Chemistry 30A Exam 1 Solutions May 2 2003 Statistics High score 94 Average score 62 0 Low score 17 Standard Deviation irrelevant as it does not control grade distribution in this class A note about exam keys The answers presented here are usually significantly longer than expected from a student taking the exam An exam key serves not only to reveal what was expected but to instruct you as well Exam key posted 10 AM Friday May 2 2003 To see the current course grade cutoffs consult If Grades Were Assigned Today at the Chem 30A Home Page 1 The number of sp2 atoms in captopril is 4 or 2 if you do not believe carbonyl oxygen atoms are sp2 2 None of the atoms in captopril have open octets 3 There are nine lone pairs in captopril 4 Influence 1 Lone pair repulsion influences molecular geometry Influence 2 Lone pairs may allow a species to be a base or nucleophile Influence 3 Lone pairs may play a role in determining formal charges 5 Carbon and sulfur both have an electronegativity of 2 5 6 From left to right Thiol amide and carboxylic acid 7 Captopril does not possess an amine group NH2 O NH2 8 H3C HS N S O S CO2H 9 The single most important factor that drives any molecule into its lowest energy conformation or geometry is the minimization of strain or electron repulsion 10 The S C C bond angle of captopril is a bit more than 109 5o H HS 11 H3C H R HS R H CH3 H H H Most stable staggered Least stable eclipsed 12 Captopril does not have ring strain 13 Shrinking the ring to four atoms causes the molecule to have significant ring strain H3C HS N CO2H O H3C 14 Enantiomer HS N CO2H O Diastereomer H3C HS N HS N or H3C HS CO2H O 15 or H3C O CO2H CH3 HS O N N O HO2 C CO2H 16 The enantiomer and diastereomer are optically active 17 Captopril has 2 stereocenters so it has four possible stereoisomers 18 There is no direct relationship between absolute stereochemistry R vs S and direction of rotation of plane polarized light so we cannot determine if captopril is dextrorotatory or levorotatory 19 a Resolution Separation of enantiomers b Racemic mixture A 1 1 mixture of enantiomers c Inductive effect The electron withdrawing or donating effect that one portion of the molecule has on another portion of the molecule d Torsional strain Increase in energy caused when two atoms or groups of atoms come too close causing electron repulsion Chem 30A Spring 2003 Exam 1 Solutions 2 O 20 R O O R O O R O 21 The single most important feature that influences the basicity of any base is its ability to share an electron pair with a proton to form a new covalent bond 22 The most acidic hydrogen of captopril is CO2H 23 Deprotonation of RCO2H leads to a conjugate base with resonance question 20 The conjugate bases formed from RSH or RCH3 do not enjoy this resonance and therefore their negative charges are not as effectively dispersed Greater charge dispersal affords greater stability and reduced basicity Thus RCO2 is a more stable weaker base than RS and RCH2 Recall that conjugate acid strength is inversely proportional to base strength so the weakest base RCO2 is derived from deprotonation of the strongest conjugate acid RCO2H O 24 O H2 O electrophile nucleophile R O HO H R O 25 Inclusion of a fluorine atom makes the RCO2H more acidic due to the inductive effect H3C HS N F CO2H O 26 Removing resonance causes a decrease in acidity H3C HS N O CH2OH lone pair 27 N R Most stable CO2H N R CO2H Least stable Chem 30A Spring 2003 Exam 1 Solutions 3