Practice problems on SN1/SN2 reactions - Answers
1.
Predict which of each pair of the following reactions will proceed at a higher rate. Provide a brief explanation.
a)
SCN- + CH3CH2Br
SCN- + CH3CH2Br
H2O / C2H5OH 50%
H2O / C2H5OH 50%
b) CH3CH2CH(CH3)CH2Br + SH −
(CH3)2CHBr + NH3
⎯
⎯→
CH3CH2CH(CH3)CH2SH + Br −
CH3CH2CH2CH2SH + Br+
CH3OH
CH3CH2CH2NH3+Br-
d) CH3Br + (CH3)3N
DMF
(CH3)4N+Br-
CH3Br + (CH3)3P
DMF
(CH3)4P+Br-
(CH3)2CHCH2 Cl + N 3−
C2H5OH
(CH3)2CHCH2I + N3-
C2H5OH
-OH
CH3I +
f)
CH3I +
-O
H2O / r.t.
2CCH3
Less steric hindrance
OH
⎯CH
⎯3⎯
→ (CH 3 ) 2 CH N H 3 Br −
CH3CH2CH2Br + NH3
e)
Stronger nucleophile
CH3CH2NCS + Br −
CH3CH2CH2CH2Br + SHc)
CH3CH2SCN + Br-
H2O / r.t.
Less steric hindrance
Stronger nucleophile
(CH3)2CHCH2 N3 + Cl −
(CH3)2CHCH2N3 + I-
Better leaving group
CH3OH + I-
Stronger nucleophile
H3CO2CCH3 + I-
H 2 O, Δ
g) (CH3)3CBr ⎯⎯
⎯
⎯→ (CH3)3COH + HBr
More stable carbocation
H 2 O, Δ
(CH3)2CHBr ⎯⎯
⎯
⎯→ (CH3)2CHOH + HBr
O
h)
OCH2CH2Cl
−
i)
H 2C
⎯→
OCH2 CH 2 CH 2 Cl ⎯
Lower energy transition state
CH2 O
CH2 CH2
OH, solvent
CH3CH2Br + CN − ⎯CH
⎯3⎯⎯⎯⎯→ CH3CH2CN + Br −
CH3CH2Br +
CN
(CH3)2NCHO (DMF), solvent
Cl
j)
CH2
CH3CO2
k) N3
Br
Better solvent for SN2 reactions
H 3CCO2
+
+
Cl
Weaker bonds
H 3CCO2
Cl
CH3CO2
CH3CH2CN +
+
+
+ (CH3)2CHCH2Br
C2H5OH
Cl
(CH3)2CHCH2Br + Br
C H 5OH
⎯→ (CH3)2CHCH2SC6H5 + Br −
C 6 H 5S − + (CH3)2CHCH2Br ⎯⎯2 ⎯
Stronger nucleophile