Chapter 4 -- Introduction to Alkenes 4.1 – Structure and Bonding in Alkenes 4.3 – Unsaturation Number 4.2 – Stereocenters and Alkene Nomenclature 4.4 – Physical Properties of Alkenes (low emphasis) 4.5 – Relative Stabilities of Alkene Isomers 4.6 – Alkene Addition Reactions (more in later chapters) 4.7 – Adding Hydrogen Halides to Alkenes: Carbocations 4.8 – Reaction Rates 4.9 – Catalysis

Alkenes Alkenes are hydrocarbons that contain one or more C=C double bonds. They are classified as unsaturated hydrocarbons (alkanes are classified as saturated). Also known as olefins. Ethylene (ethene) is the simplest alkene. 4.1 Structure and Bonding in Alkenes

Actual Ethene (Ethylene) geometry Geometry is typical of other alkenes. Ethylene is a planar molecule (trigonal planar geometry). (perfect sp2 would 120o all around) 4.1 Structure and Bonding in Alkenes

Bond Length Bonds with more s character are shorter and stronger (harder to break). 4.1 Structure and Bonding in Alkenes

Carbon Hybridization in Alkenes Alkene carbon orbitals are hybridized differently than alkane carbon orbitals. Alkene carbon – sp2 hybridized. 4.1 Structure and Bonding in Alkenes

Orbital Picture of Ethylene – the sp2 bonds Trigonal planar geometry → sp2-hybridization ~33% s character, ~66% p character More s character → electrons are held closer to the nucleus. 4.1 Structure and Bonding in Alkenes

The bonds of ethene ( s versus p ) s bond: head-to-head overlap of orbitals p bond: side-to-side overlap of p orbitals 4.1 Structure and Bonding in Alkenes

Pi Orbital Interaction Diagram for Ethylene 4.1 Structure and Bonding in Alkenes

Electron Distribution in Ethylene There is a local negative charge associated above and below the C=C double bond nodal plane. Most of the important reactions of alkenes involve the electrons in the filled  bonding orbital acting as an electron donor (nucleophile). 4.1 Structure and Bonding in Alkenes

Unsaturation Number Gives info on number of rings and/or p bonds. Maximum # of H’s in a hydrocarbon: CnH2n+2 Each ring and/or p bond reduces number of hydrogens by 2. aka degree of unsaturation 4.3 Unsaturation Number

Unsaturation Number: these eqns do work Oxygen can be ignored (CH4 CH3OH ) Halogens are counted as 1 H: (CH4 CH3X) Nitrogen increases H count by 1: ( CH3NH2 ) C = # of carbons, N = # of nitrogens, H and X similarly the number H’s and halogens in the formula. 4.3 Unsaturation Number

Rotation is restricted in alkenes 4.1 Structure and Bonding in Alkenes

Double-Bonds can lead to Stereoisomers Stereoisomers: Molecules with the same atom connectivity, but different spatial arrangement. Stereoisomers of 2-butene: Each stereoisomer of 2-butene has its own characteristic properties (i.e. different boiling points, different polarities, etc.) and they do not interconvert without a “catalyst”. 4.1 Structure and Bonding in Alkenes

A new concept: Stereocenters When an alkene can exist as double-bond stereoisomers, both carbons of the double bond are stereocenters. Stereocenter – an atom at which the interchange of two bonded groups gives a stereoisomer. 4.1 Structure and Bonding in Alkenes

IUPAC Nomenclature for Alkenes Alkene nomenclature is derived by modifying alkane nomenclature. Find principle chain with greatest number of double bonds. The alkene carbons get lower numbers. 4.2 Nomenclature of Alkenes

More Than One Pi Bond If the principal chain has two double bonds, change the –ane ending to –adiene. Three double bonds –atriene and so on. 4.2 Nomenclature of Alkenes

Substituents Containing Alkenes Some common groups that contain double bonds: 4.2 Nomenclature of Alkenes

Substituents Containing Alkenes Substituent groups are numbered from the point of attachment to the principal chain or ring. 4.2 Nomenclature of Alkenes

Traditional (common) names Some alkenes have nonsystematic traditional names that are recognized by IUPAC. For now you don’t need to know styrene, but you are expected to know isoprene 4.2 Nomenclature of Alkenes

The E, Z Nomenclature System cis vs trans is not always clear Cahn-Ingold-Prelog system: Assign relative priorities to each of the two groups attached to each alkene carbon. Z (zusammen): “together” – Higher priority groups on the same side. E (entgegen): “across” – Higher Priority groups on opposite sides. 4.2 Nomenclature of Alkenes

Priority Sequence Rules Sequence Rules are used to assign relative priorities. Examine atoms directly attached to each carbon of the double bond (level 1). Higher atomic # = higher priority Higher isotopic mass = higher priority 4.2 Nomenclature of Alkenes

Sequence Rules If level 1 atoms are the same, then work outwards until you find the first point of difference. Arrange atoms in each set by decreasing priority order. Higher atomic # (or mass isotope) at the first point of difference = higher priority 4.2 Nomenclature of Alkenes

Sequence Rules Dealing with double and triple bonds: Double bonds – rewritten as single bonds and the atoms at each end of the double bond are duplicated. Triple bonds – rewrite and triplicate 4.2 Nomenclature of Alkenes

DO SOME then skip through the next few

Sequence Rules If level 2 sets in the two group are identical, then, within each set, choose the atom of highest priority. Identify the level 3 set of atoms attached to it. Compare these level 3 sets. 4.2 Nomenclature of Alkenes

Sequence Rules 3a. If no decision at step 3, choose the atoms of next highest priority in the level 2 set and repeat the process in step 3. 4.2 Nomenclature of Alkenes

Sequence Rules 3b. If two or more atoms in any set are the same, decide on their relative priorities by continuing outward from each. 3c. If no decision is possible, move away from the double bond within each group to atoms at the next level and repeat step 3. 4.2 Nomenclature of Alkenes

Physical Properties (won’t be tested) Very similar to corresponding alkanes Dipole moments of some alkenes, though small, are greater than those of the corresponding alkanes. 4.4 Physical Properties of Alkenes

Relative Stabilities of Alkene Isomers More alkyl substituents → more stable alkene trans places larger groups farther apart. Could be obtained from heats of formation (DHf ˚), as in the text, but can also use heats of combustion or (just for alkenes) heat of hydrogenation. 4.5 Relative Stabilities of Alkene Isomers

Relative Stabilities more stable = lower potential energy Standard enthalpy change (DH˚) for a reaction is given by I’ll derive relative stabilities of alkene isomers from this relationship, a different manner than used in the text. Exothermic reactions: DH˚ < 0. If two reactants produce the same product, the difference in heat released reflects the relative Ho’s of the reactants. Endothermic reactions: DH˚ > 0 – not spontaneous. 4.5 Relative Stabilities of Alkene Isomers

The general method is illustrated by isomeric alkane combustion

Heats of combustion, some examples – n-C8H18 + 12.5 O2 → 8CO2 + 9 H2O + 1307.5 kcal/mol n-C9H20 + 12.5 O2 → 9CO2 + 10 H2O + 1463.9 kcal/mol D = 156.4 kcal/CH2 Cyclo-alkanes are nothing but (CH2)n, how do they compare ?

What makes a reaction go? Producing more stable materials more stable = lower energy Enthalpy changes (DH˚) for a reaction are one way to track this Favorable reactions are Exothermic: DH˚ < 0 and A mechanism with an achievable transition state barrier must exist ! In the case of hydrogenation a catalyst is required 4.5 Relative Stabilities of Alkene Isomers

Catalytic hydrogenation (adding H2 to an alkene) is exothermic Catalytic hydrogenation (adding H2 to an alkene) is exothermic. If two alkenes afford the same alkane, differences in the heat of reaction are due to differences in the potential energies of the alkenes. Shown diagram here

RE: Stabilities of Alkene Isomers More alkyl substituents → more stable alkene trans places larger groups farther apart. 4.5 Relative Stabilities of Alkene Isomers

Addition Reactions A characteristic reaction of alkenes The p bond is electron rich = nucleophilic HF, HCl, HBr, HI add to alkenes → alkyl halides 4.6 Addition Reactions of Alkenes

Regioselectivity of Addition Unsymmetrical alkenes can give regioisomers. Generally, the halogen goes to the most substituted carbon and H goes to the carbon with the most H. 4.7 Addition of Hydrogen Halides to Alkenes

Carbocation Intermediates Addition occurs in two successive steps: 1. e- pair in alkene is donated to the proton in H-X. The resulting species is called a carbocation (carbenium ion). Carbocations are reactive intermediates 4.7 Addition of Hydrogen Halides to Alkenes

Carbocation Intermediates Addition occurs in two successive steps: 2. The halide ion (X-) is a nucleophile and donates a lone pair of electrons to the electron deficient carbocation forming a new bond. The complete description of a reaction pathway, including any reactive intermediates, is called the mechanism of the reaction. 4.7 Addition of Hydrogen Halides to Alkenes

Back to the Regioselectivity The reaction mechanism can be used to explain regioselectivity. The tert-butyl cation is the only one that is formed, thus it leads to t-butylbromide as the only reaction product. 4.7 Addition of Hydrogen Halides to Alkenes

Structure and Stability of Carbenium Ions Carbocations are trigonal planar → sp2-hybridized. Electron deficient = Lewis acidic Stabilized by alkyl groups Stability: primary < secondary < tertiary 4.7 Addition of Hydrogen Halides to Alkenes

Hyperconjugation Hyperconjugation is an additional factor in stabilization for a carbocation (carbenium ion). The overlap of bonding electrons from the adjacent s- bond with the unoccupied p-orbital of the carbocation. 4.7 Addition of Hydrogen Halides to Alkenes

Hyperconjugation-2 (low emphasis) Hyperconjugation is sometimes shown with “no- bond” resonance structures. I am not a fan of such depictions. 4.7 Addition of Hydrogen Halides to Alkenes

Carbocation Rearrangements Can be identified by a change in carbon skeleton Rearrangement is favored if a more stable carbocation can form 4.7 Addition of Hydrogen Halides to Alkenes

Another example

How a shift actually happens

Carbocation Rearrangements Carbocation rearrangements take place in order to form a more stable carbenium ion. 4.7 Addition of Hydrogen Halides to Alkenes

Carbocation Rearrangements Carbocation rearrangements are not limited to alkyl shifts. Hydride shift - a hydrogen atom along with its two bonding electrons migrates. 4.7 Addition of Hydrogen Halides to Alkenes

RE: What makes a reaction go? Producing more stable materials more stable = lower energy Enthalpy changes (DH˚) for a reaction are one way to track this Favorable reactions are Exothermic: DH˚ < 0 Free energies (DG˚) are a better measure. and A mechanism with an achievable transition state barrier (DGǂ) must exist ! 4.5 Relative Stabilities of Alkene Isomers

Relative Reaction Rates The reactions below have the same DG˚ between reactants and products but occur at very different rates. The same considerations apply when a single reaction can yield alternative products: two reactions are in competition. 4.8 Reaction Rates

Reaction Rates Two factors govern intrinsic reaction rates and the outcome when competing reactions are possible. The sizes of the energy barriers. The temperature: reactions are faster at higher temperatures. As a result many reaction are more selective at lower temperatures. 4.8 Reaction Rates

Molecular Energy Distributions Molecules must possess enough energy to get to the transition state. Maxwell-Boltzmann distribution – characterizes the energy of a collection of molecules. 4.8 Reaction Rates

What is the “transition state”? The transition state is the structural and dynamic features present at the energy barrier to the interconversion between reactants and products. The forward and reverse of any single step of a reaction have same transition state (‡). But in the case of HX addition to an alkene, this free energy course diagram doesn’t apply. The “product” of going over the energy barrier is not the product that isolated; rather, it’s carbocation intermediate that lies at higher energy than the reactants. 4.8 Reaction Rates

What is the “transition state”? The transition state represents the structure present at the highest energy point along a free energy course diagram. For it would be something like 4.8 Reaction Rates

Hydration of Alkenes Hydration = the addition of H2O to a molecule. Alkene hydration is acid-catalyzed (shown as 1M HONO2 in this case, really H3O+). H-OH is added across the alkene. 4.9 Catalysis

End of coverage for MQ1

Multistep Reactions Multistep reactions take place with the formation of reactive intermediates. Rate-determining step: The slowest step in a chemical reaction; controls the reaction rate. 4.8 Reaction Rates

Hammond’s Postulate The structure and energy of the transition state can be approximated by the structure and energy of a high energy intermediate. 4.8 Reaction Rates

Hammond’s Postulate Two possible modes of HBr addition to 2- methylpropene: 4.8 Reaction Rates

RE: What makes a reaction go? Producing more stable materials more stable = lower energy Enthalpy changes (DH˚) for a reaction are one way to track this Favorable reactions are Exothermic: DH˚ < 0 Free energies (DG˚) are a better measure. and A mechanism with an achievable transition state barrier (DGǂ) must exist ! In some cases “catalysts” are required. 4.5 Relative Stabilities of Alkene Isomers

Catalysis Catalyst: A substance that increases the reaction rate without being consumed. Catalyzed vs un-catalyzed reaction: 4.9 Catalysis

Catalysis A catalyst increases the reaction rate. This means that it lowers the standard free energy of activation for a reaction. A catalyst is not consumed. It may be consumed in one step of a catalyzed reaction, but if so, it is regenerated in a subsequent step. A catalyst does not affect the energies of reactants and products. A catalyst accelerates both the forward and reverse reactions by the same factor. 4.9 Catalysis

Hydration of Alkenes Hydration = the addition of H2O to a molecule. Alkene hydration is Acid-catalyzed. (Homogeneous) H-OH is added across the alkene. 4.9 Catalysis

Hydration of Alkenes Mechanism Protonation of the double bond to give the more stable carbocation. H3O+ is acting as the acid catalyst. 4.9 Catalysis

Hydration of Alkenes Mechanism Water, the nucleophile, combines with the carbocation in a Lewis Acid-Base association reaction. 4.9 Catalysis

Hydration of Alkenes Mechanism A proton is lost to solvent in another acid-base reaction to give the alcohol. The acid catalyst (H3O+) is regenerated. 4.9 Catalysis

Acid catalyzed interconversion of alkene isomers cis- and trans-2-butene can be equilibrated by adding a few drop of a strong acid. 1-butene can be converted to the equilibrium mixture of cis- and trans-2-butene by adding a catalytic amount of a proton donor.

Catalysis Heterogeneous Catalyst: the catalyst and the reactants exist in separate phases. Homogeneous Catalyst: a catalyst that is soluble in the reaction mixture. 4.9 Catalysis

Catalytic Hydrogenation of Alkenes Metal catalyst allows for addition of H2. Heterogeneous Benzene  bonds are less reactive. 4.9 Catalysis

Principle of Microscopic Reversibility If a reaction occurs by a certain mechanism, the reverse reaction under the same conditions occurs by the same exact reverse of that mechanism. Dehydration is the reverse of hydration. 4.9 Catalysis

Enzymes Catalysis Biological catalysts are called enzymes. Most biological mechanisms would be too slow to be useful without enzymes. 4.9 Catalysis