Chapter 5. Organic Reaction Overview
Recommended article: 【Organic Chemistry】 Table of Contents
1. Overview
3. Reaction Thermodynamics and Reaction Kinetics
1. Overview
⑴ Reaction: The phenomenon where the bonds of the reactants break and the bonds of the products form.
⑵ Reaction equation
① Types of arrows
○ →: reaction arrow
○ ⇄: double reaction arrows (equilibrium arrows)
○ ↔: double-headed arrow. resonance structure
○ ↷: full-headed curved arrow. movement of a pair of electrons
○ ⇀: half-headed curved arrow. movement of a single electron
② Reagents are indicated on the left side of the reaction equation or above the arrow
③ Solvents and temperature in the reaction are indicated above and below the arrow
④ In multistep reactions, reagents are denoted as [1], [2], and so on
⑶ All reactions can be categorized as acid-base reactions and redox reactions, but can be further classified based on mechanisms
① Acid-base reactions specified separately in mechanisms refer to general acid-base reactions with terminal functional groups, and they are faster as the reaction occurs at the terminal site
⑷ Intramolecular reactions are much faster than intermolecular reactions
① Many questions on discrimination involve intramolecular reactions
② If conditions allow intramolecular SN2 reactions, they are mechanistically configured to occur
③ If conditions allow ring-closure reactions (e.g., epoxidation), they are mechanistically configured to occur
2. Types of Organic Reactions
⑴ Substitution reaction
Figure 1. Example of substitution reaction
⑵ Elimination reaction
Figure 2. Example of elimination reaction
⑶ Addition reaction
Figure 3. Example of addition reaction
3. Reaction Thermodynamics and Reaction Kinetics
⑴ Reaction Thermodynamics: Related to ΔG°
① Entropy
○ Reaction with more products than reactants increases entropy, and reaction with fewer products decreases entropy
○ Linear molecules becoming cyclic reduce degrees of freedom, hence decrease in entropy
② Free energy
○ Not applicable to gas-phase reactions, hence entropy values are generally low
○ Organic reactions not proceeding at high temperatures nearly equate free energy change with enthalpy change
○ Organic chemistry deals only with spontaneous reactions anyway, so the concept of free energy isn’t crucial.
⑵ Reaction Kinetics: Related to activation energy Eact = ΔG‡
① Important in organic chemistry
② Activation energy
○ Most organic reactions have activation energy of 40 to 150 kJ/mol
○ Reactions with activation energy below 80 kJ/mol proceed easily at room temperature
③ Catalyst: Reduces activation energy, thereby increasing reaction rate
⑶ Natural noise in the environment: 80 kJ/mol
① Energy involved in conformational changes (ringflip) is generally less than 80 kJ/mol
② Energy involved in entering or leaving a reaction is much larger than this
⑷ Hammond Postulate
① Introduction
○ Reaction rate is dependent on the energy of the transition state.
○ As the precise configuration of the transition state remains unknown, its arrangement is deduced from either the reactants or the products.
② Explanation
○ Transition state is energetically similar to stable chemical species
○ Transition state for endothermic reactions resembles products, while that for exothermic reactions resembles reactants
○ Reactions that produce intermediates are typically endothermic. If the intermediate is a stable compound, it is assumed that the transition state leading to the intermediate would also be stable and resemble the intermediate. Therefore, in such cases, the reaction can proceed easily to form the final product.
⑸ Hoffmann’s Law: Deriving concepts of rate control and thermodynamic control
① Experiment
Figure 4. Halogenation hydrogen addition reaction of 1,3-butadiene
○ 0 °C: 3-bromobut-1-ene (71%) + 1-bromobut-2-ene (29%)
○ 40 °C: 3-bromobut-1-ene (15%) + 1-bromobut-2-ene (85%)
② Interpretation
○ 1,2 addition: Formation of 3-bromobut-1-ene
○ 1,4 addition: Formation of 1-bromobut-2-ene
○ Mechanistically, generation of 3-bromobut-1-ene is more reasonable
○ At high temperatures, reverse reaction occurs, hence thermodynamically more stable product dominates
○ 0 °C (low temperature): Product ratio determined by rate control
○ 40 °C (high temperature): Product ratio determined by thermodynamic control
4. Reaction Mechanism
⑴ Transition State Theory: Reactions occur via transition states
① Transition state: Most unstable state during reaction
② Reasons for instability of transition state: Violation of octet rule, charge separation, etc.
③ Activation energy: Energy difference between transition state and reactants
④ Among various possible transition states, reaction proceeds along the path with the lowest activation energy
⑵ Categorization of reactions based on mechanism steps
① One-step reaction
○ Also known as concerted reaction
○ Breaking of reactant bonds and forming of product bonds occur simultaneously
○ Reaction rate analysis: Reaction rate is proportional to the product of reactant concentrations
② Stepwise reaction
○ Reaction proceeds via unstable intermediates (intermediate)
○ Reactant → Transition state → Intermediate → Transition state → Product, similar to the mechanism
○ Reaction rate analysis: Rate-determining step exists; generally, reaction rate is proportional to the concentration of one reactant
⑶ Categorization of reactions based on mechanism types
① Homolysis: Homolytic cleavage occurs in radical reactions, symmetrical bonds, and C-H bonds.
Figure 5. Homolysis
② Heterolysis (heterolytic cleavage): Heterolytic cleavage occurs due to polar reactions, differences in electronegativity, differences in polarity, etc.
○ Example: Most organic chemistry reactions, Cl-Cl reactions, Br-Br reactions, cases with double bonds
Figure 6. Heterolysis
③ Energy is required for bond cleavage
⑷ Types of intermediates
① Radical
Figure 7. Radical
○ Intermediate with unpaired electron
○ Formed through homolysis reactions
○ Highly unstable as it does not follow the octet rule and may have charge separation
○ Generally, does not possess charge
○ sp2 hybridization
② Carbocation: Violates octet rule and is unstable, trigonal planar
Figure 8. Carbocation
○ Reaction creating carbocation has the carbocation formation step as the rate-determining step
○ Carbocation allows rearrangement
○ Carbocation can undergo racemization
○ Electron-donating inductive effect: More alkyl groups around carbocation (up to 3 maximum) stabilize it
○ Alkyl groups (C) have electronegativity of 2.5, while hydrogen (H) has electronegativity of 2.1, showing different tendencies than electronegativity
○ Hydrogen has only one electron, hence showing a different tendency than electronegativity
○ Stability order
○ Benzyl carbocation ≈ Allyl carbocation ≈ Tertiary carbocation > Secondary carbocation > Primary carbocation > Methyl carbocation > Vinyl carbocation
Figure 9. Electron-donating inductive effect of carbocation
○ Tertiary allylic cation ＞ Secondary allylic cation ＞ Allyl cation
○ Tertiary benzylic cation ＞ Secondary benzylic cation ＞ Benzyl cation
○ Hyperconjugation
Figure 10. Hyperconjugation
○ Definition: Weak delocalization of alkyl group’s sp3 orbital electron onto the empty p orbital of carbocation
○ sp3 bonding acts like a sigma bond, but behaves like a pi bond, causing electron delocalization.
○ Hydrogen’s s orbital cannot delocalize onto carbocation’s empty p orbital.
○ Stability assessment: More alkyl groups around carbocation lead to stabilization
○ Cause of rearrangement reaction
○ Hyperconjugation weakens nearby carbon’s C-H bond (Type 1) or C-Me bond (Type 2)
○ Type 1. 1,2-hydride shift
Figure 11. 1,2-hydride shift
○ Type 2. 1,2-methyl shift
Figure 12. 1,2-methyl shift
○ sp2 carbocation is more stable than sp carbocation: sp carbocation has higher s-character, hence nucleus’s influence is stronger.
③ Carbanion: Satisfies octet rule, but carbon with low electronegativity bears negative charge, causing instability.
Figure 13. Carbanion
5. Energy Diagram
⑴ Bond dissociation energy
① Definition: Energy required to break covalent bonds, generally positive
② Bond strength: Higher bond strength → Greater stability → Higher bond dissociation energy
③ Enthalpy change and bond dissociation energy
④ Bond dissociation energy is measured for reactions in the gaseous state, hence approximated values are provided for organic reactions
⑵ Energy diagram
① x-axis represents reaction coordinate, y-axis represents energy
② One-step reaction: Single transition state
③ Multistep reaction: Multiple transition states, intermediates exist
④ Activation energy: Energy difference between transition state and immediate reactant or intermediate
⑤ Rate-determining step of multistep reactions: Reaction rate of the step with the highest activation energy
Input: 2019.01.09 13:28