Organic Chemistry 4th Edition

Paula Yurkanis Bruice Chapter 3

Alkenes:

Structures, Nomenclature, andan Introduction to Reactivity

Thermodynamicsand Kinetics

Irene LeeCase Western Reserve University

Cleveland, OH©2004, Prentice Hall

Hydrocarbons containing double bonds

C C

double bond

the functional group

center of reactivity

Alkenes

Noncyclic alkene: CnH2n

Cyclic alkene: CnH2n–2

Molecular Formula of Alkene

CH3CH2=CH2

Systematic Nomenclature of Alkenes

•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

•No numbering of the functional group is needed in a cyclic alkene

Special Nomenclatures

Structure of Alkene

Isomers of Alkene

Dipole Moments of Alkene Isomers

Conversion of alkene isomers requires breaking ofthe π bond between the two sp2 carbons

Cis-Trans Interconversion in Vision

E and Z isomers

Naming by the E,Z SystemRule 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.

Rule 3: Multiple bonds are treated as attachment of multiple single bonds.

Rule 4: Rank the priorities by mass number in isotopes.

An alkene is an electron-rich molecule

Nucleophile: an electron-rich atom or molecule that shares electrons with electrophiles

Examples of Nucleophiles

A nucleophile

Nucleophiles are attracted to electron-deficient atoms or molecules (electrophiles)

Examples of Electrophiles

Electrophilic Addition of HBr to Alkene

Curved Arrows in Reaction Mechanisms

Movement of a pair of electrons

Movement of one electron

Utilization of Curved Arrows

Rules for Use of Curved Arrows

A Reaction Coordinate Diagram

Transition states have partially formed bonds

Intermediates have fully formed bonds

Thermodynamics describes the properties of a system at equilibrium

Thermodynamic Parameters

Gibbs standard free energy change (∆G°)

Enthalpy (∆H°): the heat given off or absorbed during a reaction

Entropy (∆S°): a measure of freedom of motion

∆G° = ∆H° – T∆S°

If ∆S° is small compared to ∆H°, ∆G° ~ ∆H°

Exergonic Reaction–∆G°

Endergonic Reaction

+∆G°

∆H° can be calculated from bond dissociation energies

Solvation: the interaction between a solvent and a molecule (or ion) in solution

Solvation can affect ∆H° and/or ∆S°, which ultimately affects ∆G°

Kinetics deals with the rate of chemical reactions and the factors that affect those rates

The rate-limiting step controls the overall rate of the reaction

Rate of a reaction =

number of collisionsper unit time

fraction withsufficient energy

fraction withproper orientationx x

The free energy between the transition state and the reactants

∆G‡: (free energy of transition state) – (free energy of reactants)

∆G‡ = ∆H‡ – T∆S‡

∆H‡:(enthalpy of transition state) – (enthalpy of reactants)

∆S‡:(entropy of transition state) – (entropy of reactants)

Rates and Rate Constants

First-order reaction

A B

rate = k[A]

Second-order reaction

A + B C + D

rate = k[A][B]

The Arrhenius Equation

k = Ae–Ea/RT

Ea = ∆H‡ + RT

Rate Constants and the Equilibrium Constant

A Bk1

k–1

Keq = k1/k–1 = [B]/[A]

Transition State versus Intermediate

Transition states have partially formed bonds

Intermediates have fully formed bonds

intermediate

intermediate

Electrophilic Addition of HBr to 2-Butene

The rate-limiting step controls the overall rate of the reaction