Hoa huu co _phan ung 2.ppt

HOÁ HỌC HỮU CƠ
Organic Chemistry
CHƯƠNG 5 (t.t)
CÁC PHẢN ỨNG TRONG HÓA HỮU CƠ
GIỚI THIỆU CHƯƠNG
GIỚI THIỆU PHẢN ỨNG
PHÂN LỌAI PHẢN ỨNG
DẶC ĐIỂM PHẢN ỨNG
PHẢN ỨNG THẾ
PHẢN ỨNG CỘNG
PHẢN ỨNG TÁCH
PHẢN ỨNG OXI HÓA
PHẢN ỨNG THẾ ( SUBSTITUTION)
PHẢN ỨNG THẾ ÁI NHÂN (NUCLEOPHILIC)

PHẢN ỨNG THẾ ÁI ĐIỆN TỬ (ELECTRPHILIC)

PHẢN ỨNG THẾ GỐC TỰ DO (RADICAL)
PHẢN ỨNG THẾ ÁI NHÂN SN
GIỚI THIỆU

CƠ CHẾ VÀ ĐỘNG HỌC

CÁC YẾU TỐ ẢNH HƯỞNG
Nucleophilic Substitution
A nucleophile is a specie with an unshared electron pair which reacts with an electron deficient carbon
A leaving group is substituted by a nucleophile
Heterolytic cleavage







Functional Group Transformations Using SN2 Reactions
PHẢN ỨNG THẾ ÁI NHÂN SN
PHẢN ỨNG THẾ ÁI NHÂN SN
PHẢN ỨNG THẾ ÁI NHÂN SN
Nucleophilic Substitution-Mechanism
The leaving group polarizes the bond
The nucleophile reacts at the electron deficient carbon






Bond breaking and bond forming
Simultaneous: SN2
Stepwise: SN1
Mechanism of SN2 Reactions







A transition state: high energy state of the reaction
Unstable entity with a very brief existence (10-12 s)
Bonds are partially formed and broken
Both chloromethane and hydroxide involved in the transition state
the reaction is second order
Mechanism for SN1 Reactions
Step 1 is rate determining
formation of unstable ionic products
water molecules help stabilize the ionic products
Energy Diagram of SN2 Reactions
An energy barrier: bond being broken in going to the transition state (the top of the energy barrier)
Free energy of activation (DG‡ )
Free energy change of the reaction, DGo
Energy Diagram of SN2 Reactions










A reaction with DG‡ above 84 kJ mol-1 will require heating to proceed at a reasonable rate
DG‡ = 103 kJ mol-1 : heating required
Stereochemistry of SN2 Reactions
Backside attack of nucleophile results in an inversion of configuration (use your models)






Stereochemistry of SN2 Reactions
In cyclic systems a cis compound can react and become trans product
SN1 Reactions
tert-Butyl Chloride with Hydroxide Ion
Rate independent of hydroxide concentration
Rate dependent only on concentration of tert-butyl chloride






SN1 reaction: Substitution, nucleophilic, 1st order (unimolecular)
The rate depends only on the concentration of the alkyl halide
Only the alkyl halide involved in the transition state of the step that controls the rate
Kinetics of SN1 Reactions
In multistep reactions, the rate of the slowest step will be the rate of the entire reaction



Rate determining step
Mechanism for SN1 Reactions
Carbocationic intermediates
6 electrons,
sp2 hybridized
empty p orbital





Mechanism for SN1 Reactions
The more highly substituted a carbocation is, the more stable it is, The more stable a carbocation is, the easier it is to form
Stereochemistry of SN1 Reactions
In SN1 reactions, racemization occurs
achiral carbocation intermediates
Racemization: transformation of an optically active compound to a racemic mixture
Stereochemistry of SN1 Reactions








SN1 vs. SN2 Reactions
Factors affecting the respective rates of SN1 and SN2 Reactions
Structure of the Substrate
In SN2 reactions: Methyl > primary > secondary >> tertiary (unreactive)
Steric hinderance: the spatial arrangement of the atoms or groups at or near a reacting site hinders or retards a reaction
Generally only tertiary halides undergo SN1 reactions (relatively stabilized carbocations)

SN1 vs. SN2 Reactions
The Hammond-Leffler Postulate
The transition state for an exergonic reaction looks very much like starting material
The transition state for an endergonic reaction looks very much like product
Generally the transition state looks most like the species it is closest to in energy

SN1 vs. SN2 Reactions
The Hammond-Leffler Postulate
SN1 reaction: the first step transition state looks very much like carbocation
stabilized by all the factors that stabilize carbocations
Concentration Effect
The Effects of the Concentration and Strength of Nucleophile
SN1 Reaction
Rate independent on the identity or concentration of nucleophile
SN2 Reaction
Rate directly proportional to the concentration of nucleophile
Stronger nucleophiles react faster (a negatively charged nucleophile is always more reactive than its neutral conjugate acid)



When comparing nucleophiles with the same nucleophilic atom, nucleophilicities parallel basicities


Solvent Effects
Solvent Effects on SN2 Reactions:
Polar Aprotic Solvents solvate cations leave anions unsolvated (positive centers in the solvent sterically hindered)



Solvent Effects
Polar Aprotic Solvents
solvate cations
leave anions unsolvated
generate “naked” (very reactive) nucleophiles
Trends for nucleophilicity are the same as for basicity




They are excellent solvents for SN2 reactions
Solvent Effects
Solvent Effects on SN2 Reactions:
Polar Protic Solvents (hydrogen atom attached to strongly electronegative atoms)
Solvate nucleophiles (hydrogen bonds) and make them less reactive
Solvate cations and release naked ions (more reactive)






Solvent Effects
Solvent Effects on SN1 Reactions:
Polar protic solvents stabilize the carbocation-like transition state leading to the carbocation thus lowering DG‡
Polar protic solvents are excellent solvents for SN1 reactions
Water-ethanol and water-methanol mixtures are most common
Leaving Group
The Nature of the Leaving Group
Best leaving groups: weak and relatively stable bases
Leaving group ability of halides:

Trend opposite to basicity:

Other very weak bases which are good leaving groups:



The poor leaving group hydroxide can be changed into the good leaving group water by protonation
SN1 vs. SN2 Reactions
Summary SN1 vs. SN2
SN1 vs. SN2 Reactions
Summary SN1 vs. SN2
Stereochemistry can be controlled in SN2 reactions


SO SÁNH PHẢN ỨNG THẾ SN1 VÀ SN2
SO SÁNH PHẢN ỨNG TÁCH LOẠI E1 VÀ E2
GIẢN ĐỒ NĂNG LƯỢNG
DỰ ĐOÁN PHẢN ỨNG THẾ & TÁCH
CẢM ƠN SỰ THEO DÕI CỦA CÁC BẠN

CHÚC MỌI ĐIỀU TỐT ĐẸP