Last Name First Name MI Total Score Student ID Number Circle the name of your TA HEATHER CARI KAUSHIK RYAN Discussion Section Day 100 Time Chem 30A Fall 2005 MIDTERM 1 50 Min Weds October 26th INTERPRETATION OF THE QUESTIONS IS PART OF THE EXAM DO NOT ASK FOR THE QUESTIONS TO BE EXPLAINED TO YOU ONLY ANSWERS WRITTEN IN THE BOXES PROVIDED WILL BE GRADED DO NOT OPEN THIS EXAM UNTIL INSTRUCTED TO DO SO Q1 Q2 32 39 Q3 Total 44 100 Once you eliminate the impossible whatever remains no matter how improbable must be the truth Sherlock Holmes by Sir Arthur Conan Doyle Q1 Indicate whether each molecule drawn below is chiral or achiral by writing either CHIRAL or ACHIRAL in the box below each one Be sure of your choices as each correct answer is worth 2 points BUT each incorrect answer scores 1 point i e don t just guess Hint it may help you to draw the mirror image in each case a Cl b Cl H CH3 C H H Cl c H H3C d F NH2 H2N F F Cl e f HO CH3 Br Br OH g HO2C i Br h H OH Br O OH OH H O N O H j N H H3CO Question 1 is continued on the next page 1 k In the box below draw using line formula the structure of the lowest molecular mass ACYCLIC ALKANE i e no rings allowed that is chiral No isotopes allowed i e no deuterium etc Name the compound according to systematic IUPAC rules Note you will only get credit for the compound name if you have the correct compound 4 pt structure 2 pt name Name l In the box below draw using line formula the structure of the lowest molecular mass CYCLIC ALKANE that is chiral No isotopes allowed i e no deuterium etc Name the compound according to systematic IUPAC rules Note you will only get credit for the compound name if you have the correct compound 4 pt structure 2 pt name Name 2 Q2 a In the box below briefly explain the trend observed for the angle labeled as in compounds A B and C 2 pt H A Me H H B Me Me 112 2 Me C Me Me Me 111 1 Me Me 109 5 b Compounds D E and F undergo cyclization reactions to give compounds G H and I respectively it doesn t matter what X and Y are just appreciate that they react together to close the ring at Z Using an argument based solely upon bond angles explain briefly in the box below why the reaction of F to give I is faster than the reaction of E to give H which in turn is faster than D to give G 4 pt D H H H E Me Y Y X X 1 H Z H Me Y X G F Me 20 H H 250 Me I Z Me Z Question 2 is continued on the next page 3 Me Relative Reaction Rate c For compound D complete the Newman projections of the three different staggered conformations S1 S2 and S3 Draw them as if you are looking down the C1 C2 bond 6 pt H X H Y 1 1 1 1 S1 S2 S3 2 D d Assuming that the X and Y groups have greater steric demands than hydrogen H comment briefly upon the relative energies of conformers S1 S2 and S3 3 pt e For compound F complete the Newman projections of the three different staggered conformations S4 S5 and S6 Draw them as if you are looking down the C1 C2 bond 6 pt Me X Me Y 1 1 1 1 S4 S5 S6 2 F f Assuming that methyl Me X and Y groups all have the same steric demands comment briefly upon the relative energies of conformers S4 S5 and S6 3 pt Question 2 is continued on the next page 4 g After considering the relative energies of the staggered conformations associated with compounds D and F construct an argument NOT based upon bond angles to explain briefly why the reaction of F to give I is faster than the reaction of D to give G 8 pt h Now compare compound D with compound J In the box below indicate which compound you would expect to have the larger value of and why 3 pt H2 C H H D H2C J Y X CH2 Y X i You have given TWO reasons one in part b and one in part g to explain why the reaction of F to give I is faster than the reaction of D to give G In the knowledge that J cyclizes faster than D comment upon which effect is more important for this series of compounds 4 pt 5 Q3 a trans 1 Fluoro 3 methylcyclohexane can exist in two different chair conformations A and B Complete the picture of conformer B by drawing in a bond to one methyl Me group and one fluorine F atom in the appropriate positions In the two boxes to the right complete the corresponding Newman projections by filling in methyl Me groups hydrogen H atoms AND fluorine atoms F where appropriate 8 pt A Me 5 3 4 1 6 3 1 2 2 F Chair Flip B 4 5 3 6 2 6 1 4 5 b trans 1 Fluoro 3 methylcyclohexane is redrawn below Assign each chiral center either an R or S configuration by writing R or S in each small box Is this molecule the SAME as that shown in part a or is it the ENANTIOMER or a DIASTEREOISOMER 6 pt F Me Relationship with part a c In relation to the compound shown above in part b draw in the boxes below i a diastereoisomer ii a chiral constitutional isomer that still contains a cyclohexane ring and iii an achiral constitutional isomer that still contains a cyclohexane ring 9 pt i ii Diastereoisomer iii Constitutional Isomer Chiral Question 3 is continued on the next page 6 Constitutional Isomer Achiral d By replacing ONE of the hydrogen atoms in trans 1 fluoro 3 methylcyclohexane with a fluorine atom draw in the boxes below i a molecule that is achiral and ii a molecule that contains only ONE chiral center 6 pt i F ii Me trans 1 fluoro 3 methylcyclohexane Achiral One chiral center e There are five ACHIRAL isomers of trifluorocyclohexane there are two pairs of diastereoisomers and one compound that exists as a single stereoisomer In the appropriate boxes below draw these five structures you choose in what positions to add three fluorine F atoms to the cyclohexane rings drawn in the boxes below be sure to use wedged and dashed lines where appropriate 15 pt Diastereoisomers Diastereoisomers single stereoisomer 7