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CHEM 237 , Winter 2007
HOUR EXAM 1 ANSWERS
Name___________________________________________________ Student#_________________________________________________ TA name and/or section name________________________________
1. ( 5 pts) Finish the following sentence (Do not use a second sentence): “An orbital is a solution to Schrödinger’s equation (for one electron); a function of the position of an electron in space.” 2. ( 12 pts, Carey 1. 49 ) One of the following four structural formulas is not a permissible resonance contributor. (a. 3 pts) Which one? ________(C)__________
N O
H 2 C
H 3 C
(A)
N O
H 2 C
H 3 C
(B)
N O
H 2 C
H 3 C
(C)
N O
H 2 C
H 3 C
(D)
(b. 3 pts) Rank the three remaining structures in order of their contribution to the resonance hybrid? ______(B)______ > ______(D)_______ > _______(A)_______ most important least important (c. 6 pts) Use curved arrows to show the electron movement that connects the three resonance contributors and use the appropriate straight arrows to indicate these connections. Answer clockwise from A to A.
N O
H 2 C
H 3 C
(A)
N O
H 2 C
H 3 C
(B)
N O
H 3 C
H 3 C
(D)
N O
H 2 C
H 3 C
(B)
N O
H 2 C
H 3 C
(A)
3. ( 12 pts, Carey 2. 22 ) Circle isomers in the following three sets.
4. ( 12 pts) Indicate which of the following pairs do not constitute resonance structures by drawing an “X” in the space provided? O O OH O
OH
H 3 CC O
H 3 CC O
H 3 CC CH 2
H 2 CCH CH 2 H 2 CCH CH 2
HOCN HNCO
and and and and and
H 3 CC-O
O
H 3 CC-OH
H 3 CCCH 3
and
5. ( 12 pts, Carey 2. 27 ) Give the IUPAC name of each of the following. X X X X 3 , 5 - dimethylheptane 4 - ethyl- 2 , 2 - dimethylhexane 3 - ethyl- 4 , 5 , 6 - trimethyloctane
- ( 12 pts) Ozone (O 3 ) is found in the upper atmosphere where it absorbs highly energetic ultraviolet radiation and thereby provides the surface of the earth with a protective screen. One possible resonance structure for ozone is the following: (a) Assign any necessary formal charges to the atoms in this structure. O O O ::^ : .... .. .. (b) Write another equivalent resonance structure for ozone. O O O : :: .... .. .. (c) What do these resonance structures predict about the relative lengths of the two distances between pairs of bonded oxygen atoms in ozone? They are of equal length.
- ( 10 pts) You may be surprised to learn that the molecule CH 2 with divalent carbon can exist. It is of course very reactive. We can conceive of two limiting structures for this molecule with H-C-H bond angles of 180° as in BeH 2 or 120° as in BH 3. Draw the two structures. Draw the appropriate atomic and hybrid molecular orbitals for each structure, indicate what type of orbitals you are using, and indicate their electron occupancies?
O O H 3 C H N H O (a) What is the chemical formula for the caspsaicin? C 18 H 27 NO 3 (b) How many lone electron pairs are there? ____________ 7 ______________ (c) How many pi electrons are there? ________________ 10 ________________
- ( 9 pts). The following structure is missing some multiple pi bonds. Please draw them in. Indicate formal charges if and where necessary. Indicate lone electron pairs if and where necessary.
N C
CH
CH
N N C
CH
CH
N
- (a. 3 pts) Rank the following compounds left to right in order of increasing pKa? The formulas represent ethane, ethene (ethylene), and ethyne (acetylene). pKa _____ C 2 H 2 _____ < _____ C 2 H 4 _____ < ____ C 2 H 6 _____ (b. 4 pts) Justify you answer. The more s-character in the hybrid orbital following deprotonation, the lower the energy of the remaining electron pair, because s is lower in energy than p. Thus C 2 H 2 , with sp hybrids
having 50 % s-character is the most acidic while C 2 H 6 with sp 3 hybrids having 25 % s-character is the least acidic.
