《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 ringpuckering Larger rings have many more possible conformations than smaller rings and are more difficult to analyze

H- H 新轮 Planar Structure Boat Conformation Twist Boat Conformation h当tan币mation Wn情女h(120月 状n抽女当 址n★女山 og中女当 wn●dp时女h(bon) dph刺女In at bo bonds mall edipsing女n no edipsing stzain small sberic s女an sbericing of bro hpdrogens mk女当 ntk女当h

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
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