Stereochemistry of Alkanes and Cycloalkanes

Transcription

1 Stereochemistry of Alkanes and ycloalkanes onformation of ethane: Rotation is possible around - Different arrangements of atoms that can be converted into one another by rotation about single bonds are called conformations Each specific structure is called a conformer or conformational isomer onformational isomers can be represented in two ways: Newman representation Sawhorse representation

2 onformational isomers cannot be isolated (fast interconversion) Staggered Eclipsed Staggered Intermediate geometries are called skew conformations The angle between - bonds on front and back carbons is called the dihedral angle: (0 to 0 ). - eclipsed interaction = kj/mol or kcal/mol onformations of propane and butane Propane. staggered eclipsed.0 kcal/mole. kcal/mole.0 kcal/mole Eclipsed conformation: (- =.0 kcal/mol) =.0 kcal/mol - =. kcal/mol =. Total =. kcal/mol

3 n-butane Staggered conformations have lower torsional energy and are more stable than eclipsed conformation. eclipsed anti gauche eclipsed

4 The most favorable conformation for any alkane is the one in which the - bonds have staggered arrangements and in which large substituents are arranged anti to each other. Energy ost Interaction ause kj/mole kcal/mol eclipsed Torsional strain.0.0 eclipsed Mostly Torsional strain.0. eclipsed Torsional plus Steric strain.5 gauche Steric strain At room temperature, rotation around σ bonds occur rapidly, so all conformers are present. At any given instant there is greater % of the more stable isomer. onformation and Stability of ycloalkanes: The Baeyer Strain theory Baeyer suggested that, since carbon prefers tetrahedral geometry (~09 ), ring sizes other than 5 and may be too strained to exist yclopropane yclobutane yclopentane

5 Total energy of a compound - energy of a strain-free reference compound = extra energy in the compound due to strain. eat of combustion or energy released when a compound burns with oxygen helps to determine energy strain. The more strained the compound the more heat of combustion (more energy is released). yclohexane is strain free. See figure.8 and.9. ycloalkanes adopt their minimum energy conformations for a combination of three reasons: Angle strain, strain due to expansion or compression of bond angles Torsional strain, strain due to eclipsing of bonds in neighboring atoms (bonding electrons repel each other) Steric strain, strain due to repulsive interactions when atoms are too close yclopropane Poor overlap of atomic orbitals in the formation of - bonds. 09 typical - bond bent cyclopropane bond -- bond angle can not be 09.5, but instead 0 (angle strain = 9.5 ). There is less overlap therefore, the bond is weaker than the usual - bond. yclopropane has bent bonds. Total strain energy for cyclopropane =.9 kj/mol 5

6 yclobutane Less angle strain than cyclopropane, but more torsional strain. Total strain energy 0. kj/mol Is not flat, see figure. Almost eclipsed yclopentane Internal angles = 08 ( angle strain) Adopts a puckered out-of-plane conformation Total strain energy =.0 kj/mol (from torsional strain), fig.

7 yclohexane All angles = 09.5 There are different conformations: chair boat twist-boat The chair form is the most stable conformation for cyclohexane. ow to draw a chair conformation? Six axial bonds, one on each carbon, are parallel and alternate up and down Six equatorial bonds, one on each carbon, come in three sets of two Parallel lines. hair cyclohexane has no torsional strain (axial) 5 (equatorial) 5 newman projection staggered ethane hair cyclohexanes can rapidly interconvert (ring flip) axial equatorial becomes equatorial becomes axial 7

8 A substituent is always more stable in an equatorial position than in an axial position 5 5 equatorial axial,-diaxial interaction Disubstituted yclohexanes Monosubstituted cyclohexanes usually have the substituent in equatorial position. Disubstituted cyclohexanes more complex situation: Steric effect Example: is-,-dimethylcyclohexane ring flip Two - diaxial interactions = (x.8) 7. kj/mol One gauche interaction =.8 kj/mol Total strain =. kj/mol 8

9 Trans-,-Dimethylcyclohexane I ring flip (I) One gauche interaction =.8 kj/mol (II) Four - diaxial interactions = 5. kj/mol II Boat cyclohexane: Free of angle strain Less stable than chair conformation (steric and torsional strain) 5 Note eclipsing and Steric strain from, interaction Total strain = 9 kj/mol Twist-boat conformation = kj/mol 9