Chapter 4 Alkenes: Structure, Nomenclature, Stability, and an Introduction to Reactivity (Part II) Essential Organic Chemistry Paula Yurkanis Bruice

4.6 The Relative Stabilities of Alkenes Alkyl substituents that are bonded to the sp 2 carbons of an alkene have a stabilizing effect on the alkene. The more alkyl substituents bonded to the sp 2 carbons of an alkene, the greater is its stability.

Stability The stability of alkenes depends upon number of substituents The more substituents, the more stable

Stability Steric repulsion (Steric strain) is responsible for energy differences among the disubstituted alkenes

4.7 How Alkenes React ; Curved Arrows The functional group is the center of reactivity of a molecule. In essence, organic chemistry is about the interaction between electron-rich atoms or molecules and electron- deficient atoms or molecules. It is these forces of attraction that make chemical reactions happen. A very simple rule: Electron-rich atoms or molecules are attracted to electron-deficient atoms or molecules!

Electrophiles vs Nucleophiles Electrophile: electron-deficient atom or molecule that can accept a pair of electrons. Nucleophile: electron-rich atom or molecule that has a pair of electrons to share. A very simple rule restated: A Nuclophile reacts with an electrophile!

Nucleophiles : Organic molecules with double bonds (alkenes, alkynes) are also nucleophilic. Examples: Electrophiles vs Nucleophiles

Reactions  Alkenes are similar in structure and do similar reactions. All contain a double bond All contain the same functional group  Reactions are categorized through different types of mechanisms.

Reactions Typical for unsaturated systems is the addition reaction: A+B  C

Reactions A LOOK AT THE REACTANTS REACTANTS

Reactions WHAT IS THE NATURE OF THIS REAGENT?

Reactions Hydrogen bromide is a strong acid and forms hydronium ions, H 3 O +, and bromide, Br –, when dissolved in water. electrophile H 3 O + is positively charged, thus it is electron deficient it is electrophilic “electron loving”

Reactions In the presence of an electron-rich species the hydronium ion reacts: A new positively charged species is formed. electrophile

Reactions The newly formed species, a carbocation, is again electron deficient, thus it is electrophilic. electrophile

Reactions One species present that is rich in electrons is Br –. Since Br – bears a negative charge it seeks for neutralization. It is nucleophilic (nuclei are positively charged).

Reactions electrophile nucleophile The two species, electrophile and nucleophile, combine and form a new compound.

Mechanism Summarizing our reaction, we realize it is a 2-step mechanism STEP 1 STEP 2

Mechanism Step 1 reaches a carbocation “intermediate.” One new bond is formed. Step 2 completes the reaction by forming a second bond. Again, it is the interplay between positively charged (electrophilic) and negatively charged (nucleophilic) species. Intermediates are species with a very short lifetime. However, their stability (energy) often determines the outcome of a reaction.

A Few Words about Curved Arrows

4.8 Using a Reaction Coordinate Diagram (Energy Profile) to Describe a Reaction

Transition State

Transition state The chemical species that exists at the transition state, with old bonds in the process of breaking and new bonds in the process of forming: bond breaking bond forming TS 1 bond forming TS 2

Reactions Overall reaction coordinate