Last Name First Name MI Total Score Student ID Number Circle the name of your TA MIKE Discussion Section Day ROB 100 Time Chem 30A Winter 2005 MIDTERM 1 50 Min Weds February 2nd 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 Q3 24 Q4 22 18 Q5 BONUS 5 36 Q6 BONUS 10 Total 100 A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools Douglas Adams Q1 a Using LINE FORMULAE draw the four different CONSTITUTIONAL isomers with the molecular formula C4H9Cl 2 pt each b Name each isomer using SYSTEMATIC naming rules in the knowledge that chloro groups are higher in priority than methyl groups i e they get the smaller positional number if a choice has to be made 2 pt each A B Name Name C D Name Name c Which of the constitutional isomers of C4H9Cl A D has the highest boiling point 79 C 2 pt d Which of the constitutional isomers of C4H9Cl A D has the lowest boiling point 51 C 2 pt e One of the constitutional isomers of C4H9Cl can exist in two stereoisomeric forms enantiomers Draw these two non superimposable mirror images in the appropriate boxes below 2 pt each R Enantiomer S Enantiomer 1 Q2 a Above the 0 line on the graph below fill in the groups on the Newman projection of the MOST UNSTABLE eclipsed conformation E1 of 2 3 dimethylbutane as viewed from C2 to C3 i e C2 is in the front By rotating C2 and the groups attached to it 60 CLOCKWISE the first staggered conformation S1 is reached fill in the groups on this Newman projection above the 60 line Continue this clockwise rotation and complete the Newman projections of the other eclipsed E2 E3 and staggered S2 S3 conformations 2 pt each 4 3 3 2 2 E1 E2 E3 2 2 2 1 2 2 Methylbutane E1 2 S2 S3 Energy S1 1 2 3 Dimethylbutane 4 2 2 0 60 120 180 240 Dihedral Angle 300 360 b Draw short horizontal bars on the GRAPH ABOVE like those drawn for E1 at 0 360 indicating the relative energies of S1 E2 S2 E3 and S3 and complete the graph by drawing in a curve that shows how the energy changes relative to the dihedral angle 6 pt 2 Q3 a Methylcyclohexane can exist in two different chair conformations one of which is 1 8 kcal mol more stable than the other i e the A value for the methyl group is 1 8 In each of the two boxes below draw in a bond to one methyl CH3 group in the appropriate position 2 pt CH3 1 G 1 1 8 kcal mol Least stable chair 1 Most stable chair b Chlorocyclohexane also exists in two different chair conformations one of which is 0 6 kcal mol more stable than the other i e the A value for the chloro group is 0 6 In each of the two boxes below draw in a bond to one chloro Cl group in the appropriate position 2 pt Cl 1 G 1 1 0 6 kcal mol Least stable chair Most stable chair c For trans 1 chloro 4 methylcyclohexane shown below draw in bonds to CH3 and Cl groups as appropriate to indicate the least and most stable chair conformers Assuming that A values are additive and hence subtractive if necessary predict what the G value will be 6 pt G Cl 1 1 1 CH3 Least stable chair kcal mol Question 3 is continued on the next page 3 Most stable chair d For cis 1 chloro 4 methylcyclohexane shown below draw in bonds to CH3 and Cl groups as appropriate to indicate the least and most stable chair conformers Assuming that A values are additive and hence subtractive if necessary predict what the G value will be 6 pt G Cl 1 1 1 CH3 Least stable chair kcal mol Most stable chair e For the isomer of 1 chloro 3 5 dimethylcyclohexane shown below draw in bonds to C H3 and Cl groups as appropriate to indicate the least and most stable chair conformers Assuming that A values are additive and hence subtractive if necessary predict what the G value will be 8 pt G CH3 Cl 1 1 1 CH3 Least stable chair kcal mol Most stable chair f In the appropriate boxes below draw the three most stable conformations of cyclohexylcyclohexane shown to the right in which the indicated H atoms maintain an ANTI relationship i e are 180 apart Once you have drawn these three different conformers use the box below them to briefly explain your answers 12 pt H H Cyclohexylcyclohexane Cyclohexylcyclohexane Most Stable Conformer 2nd Most Stable Conformer Explanation 4 3rd Most Stable Conformer Q4 a f In each box write down either R S or X indicating that the carbon atom to which the arrow is pointing has the R configuration the S configuration or is X not a stereocenter 3 pt each a F Br H Cl c b HO d Cl H2N e SH NO2 NO2 CN f Cl NH2 H CO2H H H Br g The two enantiomers of limonene have EXACTLY the same boiling point melting point vapor pressure and appallingly bad solubility in water Interestingly one of them smells of oranges the R isomer and the other the S isomer smells like lemons Explain why they smell different 4 pt R limonene S limonene 5 Q5 BONUS Explain why trans decalin systems cannot undergo chair flips whereas cis decalin systems can examples are shown in the box below 5 pt CH3 X trans decalin CH3 cis decalin 6 NO RING FLIP CH3 Q6 BONUS You ve already seen that a chloro substituent on a cyclohexane ring prefers to be in the equatorial position i e the conformer with the Cl in the EQUATORIAL position is favored by 0 6 kcal mol Suggest why in the case of 2 chlorotetrahydropyran shown below the conformer in which the Cl atom is in the AXIAL position is favored by over 2 kcal mol 10 pt O Cl G O Cl O 2 2 kcal mol 2 chlorotetrahydropyran Cl FAVORED 7