1 / 85

Stereochemistry of organic compounds Molecules in three dimensions

Stereochemistry of organic compounds Molecules in three dimensions. A lkanes conformation. Stereochemistry concerned with the 3-D aspects of molecules  bonds are cylindrically symmetrical Rotation is possible around C-C bonds in chain molecules. Staggered conformation.

jordanf
Download Presentation

Stereochemistry of organic compounds Molecules in three dimensions

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Stereochemistry of organic compounds Molecules in three dimensions

  2. Alkanesconformation • Stereochemistry concerned with the 3-D aspects of molecules •  bonds are cylindrically symmetrical • Rotation is possible around C-C bonds in chain molecules Staggered conformation Eclipsed conformation

  3. Ethane • Conformation- Different arrangement of atoms resulting from rotation around σbond • Conformations can be represented in 2 ways: Staggered conformation

  4. Torsional Strain • We do not observe perfectly free rotation • There is a barrier to rotation, and some conformers are more stable than others • Staggered- most stable: all 6 C-H bonds are as far away as possible • Eclipsed- least stable: all 6 C-H bonds are as close as possible to each other

  5. Conformers energy

  6. Conformations of Other Alkanes • The eclipsed conformer of propane has 3 interactions: two ethane-type H-H interactions, and one H-CH3 interaction

  7. Conformations of Other Alkanes • Conformational situation is more complex for larger alkanes • Not all staggered conformations has same energy, and not all eclipsed conformations have same energy

  8. Anti conformation- methyl groups are 180˚ apart • Gauche conformation- methyl groups are 60˚ apart Which is the most energetically stable?

  9. Steric Strain • Steric strain- repulsive interaction occurring between atoms that are forced closer together than their atomic radii allow

  10. Energy cost for torsional and steric strain

  11. Cycloalkanesconformation

  12. Cycloalkanesconformation • Cycloalkanes are less flexible than chain alkanes • Much less conformational freedom in cycloalkanes

  13. Stability of Cycloalkanes: Ring Strain • Rings larger than 3 atoms are not flat • Cyclic molecules adopt 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

  14. Stability of Cycloalkanes: The Baeyer Strain Theory • Baeyer (1885): since carbon prefers to have bond angles of approximately 109°, ring sizes other than five and six may be too strained to exist • Rings from 3 to 30 C’s do exist but are strained due to bond bending distortions and steric interactions

  15. Types of Strain • Angle strain - expansion or compression of bond angles away from most stable (109º) • Torsional strain - eclipsing of bonds on neighboring atoms • Steric strain - repulsive interactions between nonbonded atoms in close proximity

  16. Cyclopropane conformation • 3-membered ring must have planar structure • Symmetrical with C–C–C bond angles of 60° • Requires that sp3based bonds are bent (and weakened) • All C-H bonds are eclipsed

  17. Bonds of cyclopropane are bent • In cyclopropane, the C-C bond is displaced outward from internuclear axis

  18. Cyclobutane conformation • Cyclobutane has less angle strain than cyclopropane but more torsional strain because of its larger number of ring hydrogens • Cyclobutane is slightly bent out of plane - one carbon atom is about 25° above • The bend increases angle strain but decreases torsional strain

  19. Cyclopentane conformation • Planar cyclopentane would have no angle strain but very high torsional strain • Actual conformations of cyclopentane are nonplanar, reducing torsional strain • Four carbon atoms are in a plane • The fifth carbon atom is above or below the plane – looks like an envelope

  20. Conformations of Cyclohexane • Substituted cyclohexanes occur widely in nature • The cyclohexane ring is free of angle strain and torsional strain • The conformation has alternating atoms in a common plane and tetrahedral angles between all carbons • This is called a chair conformation

  21. Conformations of Cyclohexane

  22. How to Draw Cyclohexane

  23. Axial and Equatorial Bonds in Cyclohexane • The chair conformation has two kinds of positions for substituents on the ring: axial positions and equatorial positions • Chair cyclohexane has six axial hydrogens perpendicular to the ring (parallel to the ring axis) and six equatorial hydrogens near the plane of the ring

  24. Axial and Equatorial Bonds • Each carbon atom in cyclohexane has one axial and one equatorial hydrogen • Each face of the ring has three axial and three equatorial hydrogens in an alternating arrangement

  25. Drawing the Axial and Equatorial Hydrogens

  26. Conformational Mobility of Cyclohexane • Chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions by a ring-flip

  27. CyclohexaneConformations Chair conformation is the most stable Boat is the least stable conformation (29 kJ/mol) because of steric and torsional strain

  28. Conformations of Monosubstituted Cyclohexanes • Cyclohexane ring rapidly flips between chair conformations at room temp. • Two conformations of monosubstituted cyclohexane aren’t equally stable. • The equatorial conformer of methylcyclohexane is more stable than the axial by 7.6 kJ/mol

  29. 1,3-Diaxial Interactions • Difference between axial and equatorial conformers is due to steric strain caused by 1,3-diaxial interactions • Hydrogen atoms of the axial methyl group on C1 are too close to the axial hydrogens three carbons away on C3 and C5, resulting in 7.6 kJ/mol of steric strain

  30. Relationship to Gauche Butane Interactions • Gauche butane is less stable than anti butane by 3.8 kJ/mol because of steric interference between hydrogen atoms on the two methyl groups • The four-carbon fragment of axial methylcyclohexane and gauche butane have the same steric interaction • In general, equatorial positions give more stable isomer

  31. Geometry of C=C bond • Carbon atoms in a double bond are sp2-hybridized • Three equivalent orbitals at 120º in plane • Fourth orbital is atomic p orbital • Combination of electrons in two sp2 orbitals of two atoms forms  bond between them • Additive interaction of p orbitals creates a  bonding orbital • Occupied  orbital prevents rotation about -bond • Rotation prevented by  bond - high barrier, about 268 kJ/mole in ethylene

  32. Rotation of  Bond Is Prohibitive • This prevents rotation about a carbon-carbon double bond (unlike a carbon-carbon single bond). • Creates possible alternative structures for substituted C=C bonds

  33. Cis-trans Isomerism in Alkenes • The presence of a carbon-carbon double bond can create two possible structures – 2 stereoisomers • cis isomer - two groups on same side of the double bond • trans isomer - two groups on opposite sides

  34. Whatmolecules can exist as cis-trans stereoisomers Are these molecules cis-trans isomers? And what about these molecules?

  35. Explain when C=C double bond exist in 2 forms: cis and trans

  36. Assigningdouble bondconfiguration • Neither compound is clearly “cis” or “trans” • Substituents on C1 are different than those on C2 • We need to define “similarity” in a precise way to distinguish the two stereoisomers • Cis, trans nomenclature only works for disubstituted double bonds • E/Z Nomenclature for 2, 3 or 4 substituted double bond

  37. E,Z Stereochemical Nomenclature High(C1)-Low(C1)-Hi(C2)-Low(C2)

  38. E,Z Stereochemical Nomenclature • Priority rules of Cahn, Ingold, and Prelog (CIP rules) are used for assigning Higherand Lowersubstituents • Compare where higher priority groups are with respect to bond and designate as prefix • E -entgegen, opposite sides • Z - zusammen, together on the same side

  39. Ranking Priorities: Cahn-Ingold-Prelog Rules RULE 1 • Must rank atoms that are connected at comparison point • Higher atomic number gets higher priority • I > Br > Cl > S > P > F > O > N > C > H

  40. RULE 2 • If atomic numbers are the same, compare at next connection point at same distance • Compare until something has higher atomic number • Do not combine – always compare

  41. RULE 3 • Substituent is drawn with connections shown and no double or triple bonds • Added atoms are valued with 0 ligands themselves

  42. Assigningdouble bondconfiguration Cl > CH3CH2OH> CH2CH3 (Z)-3-chloro-2-ethyl-2-buten-1-ol (E)-3-chloro-2-ethyl-2-buten-1-ol

  43. Cis-trans isomerism in cycloalkanes • For cycloalkanes with 2 substituents at different carbons – 2 orientations of substituents with respect to ring plane are possible • They are also cis-trans (E, Z) stereoisomers

  44. Chirality

  45. What is chirality? • Some objects are not the same as their mirror images (technically, they have no plane of symmetry) • A right-hand glove is different than a left-hand glove. The property is commonly called “handedness” • Some organic molecules have handedness that results from substitution patterns on sp3 hybridized carbon

  46. Molecules that have one carbon with 4 different substituents have a non-superimposable mirror image (these molecules are chiral) • Enantiomers = non-superimposable mirror image stereoisomers

  47. Enantiomers of lactic acid Trying to superimpose these molecules

  48. If an object has a plane of symmetry it’s the same as its mirror image • A plane of symmetry divides an entire molecule into two pieces that are exact mirror images • Achiral means that the object has a plane of symmetry • Molecules that are not superimposable with their mirror images are chiral (have handedness) • Hands, gloves are prime examples of chiral object • They have a “left” and a “right” version • Organic molecules can be Chiral or Achiral

  49. Chiral and Achiral molecules

  50. Chiral Centers • A point in a molecule where four different groups (or atoms) are attached to carbon is called a chiral center (or stereogenic center) • There are two ways that 4 different groups (or atoms) can be attached to one carbon atom • If two groups are the same, then there is only one way • A chiral molecule usually has at least one chiral center

More Related