《有机化学》课程教学课件（McMurry’s Organic Chemistry, 6th edition）Chapter 04 Stereochemistry of Alkanes and Cycloalkanes

Stereochemistry of Alkanes and Cycloalkanes Based on McMurry's Organic Chemistry,6th edition,Chapter 4

Stereochemistry of Alkanes and Cycloalkanes Based on McMurry’s Organic Chemistry, 6th edition, Chapter 4

Stereochemistry Stereochemistry is a branch of chemistry concerned with the 3-dimensionaLaspects of molecules The different arrangements of atoms resulting from the rotation of the single bonds are called conformations and a specific conformation is called a conformer Conformers interconvert rapidly and a structure is an average of conformers H H 2004 Thomson/Brooks Cole

Stereochemistry „ Stereochemistry is a branch of chemistry concerned with the 3-dimensional aspects of molecules „ The different arrangements of atoms resulting from the rotation of the single bonds are called conformations and a specific conformation is called a conformer „ Conformers interconvert rapidly and a structure is an average of conformers

Representing Conformations ■ Sawhorse representations view the C-C bond from an oblique angle Newman projections view the C-C bond directly end-on Sawhorse representation and represent the two C atoms on this particular C-C Back carbon by a circle Bonds to front carbon are lines going to the center Front carbon Bonds to rear carbon are Newman lines going to the edge of projection the circle

Representing Conformations „ Sawhorse representations view the C-C bond from an oblique angle „ Newman projections view the C-C bond directly end-on and represent the two C atoms on this particular C-C by a circle „ Bonds to front carbon are lines going to the center „ Bonds to rear carbon are lines going to the edge of the circle

Ethane's Conformations There barrier to rotation between conformations is small (12 kJ/mol)The most stable conformation of ethane has all six C-H bonds away from each other(staggered) The least stable conformation has all six C-H bonds as close as possible (eclipsed)in a Newman projection- energy due to torsional strain Eclipsed conformers 4.0 kJ/mol HH 八 12 k,J/mol 4.0 kJ/mo 4.0 kJ/mol Ethane-eclipsed conformation

Ethane’s Conformations „ There barrier to rotation between conformations is small (12 kJ/mol) The most stable conformation of ethane has all six C–H bonds away from each other (staggered ) „ The least stable conformation has all six C–H bonds as close as possible (eclipsed) in a Newman projection – energy due to torsional strain

Conformations of Propane Propane(CaHa)torsional barrier around the carbon- carbon bonds 14 kJ/mol ■ Eclipsed conformer of propane has two ethane-type H- H interactions and an interaction between C-H and C- C bond 6.0 kJ/mol CH Rotate rear carbon60° 、HH HH 4.0 k.J/mol 4.0 k.J/mol ered propane Eclipsed propane In butane,not all the staggered conformations have the same energy,and not all the eclipsed conformations have the same energy

Conformations of Propane „ Propane (C 3 H 8) torsional barrier around the carbon– carbon bonds 14 kJ/mol „ Eclipsed conformer of propane has two ethane-type H– H interactions and an interaction between C–H and C– C bond „ In butane, not all the staggered conformations have the same energy, and not all the eclipsed conformations have the same energy

Conformations of Butane 女人因术女人因 Gauche Gauche Anti 60 60 2 Ant Dihedral ne between ■ anti conformation has two methyl groups 180away from each other ■ Rotation around the C2-C3 gives eclipsed conformation ■ Staggered conformation with methyl groups 60apart (causing steric strain)is gauche conformation

Conformations of Butane „ anti conformation has two methyl groups 180° away from each other „ Rotation around the C2–C3 gives eclipsed conformation „ Staggered conformation with methyl groups 60° apart (causing steric strain) is gauche conformation

Stability of Cycloalkanes since carbon prefers to 120 have bond angles of ~109°，ring sizes other than five and six are highly strained (angle strain) 40 ■ Rings are strained due to bond bending distortions 34567891011121314 and steric interactions Ring size l09°(tetrahedral 19 *499 90 1089 Cyclopropane Cyclobutane Cyclopentane 2004 Thomson-Brooks/Cole

Stability of Cycloalkanes „ since carbon prefers to have bond angles of ~109°, ring sizes other than five and six are highly strained (angle strain ) „ Rings are strained due to bond bending distortions and steric interactions

The Nature of Ring Strain ■ Rings larger than 3 atoms are not flat Cyclic molecules can assume nonplanar conformations to minimize angle strain and torsional strain by ring- puckering (a) (b) H (c)Not quite eclipsed H H H H H H H H Not quite eclipsed Larger rings have many more possible conformations than smaller rings and are more difficult to analyze

The Nature of Ring Strain „ Rings larger than 3 atoms are not flat „ Cyclic molecules can assume nonplanar conformations to minimize angle strain and torsional strain by ring￾puckering „ Larger rings have many more possible conformations than smaller rings and are more difficult to analyze

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Summary:Types of Strain Angle strain-expansion or compression of bond angles away from most stable Torsional strain-eclipsing of bonds on neighboring atoms Steric strain-repulsive interactions between nonbonded atoms in close proximity

Summary: Types of Strain „ Angle strain - expansion or compression of bond angles away from most stable „ Torsional strain - eclipsing of bonds on neighboring atoms „ Steric strain - repulsive interactions between nonbonded atoms in close proximity