Chapter 3. Stabilizing Effects in Hydrocarbon Chemistry. The goal of this chapter: Iden=fy the presence of strained stabilized systems

Transcription

1 Stabilizing Effects in ydrocarbon Chemistry The goal of this chapter: Iden=fy the presence of strained stabilized systems Predict quan=ta=ve values of strain/stabiliza=on based on chemical equa=ons or other methods (e.g., Benson Increments)

2 f 298 (kcal/mol) Alkane heats of forma2on increase in regular pa7erns ~ - 5 kcal/mol for each C 2 Basis for Benson and other group increment schemes

3 (C 2 ) 6 6*(-4.93) = kcal/mol - Calculated kcal/mol - Experiment ow should one derive other group increments? n-octane 3 C-(C 2 ) 6 -C kcal/mol - cyclohexane (C 2 ) kcal/mol = (C 3 ) kcal/mol ~ -10 kcal/mol per C 3 Rela2onships between heats of forma2on assist in our understanding of alkane thermochemistry

4 3 C C C 3 C 3 isobutane (C 3 ) 3 C kcal/mol - 3 (C 3 ) 3 * ( kcal/mol) kcal/mol C kcal/mol ~ -2 kcal/mol group increment 3 C C 3 C C 3 C 3 neopentane (C 3 ) 4 C kcal/mol - 4 (C 3 ) 4 * ( kcal/mol) kcal/mol C kcal/mol ~ 0 kcal/mol group increment Deriva2on of addi2onal group increments

5 C 3 C 3 or 3 C "skew" interaction C 3 C 3 C 3 2 "skew" interactions This unfavorable interactions must also be corrrected in group increment schemes 2 skews 1 skew - = 1 skew kcal/mol kcal/mol kcal/mol Positive value indicative of destabilization (strain) Some alkanes are strained from unfavorable steric interac2ons, which increases their heat of forma2on

6 3 C C 3 3 C 3 C 3 C C 3 C 3 C 3 Which of these compounds is more strained? Why? What are their respective heats of formation?

7 3 C C 3 3 C C 3 2 skew interactions 3 C C 3 C 2 C 2 C kcal/mol C 3 3 C C 3 4 skew interactions 3 C C kcal/mol Remember: Think in 3- dimensions!

8 2, cat. AlBr 3 low temp. mixture of isomers -12 kcal/mol driving force expt f x -5 = x -2 = -8 Benson = -58 kcal/mol AlBr 3 higher temp. C 3 Thermal rearrangement of anthracene to tetramethyl- adamantane 4 x -10 = x -5 = x 0 = 0 Benson = -70 kcal/mol AlBr 3 even higher temp. 3 x -10 = x -5 = x -2 = -2 3 x 0 = 0 2 skew = +2 Benson = -65 kcal/mol AlBr 3 even higher temp. 1 x -10 = x -5 = x -2 = -10 Benson = -60 kcal/mol Benson helps to understand reac2ons also.

9 2 skews 2 skews 2 skews Acyclic Cyclic Polycyclic Do all of these skew interac2ons destabilize systems to the same extent?

10 2 skews 2 skews 2 skews Acyclic Cyclic Polycyclic ~1.4 kcal/mol ~2.0 kcal/mol ~3.0 kcal/mol Systems have different flexibili2es, which can minimize repulsive interac2ons. Skew interac2ons are less destabilizing in acyclic and cyclic systems because methyl groups can bend away

11 R Does the size of subs2tuents always influence the strain? R R R = C 3 R = C 2 5 R = i-c 3 7 R = t-c 4 9, kcal/mol R Why is the cis/trans isomeriza2on energy different from t- butyl than for other bulky groups like isopropyl?

12 3 C R= C 3 - bumping 3 C C 3 R=i-C bumping C 3 3 C C 3 3 C R=C bumping 3 C C 3 3 C C 3 R=t-C 4 9 C 3 -C 3 bumping C 3 Favorable arrangements exist for all subs2tuent groups except t- butyl, which has a much larger cis/trans isomer difference of 10 kcal/mol (versus 1 kcal/mol for others) Bulky subs2tuents aren t always less favorable!

13 ow would the environment affect strain for different R groups? R R R R = Me ~ Et ~ ipr << tbu R = Me ~ Et << ipr << tbu R = Me << Et << ipr << tbu R' R'' R' Different environments result in varying amounts of strain based on subs2tuent type. Degree of strain determined by the number of skew interac2ons.

14 kcal/mol kcal/mol kcal/mol kcal/mol kcal/mol kcal/mol kcal/mol kcal/mol kcal/mol Branching is known to stabilize alkanes (calimetric studies by Rossini in the 1930s). What is the physical origin of the stabiliza2on?

15 kcal/mol kcal/mol kcal/mol kcal/mol kcal/mol kcal/mol At least in part, this is due to stabilizing van der Waals interac2ons between C and atoms on 1,3- disposed carbon atoms kcal/mol kcal/mol kcal/mol Computa2onally, very sensi2ve to electron correla2on. F/6-31G* MP2/6-31G* Expt n- pentane 0.0 kcal/mol 0.0 kcal/mol 0.0 kcal/mol 2- methylbutane +0.6 kcal/mol kcal/mol kcal/mol neopentane kcal/mol kcal/mol kcal/mol

16 3 favorable branching interactions 4 favorable branching interactions 6 favorable branching interactions 1,3- alkyl- alkyl interac2ons stabilize branched alkanes over their linear counterparts. Alkanes with lower heats of forma2on tend to possess greater numbers of these interac2ons.

17 isobutane 3 favorable 1,3-interactions n-butane 2 favorable 1,3-interactions 1 branch point kcal/mol kcal/mol kcal/mol neopentane 6 favorable 1,3-interactions kcal/mol n-pentane 3 favorable 1,3-interactions kcal/mol 3 branch points kcal/mol propane 1 favorable 1,3-interaction??? kcal/mol kcal/mol per branch 1,3- alkyl- alkyl interac2ons are also present in linear alkanes. These type of branching interac2ons were termed protobranching (proto: being the parent form of)

18 1 protobranch 2 protobranches 3 protobranches 4 protobranches But how do we determine the quantitative value of a protobranching interaction? 5 protobranches Isodesmic bond seperation reaction (conceived by John Pople* in the 1970s) + C 4 2 C kcal/mol Value of 1 protobranch Seperate the compound of interest into the simplest two heavy atom bonds. Balance with methane. * John Pople was the co-recipient of the 1998 Nobel Prize in Chemistry 1,3- alkyl- alkyl interac2ons are also present in linear alkanes. These type of branching interac2ons were termed protobranching (proto: being the parent form of)

19 The energy of protobranching interac2ons are addi2ve for n- alkanes, but are a7enuated for branched systems.

20 Changes in the number of branches are par2cularly difficult to describe computa2onally. Systema2c devia2ons are apparent.

21 less stable, higher energy than "normal" "strained" and/or "anti-aromatic" 0 reference standard "conjugated" and/or "aromatic" more stable, lower energy than "normal" What standard to use? What is normal? eats of forma2on are based on elements in their standard state. (graphite for C, 2 (gas) for ) D f of alipha2cs are nega2ve [C 2 6 = kcal/mol] D f of aroma2cs are posi2ve [C 6 6 = kcal/mol]

22 "Standards" for the assessment of strain should be selected based on "strain-free" structural features. -bond angles -torsional angles -non-bonded interactions C C acetyleneic olefinic aroma=c alipha=c Illustration: (C) 6 Acetylenes: 3 C C f = kcal/mol Olefins: (hypothetical, resonance free) f = 51.3 kcal/mol Aromatics: f = 19.8 kcal/mol Aliphatics: (hypothetical, strain free) f = kcal/mol

23 3 C C 3 C 3 3 C C 3 C 3 f C kcal/mol C 3 C kcal/mol At the extreme, the aromatic graphite is 0.5 kcal/mol more stable than the aliphatic diamond! 3 C C C f kcal/mol kcal/mol

24 Unsaturation Strain C kcal/mol kcal/mol heats of formation from group increments C kcal/mol kcal/mol -5.8 kcal/mol Note that none of these compounds are strained appreciable by angle, torsional, or non- bonded interac=ons Double and triple CC bonds are unfavorable vs single CC bonds!

25 Baeyer Strain angle deviation Ideal tetrahedral value vs. cyclic rings

26 Baeyer strain of Cycloalkanes and Silacycloalkanes. Based on Baeyer s assump=on that all rings are planar.

27 Strain in Cyclobutane 2.0Å Angle Strain = 19.4 degrees Torsion Strain = Planar rings have eclipsed bonds 1.55Å -1.3 kcal/mol "puckers" 1,3 - transannular CC interactions destabilize cyclobutane (sum of van der Waals radii ~3.2Å)

28 Determining Strain Energies Cyclopropane 3 - C 2 groups 3 * (-5.0) = kcal/mol expected heat of formation Actual heat of formation = 12.7 kcal/mol (-15.0) = 27.7 kcal/mol Ring strain of cyclopropane Cyclobutane 4 - C 2 groups 4 * (-5.0) = kcal/mol expected heat of formation Actual heat of formation = 6.8 kcal/mol (-20.0) = 26.8 kcal/mol Ring strain of cyclobutane Benson group increments can be used to derive strain energies (-25.0) = 6.7 kcal/mol Ring strain of cyclopentane (-30.0) = 0.6 kcal/mol Ring strain of cyclohexane

29 omodesmotic Equations* + 3 C (-20.0) 3(-25.0) + 4 C (-20.0) 4(-25.0) + 5 C kcal/mol 26.8 kcal/mol 6.7 kcal/mol The same values can reproduced directly using chemical equa=ons coupled with experimental (or computa=onally determined) heats of forma=on (-20.0) 5(-25.0) + 6 C kcal/mol But are these equa=ons appropriate? (-20.0) 6(-25.0) * a reaction which has: (a) equal numbers of each type of carbon-carbon bond [C(sp3)-C(sp3), etc.] in reactants and products and (b) equal numbers of each type of carbon atom (sp3, sp2, sp) with one, two, or three atoms attached What about the reference compounds?

30 The "Art" of Balancing Chemical Reactions no stabilizing geminal interactions in cyclopropane + 3 C propane stabilized by protobranching (1,3-alkyl-alkyl interaction) 2 cross-ring geminal C-C interactions + 4 C protobranches 5 geminal 1,3-CC interactions + 5 C protobranches 6 geminal 1,3-CC interactions + 6 C protobranches

31 Isodesmic* vs. omodesmotic Approaches + 3 C kcal/mol The strain energy of cyclopropane Is actually lower than indicated by equa=on 1 Stabilized by 3*(2.8) = 8.4 kcal/mol + 3 C 4 3 C kcal/mol Equa=on 2 gives a revised value which is free of contamina=on No stabilization problem because reference compounds have no (proto)branching *a reaction in which all bonds between heavy (non-hydrogen) atoms are seperated into the simplest (or parent) molecule with the same type of bond The strain of the carbon skeleton may be even lower because of torsional strain (6 eclipsed C- bonds)

32 This homodesmotic equation is flawed + 4 C kcal/mol 2 protobranches 4 protobranches This isodesmic equation is flawed + 4 C 4 4 C kcal/mol 2 protobranches 0 protobranches This equation is properly balanced + 2 C kcal/mol 2 protobranches 2 protobranches

33 + 5 C kcal/mol 5 protobranches 5 protobranches + 6 C kcal/mol 6 protobranches 6 protobranches omodesmo=c equa=ons balance branching interac=ons for cyclopentane and larger cycloalkanes.

34 Conjuga=on and hyperconjuga=on are known to stabilize hydrocarbon systems containing mul=ple bonds. Prototype hyperconjugated hydrocarbons (also carbocations) Prototype conjugated hydrocarbons C C C 2 sigma pi Antibonding Partial occupation of antibonding orbital Bonding

35 + C kcal/mol + C kcal/mol yperconjuga=on is generally assessed by homodesmo=c equa=ons like these. Do the energies seem reasonable? What stereoelectronic effects are present in each equa=on?

36 + C kcal/mol 1 "ene" hyperconjugation 1 protobranch + C "yne" hyperconjugation 1 protobranch 4.9 kcal/mol omodesmo=c assessments (equa=ons 1 and 2) are imbalanced. + C kcal/mol Equa=ons 3 and 4 are more appropriate! 1 "ene" hyperconjugation + C kcal/mol 1 "yne" hyperconjugation

37 (-25.9) kcal/mol "textbook" description of diene conjugation (-69.6) 0.0 kcal/mol??? Are textbook defini=ons of conjuga=on energies correct?

38 kcal/mol kcal/mol Are hyperconjugation and protobranching interactions fully balanced? + 2 C kcal/mol + 2 C kcal/mol (27.1 kcal/mol) More reasonable value, but requires correction of 12 kcal/mol for differences in C- bond hydribidzation Balancing all interac=on is cri=cal for obtaining reasonable energies, the later values for diene and diyne conjuga=on have been confirmed by independent theore=cal methods.

39 Mini Quiz 4 1. Using what you have learned about alkane stabili2es, rank the following alkanes in order of lowest (most nega2ve) to highest heat of forma2on. 2. Write appropriate equa2ons to es2mate the cage strain of tetrahedrane and adamantane. C C C C tetrahedrane C 2 C 2 C C 2 C 2 C C C 2 adamantane int: Start with isobutane to balance C groups and propane to balance C 2 groups C 2 C