Chemistry 2050 Introduction to Organic Chemistry Fall Semester 2005, Dr. Rainer Glaser

Examination #1 “Bonding, Alkanes, Alcohols & Alkyl Halides”

Wednesday, 09/14/05, 11–11:50 am.

Name: Answer Key

Question 1. Structural Formula of Organic Molecules 20 Question 2. Atomic Structure, Lewis Structures & Bonding 20 Question 3. Isomers and Combustion 20 Question 4. Conformation and Stereoisomers 20 Question 5. Alcohols and Alkylhalides 20 Total 100

— 1 — Question 1. Structural Formula of Organic Molecules. (20 points)

(a) Draw a complete structural formula of pentane. (4 points) H H H H H

H C C C C C H

H H H H H

(b) Draw a condensed structural formula of pentane. (1 points) H3C−CH2−CH2−CH2−CH3

(c) Draw a simplified structural formula (aka bond-line formula) of pentane. (1 points)

(d) Provide the complete structural formula and the name of one structural isomer of pentane. (6 p.) Structure: Name: H H H C H H H 2-methylbutane

H C C C C H

H H H H

(e) Size and shape of acyclic alkanes. (8 p.) Approximate angle ∠(C−C−C) Typical length of a C−C single bond: _154_ pm in acyclic alkanes: > 109.5°

Approximate angle ∠(H−C−C) Typical length of a C−H bond: __105_ pm in acyclic alkanes: > 109.5°

Hybridization of C in acyclic alkanes: sp3 Tetrahedral angle: 109.5°

— 2 — Question 2. Atomic Structure, Lewis Structures & Bonding. (20 points)

(a) A neutral oxygen atom contains __8_ protons, __8_ neutrons, _2_ core electrons, and __6_ valence electrons. In the left box, complete the electron configuration of a neutral oxygen atom by provision of the exponents (e.g. the number of electrons in that AO). On the right, draw all valence electrons as “arrows”. (8 points)

2 2 2 1 1 1s 2s 2px 2py 2pz ↑↓ ↑↓ ↑ ↑

(b) Draw the complete Lewis structures (all atoms, all bonds, all lone pairs, draw all pairs as dashes, use dots only for unpaired electrons, indicate formal charges if any). (6 points)

Carbon dioxide, CO2 Nitrous Acid, HONO

H N O C O O O

- (c) Draw the three important resonance forms of nitrate anion, NO3 . Draw “resonance arrows” between the structures. Draw complete Lewis structures. (6 points)

O O O

N N N O O O O O O

— 3 — Question 3. Isomers and Combustion. (20 points)

(a) Give the molecular formula and the name for the following compound. (4 points)

C10H22

2,2-dimethyl-4-ethyl-hexane

(b) Rewrite the condensed structural formula as a bond-line drawing and give its name. (4 points)

(H3C)3CCH2CH2CH3 2,2-dimethyl-pentane

(c) Circle the isomers. (4 points) butane ⋅ cyclobutane ⋅ isobutane ⋅ 2-methylbutane

(CH3)3CCH2CH2CH3 ⋅ (CH3)3CCH2CH(CH3)3 ⋅ H3C(CH2)5CH3 ⋅ H3CCH2CH=CHCH2CH2CH3

(d) Provide the reaction formula for the complete combustion of octane (e.g. what happens in your car’s engine). (3 points)

H3C(CH2)6CH3 + _12.5_ O2 → _8_ CO2 + _9_ H2O

(e) Provide the reaction formula for the incomplete combustion of octane. Note that 10 O2 are available. [Hint: One more product is formed.] (5 points)

5 fewer O-atoms, 4 CO are formed instead of 5 CO2

H3C(CH2)6CH3 + 10 O2 → 5 CO + 3 CO2 + 9 H2O

— 4 — Question 4. Conformation and Stereoisomers. (20 points)

(a) Draw the perspective drawing of staggered H ethane. (4 points) H H

H H H

(b) Draw the Newman projection of the trans conformation of butane. (4 points) CH3 H H

H H

CH3

(c) Draw the two chair conformations of ethylcyclohexane in which the substituent is either axial or equatorial. Circle the structure that is preferred. (6 points) Axial ethyl. Equatorial ethyl. C2H5 H

H C2H5

(d) Draw the best conformation of cis-1-methyl- 4-isobutylcyclohexane. (6 points) H

C(CH3)3 H

CH3

— 5 — Question 5. Alcohols and Alkyl Halides. (20 points)

(a) Nomenclature. Draw the complete structure H Cl H H of 2-chlorobutane; draw every atom, every bond, every lone pair. (3 points) H C C C C H

H H H H

Give the IUPAC name of the structure shown. (3 points) Cl Cl Name: 2,2-dichloropentane

(b) When tert.-butanol, (CH3)3C−OH, is treated with HCl, the first step of the reaction consists in the protonation. Draw the product of this protonation. The next step is the elimination of water and the formation of the tert.-butyl cation. Draw this cation and indicate the hybridization of the central C- atom. For both structures, show all atoms, show all lone pairs, show formal charges. (6 points) Protonated tert.-butanol Tert.-butyl cation

H3C H CH3

C O C H C H C CH 2 3 H 3 3 sp H3C

(c) Complete the structure of water by adding lone pairs and indicating partial charges on all the atoms. Then draw an arrow that shows the direction of the dipole moment of water (from plus to minus). On the right, again complete the structure of the water molecule shown and then draw a second water that is hydrogen-bonded to water molecule drawn via the hydrogen atom on your right. Try to get the relative distances as right as possible. (8 points) Water Water Dimer H

δ δ O H H H O H H O δ

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