Organic Chemistry
4 th Edition
Paula Yurkanis Bruice Chapter 3
Alkenes:
Structures, Nomenclature, and an Introduction to Reactivity
Thermodynamics and Kinetics
Irene Lee
Case Western Reserve University
Cleveland, OH
©2004, Prentice Hall

Alkenes
Hydrocarbons containing double bonds
C C double bond the functional group center of reactivity

Molecular Formula of Alkene
Noncyclic alkene:
C n
H
2n
CH
3
CH
2
=CH
2
Cyclic alkene:
C n
H
2n –2

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 of the p bond between the two sp 2 carbons

Cis-Trans Interconversion in Vision

E and Z isomers

Naming by the E,Z System
Rule 1: Consider the atomic number of the atoms bonded directly to a specific sp 2 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
A nucleophile
Nucleophile: an electron-rich atom or molecule that shares electrons with electrophiles
Examples of Nucleophiles

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
D
G
°
=
D
H
° – T
D
S
°
Gibbs standard free energy change (
D
G
°
)
Enthalpy (
D
H
°
): the heat given off or absorbed during a reaction
Entropy (
D
S
°
): a measure of freedom of motion
If
D
S
° is small compared to
D
H
°
,
D
G
° ~ D
H
°

Exergonic Reaction
– D
G
°
Endergonic Reaction
+
D
G
°

D
H
° can be calculated from bond dissociation energies

Solvation: the interaction between a solvent and a molecule (or ion) in solution
Solvation can affect
D
H
° and/or
D
S
°
, which ultimately affects
D
G
°

Kinetics deals with the rate of chemical reactions and the factors that affect those rates
Rate of a reaction = number of collisions per unit time x fraction with sufficient energy x fraction with proper orientation
The rate-limiting step controls the overall rate of the reaction

The free energy between the transition state and the reactants

D
G
‡
=
D
H
‡ – T
D
S
‡
D
G
‡
:
(free energy of transition state) – (free energy of reactants)
D
H
‡
:
(enthalpy of transition state) – (enthalpy of reactants)
D
S
‡
:
(entropy of transition state) – (entropy of reactants)

Rates and Rate Constants
First-order reaction
A rate = k[A]
B
Second-order reaction
A + B C + D rate = k[A][B]

The Arrhenius Equation
k = Ae
–E a
/RT
E a
=
D
H
‡
+ RT
Rate Constants and the Equilibrium
Constant k
1
A B k
–1
K eq
= k
1
/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
The rate-limiting step controls the overall rate of the reaction