Chapter 4

Chapter 4 - Cycloalkanes Names: Cycloalkanes (CH2)n not...

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Unformatted text preview: Cycloalkanes Names: Cycloalkanes (CH2)n not CnH2n+2 Abundant in nature: "rigid scaffolding". Cyclopropane, , , etc. Substituents: Cycloalkyl. Substituted cycloalkanes: single substituent is automatically at "C1". Ethylcyclobutane (no # needed) Alkylcycloalkane or cycloalkylalkane? 1 3 2 4 5 Larger stem controls: 1-Cyclopropylpentane Stereoisomers Cycloalkanes have two sides: "up", "down". With two or more substituents, new type of isomerism: Same side: cis Opposite sides: trans Stereoisomers Br CH3 CH3 Cis-1,2-dimethylcyclopropane F Trans-1-bromo-3fluorocyclohexane Definition of stereoisomers: Same connectivity (not constitutional isomers), but differing arrangement in space. Note: This definition includes all rotamers (anti, gauche, etc.). Operational (practical) definition: Stereoisomers should be stable at room temperature. Rotamers interconvert rapidly by rotation, whereas cis,trans isomerization requires bond breaking. Ring Strain 108 60 90 120 sp3-Carbon wants 109.5 How do we quantify "ring strain"? Need an "unstrained" reference and a measure of energetic content. We get numbers by measuring heats of combustion. ~160 ~160 ~160 Isomers An Application: The Relative Heat Content of the Two Isomeric Butanes Most branched alkanes are slightly more stable than their linear isomers Are cycloalkanes "normal"? Define normal from heat of combustion Hcomb of CH3(CH2)nCH3 Every additional (CH2) increment gives an extra Hcomb ~ -157.4. We can therefore calculate Hcomb (expected) (CH2)n: n x 157.4. Any discrepancy with Hexp equals ring strain. 108 60 90 120 Ring Strain : 1. Bond angle, especially in small rings 2. Eclipsing 3. Transannular, especially in medium sized rings Cyclopropane Eclipsed Strain Relief Through "Banana" Bonds Weakened: 65 kcal/mol Trimethylene diradical Cyclobutane: "Puckering" Reduces Eclipsing Cyclopentane: Envelope Conformation Almost staggered The Unstrained Cyclohexane: A "Chair" Conformation Move C1,4 A Newman View of a Cyclohexane C-C Bond: Staggered! The Cyclohexane Boat is Strained Move C1,4 + 6.9 kcal/mol ...So it Twists. -1.4 kcal mol-1 -1.4 kcal mol-1 But this is only part of its mobility. The molecule "flips" from one chair to another chair form. Cyclohexane Ring Flip H ax H eq Ea = 10.8 G = O H eq H ax H H Complex Movement: Goes through boat Transannular strain Eclipsing strain )( H H HH )( H H Chair Boat + 6.9 kcal mol-1. Boat is a TS. The Chair-Chair Flip Manifold 100,000 times/sec How to Draw the Chair Cyclohexane "down" "up" Equatorial bonds must be parallel to the CC bond(s) "one over" [not the attached one(s), but the next one(s)] The Chair-Chair Flip Causes Equatorial-Axial Exchange G = 0 The two structures are the same. However, what happens in substituted cyclohexanes? Substituted cyclohexanes: G 0 Conformational Analysis: Interplay of energetics of ax-eq substituents. Example: Methylcyclohexane H CH3 G = +1.7 H H gauche eq transannular ) ( H CH3 ax Axial-Equatorial Conformers Anti to ring Gauche to ring Size vs bond length Note: These numbers do not reflect absolute size, but size with respect to transannular and gauche interactions in cyclohexane. The power of conformational analysis: _G may be additive. Consider the dimethylcyclohexanes: CH3 CH3 1,1-Dimethylcyclohexane G = 0 H3C CH3 CH3 CH3 CH3 G = 0 CH3 CH3 But: Cis-1,4-dimethylcyclohexane CH3 diaxial CH3 H3C Trans-1,4-dimethylcyclohexane G = +3.4! diequatorial The largest group often enforces one conformation: ax eq eq ax +1.7 +1.7 ax G = 3.4-5 = -1.6 eq -5 Large substituents, such as tert-Bu, are said to "lock" a conformation. Problem: BrCOOHH3CH3CBrCOOH G = ? BrCOOHH3CH3CBrCOOH Br Me CO2H G = +2.56 Br Me +1.70 CO2H +1.41 -0.55 All-cis-hexamethylcyclohexane: All-trans-hexamethylcyclohexane: Medium Rings Suffer Transannular Strain Bicyclic, Fused, Polycyclic, Polyhedral Alkanes Bridge Locked boat H Bicyclo[2.2.1]heptane (norbornane) H Fusion H cis Bicyclo[4.4.0]decane (decalin), trans and cis Home exercise: Make models and try the ring flip! H trans m.p. 135C ! Strain: 130 kcal mol-1 m.p. 126C Strain: 166 kcal mol-1 Maier, 1978, tetra-t-Butetrahedrane. Substituted C4H4 Eaton, 1964, cubane, C8H8 m.p. 202C m.p. 430C ! Strain: 60 kcal mol-1 Paquette, 1982, dodecahedrane, C20H20, 12 faces Maier, Sekiguchi, 2002, tetrakis(trimethylsilyl)tetrahedrane. Octanitrocubane: a New Explosive and Rocket Fuel Eaton, Adv. Mat., 2000. The Allotropes of Carbon: Cn a truncated icosahedron Benzene Carbon Nanotubes: Novel Materials for the Future ...
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This note was uploaded on 03/21/2012 for the course CHEM 140A taught by Professor Whiteshell during the Fall '04 term at UCSD.

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