有机化学7

Carbocation stability favors SN2
Governed by steric factors
favors SN1 SN1 SN2 & SN1 SN2 SN2 Access to the site of reaction
E
[R]+ + X-
SN1:
3o
>
2o
>
1o
R-X + HOH ROH2+ + XROH + H3O+ + X5-13
CH2Cl2 methylene chloride or dichloromethane
5-1
5-2
Physical Properties of Methyl Halides
-
Reactions of Alkyl Halides: Nucleophilic Substitution
Our study of organic reactions will be organized around mechanistic similarities. This method of organization ___________ allows organic chemistry to be understood - not just memorized - based on unifying principles of reactivity. reactivity by _______ analogy to rationalize reason ___ It allows you to _______ reactions that you may never have previously seen. Nucleophiles attack electrophillic centers:
HB H-Br pKa = -9
5-6
Nucleophilic Substitution Mechanism (I): the SN2 Reaction
SN2 pathway ( Substitution, The ____ ubstitution Nucleophilic, ucleophilic Bimolecular)
5-3
nucleophile
(neutral or negatively ( g y charged) g )
electrophile
(neutral or positively charged)
l leaving i group
5-4
The Nucleophile Introduces a Wide Variety of New Functional Groups
+ -
C-X bond strength decreases with the i increasing i size i of f X and db bond dl length. th
Polar C C-X X bond
Boiling B ili points i t i increase with ith th the size i of fX and are higher than those of corresponding alkanes, due to increased strength of dipole-dipole and dispersion interactions.
5-7 5-8
Comparison of SN1 and SN2 Pathways
‡ ‡
Recognizing Leaving Groups (LGs)
Leaving group reactivity correlates to basicity. g ability. y The ______ the base, the ____fix as prefix
Simple compounds (common names):
X = F: methylfluoride or fluoromethane Cl: -chloride chloroBr: -bromide bromoI: -iodide iodoCHCl3 chloroform or trichloromethane CCl4 carbontetrachloride or tetrachloromethane
• Simutaneous bond making g and bond breaking • The TS≠ involves the association two molecules (bimolecular) of ___ • reaction rate =
k [substrate] [nucleophile] mol L-1 s-1
The Nucleophiles
Nucleophilicity depends on: basicity y, p polarizability y, and solvent p properties p . more basic, more nucleophilic higher polarizability, better nucleophile Basicity is the affinity of a base for a proton; N l Nucleophilicity hili it i is th the affinity ffi it of faL Lewis i b base f for a carbon b atom. t
transition state
Nucleophilic Substitution Mechanism (II): the SN1 Reaction
SN1 pathway ( Substitution, Nucleophilic, The ____ p Unimolecular )


RDS TS‡
Substrates in SN1 Reactions: Carbocation Stability
Governed by electronic factors
ROH + H3O+ + HX
Its stability influences the of RDS and the reaction rate. G‡
<
< y basicity
<
more basic and more polarizable
Polar protic
<
<
<
Increasing Nucleophilicity
<
(slower)
(very slow)
(does not occur)
Bulky substituents hinder the attack of the nucleophile
Incre easing ba asicity X–
Solvolysis: SN1 Reaction
R-X + HOH (solvent)
Hyperconjugation yp j g stablizes positive charge.
Reaction coordinate
5-14
Substrates in SN1 Reactions: Carbocation Stability
Why are these 1o carbocations as stable as 2o carbocations?
• Bond breaking precedes bond making
one • The TS≠ of the rate-determining rate determining step involves just ___ molecule (unimolecular); reaction rate = k [substrate] mol L-1 s-1
solvent Polar aprotic
Increasing Nucleophilicity
H2O < NH3 row 2 row 3 < < < basicity < <
Within a row, nucleophilicity increases from right to left basicity and parallels with _______
more basic b i and d more polarizable l i bl
< <
5-15
<
5-16
Generalizations about Nucleophilicity (II)
Increasing Nucleophilicity
Summary on Nucleophilicity of Nucleophiles
The Substrate: Steric Effects in SN2 Reactions



1
0.1
>
>
>>
~
>
0 03 0.03
Increasing reactivity
1.3 105
methyl > primary > secondary > tertiary
5-11
(fast)
Comparing p g nucleophiles p having g the same reactive atom, , the species p with the negative charge is the more powerful nucleophile than their conjugated acid.