Alkenes & Thermodynamics/Kinetics Alkenes Structure of Alkenes
Alkenes & Thermodynamics/Kinetics • Structures & formulas of alkenes • Nomenclature of alkenes • Isomerism of alkenes • Reaction mechanisms of alkenes (introduction to “curved arrows”)
• Energetics of reactions: reaction coordinate diagrams • ∆G˚, ∆H˚ and ∆S˚ • Keq and ∆G˚ • Estimating ∆H˚ using bond dissociation energies • Rates and rate constants • Reaction mechanisms and kinetics
Alkenes “alk” = C, H “ene” = double bond
C
C
double bond the functional group center of reactivity
Structure of Alkenes
1
Molecular Formulas of Alkenes Noncyclic alkene: CnH2n
Cyclic alkene: CnH2n–2
CH3CH2=CH2
Systematic Nomenclature of Alkenes •Parent hydrocarbon = longest continuous chain containing the functional group
•Cite the substituents in alphabetical order
•Name with the lowest functional group number and then the lowest substituent numbers
2
•No numbering of the functional group is needed in a cyclic alkene (alkene is always between C1-C2) •Use the one lowest substituent number, not lowest sum
Special Nomenclatures
Cis/trans isomers of alkenes
3
Conversion of alkene isomers requires breaking of the π bond between the two sp2 carbons
Dipole Moments of Alkene Isomers
Slide 22
4
When cis & trans don’t make sense, we use E & Z
Naming by the E,Z System Rule 1: Consider the atomic number of the atoms bonded directly to a specific sp2 carbon.
Rule 2: If there is a tie, consider the atoms attached to the tie.
5
Rule 3: Multiple bonds are treated as though the atom is bonded to more than one of the atoms to which it is multiply bonded.
Rule 4: Rank the priorities by mass number in isotopes.
Reactions in organic chemistry • Fundamental concept: positive attracted to negative • Electron-rich molecules (or atoms) are attracted to electron-poor molecules (or atoms) • Functional groups are centers of reactivity
6
Electrophile: electron-deficient molecule or atom Examples of electrophiles:
Nucleophile: electron-rich molecule or atom Nucleophiles react with electrophiles Examples of nucleophiles:
Reactions of alkenes An alkene is electron-rich, thus is a nucleophile and will react with electrophiles
Back to slide 12
7
Mechanisms of alkene reactions Example: Electrophilic Addition of HBr to Alkene
Identify the electrophile and the nucleophile in this reaction
Curved Arrows in Reaction Mechanisms Movement of a pair of electrons (more common)
Movement of one electron
Utilization of Curved Arrows
8
Rules for Use of Curved Arrows Curved arrows start at the electron source (lone pair or bond) and indicate the flow of electrons
Thermodynamics & Kinetics
Transition states have partially formed bonds Intermediates (none in this diagram) have fully formed bonds
9
Thermodynamics describes the properties of a system at equilibrium
Thermodynamic Parameters ∆G° = ∆H° – T∆S° Gibbs standard free energy change ∆G°= free energy of products – free energy of reactants
Enthalpy (∆H°): the heat given off or absorbed during a reaction Entropy (∆S°): a measure of freedom of motion If ∆S° is small compared to ∆H°, ∆G° ~ ∆H°
∆G˚= -RTlnKeq
Exergonic Reaction ∆G° < 0
Endergonic Reaction ∆G° > 0
10
∆H° can be calculated from bond dissociation energies (see table 3.1, p. 129)
Solvation: the interaction between a solvent and a molecule (or ion) in solution
Solvation can affect ∆H° and/or ∆S°, which ultimately affects ∆G°
∆G‡ = ∆H‡ – T∆S‡ ∆G‡: (free energy of transition state) – (free energy of reactants) ∆H‡: (enthalpy of transition state) – (enthalpy of reactants) ∆S‡: (entropy of transition state) – (entropy of reactants)
11
Kinetics deals with the rate of chemical reactions and the factors that affect those rates Rate of a reaction = number of collisions x per unit time
fraction with x sufficient energy
fraction with proper orientation
The rate-limiting step controls the overall rate of the reaction
The free energy difference between the transition state and the reactants determines the rate
Rates and Rate Constants First-order reaction
Second-order reaction
A
A+B
rate = k[A]
B
C+D
rate = k[A][B]
12
The Arrhenius Equation –Ea/RT
k = Ae
Ea = ∆H‡ + RT
Rate Constants and the Equilibrium Constant A
k1 k–1
B
Keq = k1/k–1 = [B]/[A]
Transition State versus Intermediate intermediate
intermediate
Transition states have partially formed bonds Intermediates have fully formed bonds
Electrophilic Addition of HBr to 2-Butene
Which is the rate-limiting step?
13
• Energetics of reactions: reaction coordinate diagrams • ∆G˚, ∆H˚ and ∆S˚ • Keq and ∆G˚ • Estimating ∆H˚ using bond dissociation energies • Rates and rate constants • Reaction mechanisms and kinetics
Alkenes “alk” = C, H “ene” = double bond
C
C
double bond the functional group center of reactivity
Structure of Alkenes
1
Molecular Formulas of Alkenes Noncyclic alkene: CnH2n
Cyclic alkene: CnH2n–2
CH3CH2=CH2
Systematic Nomenclature of Alkenes •Parent hydrocarbon = longest continuous chain containing the functional group
•Cite the substituents in alphabetical order
•Name with the lowest functional group number and then the lowest substituent numbers
2
•No numbering of the functional group is needed in a cyclic alkene (alkene is always between C1-C2) •Use the one lowest substituent number, not lowest sum
Special Nomenclatures
Cis/trans isomers of alkenes
3
Conversion of alkene isomers requires breaking of the π bond between the two sp2 carbons
Dipole Moments of Alkene Isomers
Slide 22
4
When cis & trans don’t make sense, we use E & Z
Naming by the E,Z System Rule 1: Consider the atomic number of the atoms bonded directly to a specific sp2 carbon.
Rule 2: If there is a tie, consider the atoms attached to the tie.
5
Rule 3: Multiple bonds are treated as though the atom is bonded to more than one of the atoms to which it is multiply bonded.
Rule 4: Rank the priorities by mass number in isotopes.
Reactions in organic chemistry • Fundamental concept: positive attracted to negative • Electron-rich molecules (or atoms) are attracted to electron-poor molecules (or atoms) • Functional groups are centers of reactivity
6
Electrophile: electron-deficient molecule or atom Examples of electrophiles:
Nucleophile: electron-rich molecule or atom Nucleophiles react with electrophiles Examples of nucleophiles:
Reactions of alkenes An alkene is electron-rich, thus is a nucleophile and will react with electrophiles
Back to slide 12
7
Mechanisms of alkene reactions Example: Electrophilic Addition of HBr to Alkene
Identify the electrophile and the nucleophile in this reaction
Curved Arrows in Reaction Mechanisms Movement of a pair of electrons (more common)
Movement of one electron
Utilization of Curved Arrows
8
Rules for Use of Curved Arrows Curved arrows start at the electron source (lone pair or bond) and indicate the flow of electrons
Thermodynamics & Kinetics
Transition states have partially formed bonds Intermediates (none in this diagram) have fully formed bonds
9
Thermodynamics describes the properties of a system at equilibrium
Thermodynamic Parameters ∆G° = ∆H° – T∆S° Gibbs standard free energy change ∆G°= free energy of products – free energy of reactants
Enthalpy (∆H°): the heat given off or absorbed during a reaction Entropy (∆S°): a measure of freedom of motion If ∆S° is small compared to ∆H°, ∆G° ~ ∆H°
∆G˚= -RTlnKeq
Exergonic Reaction ∆G° < 0
Endergonic Reaction ∆G° > 0
10
∆H° can be calculated from bond dissociation energies (see table 3.1, p. 129)
Solvation: the interaction between a solvent and a molecule (or ion) in solution
Solvation can affect ∆H° and/or ∆S°, which ultimately affects ∆G°
∆G‡ = ∆H‡ – T∆S‡ ∆G‡: (free energy of transition state) – (free energy of reactants) ∆H‡: (enthalpy of transition state) – (enthalpy of reactants) ∆S‡: (entropy of transition state) – (entropy of reactants)
11
Kinetics deals with the rate of chemical reactions and the factors that affect those rates Rate of a reaction = number of collisions x per unit time
fraction with x sufficient energy
fraction with proper orientation
The rate-limiting step controls the overall rate of the reaction
The free energy difference between the transition state and the reactants determines the rate
Rates and Rate Constants First-order reaction
Second-order reaction
A
A+B
rate = k[A]
B
C+D
rate = k[A][B]
12
The Arrhenius Equation –Ea/RT
k = Ae
Ea = ∆H‡ + RT
Rate Constants and the Equilibrium Constant A
k1 k–1
B
Keq = k1/k–1 = [B]/[A]
Transition State versus Intermediate intermediate
intermediate
Transition states have partially formed bonds Intermediates have fully formed bonds
Electrophilic Addition of HBr to 2-Butene
Which is the rate-limiting step?
13
