ERT 102/4 Organic Chemistry
Tutorial questions
Chapter 4: Stereochemistry of alkanes and cycloalkanes
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Part A : Multiple choice questions (2 marks for each questions (40%))
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1.
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The eclipsed and staggered forms of ethane are said to differ in:
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A)
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molecular formula.
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B)
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configuration.
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C)
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conformation.
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D)
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constitution.
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E)
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structure.
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2.
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Which of the following is the staggered conformation for rotation about the C1-C2 bond in the following structure?
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A)
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I
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B)
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II
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C)
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III
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D)
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IV
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E)
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V
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3)
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Among the butane conformers, which occur at energy minima on a graph of potential energy versus dihedral angle?
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A)
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gauche only
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B)
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eclipsed and totally eclipsed
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C)
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gauche and anti
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D)
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eclipsed only
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E)
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anti only
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4)
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Which of the following best explains the relative stabilities of the eclipsed and staggered forms of ethane? The ______form has the most ______strain.
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A)
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eclipsed; steric
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B)
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eclipsed; torsional
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C)
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staggered; steric
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D)
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staggered; torsional
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5)
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Which of the following best explains the reason for the relative stabilities of the conformers shown?
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A)
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I has more torsional strain.
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B)
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I has more steric strain.
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C)
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II has more torsional strain.
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D)
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II has more steric strain.
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6)
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Which of the following statements about the conformers that result from rotation about the C2-C3 bond of butane is correct?
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A)
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The highest energy conformer is one in which methyl groups are eclipsed by hydrogens.
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B)
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The gauche conformer is an eclipsed one.
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C)
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Steric strain is absent in the eclipsed forms.
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D)
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Torsional strain is absent in the eclipsed forms.
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E)
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none of the above
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7)
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Draw the Newman projection that represents the most stable conformation of 3,3-dimethylhexane viewed along the C3-C4 bond.
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8)
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Draw the Newman projection that represents the least stable conformation of 3,3-dimethylhexane viewed along the C3-C4 bond.
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9)
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Draw the Newman structure for the most stable conformation of 1-bromopropane considering rotation about the C1-C2 bond.
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10)
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Draw a Newman projection of the most stable conformation of 2-methylpropane.
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11)
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Define the term conformation.
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12)
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Use a sawhorse structure to depict the eclipsed conformer of ethane.
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13)
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View a butane molecule along the C2-C3 bond and provide a Newman projection of the lowest energy conformer.
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14)
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Provide a representation of the gauche conformer of butane.
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15)
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Draw the Newman projection of the most stable conformation that results due to rotation about the C2-C3 bond in 2,3-dimethylbutane.
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16)
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Which of the following correctly lists the conformations of cyclohexane in order of increasing energy?
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A)
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chair < boat < twist-boat < half-chair
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B)
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half-chair < boat < twist-boat < chair
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C)
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chair < twist-boat < half-chair < boat
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D)
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chair < twist-boat < boat < half-chair
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E)
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half-chair < twist-boat < boat < chair
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17)
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Which of the following is the most stable conformation of bromocyclohexane?
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A)
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I
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B)
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II
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C)
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III
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D)
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IV
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E)
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V
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18)
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In the boat conformation of cyclohexane, the "flagpole" hydrogens are located:
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A)
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on the same carbon.
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B)
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on adjacent carbons.
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C)
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on C-1 and C-3.
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D)
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on C-1 and C-4.
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E)
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none of the above
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19)
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Which conformer is at a local energy minimum on the potential energy diagram in the chair-chair interconversion of cyclohexane?
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A)
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half-chair
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B)
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planar
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C)
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boat
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D)
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twist-boat
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E)
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fully eclipsed
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20)
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Draw the chair conformer of cyclohexane. Label the axial hydrogens (Ha) and the equatorial hydrogens (He).
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Part B:Structured questions (60%)
1. Consider rotation around a C-C bond in neopentane, (CH3)4C.
a) Draw the most stable and least stable conformers that result from rotation around this bond using Newman projections.
b) Draw a graph of energy versus rotation for rotation around this bond.
2. a) Draw the three staggered and three eclipsed conformations that result from rotation around the designated bond using Newman projection.
b) Label the most stable and the least stable conformation.
3.Draw a second chair conformer for each cyclohexane. Then decide which conformer is present in higher concentration at equilibrium.
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