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Unformatted text preview: 10/21/10 The (E)-(Z) System for Designating Alkene Diastereomers" The Cahn-Ingold-Prelog convention is used to assign the groups of highest priority on each carbon" Chapter 7 Alkenes and Alkynes I: Properties and Synthesis Elimination Reactions of Alkyl Halides" PRIORITY IS DETERMINED BY ATOMIC NUMBER" If the group of highest priority on one carbon is on the same side as the group of highest priority on the other carbon the double bond is Z (zusammen)" If the highest priority groups are on opposite sides the alkene is E (entgegen)" examples Chapter 7 2 Relative Stabilities of Alkenes" Generally Z alkenes are less stable than E alkenes because of steric hinderance" Heats of hydrogenation of three butene isomers:" Heat of Hydrogenation" The relative stabilities of alkenes can be measured using the exothermic heats of hydrogenation" The same alkane product must be obtained to get comparable results" Overall Relative Stabilities of Alkenes" The greater the number of attached alkyl groups (i.e. the more highly substituted the carbon atoms of the double bond), the greater the alkenes stability" Chapter 7 3 Chapter 7 4 1 10/21/10 Elimination Reactions of Alkyl Halides" Dehydrohalogenation" Used for the synthesis of alkenes" Elimination competes with substitution reaction" Strong bases such as alkoxides favor elimination" NB the double bond formed can be under kinetic or thermodynamic control" The alkoxide bases are made from the corresponding alcohols" Heat favours elimination The solvents here cannot be water. WHY? Chapter 7 5 Chapter 7 6 The E2 Reaction (WE ADDED SN2)" E2 reaction involves concerted removal of the proton, formation of the double bond, and departure of the leaving group" Both alkyl halide and base concentrations affect rate and therefore the reaction is 2nd order" E2 vs SN2" Thermodynamically favoured SN2 lose and form bonds"
Compare with SN2 With alkene form C=C ( bonds) and lose bonds" E2 is favoured when SN2 is "difficult" for steric reasons (base/nucleophile cant find the C-X" E2 favoured by heating and increased concentration" Greater increase in entropy in an elimination than in a substitution" Chapter 7 7 Chapter 7 8 2 10/21/10 The E1 Reaction" The E1 reaction competes with the SN1 reaction and likewise goes through a carbocation intermediate ONLY FOR 3o CARBOCATIONS" Synthesis of Alkenes via Elimination Reactions" Dehydrohalogenation" Reactions by an E2 mechanism are most useful FOCUS ON THIS" E1 reactions can be problematic" E2 reactions are favored by:" Secondary or tertiary alkyl halides" Alkoxide bases such as sodium ethoxide or potassium tert-butoxide" Bulky bases such as potassium tert-butoxide should be used for E2 reactions of primary alkyl halides" Chapter 7 9 Chapter 7 10 Zaitsevs Rule: Formation of the Most Substituted Alkene is Favored with a Small Base" Some hydrogen halides can eliminate to give two different alkene products" The transition state in this E2 reaction has double bond character" The trisubstituted alkene-like transition state will be most stable and have the lowest G" Kinetic control of product formation: When one of two products is formed because its free energy of activation is lower and therefore the rate of its formation is higher" This reaction is said to be under kinetic control" Zaitzevs Rule: when two different alkene products are possible in an elimination, the most highly substituted (most stable) alkene will be the major product" This is true only if a small base such as ethoxide is used" Chapter 7 11 Chapter 7 12 3 10/21/10 Formation of the Least Substituted Alkene Using a The Stereochemistry of E2 Reactions: The Orientation of Bulky Base (ie to go against Zaitsevs rule)" Bulky bases such as potassium tert-butoxide have difficulty removing sterically hindered hydrogens and generally only react with more accessible hydrogens (e.g. primary hydrogens)" Groups in the Transition State anti! All four atoms involved must be in the same plane" Anti coplanar orientation is preferred because all atoms are staggered" In a cyclohexane ring the eliminating substituents must be diaxial to be anti coplanar" Chapter 7 13 Chapter 7 14 In menthyl chloride the molecule must first change to a less stable conformer to produce an axial chloride" Elimination is slow and can yield only the least substituted (Hoffman) product Acid Catalyzed Dehydration of Alcohols" from anti elimination" Recall that elimination is favored over substitution at higher temperatures" Typical acids used in dehydration are sulfuric acid and phosphoric acid" The temperature and concentration of acid required to dehydrate depends on the structure of the alcohol" Primary alcohols are most difficult to dehydrate, tertiary are the easiest" Please note that an E1 reaction would give BOTH 1- and 2-menthene WHY DOES THE SECOND REACTION GO AT LOWER TEMPS?" Rearrangements of the carbon skeleton can occur"
Chapter 7 15 Chapter 7 16 4 10/21/10 Mechanism for Dehydration of Secondary and Tertiary Alcohols: An E1 Reaction (this is less important)" Only a catalytic amount of acid is required since it is regenerated in the final step of the reaction" Carbocation Stability and the Transition State" Recall the stability of carbocations is:" The second step of the E1 mechanism in which the carbocation forms is rate determining" The transition state for this reaction has carbocation character " Tertiary alcohols react the fastest because they have the most stable tertiary carbocation-like transition state in the second step"
Chapter 7 17 Chapter 7 18 The relative heights of G for the second step of E1 dehydration indicate that primary alcohols have a prohibitively large energy barrier" Synthesis of Alkynes by Elimination Reactions" Alkynes can be obtained by two consecutive dehydrohalogenation reactions of a vicinal dihalide" They proceed through an E2 like mechanism Chapter 7 19 Chapter 7 20 5 10/21/10 Alkenes can be converted to alkynes by bromination and two consecutive dehydrohalogenation reactions" The Acidity of Terminal Alkynes" Recall that acetylenic hydrogens have a pKa of about 25 and are much more acidic than most other C-H bonds" The relative acidity of acetylenic hydrogens in solution is:" Geminal dihalides can also undergo consecutive dehydrohalogenation reactions to yield the alkyne" Acetylenic hydrogens can be deprotonated with relatively strong bases (sodium amide is typical) (NO use NaH, LDA (LiNiPr2))" The products are called alkynides" Chapter 7 21 Chapter 7 22 Replacement of the Acetylenic Hydrogen Atom of Hydrogenation of Alkenes" Hydrogen adds to alkenes in the presence of metal catalysts" Heterogeneous catalysts: finely divided insoluble platinum, palladium or nickel catalysts" Homogeneous catalysts: catalyst(typically rhodium or ruthenium based) is soluble in the reaction medium " Wilkinsons catalyst is Rh[(C6H5)3P]3Cl" Terminal Alkynes" Sodium alkynides can be used as nucleophiles in SN2 reactions" RMgBr, RLi CANNOT!!" New carbon-carbon bonds are the result" Only primary alkyl halides can be used or else elimination reactions predominate" This process is called a reduction or hydrogenation" An unsaturated compound becomes a saturated (with hydrogen) compound" Chapter 7 23 Chapter 7 24 6 10/21/10 Hydrogenation: The Function of the Catalyst" The catalyst provides a new reaction pathway with lower G values" In heterogeneous catalysis the hydrogen and alkene adsorb to the catalyst surface and then a step-wise formation of C-H bonds occurs" Both hydrogens add to the same face of the alkene (a syn addition)" Addition to opposite faces of the double bond is called anti addition" Chapter 7 25 Chapter 7 26 Hydrogenation of Alkynes" Reaction of hydrogen using regular metal catalysts results in formation of the alkane" Lindlars catalyst also produces Z-alkenes from alkynes" syn Addition of Hydrogen: Synthesis of Z-Alkenes" The P-2 catalyst nickel boride results in syn addition of one equivalent of hydrogen to a triple bond" An internal alkyne will yield a Z double bond" Anti Addition of Hydrogen: Synthesis of E-Alkenes" A dissolving metal reaction which uses lithium or sodium metal in low temperature ammonia or amine solvent produces E-alkenes" Net anti addition occurs by formal addition of hydrogen to the opposite faces of the double bond" Chapter 7 27 Chapter 7 28 7 10/21/10 Substitution versus Elimination" SN2 versus E2"
Requires basic nucleophile" e.g., RO-, R = alkyl/H" Non-basic nucleophile" Br-, I-, NC-, etc. Primary substrate with base" If the base is small, SN2 competes strongly because approach at carbon is Substitution versus Elimination" SN1 versus E1" Primary substrate ALMOST NEVER SN1 OR E1 NEED VERY STRONG ACID" If the base is small, SN2 competes strongly because approach at carbon is unhindered" unhindered" Secondary substrate (TO FAVOUR SN REACTION, COOL DOWN!)" Approach to carbon is sterically hindered and E2 elimination is favored" Secondary substrate" Approach to carbon is sterically hindered and E2 elimination is favored" Tertiary Substrate" "NO SN2 ONLY E2" Chapter 7 29 Chapter 7 30 SUMMARY SLIDE, E1 v E2" SUMMARY SLIDE, E1 v E2 v SN1 v SN2 " Chapter 7 31 Chapter 7 32 8 10/21/10 SUMMARY SLIDE, E1 v E2 v SN1 v SN2 " END SLIDES " Chapter 7 33 Chapter 7 34 9 ...
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