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24 Cards in this Set
- Front
- Back
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equilibruim constant
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Keq=[products]/[reactants]
Keq>1=reaction favored as written. Keq<1=reverse rxn favored Keq=e^(-G/RT) R=8.314 e=2.718 |
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Gibbs free energy
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G=free energy of products-reactants
G=-RT(lnKeq) G=-2.303RT(logKeq) rxn is favored if G is negative bc of net decrease in energy |
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free energy, entropy and enthalpy
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^G=^H-T^S (^means: change in)
^G=free energy of products - reactants. ^H=enthalpy of products - reactants ^S=entropy of products - reactants |
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enthalpy
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H. the heat of reaction. if weaker bonds broken and stronger bonds formed, rxn is exothermic, heat evolved H is negative.
energy consumed, rxn is endotermic, H is positive. enthalpy is usually the most important component in driving force of rxn. |
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entropy
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S.freedom of motion.
rxns favo products with the greatest entropy. products have more freedom of motion, positive S, makes negative contribution to G. |
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bond dissociation enthalpy
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the amount of enthalpy required to break a particular bond homolytically (each atom retains one of the bond's two electrons). BDE always positive(endothermic) bc energy is consumed to break bonds.
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overall enthalpy change for a rxn
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^H=(sum of BDE of all bonds broken)-(sume of BDE of all bonds formed)
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rate law
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rate=Kr[A]^a[B]^b
A+B-->C+D Kr=rate constant Kr=Ae^(Ea/RT) |
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Activation energy
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Kr=Ae^(-Ea/RT)
A=constant (frequency factor) Ea=activation energy R=8.314 rate of rxn depends on the fraction of molecules with kinetic energy of at least Ea. represents the energy difference between the reactants and transition state |
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rate limiting step
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the highest energy step determines the overall rate. this is the highest point in an energy diagram, transition state.
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halogenation
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R-H +X2 ---> R-X + H-X
a halogen atom replaces a H |
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primary, secondary, tertiary hydrogens
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primary: RCH3. secondary: R2CH2. tertiary: R3CH.
secondary H is 4.5x as reactive as each primary for chlorination. decreasing stability: tertiary, secondary, primary, methyl |
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selectivity
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Bromination is more selective than chlorination bc the major reaction is favored 97:1 rather than 4.5:1
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transition state
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in endothermic rxn, the transition state is closer to the products in energy and structure. in exothermic rxn, the transition state is closer to the reactants in energy and structure.
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Hammond postulate
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related species that are closer in energy are also closer in structure. the structure of a transition state resembles the structure of the closest stable species.
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solving free radical reaction problems
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1. draw a step that breaks the weak bond in the initator
2. draw a rxn of the initator with one of the starting materials. 3. draw a rxn of the free-radical version of the starting material with another starting-material molecule to form a bond needed in the product and generate a new radical intermediate. 4. draw termination steps. |
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reactive intermediates
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carbocation (CH3+), radical (CH3.), carbanion(CH3-), carbene (CH2:)
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carbocations
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CH3+ stability high 3>2>1>CH3+. electrophilic, strong acids
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radicals
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CH3.
stability high 3>2>1> CH3. electron deficient |
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carbanions
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:CH3-
stability high :CH3->1>2>3 nucleophilic, strong bases |
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carbenes
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:CH2
both nucleophilic and electrophilic |
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alkyl halide
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a halogen atm bonded to one of the sp3 hybrid carbon atoms of an alkyl. polar bc halogen more electronegative than C atoms, C atom somewhat electrophilic. allows to serve as intermediates.
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vinyl halide
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halogen atom bonded to one of the sp2 hybrid carbon atoms of an alkene (C=C)
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aryl halide
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halogen atom bonded to one of the sp2 hybrid carbon atoms of an aromatic ring (cyclopentane with three double bonds)
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