© 2016 Pearson Education, Inc. Alkenes Structure, Nomenclature, and an introduction to Reactivity Thermodynamics and Kinetics Paula Yurkanis Bruice University of California, Santa Barbara Chapter 5

© 2016 Pearson Education, Inc. Saturated and Unsaturated Hydrocarbons Saturated hydrocarbons have no double bonds. Unsaturated hydrocarbons have one or more double bonds.

© 2016 Pearson Education, Inc. Introduction sp 2 -sp 2  bond p-p  bond Alkenes contain a C=C double bond C=C double bond consists of

© 2016 Pearson Education, Inc. Introduction compared to alkanes, bond lengths decrease in alkenes compared to alkanes, bond angles increase in alkenes

© 2016 Pearson Education, Inc. Alkenes

© 2016 Pearson Education, Inc. Introduction Typical representatives are –Ethene, plant growth hormone, affects seed germination, flower maturation, and fruit ripening.

© 2016 Pearson Education, Inc. Introduction Typical representatives are –citronellol, in rose and geranium oils OH Geranium “Mavis Simpson”

© 2016 Pearson Education, Inc. Introduction Typical representatives are –limonene, in lemon and orange oils Citrus limon

© 2016 Pearson Education, Inc. Introduction Typical representatives are –  -phellandrene, in oil of eucalyptus Eucalyptus globulus

© 2016 Pearson Education, Inc. 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

© 2016 Pearson Education, Inc. 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

© 2016 Pearson Education, Inc. Molecular Formulas Alkene: C n H 2n+2- 2P-2R P = number of double bonds R = number of rings.

© 2016 Pearson Education, Inc. 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.

© 2016 Pearson Education, Inc. Nomenclature of Alkenes 1-1. Find the longest carbon chain Enumerate the carbons such that the functional group, here the double bond, gets the lowest possible number.

© 2016 Pearson Education, Inc. Stereoisomers of an Alkene are named using a cis or trans Prefix

© 2016 Pearson Education, Inc. Nomenclature of Alkenes 2. Substituents are cited before the parent longest chain, along with a number indicating its position at the chain.

© 2016 Pearson Education, Inc. 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.

© 2016 Pearson Education, Inc. Nomenclature of Dienes two double bonds = diene

© 2016 Pearson Education, Inc. 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.

© 2016 Pearson Education, Inc. 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 Numbers are needed if the ring has more than one double bond.

© 2016 Pearson Education, Inc. 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]

© 2016 Pearson Education, Inc. Nomenclature of Alkenes

© 2016 Pearson Education, Inc. Nomenclature of Cyclic Alkenes A number is not needed to denote the position of the functional group; it is always between C1 and C2.

© 2016 Pearson Education, Inc. Vinylic and Allylic Carbons vinylic carbon: the sp 2 carbon of an alkene allylic carbon: a carbon adjacent to a vinylic carbon

© 2016 Pearson Education, Inc. Reactions of Organic Compounds Organic compounds can be divided into families. All members of a family react in the same way. The family a compound belongs to depends on its functional group.

© 2016 Pearson Education, Inc. Each family can be put in one of four Groups The families in a group react in similar ways.

© 2016 Pearson Education, Inc. 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!

© 2016 Pearson Education, Inc. 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!

© 2016 Pearson Education, Inc. Nucleophiles : Organic molecules with double bonds (alkenes, alkynes) are also nucleophilic. Examples: Electrophiles vs Nucleophiles

© 2016 Pearson Education, Inc. Electrophiles

© 2016 Pearson Education, Inc. Nucleophiles A nucleophile has a negative charge a lone pair a double bond

© 2016 Pearson Education, Inc. A Nucleophile reacts with an Electrophile Your first organic reaction is a two-step reaction.

© 2016 Pearson Education, Inc. Curved Arrows The mechanism of a reaction is the step-by-step description of the process by which reactants are converted into products. first step of the reaction Curved arrows are used to show the mechanism of a reaction.

© 2016 Pearson Education, Inc. second step of the reaction The curved arrow shows where the electrons start from and where they end up. Curved Arrows

© 2016 Pearson Education, Inc. How to draw Curved Arrows

© 2016 Pearson Education, Inc. How to draw Curved Arrows

© 2016 Pearson Education, Inc. How to draw Curved Arrows

© 2016 Pearson Education, Inc. How to draw Curved Arrows

© 2016 Pearson Education, Inc. A Reaction Coordinate Diagram A reaction coordinate diagram shows the energy changes that take place in each step of a reaction.

© 2016 Pearson Education, Inc. Thermodynamics and Kinetics

© 2016 Pearson Education, Inc. The Equilibrium Constant The equilibrium constant gives the concentration of reactants and products at equilibrium.

© 2016 Pearson Education, Inc. Exergonic and Endergonic Reactions

© 2016 Pearson Education, Inc. Gibbs Free-Energy Change (∆G°) ΔG° = free energy of the products – free energy of the reactants ΔH° = heat required to break bones – heat released from forming bonds ΔS° = freedom of motion of the products – freedom of motion of the reactants

© 2016 Pearson Education, Inc. A Reduction Reaction

© 2016 Pearson Education, Inc. Catalytic Hydrogenation

© 2016 Pearson Education, Inc. Using ∆H° Values to determine the Relative Stabilities of Alkenes

© 2016 Pearson Education, Inc. Using ∆H° Values to determine the Relative Stabilities of Alkenes The relative energies (stabilities) of three alkenes that can be catalytically hydrogenated to 2-methylbutane. The most stable alkene has the smallest heat of hydrogenation. (Notice that when a reaction coordinate diagram shows ∆H° values, the y-axis is potential energy; when it shows ∆G° values, the y-axis is free energy [Figure 5.2].)

© 2016 Pearson Education, Inc. 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.

© 2016 Pearson Education, Inc. Stability The stability of alkenes depends upon number of substituents The more substituents, the more stable

© 2016 Pearson Education, Inc. Relative Stabilities of Cis and Trans Alkenes

© 2016 Pearson Education, Inc. Relative Stabilities of Dialkyl-Substituted Alkenes

© 2016 Pearson Education, Inc. Stability Steric repulsion (Steric strain) is responsible for energy differences among the disubstituted alkenes

© 2016 Pearson Education, Inc. Kinetics: How fast is the product formed? ∆G ‡ = free energy of the transition state - free energy of the reactants The greater the energy barrier, the slower the reaction.

© 2016 Pearson Education, Inc. Rate of a Chemical Reaction Increasing the concentration increases the rate. Increasing the temperature increases the rate. The rate can also be increased by a catalyst.

© 2016 Pearson Education, Inc. An Electrophilic Addition Reaction Transition states have partially formed bonds.

© 2016 Pearson Education, Inc. Reaction Coordinate Diagram for each step of the addition of HBr to 2-Butene

© 2016 Pearson Education, Inc. Reaction Coordinate Diagram for the addition of HBr to 2-Butene The rate-limiting step of the reaction is the step that has its transition state at the highest point of the reaction coordinate diagram.

© 2016 Pearson Education, Inc. Transition State

© 2016 Pearson Education, Inc. 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

© 2016 Pearson Education, Inc. A Catalyst A catalyst provides a pathway for a reaction with a lower energy barrier. A catalyst does not change the energy of the starting point (the reactants) or the energy of the end point (the products).

© 2016 Pearson Education, Inc. Enzymes Most biological reactions require a catalyst. Most biological catalysts are proteins called enzymes. The reactant of a biological reaction is called a substrate.

© 2016 Pearson Education, Inc. The Active Site of an Enzyme An enzyme binds its substrate at its active site.

© 2016 Pearson Education, Inc. Enzyme Side Chains that bind the Substrate Some enzyme side chains bind the substrate.

© 2016 Pearson Education, Inc. Some enzyme side chains are acids, bases, and nucleophiles that catalyze the reaction. Enzyme Side Chains that catalyze the Reaction