Organic Chemistry Chapter 8 Elimination; Alkenes PDF

Organic Chemistry Chapter 8 Elimination; Alkenes 1  Elimination Elimination reactions involve the loss of elements from the starting material to form a new bond in the product. Generally: removal of “H” and a “goo...

Organic Chemistry Chapter 8 Elimination; Alkenes 1  Elimination Elimination reactions involve the loss of elements from the starting material to form a new bond in the product. Generally: removal of “H” and a “good leaving group” to generate a bond ©2020 McGraw-Hill Education. 2  Elimination Elimination reactions involve the loss of elements from the starting material to form a new bond in the product. ©2020 McGraw-Hill Education. 3  Elimination of HX A base removes the elements of an acid, HX, from the organic starting material…..dehydrohalogenation ©2020 McGraw-Hill Education. 4  Dehydrohalogenation Removal of the elements HX Dehydrohalogenation is an example of elimination. β carbon has the proton α carbon has the halide or leaving group formed broken Text says: “The curved arrow formalism shown below illustrates how four bonds are broken or formed in the process” what it means is that a total of four bonds are involved ©2020 McGraw-Hill Education. 5  Common Bases for Dehydrohalogenation most common bases used in elimination reactions …alkoxides. negatively charged oxygen compounds, such as HO− and its alkyl derivatives, RO−, From hydroxides or alcohols ©2020 McGraw-Hill Education. 6  Determining the reaction path Find the α carbon. Identify all β carbons with H atoms. Remove the elements of H and X from the α and β carbons and form a π bond. ©2020 McGraw-Hill Education. 7  Other example of elimination reaction…..not limited to double bond formation Ref: http://www.chem.ucla.edu/~harding/IGOC/B/beta_elimination.html ©2020 McGraw-Hill Education. 8  These elimination reactions produce Alkenes carbon-carbon double bond sp2 hybridized carbon trigonal planar carbons …..bond angles are 120 degrees restricted rotation about double bonds ©2020 McGraw-Hill Education. 9  Alkene Structure The double bond of an alkene consists of a σ bond and a π bond. ©2020 McGraw-Hill Education. 10  Classifying Alkenes Alkenes are classified according to the number of carbon atoms bonded to the carbons of the double bond. Figure 8.1 ©2020 McGraw-Hill Education. 11  Stereoisomers of Alkenes Ex: 2-butene restricted rotation, two stereoisomers of 2-butene are possible. cis-2-Butene and trans-2-butene are diastereomers (i.e., non-mirror image stereoisomers). ©2020 McGraw-Hill Education. 12  Alkene Diastereomers cis-trans isomers are possible if the two groups on each end of a carbon-carbon double bond are different from each other, ©2020 McGraw-Hill Education. 13  Alkene Diastereomers..examples Notice Naming: parent chain contains the “ene” with cis, trans, in front; rest of rules apply as before Ref: https://canvas.instructure.com/courses/954116/pages/geometric- isomerism-in-alkenes?module_item_id=7704637 cis trans Ref: https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Book%3A_The_Basics_of_GOB_Chemistry_(Ball_et_al.)/ 13%3A_Unsaturated_and_Aromatic_Hydrocarbons/13.02%3A_Cis-Trans_Isomers_(Geometric_Isomers) ©2020 McGraw-Hill Education. 14  Stability of Alkenes In general, trans alkenes are more stable than cis alkenes because of reduced steric hinderence of the groups on the double bond. ©2020 McGraw-Hill Education. 15  Stability in Alkenes The stability of an alkene increases as the number of R groups bonded to the double bond carbons increases. sp2 carbons are more able to accept electron density sp3 carbons are more able to donate electron density Increasing the number of electron donating groups on a carbon atom able to accept electron density makes the alkene more stable. ©2020 McGraw-Hill Education. 16  What this means…example 2-butenes are disubstituted and more stable than 1-butene (monosubstituted). trans-2-Butene is more stable than cis-2-butene (less crowding). ©2020 McGraw-Hill Education. 17  Practice HW: 8.28, 8.29a, 8.30, 8.31 ©2020 McGraw-Hill Education. 18  Elimination Mechanisms E2 mechanism; bimolecular elimination. TE1 mechanism; unimolecular elimination. E2 and SN2 reactions have some features in common, as do E1 and SN1 reactions. ©2020 McGraw-Hill Education. 19  E2 Mechanism most common mechanism for dehydrohalogenation The reaction is concerted—all bonds are broken and formed in a single step..base and alkyl halide are in the rate reaction second-order kinetics, both the alkyl halide and the base appear in the rate equation. E2 and SN2 mechanisms are similar in how the identity of the base, the leaving group, and the solvent affect the rate. ©2020 McGraw-Hill Education. 20  E2 Mechanism ©2020 McGraw-Hill Education. 21  Energy Diagram for an E2 Reaction Compare to Sn2 ©2020 McGraw-Hill Education. 22  Effect of bases in E2 E2 reactions are generally run with strong, negatively charged bases like −OH and −OR. (see previous slides) The base appears in the rate equation, so the rate of the E2 reaction increases as the strength of the base increases. Other example of strong bases DBN and DBU (diaza bicyclo compounds) ©2020 McGraw-Hill Education. 23  E2 Reaction with DBN Recognize a chemical by its structure and see Only change what it may do….DBN as a base Look for what a base can do in the reaction…find a proton and a halide….therefore elimination; because the base has not acted like a nucleophile ©2020 McGraw-Hill Education. 24  Effects of Leaving Group and Solvent on E2 Reactions the better the leaving group the faster the E2 reaction. Polar aprotic solvents increase the rate of E2 reactions. (similar to SN2) ©2020 McGraw-Hill Education. 25  Effect of Alkyl Halide Structure on E2 Reactions As the number of R groups on the carbon with the leaving group increases, the rate of the E2 reaction increases. Opposite of SN2) ©2020 McGraw-Hill Education. 26  Transition States in E2 Mechanisms increase in E2 reaction rate due to increasing alkyl substitution alkyl substituents stabilize a double bond; hence stabilizing the transition state…Ea is lowered…rate increases ©2020 McGraw-Hill Education. 27  Product Stability and Rate of E2 Reactions Increasing the number of R groups on the carbon with the leaving group forms more highly substituted, more stable alkenes in E2 reactions. Ex. disubstituted alkene is more stable, the 3o alkyl halide reacts faster than the 1o alkyl halide. ©2020 McGraw-Hill Education. 28  Ref: https://www.masterorganicchemistry.com/2012/08/31/elimination-reactions-2-zaitsevs-rule/ ©2020 McGraw-Hill Education. 29  E2 Mechanism Summary ©2020 McGraw-Hill Education. 30  The Zaitsev (Saytzeff) Rule more than one alkene product can be formed if alkyl halides have two or more different carbons, The Zaitsev rule predicts that the major product in elimination has the more substituted double bond. ©2020 McGraw-Hill Education. 31  The Zaitsev (Saytzeff) Rule, example Ref:chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Mod ules_(Organic_Chemistry)/Alkenes/Synthesis_of_Alkenes/Zaitsev%27s_Rule Ref: https://www.masterorganicchemistry.com/2017/10/18/the- hofmann-elimination/ ©2020 McGraw-Hill Education. 32  A reaction is regioselective when it yields predominantly or exclusively one constitutional isomer when more than one is possible. Thus, the E2 reaction is regioselective because a major isomer forms ©2020 McGraw-Hill Education. 33  A reaction is stereoselective when it forms predominantly or exclusively one stereoisomer when two or more are possible. The E2 reaction is stereoselective because the more stable stereoisomer is formed preferentially. ©2020 McGraw-Hill Education. 34  The E2 reaction is stereoselective because the more stable stereoisomer is formed preferentially. Examples REF; https://openwetware.org/wiki/Todd:Chem3x11_ToddL11 ©2020 McGraw-Hill Education. 35  Some of the following slides are out of sequence with the text to keep E2 reactions together. ©2020 McGraw-Hill Education. 36  Stereochemistry of E2 Reactions The transition state of an E2 reaction consists of four atoms from an alkyl halide—one hydrogen atom, two carbon atoms, and the leaving group (X)—all aligned in a plane. There are two ways for the C—H and C—X bonds to be coplanar. Anti is preferred geometry..because of staggered conformation ©2020 McGraw-Hill Education. 37  Anti Periplanar Geometry: 6 member rings anti periplanar geometry in an E2 reaction of six-membered rings. Ex. Chlorocyclohexane exists as two chair conformations. Conformation X is preferred since the bulkier Cl group is in the equatorial position. ©2020 McGraw-Hill Education. 38  Trans Diaxial Geometry for E2 Reactions trans diaxial geometry means that elimination must occur from the less stable conformer…..(location of the Cl) For E2 elimination, 1. the C—Cl bond must be anti periplanar to the C—H bond on a carbon, 2. this occurs only when the H and Cl atoms are both in the axial position. ©2020 McGraw-Hill Education. 39  E2 Reactions of Cis and Trans Isomers E2 dehydrohalogenation of cis- and trans-1-chloro-2-methylcyclohexane. The cis isomer exists as two conformations, A and B, each of which has one group axial and one group equatorial. E2 reaction must occur from conformation B, which contains an axial Cl atom. ©2020 McGraw-Hill Education. 40  Continued: Because conformation B has two different axial hydrogens, labeled Ha and Hb, E2 reaction occurs in two different directions to afford two alkenes. The major product contains the more stable trisubstituted double bond, as predicted by the Zaitsev rule. ©2020 McGraw-Hill Education. 41  Axial Leaving Groups for E2 Reactions The trans isomer of 1-chloro-2-methylcyclohexane exists as two conformers: C, having two equatorial substituents, and D, having two axial substituents. E2 reaction must occur from D, since it contains an axial Cl atom. ©2020 McGraw-Hill Education. 42  What does all this mean: Because conformer D has only one axial H, the E2 reaction occurs only in one direction to afford a single product. The most substituted, “Zaitsev” alkene is not the major product in this case. These E2 Reactions result in Anti Zaitsev Products (IMPORTANT: this is not an exception to the Zaitsev rule; it is a function of how the reaction is set up….the basics are the same as before… hydrogens are needed) ©2020 McGraw-Hill Education. 43  E1 Mechanism The E1 reaction proceeds via a two-step mechanism: the bond to the leaving group breaks first before the π bond is formed. The slow step is unimolecular, involving only the alkyl halide. E1, the leaving group comes off before the proton is removed, and the reaction occurs in two steps ©2020 McGraw-Hill Education. 44  E1 Mechanism ©2020 McGraw-Hill Education. 45  Energy Diagram for an E1 Reaction ©2020 McGraw-Hill Education. 46  Effect of Alkyl Halide Structure on E1 Reactions E1 reaction rate increases as the number of R groups on the carbon with the leaving group increases Stability of carbocation increases. ©2020 McGraw-Hill Education. 47  Effect of Base on the E1 Reaction The strength of the base usually determines whether a reaction follows the E1 or E2 mechanism. Strong bases like −OH and −OR favor E2 reactions. Weaker bases like H2O and ROH favor E1 reactions. ©2020 McGraw-Hill Education. 48  Zaitsev’s rule applies to E1 reactions also. E1 reactions are regioselective, favoring formation of the more substituted, more stable alkene. ©2020 McGraw-Hill Education. 49  E1 Mechanism Summary E1 reactions often occur with a competing SN1 reaction ©2020 McGraw-Hill Education. 50  and E1 Reactions SN1 and E1 reactions have the formation of a carbocation as the first step They differ in what happens to the carbocation. SN1 reaction, a nucleophile attacks the carbocation, forming a substitution product. E1 reaction, a base removes a proton, forming a new pi bond. ©2020 McGraw-Hill Education. 51  Comparison of E1 and E2 Mechanisms The strength of the base is the most important factor in determining the mechanism for elimination. ©2020 McGraw-Hill Education. 52  E2 Reactions and Alkyne Synthesis single elimination reaction produces a π bond of an alkene. Two consecutive elimination reactions produce two π bonds……alkyne…triple bond ©2020 McGraw-Hill Education. 53  E2 Reactions and Alkyne Synthesis Two elimination reactions are needed to remove two moles of HX from a dihalide substrate. Two different starting materials can be used—a vicinal dihalide or a geminal dihalide. ©2020 McGraw-Hill Education. 54  Bases for Alkyne Synthesis Stronger bases are needed to synthesize alkynes than those needed to synthesize alkenes. typical base: used is −NH2 (amide), used as NaNH2. KOC(CH3)3 can also be used with DMSO as solvent. The second elimination reaction requires the breaking of an sp2 hybridized C—H bonds (next slide) ©2020 McGraw-Hill Education. 55  Dehydrohalogenation of Dihalides ©2020 McGraw-Hill Education. 56  How to determine if the Reaction is 𝐍 , 𝐍 , E1, or E2? Good nucleophiles that are weak bases favor substitution over elimination. These include I−, Br−, HS−, −CN, and CH3COO−. ©2020 McGraw-Hill Education. 57  Bulky non-nucleophilic bases favor elimination over substitution. KOC(CH3)3, DBU, and DBN are too sterically hindered to attack tetravalent carbon. They are, however, able to remove a small proton, favoring elimination over substitution. ©2020 McGraw-Hill Education. 58  Predicting Reaction Mechanisms ( 𝐍 , 𝐍 , E1, or E2) Tertiary Alkyl Halides: Access the text alternative for slide images. ©2020 McGraw-Hill Education. 59  Predicting Reaction Mechanisms ( 𝐍 , 𝐍 , E1, or E2) Primary Alkyl Halides: ©2020 McGraw-Hill Education. 60  Predicting Reaction Mechanisms ( 𝐍 , 𝐍 , E1 or E2) Secondary Alkyl Halides Part 1: ©2020 McGraw-Hill Education. 61  Predicting Reaction Mechanisms ( 𝐍 , 𝐍 , E1 or E2) Secondary Alkyl Halides Part 2: ©2020 McGraw-Hill Education. 62  Predicting Reaction Mechanisms ( 𝐍 , 𝐍 , E1 or E2) ©2020 McGraw-Hill Education. 63  Summary Ref: https://tophat.com/marketplace/science-&-math/chemistry/full-course/organic-chemistry-i-&-ii-steven-forsey/294/68275/ ©2020 McGraw-Hill Education. 64  End Chapt 8 HW: See slide 18 for part 1 problems 8.8, 8.12 b,c; 8.14, 8.34, 8.36a, 8.39 a, d; 8.41b 8.52a, f; 8.56c; 8.57 b ©2020 McGraw-Hill Education. 65