Alkanes Ethane Carbons are sp3 hybridized Bonds are s bonds C C bonds 1 54 C H bonds 1 10 Bond angles 109o H H H C C H H H H H H H H H sawhorse projection Newman projection Different arrangements of atoms in a molecule convertible into one another by rotation of groups of atoms about single bonds are called conformations If the energy barrier to the rotation is nil or small the rotation is said to be free or almost free The rotation of the methyl groups around the C C bond in ethane is almost free therefore ethane can exist in an infinite number of conformations As one methyl group rotates relative to the other the energy of the molecule does change the staggered conformation has the lowest energy and the eclipsed conformation has the highest energy 1 Eclipsed conformation has torsional strain probably due to repulsions between electrons in the C H bonds on C 1 and on C 2 Propane C3H8 Rotational barrier 14kJ mole 3 3 kcal mole torsional strain Butane C4H10 Two structural isomers H3C CH2 CH2 CH3 H3C CH CH3 butane bp 0o CH3 isobutane bp 12o 2 Let s examine butane by sighting down the C 2 to C 3 bond of the staggered conformation in which the methyl groups are 180o apart Then we will rotate the rear ethyl group clockwise around the C 2 to C 3 bond until the methyl groups are again 180o apart There will be an infinite number of conformations but we will focus our attention on the staggered and eclipsed ones Sometimes the different conformations are named as follows H H H H H C C H H H H H H H C H C H H H H H H H H H H C H H C H H H H H H C C H H H H H H H H H H H H C H H H C H H H H anti clinal syn periplanar H H C H H H H C H H H H anti clinal H H H H H C C H H H H H anti periplanar syn clinal syn clinal anti periplanar Strain anti periplanar none anti clinal torsional propane synclinal steric methyl groups begin to approach each other less strain than anti clinal syn periplanar steric and torsional most strained conformation 3 Cycloalkanes Adolf von Baeyer cycloalkanes and angle strain The interior angles of regular polygons are given by the following formula angle 180 n 2 n where n is the number of sides to the polygon Sizes of Interior Angles in Degrees 60 90 108 120 129 135 180 The natural angle for tetrahedral carbon is 109o Rings of 5 and 6 carbon atoms occur commonly in nature rings of other sizes are less common Baeyer reasoned that five and six membered rings were common because they did not have much angle strain and therefore were more stable contained less energy than the other rings which deviated from 109o by greater amounts Bayer was right about the 3 and 4 member rings having angle strain They also have torsional strain because the C H bonds are eclipsed or partially eclipsed However rings larger than this are not planar they pucker even the 4 member ring does this to some extent By doing this rings of 5 or more carbons actually have bond angles very close to 109o and no angle strain Examples follow 4 Five member ring Six member ring Eight member ring 5 Ring Strain Strain Energy Per CH As A Function Of Ring Size 2 10 9 2 6 6 2 H C r e p e o m l l a c k y g r e n E n a r t S i 8 6 4 2 0 0 e n o N Angle 1 3 0 9 1 2 1 4 1 2 1 0 2 0 4 0 0 Ring Size Torsional Torsional Steric None Types of Strain The strain that does exist in rings of size 7 13 is a combination of torsional and steric strain All of this is best seen using molecular models 6 Configurational Stereoisomers Different conformations of a molecule are in a sense isomers they have the same molecular formula but are not identical They differ in regard to the orientation of atoms in space not atomic connections stereoisomers Stereoisomers that interconvert by rotation around single bonds are conformational stereoisomers Cyclic Compounds Because of the ring there is not 360o free rotation around single bonds which are part of the ring This leads to stereoisomers that do not interconvert configurational stereoisomers The term stereoisomer without modifier means configurational H C H H C H Br C H H C Br H C H H C H Br C H Br C H cis 1 2 dibromocyclobutane trans 1 2 dibromocyclobutane 7 Cyclohexane Chair Conformation No angle strain 109o No torsional strain totally staggered See McMurry 4th ed Fig 2 14 pg 63 or better make a model Two types of bonds axial and equitorial Axial parallel to axis of circumscribed sphere three up three down alternating Equitorial projecting from equator of sphere See McMurry 4th ed Fig 2 15 pg 64 or better make a model Draw it 1 parallel lines 2 3 4 equitorial bonds parallel to ring bonds 5 axial bonds up and down to make tetrahedral carbons on top on bottom Ring flip a conformational change Axial groups become equitorial equitorial become axial See McMurry 4th ed Fig 2 16 pg 65 or much better make a model 8 Substituted cyclohexanes substituent groups are more stable in equitorial positions owing to 1 3 diaxial interactions steric strain when they are axial The larger the group the greater the 1 3 diaxial interactions In most cases if a cyclohexane has a t butyl substituent almost all molecules will have the t butyl equitorial See McMurry 4th ed Fig 2 17 pg 66 9