Counterclockwise (Leftward turning) S. 3 2 Clockwise (Rightward turning) R

Transcription

1 Stereochemistry I and S in Alkanes A chiral carbon has four different groups attached to it, the differences may be several bonds away, but so long as they are different in any way the groups are considered different. o Stereocenters are chiral carbons and vice versa hiral molecules are asymmetric molecules o They do not have an internal mirror plane o They nearly always have at least one stereocenter hiral molecules may lack a mirror plane and have no chiral centers, they exist but are beyond the scope of this course (i.e. helicene) o They are optically active and rotate plane-polarized light Left (counterclockwise) rotation is (-) and levorotory ight (clockwise) rotation is (+) and dextrorotory Individual chiral centers are classified as either or S. o /S designation is completely unrelated to the direction of rotation of plane polarized light. Assigning vs. S stereocenters The Absolute onfiguration: o Working outward from the chiral carbon to each attached group. (ahn-ingold-prelog Priority ules) st priority is assigned by atomic number, heavier atoms get priority In order st then l then and lastly th if isotopes are encountered the heavier isotope gets priority then then If two of the same atom are encountered priority is assigned by the first difference in bonded atoms Multiple bonds count as multiple single bonds to the same atom o nce priority is assigned rotate the lowest priority to the back If proceeds clockwise the center is (right turning) If proceeds counterclockwise the center is S (left/sinister turning) lockwise (ightward turning) ounterclockwise (Leftward turning) S

2 Examples: ompound A: low priority clockwise = ()--bromo--butanol ompound B: low priority counterclockwise = S (S)--fluoro--hydroxypropanoic acid ompound : low priority counterclockwise = S (S)--bromo--methylbutane ompound D: low priority counterclockwise = S (S)--formyl--methylbutanenitrile ompound E: low priority flip over clockwise = (,Z)-cyclohex--en--ol

3 Stereochemistry II E and Z in Alkenes Alkenes can have multiple geometric isomers (non-superimposable, nonmirror images) If there are exactly two substituents and two hydrogens attached an alkene the isomer may be labeled as cis- or trans-. a trans alkene opposite ' vs. ' a cis alkene on same side If there are two or more substituents attached to an alkene the isomer may be labeled as E- or Z- o All cis- are Z-, but not all Z- are cis-; all trans- are E-, but not all E- are trans-. is- and trans- are accepted when appropriate, but E and Z always work. Assigning elative onfiguration: E vs. Z o or each sp carbon in the alkene identify the two substituents and prioritize them and using the ahn-ingold-prelog ules. st priority is assigned by atomic number, heavier atoms get priority In order st then l then and lastly th if isotopes are encountered the heavier isotope gets priority then then If two of the same atom are encountered priority is assigned by the first difference in bonded atoms Multiple bonds count as multiple single bonds to the same atom o If both high priority groups are on the same side of the alkene the molecule is Z. (similar to a cis) This can be remembered by Z is on the Zame Zide. o If the high priority groups are on opposite sides of the alkene the molecule is E. (similar to trans) o If the there is no difference between the groups then the molecule is symmetrical and thus E/Z is unnecessary. (e.g. monosubstituted alkenes) an E alkene st priority opposite Y W Z X vs. Y W Z X a Z alkene st priority on the same side

4 Examples: ompound A: igh priority on the same side (Z)--bromo--pentenal ompound B: igh priority on opposite sides (E)--methyl--hexene ompound : l igh priority l (E)--chloro--ethyl-5-methyl--hexene on opposite sides ompound D: igh priority on the same side (Z)--bromo--butenoic acid ompound E: B A A B (Z,E)-,-heptadiene or cis,trans-,-heptadiene

5 ahn-ingold-prelog Examples: Example : Larger Atomic umber gets Priority < The is attached to The is attached to,,,, < xygen is larger than carbon so the alcohol gets priority, functional groups usually get priority as they tend to have larger atoms. The is attached to,, vs. Example : Longer hains Get Priority < so we go to the next The is attached to The is attached to,,,, < The is attached to,, Longer chains will get priority as any atom will win over. Example : anched hains get Priority The is attached to,, > The is attached to,, > The first attached to each is the same, but the second is larger than. anched chains will usually get priority. Example : irst Difference gets Priority > The is attached to The is attached to,,,, > The is too far away, it is not the first difference. loser differences take priority not the ultimately largest atom. Example 5: Multiple Bonds ount Multiple Times The is attached to,, > > The is attached to,, The triple bond counts as bonds to so is larger than. The is attached to,, = Example 6: Mulitple Bonds ount Extra for Both Atoms The is attached to,, next atom The carbonyl counts as two - bonds from the carbons perspective so we need to go on to the next atom. The is attached to in the diol and back through the second bond to the in the carbonyl. The is attached to > > The is attached to

6 Practice Examples: Assign, S, E, or Z to each molecule. Assign the correct IUPA name to molecules whose functional groups youʼve learned. l A l B S D E G l I J K L M P l l Q S T l I U l V W X

7 Practice Examples Key: A) (Z)-cyclohexene or cis-cyclohexene B) (Z,5S)-5-bromo--chloro--heptene ) (S,S)--bromo--pentanol D) (E)--chloro--isopropyl--methoxy--hexene or (E)--chloro--(-methylethyl)--methoxy--hexene) E) (E,)-,5-dimethyl--hexene ) ()--methyl--methylthio--pentene G) (Z,)--ethyl--methylcyclopentene ) (,S)-,-dibromopentane I) ()--methylhexane J) (Z)--cyano--methyl--pentenoic acid K) (E)-,5,6,6-tetramethyl-,-heptadiene L) (E,S)--(-chloroethyl)--methyl--hexene M) (,)--methoxy--pentanol ) (S)--methylbutanenitrile ) (Z,)--tert-butylcyclohexene or (Z,)--(,-dimethylethyl)cyclohexene P) (S,S)--isopropyl--propylcyclopentane or (S,S)--(-methylethyl)--propylcyclopentane Q) (S,E)--tert-butyl--cycloheptenecarbaldehyde or (S,E)--(,-dimethylethyl)--cycloheptenecarbaldehyde ) ()--butanol S) (,S)--bromo--chlorocyclohexane T) (E,E,7)-7-chloro-5-ethyl--methyl-,-octadiene U) (E,S)-,-dichlorocyclopentene V) (E,6S)--ethyl-,,5,6-tetramethyl--octene W) (E)-,-dimethyl--hexene X) (,S)--iodo--(,,-tribromomethyl)cyclohexane