Ch.6 Alkenes: Structure and Reactivity

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1 alkene = olefin 2 2 Ethylene α-pinene 3 β-arotene (orange pigment and vitamin A precursor)

2 6.1 Industrial Preparation and Use of Alkenes ompounds derived industrially from ethylene 2 2 Ethylene (26 million tons / yr) 3 2 O 3 O 3 OO O 2 2 O l 2 2 l 2 =l O O O Ethanol Acetaldehyde Acetic acid Ethylene glycol Ethylene dichloride Vinyl chloride Ethylene oxide Vinyl acetate Polyethylene

3 ompounds derived industrially from propylene 3 2 Propylene (14 million tons / yr) O 3 3 O 3 Isopropyl alcohol Propylene oxide umene 3 3 Polypropylene

4 Ethylene, propylene, and butene are synthesized industrially by thermal cracking of natural gas ( 1-4 alkanes) and straight-run gasoline ( 4-8 alkanes). 3 ( 2 ) n o steam = = = 2 - the exact processes are complex; involve radical process 900 o = 2 + 2

5 Thermal cracking is an example of a reaction whose energetics are dominated by entropy ( S o ) rather than enthalpy ( o ) in the free-energy equation ( G o = o -T S o ). ; - bond cleavage (positive o ) ; high T and increased number of molecules larger T S o

6 6.2 alculating Degree of Unsaturation unsaturated: formula of alkene n 2n ; formula of alkane n 2n+2 in general, each ring or double bond corresponds to a loss of two hydrogens from alkane formula degree of unsaturation: the number of rings and/or multiple bonds

7 unknown hydrocarbon with molecular weight 82; 6 10 corresponding alkane; = 4 = 2 2 therefore, degree of unsaturation= 2 possible structures:

8 degree of unsaturation: containing elements other than just, Organohalogen compounds (,, X, X= F, l, Br, I) Add the number of halogens to the number of hydrogens ; a halogen is simply a replacement of hydrogen Br 2 = 2 Br 2 = Br 2 = " 4 8 " add one unsaturation: one double bond or one cycle

9 Organooxygen compounds (,, O) Ignore the number of oxygens ; oxygen forms two bonds; - vs -O- or - vs -O- 2 == 2 O 5 8 O= " 5 8 " 2 == 2 - two unsaturation: two double bonds

10 Organonitrogen compounds (,, N) Subtract the number of nitrogens from the number of hydrogens ; nitrogen forms three bonds; - vs -N- or - vs -N 2 N N= " 5 8 " two unsaturation: one double bond and one ring

11 6.3 Naming Alkenes Step 1 Name the parent hydrocarbon: Find the longest carbon chain containing the double bond and name the compound accordingly, using the suffix -ene: NOT pentene hexene

12 Step 2 1 Numbering: Begin at the end nearer the double bond or, if the double bond is equivalent from the two ends, begin at the end nearer the first branch point. This rule ensures that the double bond carbons receive the lowest possible numbers: 6 3 NOT NOT

13 Step 3 Write the full name: list substituents alphabetically ; indicate the position of double bond (the number of the first alkene carbon) immediately before the parent name ; more than one double bonds: -diene, triene exene 2-Methyl-3-hexene Ethyl-1-pentene 2-Methyl-1,3-butadiene

14 cycloalkanes are named similarly, but double bond is between 1 and 2 and the first substituent has as low a number as possible ; it's not necessary to indicate the position of the double bond in the name (always 1 and 2) Methylcyclohexene 1,4-yclohexadiene ,5-Dimethylcyclopentene NOT

15 ommon names IUPA name ommon name Ethene Ethylene Propene Propylene 2-Methylpropene Isobutylene 2-Methyl-1,3-butadiene Isoprene 1,3-Pentadiene Piperylene

16 Substituent Names 2 2 A methylene group A vinyl group An allyl group 2 Μethylenecyclopentane Br Vinyl bromide Br Allyl bromide

17 6.4 Electronic Structure of Alkenes Rotation around double bond is restricted: The π-bond must break for rotation to take place around a = double bond kj/mol (64 kcal/mol) is required to break the π-bond - rotational energy barrier for ethane: only 12 kj/mol 90 o rotation π-bond (p-orbitals are parallel) broken π-bond after rotation (p-orbitals are perpendicular)

18 6.5 is-trans Isomerism in Alkenes 3 3 X 3 3 cis-2-butene trans-2-butene cis-trans isomerism: when both carbons are bonded to two different groups A B D D B A D D these two compounds are identical; they are not cis-trans isomers A B D E B A D E these two compounds are not identical; they are cis-trans isomers

19 6.6 Sequence Rules: The E,Z Designation cis-trans isomerism: describe the disubstituted double bond geometries ; tri-and tetrasubstituted double bonds- a general method is needed cis or trans? cis or trans?

20 E, Z isomerism: a more general method for describing double-bond geometry ; E (entgegen, "opposite"); Z (zusammen, "together") igh igh igh Low Low Low Low igh Z the higher priority groups on each carbon are on the same side of the double bond E the higher priority groups on each carbon are on the opposite side of the double bond

21 Sequence Rule (ahn-ingold-prelog rule; IP rule) ; priority of substituents Rule 1 onsidering each of the double-bond carbons separately, identify the two atoms directly attached and rank them according to atomic number Br > l > O > N > > l 3 l (Z)-2-hloro-2-butene (E)-2-hloro-2-butene

22 Rule 2 If a decision can't be reached by ranking the first atoms in the substituents, look at the second, third, or fourth atoms away from the double-bond carbons until the first difference is found. < 3 O < O < 3 3 N 2 < l

23 Rule 3 Multiple-bonded atoms are equivalent to the same number of singlebonded atoms. O O O

24 3 3 (E)-3-Methyl-1,3-pentadiene Br (E)-1-Bromo-2-isopropyl-1,3-butadiene 3 O O O (Z)-2-ydroxymethyl-2-butenoic acid

25 6.7 Stability of Alkenes Relative stability from equilibrium constant: - cis-trans isomers interconvert under strong acid condition 3 3 acid 3 catalyst 3 cis (24 %) trans (76%) E rel = kj/mol (0.66 kcal/mol) E rel = 0.0 kcal/mol

26 cis trans

27 From heat of combustion o combustion= kj/mol E rel = +3.3 kj/mol o combustion= kj/mol E rel = +0.0 kj/mol

28 From heat of hydrogenation 3 3 Pd Pd 3 3 o hydro = -120 kj/mol o hydro = -116 kj/mol 4 kj/mol difference

29 Energy profile for hydrogenation Energy is Trans G o trans G o cis Butane Reaction progress

30 Stabilities of alkenes: increasing the degree of substitution leads further stabilization R R R R > R R R > R R ~ R R > R tetrasubstituted trisubstituted disubstituted monosubstituted

31 Explanations of alkene stabilities 1. yperconjugation: a stabilizing interaction between the unfilled antibonding = p bond and a filled - s bond orbital on a neighboring substituent. The more substutuents that are present, the more opportunities exist for hyperconjugation, and the more stable the alkene. bonding - σ orbital (filled) π* σ antibonding - π orbital (unfilled)

32 2. Bond strength: sp 2 -sp 3 - bond is stronger than sp 3 -sp 3 - bond ; more highly substituted alkenes always have a higher ratio of sp 2 -sp 3 bonds to sp 3 -sp 3 bonds sp 3 -sp sp 3 -sp 2 sp 3 -sp 2 sp 3 -sp 2

33 3 3 h.6 Alkenes: Structure and Reactivity 6.8 Electrophilic Addition of X to Alkenes alkenes: electron rich, nucleophilic Electrophilic addition reaction: addition of electrophiles to nucleophilic alkenes Br Br carbocation intermediate Br 3 3 The electrophile Br is attacked by the p-electrons of the double bond, and a new - σ-bond is formed. This leaves the other carbon atom with a + charge and a vacant p orbital The Br - donates an electron pair to the positively charged carbon atom, forming a -Br σ-bond and yielding the neutral addition product.

34 Reaction energy diagram for the two-step electrophilic addition of Br to 2-methylpropene. TS 1 TS 2 carbocation intermediate Energy G Br G 2 G 1 > G 2 The first step is slower than the second step. reactants Br G o 3 Br 3 3 Reaction progress

35 Writing Organic Reactions A + B both reactants (A and B) are equally emphasized Ether l + l R 25 o R 3 A B reactants A is of greater interest than B l l R Et 2 O, 25 o R 3 solvents, temperature and other reaction conditions are written either above or below the reaction arrow

36 Electrophilic addition of X: l, Br, I (I is generated in the reaction mixture) + l Ether l 2-Methylpropene (94 %) 2-hloro-2-methylpropane KI I 3 PO 4 1-Pentene 2-Iodopentane

37 6.9 Orientation of Electrophilic Addition: Markovnikov's Rule regiospeccific: only one of two possible orietation of additions occurs regioselective: one of two possible orientation of additions preferred + l l + l sole product NOT formed

38 Markovnikov's rule: In the addition of X to an alkene, the attaches to the carbon with fewer alkyl substituents and the X attaches to the carbon with more alkyl substituents. less substituted carbon + l l more substituted carbon sole product more substituted carbon 3 + Br Ether 3 Br

39 When both ends of the double bond have the same degree of substitution, a mixture of products formed. + Br Br + Br

40 Interpretation of Markovnikov's rule: In the addition of X to an alkene, the more highly substituted carbocation is formed as the intermediate l - l + l 3 o carbocation X l - l 1 o carbocation NOT formed

41 3 Br - 3 Br 3 + Br X 3 o carbocation 3 Br - 3 Br 2 o carbocation NOT formed Why should this be?

42 6.10 arbocation Structure and Stability arbocation: planar, sp 2 -hybridized, electron deficient R R R 120 o vacant p orbital sp 2

43 Stability of carbocation: measure the amount of energy required to form the carbocation from its alkyl halide: R-X R + + :X - 3 l 3 + l D = 351 kj/mol (Bond dissociation energy) 3 + e - 3 E i = 948 kj/mol (Ionization energy) l + e - l - E ea = -348 kj/mol (Electron Affinity) 3 l 3 + l kj/mol (Dissociation enthalpy)

44 tertiary halides dissociate to give carbocation much more easily than secondary or primary halides ; tertiary carbocations are more stable than secondary or primary ones Dissociation enthalpy: Methyl halide > 1 o alkyl halide > 2o alkyl halide > 3o alkyl halide arbocation stability: < R R < R < R R R 1 o 2 o 3 o

45 Why? 1. Inductive effect: result from the shifting of electrons in a σ-bond in response to the electronegativity of nearby atom ; Electrons from a relatively large and polarizable alkyl group can shift toward a neighboring positive charge more easily than the electron from a hydrogen. ; alkyl grpups donates electrons inductively and stabilize carbocations inductive effect of alkyl groups: R + R + R + R + R R methyl 1 o 2 o 3 o

46 2. yperconjugation: interaction of nearby - σ-rebital with the vacant carbocation p orbital stabilizes the cation and lowers its energy stabilization carbocation through hyperconjugation bonding - σ orbital (filled) empty p orbital (unfilled)

47 Two different - bonds in t-butyl cation: ( 3 ) σ orbitals above and below the plane: nearly parallel to the cation p orbital (hyperconjugation) 2 - σ orbitals are in plane: perpendicular to the cation p orbital (no hyperconjugation)

48 staggered conformation eclipsed conformation σ* σ stabilizing hyperconjugation destabilizing torsional strain

49 6.11 The ammond Postulate Summary about electrophilic addition: Electrophilic addition to an unsymmetrically substituted alkene gives the more highly substituted carbocation. A more highly substituted carbocation forms faster than a less highly substituted one and, once formed, rapidly goes to give the final product. A more highly substituted carbocation is more stable than a less highly substituted one. - ow are these two points (stability and rate) related? - Why does the stability of the carbocation intermediate affect the rate at which it's formed and thereby determine the structure of the final product?

50 reaction rate ~ activation energy ( G ) stability ~ G o slower reaction slower reaction less stable intermediate faster eaction more stable intermediate less stable intermediate faster eaction more stable intermediate

51 ammond Postulate h.6 Alkenes: Structure and Reactivity The geometry of the transition state for a step most closely resembles the side (i.e. reactant or product) to which it is closer in energy. transition state transition state product reactant reactant product product-like TS reactant-like TS

52 The hypothetical structure of a transition state for alkene protonation. TS 1 carbocation intermediate alkene The transition state is closer in both energy and structure to the carbocation than to the alkene. Thus, an increase in carbocation stability (lower G o ) also causes an increase in transition state stability (lower G ), therefore, increase the reaction rate.

53 δ + δ - R R Br R alkene R R R δ + R R Br R R R R carbocation product-like transition state

54 More stable carbocations form faster because their stability is reflected in the transition state leading to them. primary TS ( 3 ) l - tertiary TS Energy Gprim Gtert ( 3 ) 3 + l - G o ( 3 ) l ( 3 ) 2 2 l ( 3 ) 3 l Reaction progress

55 6.12 Evidence for the Mechanism of Electrophilic Addition: arbocation Rearrangement ow do we know that the carbocation mechanism for addition of X to alkenes is correct? The answer is we never know the correct mechanism entirely proven. The best we can do is to show that a proposed mechanism is consistent with all known facts.

56 The evidence of two step, carbocation mechanism: structural rearrangements l l l ~50 % ~50 % ow is it formed? It is difficult to explain it with one step mechanism.

57 two step mechanism can explain the rearrangements:( F.. Whitmore, 1930s) l 3 3 ydride shift o carbocation 3 o carbocation l - l l 3 l - involve hydride shift: rearrangement of adjacent hydride ion ( - ) to form more stable carbocation

58 alkyl group can also rearrange with its electron pair l methyl shift o carbocation 3 o carbocation l - l l 3 3 l 3 Rearrangements of carbocations are common. a group (: - or : 3- ) moves to an adjacent positively charged carbon; - taking its bonding electron pair with it - a less stable carbocation rearranges to a more stable ion

59 Work Terpenes: Naturally Occuring Alkenes Essential oils: fragrant mixtures of liquids from plant materials Terpenes: plant essential oils consist largely of mixtures of compounds O Myrcene (oil of bay) α-pinene (turpentine) arvone (spearmint oil) Isoprene rule: head-to-tail joining of five-carbon isoprene Tail ead Isoprene Myrecene

60 Work Terpenes: Naturally Occuring Alkenes Monoterpene: 10 carbons (two isoprene units) Sesquiterpene: 15 carbons (three isoprene units) Diterpene: 20 carbons (two isoprene units) Monoterpenes and sesquiterpenes are found primarily in plants, but the higher terpenes occur in both plants and animals. O Lanosterol, a triterpene ( 30 ) - precusors for steroid hormons Ture biological precursor of terpenes is not isoprene itselt but iosprene equivalents made from acetic acid. O 3 O Acetic acid O O O P O P O O- O- O O O P O P O O- O- Isopentenyl diphosphate Dimethylallyl diphosphate