bond strength a) bond strength e) solubility f) size d) DH
Slide 3 : STRUCTURE OF THE SUBSTRATE SN1 STRUCTURE STRUCTURE
Slide 4 : SN1 - SUBSTRATE AND CARBOCATION R-X R+ + X- slow R+ + Nu- R-Nu fast The better ion
will have the
lower energy
pathway. The energy of the carbocation
intermediate is an important
factor for an SN1 reaction. 3o < 2o < 1o
Slide 5 : SOLVOLYSIS OF tert-BUTYL BROMIDE acetone SN1
Slide 6 : 1935: Hughes & Ingold SOLVOLYSIS OF t-BUTYL BROMIDE + H3O+ + Br- tertiary
Slide 7 : relative
rate 1.0 1.7 45 RBr + H2O ROH + HBr 100% HCOOH increasing rate rel rate = rate CH3Br rate EFFECT OF INCREASING SUBSTITUTION - SN1 methyl primary secondary tertiary Guess ? EFFECT OF SUBSTRATE ON RATE
Slide 8 : HYPERCONJUGATION No bond resonace Stability carbocation
Slide 9 : CARBOCATION STABILITY HYPERCONJUGATION C C H .. H H + R R electrons in an adjacent
C-H s bond help to stabilize
the positive charge of the
carbocation by proximity
(overlap) tertiary secondary primary << < lowest
energy highest
energy REVIEW
Slide 10 : BENZYL AND ALLYL
CARBOCATIONS THESE ARE GOOD CARBOCATIONS
Slide 11 : BENZYL ALLYL BENZYL AND ALLYL CARBOCATIONS (benzylic) (allylic) RESONANCE
STABILIZATION Systems that are similar in form to benzyl and allyl are called benzylic and allylic. isoelectronic
with allylic
Slide 12 : k rel Ethyl chloride Isopropyl chloride Allyl chloride Benzyl chloride tert-Butyl chloride very small 1 74 140 12,000 80% Ethanol-water at 50° SOLVOLYSIS RATES good but
not as good
as tertiary both reactions take place
Slide 13 : SOME CARBOCATIONS
CANNOT FORM
Slide 14 : WILL NOT REACT - SN1 + + cannot become
planar X “steric rigidity”
Slide 15 : SN2 - SUBSTRATE C R Br : H O : .. .. R R large groups
introduce steric
hindrance C H Br : H H easy access
no steric
hindrance
Slide 16 : decreasing rate EFFECT OF DEGREE OF SUBSTITUTION - SN2 methyl primary secondary tertiary EFFECT OF SUBSTRATE ON RATE
Slide 17 : decreasing rate EFFECT OF BULKY SUBSTITUENTS - SN2 NEOPENTYL
slower than
t -butyl ALL ARE PRIMARY ! ( substitution on the a carbon )
Slide 18 : Steric hindrance
Slide 19 : isopropyl t-butyl neopentyl COMPARATIVE STERIC HINDRANCE
Slide 20 : WILL NOT REACT - SN2 backside
attack is
difficult
Slide 21 : BENZYL ( GOOD FOR SN1 )
IS ALSO A GOOD SN2 SUBSTRATE primary, but faster
than other primary overlap in
the activated
complex
lowers the
activation
energy I Br H H critical
overlap
Slide 22 : SN2 increasing
rate
Slide 23 : SN1 SN2 tertiary methyl** benzyl benzyl allyl allyl secondary primary primary secondary tertiary neopentyl Notice that benzyl
and allyl are good
for both SN1 and SN2 BEST WORST (fastest) (slowest) bridgehead bridgehead (bicyclic) (bicyclic) APPROXIMATE
RATE ORDERS BEST (fastest) WORST (slowest) SUMMARY ** In SN2 reactions
benzyl is actually
better than methyl,
but allyl is not. For
illustration purposes
benzyl and allyl
were grouped in the
yellow box.
Slide 24 : AND DON’T FORGET: SN1 and SN2 reactions occur only at sp3 (tetrahedral) carbon atoms. yes no sp3 sp2 sp (attempted SN2 reaction) (attempted SN1 solvolysis)
Slide 25 : R-Cl 80% EtOH 50oC “nil” 1 74 140 1.2 x 104 2.4 x 105 6.4 x 106 ) R-Cl 80% EtOH 25oC 1 1.7 2.6 3 13.6 Relative Rate Relative Rate SN1 SOLVOLYSIS RATES