Chapter 3 Alkenes: Structures, Nomenclature and an Introduction

Chapter 3 Alkenes: Structures, Nomenclature and an Introduction

Chapter 3 Alkenes: Structures, Nomenclature and an Introduction to Reactivity Thermodynamics and Kinetics Adapted from Profs. Turro & Breslow, Columbia University and Prof. Irene Lee, Case Western Reserve University 1

Ever put an apple into a bag with green tomatoes or a green banana? Natural Products: CH2=CH2 What might account for the difference between lemon and orange in the limonene structure?

2 Molecular Formulas of Alkenes Saturated vs. Unsaturated: Missing Hydrogens Alkanes are completely saturated i.e. only single bonds Each double bond has 1 degree of unsaturation. Each ring has 1 degree of unsaturation. Each triple bond is 2 degrees of unsaturation.

Compare a molecular formula to an alkanes: every TWO Hydrogens less = I degree of unsaturation 3 Molecular Formulas of Alkenes Saturated vs. Unsaturated: Missing Hydrogens Noncyclic alkene: CnH2n 1 degree of unsaturation

Cyclic alkene: CnH2n-2 (Same as an alkyne; 2 degrees of unsaturation) 4 Systematic Nomenclature of Alkenes Follows alkane rules; treats double bond as a function:

Think of alcohols 5 Substituents in alphabetical order with lowest numbers 6 Cyclic alkenes:

7 Important Special Terms Vinyl Hs: bonded to the double bond. Allylic Hs: on sp3 carbons next to the double bond.

8 Structure of Alkenes 9 Isomers of Alkene 10

Dipole Moments of Alkene Isomers 11 Cis-Trans Interconversion in Vision 12 QuickTime and a Video decompressor

are needed to see this picture. 13 E and Z isomers Priority is in order of atomic number. Rank H vs. Br and C vs. Cl. 14

Naming using E, Z E (entgegen:opposite): Z (zusammen; same) Consider the atomic number of the atoms bonded directly to a specific sp2 carbon 1,1 on same side = Z 1

2 1 2 1,1 on opposite sides = E 1

1 15 If the atoms are the same, eg. the carbon atoms: then consider the other atoms that are attached to them. 1 1

1 1 O vs C 16 Multiple bonds are treated as attachments

of multiple single bonds C | C- C C-C-C | C 17

Rank the priorities by mass number C C in isotopes | | C- C C- C 18

An alkene is an electron rich molecule, a nucleophile. nucleophile- likes nuclei (likes protons: H+) Nucleophiles: electron-rich atoms or molecules that react with electrophiles. electrophile- likes electrons (likes minus: eand anions) Examples of nucleophiles 19

Nucleophiles are attracted to electron-deficient atoms or molecules (electrophiles) Examples of Electrophiles 20 Electrophilic Addition of HBr to Alkene A two step reaction. Mechanistic path of a reaction: how reactants form products.

How can a mechanism be illustrated? i.e. bond making & bond breaking 21 Using Curved Arrows in Reaction Mechanisms Movement of a pair of electrons: START arrows from electrons pointing to electrophile Use 1/2 arrow for the movement of one electron

22 Using Curved Arrows 23 A Reaction Coordinate Diagram Transition states have partially formed bonds

Intermediates have fully formed bonds 24 Thermodynamic Parameters Go = Ho - TSo Gibbs standard free energy change (Go) Enthalphy (Ho): the heat given off or absorbed during a reaction Entropy (So): a measure of freedom of motion

If So is small compared to Ho, Go ~ Ho 25 Exergonic Reaction -Go Endergonic Reaction +Go 26

H o for any reaction can be calculated from bond dissociation energies

27 Kinetics deals with the rate of chemical reactions and the reaction mechanism Rate of a reaction =

number of collision per unit time fraction with fraction with X sufficient energy X proper orientation The rate-limiting step controls the overall rates of the reaction

28 The free energy of activation & the transition state and the reactants 29 G = H -TS 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) 30 Rates and Rate Constants First-order reaction

Second-order reaction A A+B rate = k [A]

B C+D rate = k [A][B] 31 The Arrhenius Equation -Ea/RT

k = Ae Ea = H +RT Rate Constants and the Equilibrium Constant A k1

B k-1 Keq = k1/k-1 = [B]/[A] 32 Transition State Versus Intermediate intermediate

intermediate Transition states have partially formed bonds Intermediates have fully formed bonds 33 Electrophilic Addition of HBr to 2-Butene

G0 The rate-limiting step controls the overall rates of the 34 Reaction. It has the highest activation energy. QuickTime and a Cinepak decompressor are needed to see this picture.

35 36

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