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Carlos Montanari
http://www.nequimed.iqsc.usp.br/
UNIVERSIDADE DE SÃO PAULO - USP
Instituto de Química de São Carlos - IQSC
Grupo de Estudos em Química Medicinal NEQUIMED
Conteúdo
• Ligações químicas localizadas
• Ligações químicas deslocalizadas
• Ligações químicas mais fracas que a covalente
• Estereoquímica
• Carbocátions, carbânions, radicais livre
• Mecanismos e métodos de determinação de mecanismos de reações
– March, Carey, Evans, Kwan
• Ácidos e bases Efeitos da estrutura sobre a reatividade
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Mechanisms and Methods of Determining Them
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Pericyclic
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TYPES OF REACTION
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THERMODYNAMIC REQUIREMENTS FOR REACTION
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KINETIC REQUIREMENTS FOR REACTION
ΔGfo (kcal mol-1)
-56.73
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THE BALDWIN RULES FOR RING CLOSURE
Baldwin’s rules can be applied to ketone enolates
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Exercício do dia.
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Exercício do Dia
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The Bürgi–Dunitz angle
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LiTMP
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KINETIC AND THERMODYNAMIC CONTROL
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THE HAMMOND POSTULATE
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MICROSCOPIC REVERSIBILITY Chemical equilibrium
MICROSCOPIC REVERSIBILITY…
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(de Dios)
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Por exemplo:
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MARCUS THEORY
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SN2
METHODS OF DETERMINING MECHANISMS
Identification of Products
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The von Richter Rearrangement
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Determination of the Presence of an Intermediate
1. Isolation of an Intermediate.
2. Detection of an intermediate.
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3. Trapping of an Intermediate.
Isotopic Labeling
Despite appearances, this is not a substitution reaction!
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Stereochemical Evidence
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The Walden Inversion
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c
Kinetic Evidence
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slow
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Assumption of the steady state: d[I]/dt = 0
We now insert this value for [I] into the original rate expression to obtain:
But, our original hypothesis:
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If the first step is rate determining…
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A pseudo-first-order reaction
First order reactions always have an exponential decay behavior.
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Why?
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As before, separate variables and integrate.
k1 + k-1 = 1.1 So, t1/2 = 0.63 s
That for 5 full conversions ~ 3 s!
Measuring the change in concentration of a product or a
reactant with time. 1. Periodic or Continuous Spectral Readings:
IR and UV spectroscopy, polarimetry, NMR, and ESR.
2. Quenching and Analyzing and/or Removal of Aliquots at Intervals : The materials are analyzed by spectral readings, titrations,
chromatography, polarimetry, or any other method.
4. Measurement of Changes in Total Pressure, for Gas-Phase Reactions.
5. Calorimetric Methods. The output or absorption of heat can be measured at time intervals.
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Mechanistic information: rate!
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Isotope Effects
For instance,
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Secondary isotope effects
The solvent isotope effect
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