Isomers and Stereochemistry Essential Organic Chemistry Paula Yurkanis Bruice Chapter 6 Isomers and Stereochemistry

Figure: 08-00-03UN Title: Isomer Flowchart Caption: The relationship between different kinds of isomers. Notes: Stereoisomers fall into two major categories: geometric and three-dimensional.

Review of Isomerism Isomers – Compounds that have the same molecular formula but do not have identical structures. Constitutional Isomers – differ in the way their atoms are connected. Stereoisomers – differ in the way their atoms are arrange in space.

Constitutional Isomers Figure: 08-00-02UN Title: Constitutional Isomers Caption: Constitutional isomers are compounds that have the same molecular formula but differ in the connections of the atoms. Notes: Constitutional isomers generally have different physical properties.

6.1 Cis-trans isomers Differ in the arrangement of their atoms in space (cannot interconvert)- Alkenes.

6.1 Cis-trans isomers Cyclic structure.

6.2 Chirality Chiral – Nonsuperimposable on its mirror image. Achiral – Superimposable on its mirror image. If a molecule (or object) has a mirror plane or an inversion center, it cannot be chiral.

Figure: 08-01 Title: Examples of Chiral and Achiral Objects Caption: Original and mirror images of a hand and a chair. A chiral object is not the same as its mirror image; they are nonsuperimposable. Notes: An achiral object is the same as its mirror image; they are superimposable.

Copyright © 2010 Pearson Education, Inc. Chiral or Achiral? Brandy snifter achiral achiral Copyright © 2010 Pearson Education, Inc.

Copyright © 2010 Pearson Education, Inc. Chiral or Achiral? Shears chiral Copyright © 2010 Pearson Education, Inc.

Copyright © 2010 Pearson Education, Inc. Chiral or Achiral? Beer mug achiral Copyright © 2010 Pearson Education, Inc.

Copyright © 2010 Pearson Education, Inc. Chiral or Achiral? Hiking boot chiral Copyright © 2010 Pearson Education, Inc.

Copyright © 2010 Pearson Education, Inc. Chiral or Achiral? Baseball glove chiral Copyright © 2010 Pearson Education, Inc.

Copyright © 2010 Pearson Education, Inc. Chiral or Achiral? Boat propeller chiral Copyright © 2010 Pearson Education, Inc.

Copyright © 2010 Pearson Education, Inc. Chiral or Achiral? Desk chair achiral Copyright © 2010 Pearson Education, Inc.

Copyright © 2010 Pearson Education, Inc. Chiral or Achiral? School desk chiral Copyright © 2010 Pearson Education, Inc.

Copyright © 2010 Pearson Education, Inc. Chiral or Achiral? cis-1,3-dimethylcyclopentane achiral mirror plane Copyright © 2010 Pearson Education, Inc.

Copyright © 2010 Pearson Education, Inc. Chiral or Achiral? trans-1,3-dimethylcyclopentane chiral Copyright © 2010 Pearson Education, Inc.

Copyright © 2010 Pearson Education, Inc. Chiral or Achiral? 1,1-dimethylcyclohexane achiral Copyright © 2010 Pearson Education, Inc.

6.3 Asymmetric centers An asymmetric center is an atom that is bonded to four different groups.

Chiral or Achiral? Asymmetric center 2-butanol chiral mirror image

Chiral or Achiral? 2-bromopropane achiral

6.4 Isomers with one asymmetric center

6.4 Isomers with one asymmetric center

Enantiomers A chiral compound and its mirror image are called enantiomers. 2-butanol: enantiomers

Enantiomers Asparagine: mirror plane enantiomers Vetch photo: http://www.kaweahoaks.com/html/common_vetch.html L-asparagine (from asparagus) bitter taste D-asparagine (from vetch) sweet taste enantiomers

6.5 How to draw enantiomers Figure: 08-01-11UN Title: Perspective Formulas of 2-Bromobutane Caption: Perspective formulas show that two of the bonds are in the plane of the paper (the solid lines) and that the other two bonds are in front of and behind the paper. The wedge shows that the bond is coming out of the paper at you and the hatched bond is going behind the paper. Notes: When you draw these formulas, the hatch and the wedge need to be adjacent to each other. The first image generally places the four groups in any order and the second image is the mirror image of the other one.

6.6 Naming enantiomers: the R,S system Figure: 08-01-14UN Title: Determining Configuration of Chiral Center Step 1 Caption: The first step in determining the configuration of a stereocenter is to number attached groups in descending order of priority using the Cahn-Ingold-Prelog rules. Notes: The higher the atomic number, the higher the priority. This is similar to the E,Z system learned earlier.

Absolute Configuration R and S Assign priorities to the remaining groups based on atomic numbers. Clockwise (highest to lowest priority)  R Counterclockwise  S (R)-2-butanol

Absolute Configuration Assign priority: Atomic number of atom directly bonded. If the same atom is bonded, go to the next atom, etc. Groups containing multiple bonds are treated as though multiple atoms were attached:

Absolute Configuration Determine the absolute configuration of the following compounds: Copyright © 2010 Pearson Education, Inc.

Absolute Configuration Determine the absolute configuration of the following compounds: Copyright © 2010 Pearson Education, Inc.

Figure: 08-01-20UN Title: Assignment of Configuration for 2-Bromobutane Caption: Using the rules outlined previously, first assign the priorities to each structure. Then make sure the group with the lowest priority is pointing toward the back (in this case, the hatched wedge). Draw an arrow from the highest group to the lowest group and determine if it is going to the right or to the left. Notes: If the arrow points to the right (clockwise), the configuration is R and if the arrow points to the left (counterclockwise), the configuration is S.

Figure: 08-01-21UN Title: Configuration of Chiral Center Caption: If the lowest priority is not in the back, switch the two groups (in this case the methyl and the H). Then when you determine the direction of the arrow the actual configuration of the original molecule is the opposite of what you determine. Notes: If in the new diagram the configuration is R, the original was S. If in the new diagram the configuration is S, then the original was R.

6.7 Chiral compounds are optically active Plane-polarized light is produced by passing normal light through a polarizer. Copyright © 2010 Pearson Education, Inc.

Copyright © 2010 Pearson Education, Inc. Optical Activity When plane-polarized light passes through a solution of achiral molecules, the light emerges from the solution with its plane of polarization unchanged. Copyright © 2010 Pearson Education, Inc.

Copyright © 2010 Pearson Education, Inc. Optical Activity However, when plane-polarized light passes through a solution of a chiral compound, the light emerges with its plane of polarization changed. I poached this from the internet a number of years ago, and I can’t find it now. We may need to add our own illustration here. Copyright © 2010 Pearson Education, Inc.

Copyright © 2010 Pearson Education, Inc. Optical Activity Optical Activity – The ability of a compound to rotate the plane of polarized light. A compound that rotates the plane of polarization is said to be optically active. Chiral compounds are optically active and achiral compounds are optically inactive. A polarimeter is used to make such measurements: Copyright © 2010 Pearson Education, Inc.