Cycloalkanes

Name ______________________________________ Today's class will focus on the conformations and energies of cycloalkanes. Section 6.4 of the text provides an introduction. 1 Make a model of cyclopentane. Use the 2-piece "lockable" bond connectors — these rotate more easily than the smaller ones. You may need to twist them to "lock" so that they don't pop apart. The text shows two conformations of this ring — the envelope and the half-chair. Why is cyclopentane non-planar? The two conformations are below. In the envelope, 4 Cs are in a plane; in the half-chair 3 Cs are in a plane. Th first conformer is easy. To get your model to look like the second one, start with a planar ring, keep the "front" 3 Cs in the plane, and twist the "back" C–C bond so that one C goes above that plane and the other goes below. Add Hs to each of these structures — no wedges and dashes here — these are "perspective" drawings (like the drawings of chair cyclohexane rings on p 197). Just use your model and sketch the Hs where they appear to be. fast If we were to replace one H in each of these conformers with a methyl group, what do you think would be the most favorable position for it? Circle the one H on each structure above that you would replace with a CH3. In general, what happens to the ring C–C–C angles when a ring goes from planar to non-planar? Lecture outline Structures of cycloalkanes — C–C–C angles if planar: 60° actual struct: planar 90° 108° |— slightly non-planar —| 120° |— substantially non-planar —> A compound is strained if it is destabilized by: abnormal bond angles — van der Waals repulsions — eclipsing along σ-bonds — A molecule's strain energy (SE) is the difference between the enthalpy of formation of the compound of interest and that of a hypothetical strain-free compound that has the same atoms connected in exactly the same way. Cycloalkanes (CH2)n SE (kcal/mol) n= 27.6 26.3 6.3 0.1 6.2 9.7 12.6 12.3 11.3 4.1 5.2 1.9 1.9 3 4 5 6 7 8 9 10 11 12 13 14 15 some unsaturated rings 53.7 29.8 5.9 6.0 1.4 4.8 0.5 (data from EVAnslyn, DADougherty, Modern Physical Organic Chemistry, Univ Science Books, 2006, and THLowry, KSRichardson, Mechanism and Theory in Organic Chemistry, 3rd ed, Harper and Row, 1987.) Cyclopropane C–C bonds are "bent" ("banana bonds") H H C C C H H H H bent bonds, i.e., bonds with poor orbital overlap, are the source of "angle" strain in small rings. Cyclobutane Cyclopentane fast Cyclohexane — Lowest-energy conformer is the chair H H H H H H H H H H H H H H H H H "ring-flip" — fast at room temp (! 4 µs) H H H H H H H The activation energy (Ea) for this process is 10.8 kcal/mol The chair conformer has two distinct sets of Hs — those that point directly "up" and "down" (along the rotational symmetry axis of the ring) are called axial; those that point along the "equator" of the ring are called equatorial. The cyclohexane chair has almost perfect staggering along each C–C bond. This can be seen by looking from the equatorial direction — here's what you see... — "double" Newman projections (sighting along two parallel bonds of the ring at the same time) H H H H CH2 CH 2 H H H H H H H CH 2 CH2 H H H H H Mono-substituted cyclohexanes — A substituent can be equatorial or axial... H H CH3 CH3 The steric repulsion between the axial methyl and one axial H is essentially the same as the gauche interaction between two methyls in butane. In each case we refer to the CHn groups as being "gauche" to each other, but the problem is really a single H - - - H contact in each case. H CH3 H CH2 ring H H CH2 CH3 H CH3 H H In the modeling lab you saw that the magnitude of the 1,3-diaxial interactions depend not on the total mass or "size" of the axial substituent, but on its structure, its preferred conformation and the extent of its steric interaction with the axial Hs. The increase in free energy, ΔG°, that accompanies the eq to ax ring flip has been measured for a large number of monosubstituted cyclohexanes. (As long as the ΔG° is not too large, it's fairly easy to measure the amount of each conformer present at equilibrium. From their ratio, Keq, the ΔG° can be calculated.) This energy increase is essentially a measure of the steric strain caused by the axial substituent clashing with the axial Hs on the same face of the ring. (Of course you found during the modeling lab that some equatorial substituents experience gauche interactions with the nearby ring CH2s, but those same interactions are present in both conformers — make a model and you'll see this.) X as written, this is "uphill" (!G° is positive) X –X = –CH3 –CH2CH3 –iPr –tBu –NH2 –OCH3 –I –Br –Cl –CN –CCH –CH=CH2 –Ph ΔG° = 1.7 kcal/mol 1.8 2.1 > 4.5 1.4 0.6 0.47 0.48 0.53 0.2 0.5 1.7 2.9 (data from FACarey, RJSundberg, Advanced Organic Chem, Part A, 4th ed, Kluwer/Plenum, 2000.) In the lab, you should have seen why the axial steric strain increases very little on going from methyl to ethyl to isopropyl, and then suddenly jumps for tBu. Notice that the values for the halogens are very similar — why do you think that is? Why are the –CN and –OCH3 groups so "small" compared to the methyl? What steric strains would you predict for the following axial substituents? –CCC(CH3)3 –CH2CH2CH2CHClCH(Br)2 –OCH2CH=CH2 –NH3+ Cyclohexane chairs interconvert via "twist-boat" conformation — H H H H chair boat — Energy maximum — unstable due to eclipsing and steric repulsion between "flagpole" Hs chair twist-boat — H H H H about 1.5 kcal/mol more stable than boat, but still about 5.5 kcal/mol less stable than chair Potential energy diagram for ring-flip — Label the energy minima on this diagram with the structures of the two chairs and the twist-boat (the boat is not involved in the chair-to-chair ring flip). E kcal/mol 10.8 5.5