2058 final review key

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2058

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Chemistry

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Jun 26, 2024

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Chem 2058 Final Review key 4) Draw the shape of a 2p orbital. Answer: 10) Draw a correct Lewis structure for acetonitrile, CH 3 CN. Answer: 12) Covalent bonds may be polar or nonpolar. What property of the atoms forming a given bond determines this? Answer: Electronegativity 13) The compound methylamine, CH 3 NH 2 , contains a C – N bond. In this bond, which of the following best describes the charge on the carbon atom? A) +1 B) slightly positive C) uncharged D) slightly negative E) -1 Answer: B 27) Write a completed equation for the acid-base pair shown below. HCN + NaOH → HCN + NaOH → NaCN + H 2 O 30) Methanesulfonic acid, CH 3 SO 3 H, has a pKa of -7 while ethanol, CH 3 CH 2 OH, has a pKa of 15.9. Which is the stronger acid and what accounts for this large difference in relative acidity? Answer: Methanesulfonic acid is the stronger acid. The lower the pKa, the stronger the acid. A lower pKa is associated with a larger Ka which signifies greater dissociation. The large relative difference in acidity in this case can be most easily seen by gauging the relative basicities of the conjugate bases. The weaker the base, the stronger the corresponding conjugate acid. Methanesulfonate, CH 3 SO 3 - , is considerably stabilized by resonance delocalization which is not found in ethoxide, CH 3 CH 2 O - . This effect greatly reduces the basicity of methanesulfonate relative to ethoxide. 31) Would you predict trifluoromethanesulfonic acid, CF 3 SO 3 H, to be a stronger or weaker acid than methanesulfonic acid, CH 3 SO 3 H? Explain your reasoning. Answer: Trifluoromethanesulfonic acid is a stronger acid. Compare the strengths of the conjugate bases and remember that the weaker the base, the stronger the conjugate acid. Both bases are stabilized by resonance, but in the case of the trifluoro derivative, the presence of the highly electronegative fluorine atoms serves to delocalize the negative charge to an even greater extent. This additional delocalization makes trifluoromethanesulfonate a weaker base. 42) Consider the species CH 3 O - , NH 2 - , and CH 3 COO - . Rank these ions in order of increasing basicity, and explain your rationale. Answer: CH 3 COO - < CH 3 O - < NH 2 - The first factor to consider is the nature of the atom which bears the negative charge. The more electronegative the

2 atom that bears the negative charge, the more stable the anion. Stable anions are less reactive and are hence weaker bases. Since O is more electronegative than N, the NH 2 - is the strongest base in the set. In the remaining two species, the negative charge is on the O, but in the case of CH 3 COO - , the negative charge is also delocalized by resonance. 49) Draw the important resonance forms for the structure shown below. Answer: 3) When orbitals on different atoms interact, molecular orbitals are produced. 11) Which of the followi ng statements about π molecular orbitals is/are correct? A) π molecular orbitals are cylindrically symmetric. B) Most of the electron density in a π molecular orbital is centered above and below the internuclear axis. C) When two atoms are connected by a double bond, both of these bonds are π bonds. D) Both statements B and C are correct. E) Statements A, B, and C are all correct. Answer: B 17) Structures which differ only in rotations about a single bond are called ____ Conformations _. 21) Choose the correct hybridization for the atom indicated in the molecule below. A) sp B) sp 2 C) sp 3 D) none of the above Answer: B 26) The structure of vitamin C is shown below. Which one of the following statements concerning this structure is not correct? A) The molecule contains 2 pi bonds. B) The molecule contains 1 sp 2 hybridized oxygen atom.

3 C) The molecule contains 3 sp 2 hybridized carbon atoms. D) The molecule can be classified as an aldehyde. E) The molecule contains more than one hydroxyl group. Answer: D 32) Which of the molecules below has the higher boiling point? Briefly explain your choice. (CH 3 ) 3 N or CH 3 CH 2 CH 2 NH 2 Answer: CH 3 CH 2 CH 2 NH 2 has the higher boiling point since it is capable of intermolecular hydrogen bonding. 34) Would you expect sodium chloride (NaCl) to be highly soluble in the organic solvent hexane (CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 )? Briefly explain your answer. Answer: One would not expect NaCl to be highly soluble in hexane. NaCl is an ionic solid (i.e., a very polar material) while hexane is nonpolar. Nopolar solvent molecules do not solvate ions well. The attractions of oppositely charged ions to each other are vastly greater than the weak attractions of the ions for the solvent. 8) Provide an acceptable name for the alkane shown below. Answer: 2,5-dimethylheptane 10) Provide an acceptable name for the alkane shown below. Answer: 4-isopropyldecane or 4-(1-methylethyl)decane 11) Provide an acceptable name for the alkane shown below. Answer: 3-ethyl-7-methyl-5-propylnonane 22) Why are alkanes described as hydrophobic? Answer: Alkanes are nonpolar and as such they do not dissolve to any appreciable extent in water. 25) Consider the three isomeric alkanes n-hexane, 2,3-dimethylbutane, and 2-methylpentane. Which of the following correctly lists these compounds in order of increasing boiling point? A) 2,3-dimethylbutane < 2-methylpentane < n -hexane B) 2-methylpentane < n -hexane < 2,3-dimethylbutane C) 2-methylpentane < 2,3-dimethylbutane < n -hexane D) n -hexane < 2-methylpentane < 2,3-dimethylbutane E) n -hexane < 2,3-dimethylbutane < 2-methylpentane Answer: A 34) Draw a Newman projection of the most stable conformation of 2-methylpropane. Answer:

4 35) The structures below are: A) not isomers. B) conformational isomers. C) cis-trans isomers. D) structural isomers. E) both B and D Answer: D 36) Define the term conformation. Answer: Conformations are different arrangements of the same molecule formed by rotations about single bonds. 37) Use a sawhorse structure to depict the eclipsed conformer of ethane. Answer: 38) View a butane molecule along the C 2 – C 3 bond and provide a Newman projection of the lowest energy conformer. Answer: 45) Describe the sources of angle strain and torsional strain present in cyclopropane. Answer: The angle strain arises from the compression of the ideal tetrahedral bond angle of 109.5° to 60°. The large torsional strain occurs since all C – H bonds on adjacent carbons are eclipsed. 47) Draw the most stable conformation of trans -1,2-dimethylcyclohexane. Answer: 48) Draw the most stable conformation of cis -1,2-dimethylcyclohexane. Answer:

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