Alkenes: Structure and Reactivity - Lecture Slides | CHEM 2010, Study notes of Organic Chemistry

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Alkenes: Structure and Reactivity Based on McMurry’s Organic Chemistry, 6th edition 2 Alkene - Hydrocarbon With Carbon- Carbon Double Bond  Includes many naturally occurring materials Flavors, fragrances, vitamins  Important industrial products These are feedstocks for industrial processes 5 Example: C6H10  Saturated is C6H14  Therefore 4 H's are missing  This has two degrees of unsaturation  Two double bonds?  or triple bond?  or two rings  or ring and double bond H3C C C C C CH3 H H H H H H H H Other Examples of C,H,,: Dp Aas of 7 Degree of Unsaturation With Other Elements  Organohalogens (X: F, Cl, Br, I)  Halogen replaces hydrogen  C4H6Br2 and C4H8 have one degree of unsaturation  Oxygen atoms: these don't affect the total count of H's C4H8O3 10 If C-N Bonds Are Present  Nitrogen has three bonds  So if it connects where H was, it adds a connection point, and there an extra H  Subtract one H for equivalent degree of unsaturation in hydrocarbon 11 Count pairs of H's below CnH2n+2 Add number of halogens to number of H's (X equivalent to H) Don't count oxygens (oxygen links H) Subtract N's - they have three connections Summary - Degree of Unsaturation 12 6.3 Naming of Alkenes  Find longest continuous carbon chain for root  Number carbons in chain so that double bond carbons have lowest possible numbers  Double bond carbons must be numbered consecutively Cycloalkene nomenclature ae CH, 6 5 5 5 ,_ CH; *_CH; ‘ 4 2 2 3 1,5-Dimethylcyclopentene 1-Methylcyclohexene 15 1,4-Cyclohexadiene 16 Cycloalkene nomenclature CH3 CH3CH31 2 1 2 3 1 2 4 1-methylcyclohexene 3-methylcyclohexene 4-methylcyclohexene Problem 6.4 (Page 175) | HC CH 1" (a) H,C—=CHCHCCH, (b) CH;CH,CH=CCH,CH; | CH; CHz CHs | | (c) CH;CH=CHCHCH =CHCHCH, ©2004 Thomson - Brooks/Cole 17  TABLE 6.1 Common Names of Some Alkanes? Compound Systematic name Common name H,C—=CH, Ethene Ethylene CH,CH=CH, Propene Propylene CH, ane =CH, 2-Methylpropene Isobutylene CH, H,C= i CH=CH, 2-Methyl-1,3-butadiene Isoprene CH,CH=CHCH=CH, 1,3-Pentadiene Piperylene “Both common and systematic names are recognized by IUPAC for these compounds. ©2004 Thomson - Brooks/Cole 20 Alkene Group Names ae HO H,C=CH+ Ametiglene group =A vinyl group H,C=CH— CH, + An allyl group 22 6.4 Electronic Structure of Alkenes  Carbon atoms in a double bond are sp2- hybridized  Three equivalent orbitals at 120º separation in plane  Fourth orbital is an unhybridized p orbital  Combination of an electron pair in an orbital formed by the overlap of two sp2 orbitals of two atoms forms  bond between them JA \ r a \ ee yy" : _< es : rotation me ‘s ‘c | y aw bond Broken 7 bond after rotation (p orbitals are parallel) (p orbitals are perpendicular) Aj 25 26 6.5 Cis-Trans Isomerism in Alkenes  The presence of a carbon- carbon double can create two possible structures  cis isomer - two similar groups on same side of the double bond  trans isomer similar groups on opposite sides  Each carbon must have two different groups for these isomers to occur 27 Cis, Trans Isomers Require That End Groups Must Differ in Pairs  180°rotation results in identical (superimposable) structures  Bottom pair cannot be superimposed without breaking C=C (ie. rotating around the double bond) X 30 Develop a System for Comparison of Priority of Substituents  Assume a valuation system  If Br has a higher “value” than Cl  If CH3 is higher than H  Then, in A, the higher value groups are on opposite sides  In B, they are on the same side  Requires a universally accepted “valuation” 31 Ranking Priorities: Cahn-Ingold- Prelog Rules  Must rank atoms that are connected at comparison point  Higher atomic number gets higher priority  Br > Cl > O > N > C > H In this case,The higher priority groups are opposite: (E )-1-bromo-1-chloro- propene 32 E,Z Stereochemical Nomenclature  Priority rules of Cahn, Ingold, and Prelog  Compare where higher priority group is with respect to bond and designate as prefix  E -entgegen, opposite sides  Z - zusammen, together on the same side 35  If atomic numbers are the same, compare at next connection point at same distance  Compare until something has higher atomic number  Do not combine – always compare Extended Comparison H H dH - pe ees Lower Higher 1 i CCH: OCH H H Higher Lower ©2004 Thomson - Brooks/Cole H 2+O—H 20—0-—n ra i Higher CH; H Facaurr, a i i Lower Higher 37  Substituent is drawn with connections shown and no double or triple bonds  Added atoms are valued with no ligands themselves Dealing With Multiple Bonds Some Examples: H \ H C=CH, \ c=C \ H,C CH; E)-3-Methy1-1,3-pentadiene ©2004 Thomson - Brooks/Cole iL H,C—CH Br \ / c=c / \ H,C=cC H \ H (E)-1-Bromo-2-isopropyl- 1,3-butadiene / \ H CH,OH (Z)-2-Hydroxymethyl- 2-butenoic acid 40 Practice problem 1 p. 178 H CH(CH,), \ / C=C / \ ©2004 Thomson - Brooks/Cole Solution ee C, C, H bonded L to this carbon Low H re Low C=C fF * High H,C CH,OH High a O, H, H bonded to this carbon Z configuration ©2004 Thomson - Brooks/Cole 42 45 6.7 Alkene Stability  Cis alkenes are usually less stable than trans alkenes cis-2-Butene trans-2-Butene ©2004 Thomson - Brooks/Cole 46 47 6.7 Alkene Stability  Compare heat given off on hydrogenation: Ho  Less stable isomer is higher in energy and gives off more heat  tetrasubstituted > trisubstituted > disubstituted > monosusbtituted Reaction progress ©2004 Thomson - Brooks/Cole Equilibration of 2-butenes eo H CH, H,C CH, \ Acid ye i \ catalyst / a \ 3C H H H Trans (76%) Cis (24%) ©2004 Thomson - Brooks/Cole 52 6.7 Alkene Stability  Compare heat given off on hydrogenation: Ho  Less stable isomer is higher in energy and gives off more heat  tetrasubstituted > trisubstituted > disubstituted > monosusbtituted 55 Hyperconjugation  Electrons in neighboring filled  orbital stabilize vacant antibonding  orbital – net positive interaction  Alkyl groups are more stabilizing than H 56 Bond strengths/hybridization effects  sp3-sp3 bond is weaker than sp3-sp2, sp2-sp2 57 Name each; which is more stable? Problem 13, p. 184 Solvent ff ,C=CH, + HBr —S> CH,CH,Br Reactant Lo _ HBr oC — CH, Ether, ye CH,CH,Br NL Solvent ©2004 Thomson - Brooks/Cole 60 61  Two step process  First transition state is high energy point Electrophilic Addition Energy Diagram: First transition state Carbocation intermediate Second transition state AG] CH, CH\C — Gly + HBr | a a Chi, CHAC —Br CH 3 Reaction progress ———» 62  First transition state Energy Carbocation intermediate Second transition state Reaction progress ————> ©2004 Thomson - Brooks/Cole 65 66 Electrophilic Addition for Syntheses  The reaction is successful with HCl and with HI as well as HBr. Note that HI is generated from KI and phosphoric acid 67 6.9 Orientation of Electrophilic Addition: Markovnikov’s Rule  In an unsymmetrical alkene, HX reagents can add in two different ways, but one way may be preferred over the other  If one orientation predominates, the reaction is regiospecific  Markovnikov observed in the 19th century that in the addition of HX to alkene, the H attaches to the carbon with the most H’s and X attaches to the other end (to the one with the most alkyl substituents)  This is Markovnikov’s rule No alkyl groups on this carbon Cl alkyl groups — 3 i | Eth mn this sarbon ~C= CH, + Hcl —“- CH; —0— CH CHs CH, 2-Methylpropene 2-Chloro-2-methylpropane 2 alkyl groups on this carbon je H,C CH, Br + HBr Ether H H ie i 1 alkyl group on this carbon 1-Methylcyclohexene 1-Bromo-1-methylcyclohexane ©2004 Thomson - Brooks/Cole Practice Problem 6.2 (p. 188): CH,CH; + HCl — ? ©2004 Thomsen - Brooks/Cole  Solution: ee 2 alkyl groups on this carbon / CH,CH, CH,CHs; 1 + HCl —~— Cl 1-Chloro-1-ethylcyclopentane 1 alkyl group on this carbon ©2004 Thomson - Brooks/Cole 72 Problem 6.14: Major products? | o(. J+na —* “2 (c) CH;,CH,CH,CH = CH, EEO 2 ©2004 Thamson - Bronks/Cole (b) (CHs),C—=CHCH,CH,; > ? CH. (d) Cy + HBr — ? 75 Problem 6.15: Which alkene? Br Cl | (c) CH3;CH,»,CHCH»CH».CHs (d) ©2004 Thomson - Brooks/Cole 76 77 Stability of Carbocations and Markovnikov’s Rule  More stable carbocation forms faster  Tertiary cations and associated transition states are more stable than primary cations 80 Mechanistic Source of Regiospecificity in Addition Reactions  If addition involves a carbocation intermediate  and there are two possible ways to add  the route producing the more alkyl substituted cationic center is lower in energy  alkyl groups stabilize carbocation 81 6.10 Carbocation Structure and Stability Carbocations are planar  The positively charged carbon is surrounded by only 6 electrons in three sp2 orbitals The fourth orbital on carbon is a vacant p-orbital ©2004 Thomson - Brooks/Cole Vacant p orbital sp Ee R’ 120° 82 Heterolytic bond dissociations energies: 10005 CHCl i CH3CH,Cl = 800+ (CH3)9CHCl - 191 s - (CH3)3CCl 82 600- | 143 gs Ss 4 400 - 96 o 200 as 2 0 Methyl Ne 22 3° @ 2004 Thomson/Brooks Cole (keal/mol) 85 Stabilizing Carbocations: a Vacant p orbital on this carbon. C-H bond more parallel to p orbital. C-—H bond perpendicular to p orbital. C—H bond more parallel to p orbital. © 2004 Thomson/Brooks Cole 86 Problem 6.16 : carbocation structure? CH, CH, | | (a) CH;3CH,C—=CHCHCH, + HBr —> ? (b) [-cuon, +HI — ? ©2004 Thomson - Brooks/Cole 87 90 Statement of the Hammond Postulate  A transition state should be similar to an intermediate that is close in energy  Sequential states on a reaction path that are close in energy are likely to be close in structure - G. S. Hammond carbocation G Reaction In a reaction involving a carbocation, the transition states look like the intermediate 91 Competing Reactions and the Hammond Postulate  Normal Expectation: Faster reaction gives more stable intermediate  Intermediate resembles transition state 92 “Non-Hammond” Behavior  More stable intermediate from slower reaction  Conclude: transition state and intermediate must not be similar in this case – not common 95 6.12 Mechanism of Electrophilic Addition: Rearrangements of Carbocations Carbocations undergo structural rearrangements following set patterns 1,2-H and 1,2-alkyl shifts occur Goes to give more stable carbocation Can go through less stable ions as intermediates Carbocation rearrangements: ee in ii 1 I C HC H HC H pos, Any t Ha DO, A + Do, J 4C C H H.C C H HC C H | /\ /\ H H (Cl H dH 3-Methyl-1-butene 2-Chloro-3-methylbutane 2-Chloro-2-methylbutane (approx. 50%) (approx. 50%) ©2004 Thomson - Brooks/Cole 96 Hydride Shifts —— rt " ae an H,C \ H,C H Hydride H = Notthy 7 PO eee ay “ain ng? ig ag 3 H H H H 3-Methyl-1-butene A 2° carbocation A3?° carbocation |e lor CH, 4H CH, H H,C._| | LH H,C. | | -H H~ So SH cl” “se “A \ /\ H Cl H H 2-Chloro-3-methylbutane 2-Chloro-2-methylbutane ©2004 Thomson - Brooks/Cole 97