Unsaturation. Degree of Unsaturation (Index of Hydrogen Deficiency) Copyright 2004 by Carl H. Snyder, University of Miami. All rights reserved.

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Chemistry 201 C Alkenes: Structure and Reactivity This presentation was created by Professor Carl H. Snyder Chemistry Department University of Miami Coral Gables, FL 33124 CSnyder@miami.

Unsaturation Degree of Unsaturation (Index of Hydrogen Deficiency) unsaturated saturated An alkane -- C n H 2n+2 -- is fully saturated with hydrogens.

1 Chemistry 201 C Alkenes: Structure and Reactivity This presentation was created by Professor Carl H. Snyder Chemistry Department University of Miami Coral Gables, FL CSnyder@miami.edu Copyright 2004 by Carl H. Snyder, University of Miami. All rights reserved. Unsaturation Degree of Unsaturation (Index of Hydrogen Deficiency) unsaturated saturated An alkane -- C n H 2n+2 -- is fully saturated with hydrogens. A molecule is unsaturated when it contains fewer hydrogens (and/or halogens and oxygens) than an alkane of corresponding carbon content. The degree of unsaturation or index of hydrogen deficiency equals the number of H 2 molecules it would take to saturate the molecule. What is the index of hydrogen deficiency of each? IUPAC Rules IUPAC Rules 1

IUPAC Rules Cycloalkenes Special Names for Special Groups Common Names of Some Alkenes (note error in title of table) The term methylene also applies to the -CH 2 - group in

sp 2 Hybridization Combination of Two sp 2 Carbons to Form a π-bond Two of the three p orbitals combine with the single s orbital to form three sp 2 orbitals, leaving one p

2 IUPAC Rules Cycloalkenes Special Names for Special Groups Common Names of Some Alkenes (note error in title of table) The term methylene also applies to the -CH 2 - group in alkanes, cycloalkanes, cycloalkenes, and other organic molecules. sp 2 Hybridization Combination of Two sp 2 Carbons to Form a π-bond Two of the three p orbitals combine with the single s orbital to form three sp 2 orbitals, leaving one p oribtal unhybridized. The three sp 2 orbitals point to the apexes of an equilateral triangle. The unhybridized p orbital is perpendicular to the plane of the triangle. 2 sp 2 orbitals, one from each carbon, overlap to form a σ-bond. 2 p orbitals, one from each carbon, overlap to form a π (pi) bond. 2

3 Ethene or Ethylene No Rotation About The π-bond Rotation about the double-bond requires breaking then reforming the π-bond. Breaking the π- bond requires about 268 kj/mol The rotational barrier in ethane is only 12 kj/mole. 2-Butene exists as two geometric isomers: cis-2-butene, with both methyl groups on the same side of the double bond, and trans-2-butene, with the methyl groups on opposite sides of the double bond. Geometric Isomerism in 2-Butene Identical and Nonidentical Alkenes The top two structures are mutually identical. The bottom two structures differ from one another. If either sp 2 carbon bears two identical substituents, geometric isomerism is not possible. Identify These Geometric Isomers Chemistry Reaches A Higher Plane Are these two geometric isomers? Which is cis, which is trans? Are these two geometric isomers? Which is cis, which is trans? 3

4 The Easy E,Z-System A general system of geometric designation Substituents on each sp 2 carbon are ranked by atomic number. Z-isomer: Substituents of similar rank are on the same side of the C=C double bond (zusammen). E-isomer: Substituents of similar rank are on opposite sides of the C=C double bond (entgegen). Cahn-Ingold-Prelog Sequence Rules Cahn-Ingold-Prelog Sequence Rules Cahn-Ingold-Prelog Sequence Rules Is This An E Or A Z Isomer? Chemistry Reaches A Higher Plane Are these two geometric isomers? Which is E, which is Z? 4

5 Stability of Alkenes Thermodynamic stability - Refers to the energy content of an alkene. If alkene A is thermodynamically more stable than alkene B, then alkene A is lower in energy than alkene B Kinetic stability - Refers to the chemical reactivity of an alkene If alkene A is kinetically more stable than alkene B, then alkene A is less likely to react (or less likely to react rapidly) than alkene B. Thermodynamic Stabilities of Alkenes Thermodynamic stabilities of alkenes can be determined by: 1) equilibrium constants 2) heats of combustion 3) heats of hydrogenation Equilibria trans-2-butene is favored over cis-2-butene in acid catalyzed equilibrium. The trans isomer is thermodynamically more stable, by a calculated value of 2.8 kj/mol. Combustion Heat released on combustion of both cisand trans-2-butene show that the trans isomer is more stable by 3.3 kj/mol, in close experimental agreement with the value of 2.8 kj/mol found through acidcatalyzed equilibrium studies. Catalytic Hydrogenation of An Alkene Addition of H 2 to an alkene produces an alkane. Since both isomers of 2-butene produce the same alkane, butane,... any difference in heats evolved reflects difference in the energy-contents of the two geometric isomers. Heats of Hydrogenation cis-2-butene loses 120 kj/mol, while... trans-2-butene loses only 116 kj/mol. trans-2-butene must contain 4 kj/mol less energy than does cis-2-butene. 5

6 Energy Difference: 4 kj/mole Origin of The Energy Difference trans-2-butene is more stable than cis-2-butene by 4 kj/mol. The energy difference.originates in the CH 3 / CH 3 crowding present in the cis isomer but not in the trans. Heats of Hydrogenation A Generalization In general, alkyl group subsitution on the carbons of a C=C double-bond, stabilizes the alkene (lowers its energy). Electrophilic Additions of HX To C=C A Reaction Mechanism The C=C bond of alkenes and cycloalkenes readily undergoes electrophilic addition. 6

Nucleophilic π-electrons of C=C Pi-electrons of the typical C=C double bond are more subject to attack by an electrophile -- are themselves more nucleophilic than.

7 Electrophile and Nucleophile The reagent that seeks electrons is an electrophile. An electron-rich center that is subject to attack by an electrophile (and that seeks a center of positive charge) is a nucleophile. Nucleophilic π-electrons of C=C Pi-electrons of the typical C=C double bond are more subject to attack by an electrophile -- are themselves more nucleophilic than... Sigma-electrons of a typical C-C single bond. The 2-Step Mechanism The 2-Step Mechanism The two-step mechanism of electrophilic addition of HBr to isobutylene A carbocation forms as an intermediate. The two-step mechanism of electrophilic addition of HBr to isobutylene A carbocation forms as an intermediate. a carbocation sp 2 Carbon and Carbocation Step 1 p electron sp 2 Hybridized carbon Carbocation, also sp 2 hybridized Ionization of HBr to H + and Br - Conversion of C-C π electrons to C-H σ electrons Formation of a 3 o carbocation 7

8 Step 2 Reaction of the Br - anion (a nucleophile) with the carbocation (an electrophile) to form tertbutyl bromide. A Review of The Two Steps The 2-step mechanism of the electrophilic additon of HX to a C=C double bond... as applied to HBr and isobutylene. A Reaction Coordinate Diagram Energy vs. reaction coordinate Think of the reaction coordinate (x-axis) as a strip of movie film, with each frame showing the atomic and molecular structures as they exist at each given moment. Transition State transtion states The transition state represents the highest-energy structure involved in each step of a reaction. It is unstable and cannot be isolated. Carbocation Formation In the first step, isobutylene and HBr react to form the t-butyl carbocation. The transition state for the first step resembles a structure intermediate between isobutylene and the carbocation. Transition State For Carbocation Formation δ+ represents a partial positive charge. The single partial charge of the H + is now distributed between the H and the C. Dashed or dotted lines represent covalent bonds being formed and being broken. 8

9 Product Formation In the second step, the carbocation reacts with a bromide ion to form t-butyl bromide. The transition state for the second step is intermeidate between the carbocation and the product. Transition State For Product Formation The Br s negative charge and the carbocation s positive charge are in the process of being neutralized. The dashed or dotted line represents a bond being formed. Orbital View of The Reaction sp 3 /sp 2 The C receiving the H + is sp 2 in the alkene, intermediate between sp 2 and sp 3 in the transition state, and sp 3 in the carbocation intermediate. activation energy Activation Energy The activation energy of each step represents the energy required to convert the reactant(s) to the transition state. There is an inverse relationship between the size of the activation energy and the rate of the reaction. Electrophilic Addition to Alkenes Can Be Regiospecific Markovnikov s Rule Regiospecific: Addition of XY to a C=C double bond occurs with only one of two possible orientations. Regioselective: Addition of XY to a C=C double bond occurs predominantly but not exclusively with one orientation. 9

10 Markovnikov s Rule Does Not Operate When... The Basis of Markovnikov s Rule The Basis of Markovnikov s Rule: Stability of The Intermediate Carbocation Q: Why Should The Energy of The Intermediate Affect The Rate of The Reaction? A: Hammond s Postulate The basis of Markovnikov s Rule lies in the stability of the intermediate carbocation. Stability of carbocations: 3 o > 2 o > 1 o > CH 3 The more stable the carbocation, the lower the activation energy for its formation and the faster it is formed. Hammond s Postulate In an endergonic reaction -- one in which energy flows into the system -- the transition state looks like the product(s). In an exergonic reaction -- one in which energy flows out of the system -- the transition state looks like the reactant(s) Hammond s Postulate in Action Formation of the intermediate carbocation is endergonic The T.S. resembles the carbocation 3 o carbcation is more stable than 1 o carbocation. T.S. for formation of 3 o is more stable and forms faster. 10

An Unexpected Product Carbocation Rearrangement: Hydride Shift Formation of 2-chloro-3-methylbutane is the expected product of electrophilic addition of HCl to 3-methyl-1- butene.

11 An Unexpected Product Carbocation Rearrangement: Hydride Shift Formation of 2-chloro-3-methylbutane is the expected product of electrophilic addition of HCl to 3-methyl-1- butene. Formation of 2-chloro-2-methylbutane is unexpected. A shift of a H - converts a less stable 2 o to a more stable 3 o carbocation. Carbocation Rearrangement: Methyl Shift End Alkenes: Structure and Reactivity A shift of a CH 3 converts a less stable 2 o to a more stable 3 o carbocation. 11

Electrophilic Addition Reactions

Electrophilic Addition Reactions Electrophilic addition reactions are an important class of reactions that allow the interconversion of C=C and C C into a range of important functional groups. Conceptually,