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Cyclohexane: a Strain-Free Cycloalkane

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Cyclohexane: a Strain-Free Cycloalkane Organic Chemistry I Mohammad Jafarzadeh Faculty of Chemistry, Razi University Organic Chemistry, Structure and Function (7th edition) By P. Vollhardt and N. Schore, Elsevier, 2014 1 CYCLOHEXANE: A STRAIN-FREE CYCLOALKANE The chair conformation of cyclohexane is strain free A hypothetical planar cyclohexane would suffer from 12 H–H eclipsing interactions and sixfold bond-angle strain (a regular hexagon requires 120o bond angles). One conformation of cyclohexane, obtained by moving carbons 1 and 4 out of planarity in opposite directions, is in fact strain free. This structure is called the chair conformation of cyclohexane (because it resembles a chair), in which eclipsing is completely absent, and the bond angles are very nearly tetrahedral. o -1 The calculated �H comb of cyclohexane (-944.4 kcal mol ) based on a strain-free (CH2)6 model is very close to the experimentally determined value (-944.5 kcal mol-1). Indeed, the C–C bond strength, �Ho = 88 kcal mol-1 (368 kJ mol-1), is normal. The molecular model of cyclohexane enables to recognize the conformational stability of the molecule. If we view it along (any) one C–C bond, we can see the staggered arrangement of all substituent groups along it. Because of its lack of strain, cyclohexane is as inert as a straight-chain alkane. 2 140 CHAPTER 4 Cycloalkanes 140 CHAPTER 4 Cycloalkanes Model Building 4-3 CYCLOHEXANE: A STRAIN-FREE CYCLOALKANE Model Building The cyclohexane4-3 ringCYCLOHEXANE: is one of the most common A STRAIN-FREE and important CYCLOALKANE structural units in organic chemistry. Its substituted derivatives exist in many natural products (see Section 4-7), and an understandingThe cyclohexane of its conformational ring is one of mobility the most is ancommon important and aspect important of organic structural chemistry. units in organic Table 4-2chemistry. reveals that, Its substitutedwithin experimental derivatives error, exist cyclohexane in many natural is unusual products in that(see itSection is free 4-7), and of bond-anglean understanding or eclipsing ofstrain. its conformational Why? mobility is an important aspect of organic chemistry. Table 4-2 reveals that, within experimental error, cyclohexane is unusual in that it is free The chairof bond-angle conformation or eclipsing of strain. cyclohexane Why? is strain free kcal mol؊1 88 ؍ DH؇ A hypotheticalThe chairplanar cyclohexaneconformation would sufferof cyclohexane from 12 H–H eclipsing is strain interactions free and kcal mol؊1 sixfold bond-angle strain (a regular hexagon requires 1208 bond angles). However, one 88 ؍ DH؇ conformationA hypothetical of cyclohexane, planar obtainedcyclohexane by movingwould suffercarbons from 1 and12 H–H4 out eclipsing of planarity interactions in and opposite sixfolddirections, bond-angle is in fact strain strain (a free regular (Figure hexagon 4-5). requiresThis structure 1208 bondis called angles). the chairHowever, one conformationconformation of cyclohexane of cyclohexane, (because obtainedit resembles by movinga chair), carbonsin which 1 eclipsingand 4 out is ofcom- planarity in pletely absent,opposite and directions, the bond anglesis in fact are strainvery nearlyfree (Figure tetrahedral. 4-5). AsThis seen structure in Table is 4-2,called the the chair 21 calculatedconformation DH 8comb of cyclohexane of cyclohexane (2944.4 (because kcal mol it resembles) based on a strain-freechair), in (CHwhich2)6 modeleclipsing is is com- very closepletely to the absent,experimentally and the determined bond angles value are ( 2very944.5 nearly kcal moltetrahedral.21). Indeed, As theseen C–C in Tablebond 4-2, the strength, calculatedDH 8 5 88 D kcalH 8 mol of2 cyclohexane1 (368 kJ mol (221944.4), is normal kcal mol (Table21) based 3-2). on a strain-free (CH ) model is Chair comb 2 6 Lookingvery at close the molecularto the experimentally model of cyclohexane determined value enables (2 944.5us to kcalrecognize mol2 1the). Indeed, conforma- the C–C bond tional stabilitystrength, of DHthe 8molecule. 5 88 kcal If mol we2 1view (368 it kJ along mol2 (any)1), is normalone C–C (Table bond, 3-2). we can see the Chair staggered arrangementLooking at of the all molecular substituent model groups of along cyclohexane it. We can enables visualize us to this recognize arrangement the conforma- by drawingtional a Newman stability projectionof the molecule. of that Ifview we (Figureview it 4-6).along Because(any) one of C–Cits lack bond, of strain,we can see the cyclohexanestaggered is as inertarrangement as a straight-chain of all substituent alkane. groups along it. We can visualize this arrangement by drawing a Newman projection of that view (Figure 4-6). Because of its lack of strain, Cyclohexanecyclohexane also is as has inert several as a straight-chain less stable alkane. conformations Cyclohexane also adopts other, less stable conformations. One is the boat form, in which car- bons 1 andCyclohexane 4 are out of the plane also in thehas same several direction less (Figure stable 4-7). The conformations boat is less stable than Boat the chair Cyclohexaneform by 6.9 kcal also mol adopts21. One other, reason less stablefor this conformations. difference is the One eclipsing is the boatof eight form, hydro- in which car- A chair and a boat. Do you see gen atomsbons at the1 and base 4 areof theout boat.of the Another plane in isthe steric same hindrance direction (Section(Figure 4-7). 2-9) Thedue boatto the is lessclose stable than them in cyclohexane? Boat the chair form by 6.9 kcal mol21. One reason for this difference is the eclipsing of eight hydro- A chair and a boat. Do you see gen atoms at the base of the boat. Another is steric hindrance (Section 2-9) due to the close them in cyclohexane? H H HH 6 120° H Staggered 5 H H H HHH HHH H H 6 120° H Staggered 4 5 111.4°: 1 H H H H HH 1.536HH Å H H No angle H H strain H H 107.5° 2 3 4 1.121 Å H 111.4°: Staggered 1 H H H H 1.536 Å H No angle H H H H H strain 107.5° AB2 3 1.121 Å H CStaggered Planar cyclohexaneH H Chair Hcyclohexane H 120؇ bond angles; AB(Nearly tetrahedral bond angles; C) 12 eclipsing hydrogens) no eclipsing hydrogens) Planar cyclohexane Chair cyclohexane ,120؇ bondFigure angles; 4-5 Conversion of(Nearly the (A) tetrahedral hypothetical bond planar angles; cyclohexane into the (B) chair conformation) 12 eclipsing hydrogens)showing bond lengths and angles;no eclipsing (C) molecular hydrogens) model. The chair conformation is strain free. Figure 4-5 Conversion of the (A) hypothetical planar cyclohexane into the (B) chair conformation, showing bond lengthsH and angles; (C) molecular model. The chair conformation is strain free. H H H H H CH2 H H H H H H CH2 H Figure 4-6 View along one of the H CH2 H C – C bonds in the chair conforma- tion of cyclohexane. Note the H H H H staggeredFigure arrangement 4-6 Viewof all along one of the H CH2 H substituents.C – C bonds in the chair conforma- tion of cyclohexane. Note the H H H staggered arrangement of all substituents. 3 Cyclohexane also has several less stable conformations Cyclohexane also adopts other, less stable conformations. In boat form, carbons 1 and 4 are out of the plane in the same direction. The boat is less stable than the chair form by 6.9 kcal mol-1. Reason: (a) eclipsing of eight hydrogen atoms at the base of the boat, (b) steric hindrance due to the close proximity of the two inside hydrogens in the boat framework. The distance between these two hydrogens is only 1.83 Å, small enough to create an energy of repulsion of about 3 kcal mol-1 (13 kJ mol-1). This effect is an example of transannular strain, that is, strain resulting4-3 Cyclohexane:from steric A Strain-Freecrowding Cycloalkaneof two groupsCHAPTERacross 4 a ring141 (trans, Latin, across; anulus, Latin, ring). Steric repulsion: transannular strain H H HH HH 1 4 6 5 65 H H 23 H H H H 4 1 H H H H H H 2 3 H H Eclipsing Planar cyclohexane Boat cyclohexane 4 Figure 4-7 Conversion of the hypothetical planar cyclohexane into the boat form. In the boat form, the hydrogens on carbons 2, 3, 5, and 6 are eclipsed, thereby giving rise to torsional strain. HHH H The “inside” hydrogens on carbons 1 and 4 interfere with each other sterically in a transannular interaction. The space-" lling size of these two hydrogens, re! ecting the actual size of their respec- tive electron clouds, is depicted in the ball-and-stick model on the right. proximity of the two inside hydrogens in the boat framework. The distance between these two hydrogens is only 1.83 Å, small enough to create an energy of repulsion of about 3 kcal mol21 (13 kJ mol21). This effect is an example of transannular strain, that is, strain resulting from HHH H steric crowding of two groups across a ring (trans, Latin, across; anulus, Latin, ring). Boat cyclohexane is fairly ! exible. If one of the C–C bonds is twisted relative to another, this form can be somewhat stabilized by partial removal of the transannular interaction. The or or new conformation obtained is called the twist-boat (or skew-boat) conformation of cyclo- hexane (Figure 4-8). The stabilization relative to the boat form amounts to about 1.4 kcal mol21. Boat As shown in Figure 4-8, two twist-boat forms are possible. They interconvert rapidly, with the boat conformer acting as a transition state (verify this with your model). Thus, the boat cyclohexane is not a normally isolable species, the twist-boat form is present in very small amounts, and the chair form is the major conformer (Figure 4-9).
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