cook.chem.ndsu.nodak.edu/chem341 Chapter 03 Alkanes and Cycloalkanes: Conformations and cis-trans Stereoisomers CHEM 341: Spring 2012 Prof. Greg Cook cook.chem.ndsu.nodak.edu/chem341 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 Conformations • All single bonds freely rotate at room temperature (unless constrained by a ring). • • Thus, linear alkanes are in constant motion. If the molecules were frozen to absolute zero you could see different arrangements of the groups depending on the state of the bond rotations. • Conformers: Different rotational isomers (conformations) of a molecule. • Conformational Analysis: Study of how conformations affect a molecule. 2 ©2012 Gregory R Cook Sunday, January 29, 12 Representation of 3D Structures Sawhorse Projection: A view of a molecule showing wedges and dashes for bonds coming out or going into the plane of the paper - resembles a sawhorse. cook.chem.ndsu.nodak.edu/chem341 • 3 ©2012 Gregory R Cook Sunday, January 29, 12 Representation of 3D Structures Newman Projection: A view of a molecule looking straight down one C-C single bond (see below). cook.chem.ndsu.nodak.edu/chem341 • 4 ©2012 Gregory R Cook Sunday, January 29, 12 Ethane Conformations Ethane Conformations HH C H H H C H H cook.chem.ndsu.nodak.edu/chem341 sawhorse view down this axis to see Newman Projection 60° H H H H 0° rotate front C by 60° H H HH Newman Projection eclipsed higher in energy by 2.9 kcal/mol H Newman Projection staggered Eclipsed H H Eclipsed Eclipsed E 2.9 kcal/mol Staggered 0° Staggered 60° 120° Staggered 180° 240° Staggered 300° 360° rotation 5 ©2012 Gregory R Cook Sunday, January 29, 12 A few more terms cook.chem.ndsu.nodak.edu/chem341 • • 60° H Staggered Conformation: one in which H H the relationship of the groups on H H one carbon versus an adjacent H carbon (front and back on a Newman Projection staggered Newman projection) are aligned 60° apart. Eclipsed Conformation: one in which the relationship of the groups on one carbon versus an adjacent carbon are aligned 0° apart. 0° H H H H HH Newman Projection eclipsed 6 ©2012 Gregory R Cook Sunday, January 29, 12 A few more terms cook.chem.ndsu.nodak.edu/chem341 • • 60° H Torsional Strain: The strain H H introduced by electron repulsion of H H bonds on adjacent carbons. This is H highest when the bonds are eclipsed Newman Projection staggered and lowest when staggered. Steric Strain: The strain introduced when atoms are forced to become close to each other (they bump into each other). 0° H H H H HH Newman Projection eclipsed 7 ©2012 Gregory R Cook Sunday, January 29, 12 anti Butane Conformations Butane Conformations HH H3C cook.chem.ndsu.nodak.edu/chem341 H C H H C CH3 sawhorse view down this axis to see Newman Projection CH3 H anti-methyls - 180° H H CH3 Newman Projection anti-staggered 8 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 anti-Butane 9 ©2012 Gregory R Cook Sunday, January 29, 12 eclipsed Butane Conformations Butane Conformations H CH3 H H H cook.chem.ndsu.nodak.edu/chem341 CH3 Newman Projection anti-staggered CH3 and H rotate front C by 60° CH H3 H3C CH3 and H H H H Newman Projection eclipsed higher in energy by 3.8 kcal/mol 10 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 Eclipsed Butane 11 ©2012 Gregory R Cook Sunday, January 29, 12 gauche Butane Conformations Butane Conformations CH H3 cook.chem.ndsu.nodak.edu/chem341 H3C H rotate front C by 60° H H Newman Projection eclipsed 60° H3C CH3 H H H H 0.9 kcal/mol higher in energy than the antistaggered (lowest energy) conformation Newman Projection gauche-staggered 12 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 Gauche Butane 13 ©2012 Gregory R Cook Sunday, January 29, 12 eclipsed Butane Conformations Butane Conformations H3C CH3 H H rotate front C by 60° H cook.chem.ndsu.nodak.edu/chem341 H Newman Projection gauche-staggered CH3 and CH3 CH CH33 H H H H 4.5 kcal/mol higher in energy than the anti-staggered conformation Newman Projection eclipsed 14 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 syn-eclipsed Butane 15 ©2012 Gregory R Cook Sunday, January 29, 12 Butane Energy Profile H CH3 H cook.chem.ndsu.nodak.edu/chem341 CH3 60° CH H3 H H3C H H3C H H H CH3 H CH CH33 H H H 60° CH3 H3C H H H H H CH H3 H H H H3C H H CH3 H H H H H CH3 Eclipsed Methyls Aligned Eclipsed Eclipsed E 3.8 kcal/mol Staggered anti 0° 4.5 kcal/mol Staggered gauche 60° 120° Staggered gauche 180° rotation ©2012 Gregory R Cook Sunday, January 29, 12 0.9 kcal/mol 240° Staggered anti 300° 360° 16 A few more terms cook.chem.ndsu.nodak.edu/chem341 • • Anti Conformation: one in which the two groups on adjacent carbons are as far apart as possible (180°) in a staggered conformation. Gauche Conformation: one in which the two groups on adjacent carbons are still staggered, but closer (60° apart). H CH3 H H H CH3 Newman Projection anti-staggered 60° H3C CH3 H H H H Newman Projection gauche-staggered 17 ©2012 Gregory R Cook Sunday, January 29, 12 Higher Alkanes • The most stable conformation in longer chain alkanes is the all-anti conformation H H cook.chem.ndsu.nodak.edu/chem341 H H C H H C C H C C H C H H H H H H 18 ©2012 Gregory R Cook Sunday, January 29, 12 Cyclic Compounds cook.chem.ndsu.nodak.edu/chem341 • Ring Strain • Angle Strain: the strain due to bond angles being forced to expand or contract from their ideal. • Torsional Strain: the strain due to electron repulsion of eclipsing bonds. • Steric Strain: the strain due to atoms coming too close. 19 ©2012 Gregory R Cook Sunday, January 29, 12 Cyclic Compounds Heat of Combustion: the amount of heat (energy) released when a molecule burns completely with oxygen. Ring Strain cook.chem.ndsu.nodak.edu/chem341 • 3 4 5 6 7 8 Ring Size 9 10 11 12 13 14 20 ©2012 Gregory R Cook Sunday, January 29, 12 Cyclopropane cook.chem.ndsu.nodak.edu/chem341 • Highest amount of angle strain 60° 21 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 Cyclopropane 22 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 Cyclobutane 23 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 Cyclopentane 24 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 Cyclohexane 25 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 Axial and Equatorial Positions of Cyclohexane Pink - Axial Blue - Equatorial 26 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 Ring Flips in Cyclohexane 27 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 Energy of Chair Flip Conformations 28 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 Cyclohexane Axial Positions More Crowded 1,3-diaxial interaction axial methyl H CH3 ring flip H H H H equatorial methyl H CH3 H H H H 1.8 kcal/mol more stable conformation 29 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 axial Methyl Cyclohexane 30 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 equatorial Methyl Cyclohexane 31 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 methylcyclohexane Ring Flips 32 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 A few more terms • Stereoisomers: Isomers (different compounds) that have all the same number and kind of atoms that are all connected the same, but differ in their arrangement in three dimensions. • Due to the restricted rotation in cycloalkanes, molecules with more than one substituent could have the groups either on the same side (cis) or opposite sides (trans) of the plane of the ring. These are stereoisomers. 33 ©2012 Gregory R Cook Sunday, January 29, 12 cis and trans Cycloalkanes cook.chem.ndsu.nodak.edu/chem341 Br Br H H Br trans-1,2-dibromocyclopropane Br Br H Br H cis-1,2-dibromocyclopropane 34 ©2012 Gregory R Cook Sunday, January 29, 12 cis and trans dimethylcyclopentane H3C H3C CH3 CH3 = H H cook.chem.ndsu.nodak.edu/chem341 cis-1,3-dimethylcyclopentane H3C H3C CH3 H = H CH3 trans-1,3-dimethylcyclopentane 35 ©2012 Gregory R Cook Sunday, January 29, 12 dimethylcyclohexane ax CH3 cis-1,2-dimethylcyclohexane 1 2 CH3 2 CH3 cook.chem.ndsu.nodak.edu/chem341 1 H CH3 1 2 CH3 2 H CH3 ax eq H H ring flip H 1 CH3 H same interactions in both conformations -equal in energy ax CH3 trans-1,2-dimethylcyclohexane 1 2 H CH3 eq ring flip ax CH3 2 1 CH3 eq CH3 eq H lower in different interactions in both energy conformations -- NOT equal in energy 36 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 cis-1,2-dimethylcyclohexane 37 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 cis-1,2-dimethylcyclohexane 38 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 trans-1,2-dimethylcyclohexane 39 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 trans-1,2-dimethylcyclohexane 40 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 trans-1,2-dimethylcyclohexane 41 ©2012 Gregory R Cook Sunday, January 29, 12 cis-1,3-dimethylcyclohexane cis-1,3-dimethylcyclohexane 1 ax CH3 CH3 cook.chem.ndsu.nodak.edu/chem341 H 3 CH3 trans-1,3-dimethylcyclohexane CH3 H 1 CH3 1 ring flip H 3 3 ax CH3 3 3 H different interactions in both conformations -- NOT equal in energy 1 CH3 H eq lower in energy ax CH3 ring flip 1 H eq H3C eq H3C H 3 ax CH3 1 CH3 eq H same interactions in both conformations -equal in energy 42 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 cis-1,3-dimethylcyclohexane 43 ©2012 Gregory R Cook Sunday, January 29, 12 1,4-dimethylcyclohexane cis-1,4-dimethylcyclohexane 1 CH3 4 1 H3C eq H3C cook.chem.ndsu.nodak.edu/chem341 ax CH3 4 H3C 4 H 1 CH3 eq same interactions in both conformations -- H equal in energy H ax CH3 CH3 1 H ring flip H 4 trans-1,4-dimethylcyclohexane 1 ax CH 3 4 ax CH3 H H ring flip H3C eq different interactions in both conformations -- NOT equal in energy 4 1 CH3 lower in energy eq H 44 ©2012 Gregory R Cook Sunday, January 29, 12 1,4-dimethylcyclohexane cis-1-isopropyl-4-methylcyclohexane CH3 CH 1 4 cook.chem.ndsu.nodak.edu/chem341 H3C H3C ax CH3 CH ax CH 3 ring flip 1 CH3 4 H3C eq H CH3 4 H 1 CH H The larger group would prefer to be in the equatorial position -NOT equal in energy H lower in energy eq CH3 H Y 1,3-diaxial interaction Y kcal/mol strain Y kcal/mol strain -F 0.12 -CH3 0.9 -Cl 0.25 -CH2CH3 0.95 -Br 0.25 -CH(CH3)2 1.1 -OH 0.5 -C(CH3)3 2.7 45 ©2012 Gregory R Cook Sunday, January 29, 12 Boat Conformations cook.chem.ndsu.nodak.edu/chem341 boat cyclohexane H H H H H H H H norbornane 46 ©2012 Gregory R Cook Sunday, January 29, 12 cook.chem.ndsu.nodak.edu/chem341 Sugar Structure • Glucose H O H HO OH H H OH H OH H OH H O HO HO H H OH H OH CH2OH 47 ©2012 Gregory R Cook Sunday, January 29, 12 Polysaccharides Starch cook.chem.ndsu.nodak.edu/chem341 • 48 ©2012 Gregory R Cook Sunday, January 29, 12 Polycyclic Molecules cook.chem.ndsu.nodak.edu/chem341 fused spiro bridged CO2R O H H R H OR H H H R 49 ©2012 Gregory R Cook Sunday, January 29, 12 Heterocyclic Molecules cook.chem.ndsu.nodak.edu/chem341 ethylene oxide aziridine O furan O pyrrolidine tetrahydropyran piperidine O H N O H N H N tetrahydrofuran H N pyrrole O pyran N H N pyridine O morpholine 50 ©2012 Gregory R Cook Sunday, January 29, 12