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

Introduction sp 2 -sp 2  bond p-p  bond Alkenes contain a C=C double bond C=C double bond consists of

Introduction compared to alkanes, bond lengths decrease in alkenes compared to alkanes, bond angles increase in alkenes

Introduction  Typical representatives are Ethene, plant growth hormone, affects seed germination, flower maturation, and fruit ripening.

Introduction  Typical representatives are citronellol, in rose and geranium oils OH Geranium “Mavis Simpson”

Introduction  Typical representatives are limonene, in lemon and orange oils Citrus limon

Introduction  Typical representatives are  -phellandrene, in oil of eucalyptus Eucalyptus globulus

4.1 Molecular Formulas  Alkane: C n H 2n+2  Alkene: C n H 2n or C n H 2n+2- 2P P = number of double bonds + 2H

Molecular Formulas  Alkane: C n H 2n+2  Ring: C n H 2n or C n H 2n+2- 2R R = number of rings + 2H

Molecular Formulas  Alkene: C n H 2n+2- 2P-2R P = number of double bonds R = number of rings.

4.2 Nomenclature of Alkenes  The functional group is the center of reactivity in a molecule.  The IUPAC system uses a suffix to denote certain functional groups.

Nomenclature of Alkenes  1-1. Find the longest carbon chain.  1-2. Enumerate the carbons such that the functional group, here the double bond, gets the lowest possible number.

Nomenclature of Alkenes  2. Substituents are cited before the parent longest chain, along with a number indicating its position at the chain.

Nomenclature of Alkenes  3. If a chain has more than one double bond, we first identify the chain by its alkane name, replacing the “ne” ending with the appropritate suffix: diene, triene, etc.  4. If a chain has more than one substitutent, substituents are cited in alphabetical order.

Nomenclature of Alkenes  5. If the same number for alkene is obtained in both directions, the correct name is the name that contains the lowest substituent number.

Nomenclature of Alkenes  6. A number is not needed to denote the position of the double bond in a cyclic alkene because the double bond is always placed between carbons 1 and 2.  7. Numbers are needed if the ring has more than one double bond.

Nomenclature of Alkenes  Remember that the name of a substituent is stated before the name of the parent hydrocarbon, and the functional group suffix is stated after that. [substitutent] [parent hydrocarbon] [fucntional group suffix]

Nomenclature of Alkenes

4.3 The Structure of Alkenes  All six atoms of the double bond system are in the same plane.

4.4 Cis-Trans Isomerism  Because rotation about a double bond does not readily occur, an alkene such as 2- butene can exist in two distinct forms.

cis/trans Isomers sp 2 -sp 2  bond p-p  bond side view p-p  bond front view The p-p  bond restricts free rotation.

cis/trans Isomers Upon rotation we lose p-p overlap, thus rotation doesn’t happen (easily). Consequently, geometrical isomers exist.

cis/trans Isomers All substituents are on one side of  bond All substituents are on different sides of  bond cis trans

cis/trans Isomers cis-2-pentene

cis/trans Isomers trans-3-heptene

4.5 The E,Z System of Nomenclature For more than two substituents the cis/trans system cannot be used. To use the E/Z system we need to assign priorities to each substituent on each carbon. A new system, the E/Z system is introduced.

E/Z System In case high priorities are on the same side, we assign a Z configuration. In case high priorities are on opposite sides, we assign an E configuration.

E/Z System- Rule 1 The relative priorities of the two groups depend on the atomic numbers of the atoms bonded directly to the sp 2 carbon. The greater the atomic number, the higher is the priority of the group.

E/Z System Priorities are first assigned based on atomic numbers. I > C 1 2 F > H 1 2 I > C 1 2 F > H 1 2 Z-configuration E-configuration

E/Z System- Rule 2 If the two substituents attached to the sp 2 carbon start with the same atom, you must move outward and consider the atomic numbers that are attached to the “tied” atoms.

E/Z System If you can’t decide using the first atoms attached, go out to the next atoms attached. If there are nonequivalent paths, always follow the path with atoms of higher atomic number. path goes to O, not H path goes to C, not H comparison stops here C H O H C H C H Z-configuration

E/Z System- Rule 3 If an atom is doubly (or triply) bonded to another atom, the priority system treats it as if it were singly bonded to two (or three) of those atoms.

E/Z System Atoms in double bonds are “replicated” at either end of the double bond. path goes to C, not H E-configuration

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 Nucleophile 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