20. What is the structural relationship between the two molecule shown below?

(A)constitutional isomers
(B)enantiomers
(C) diastereomers
(D)conformational isomers

16. Which is a measure of the randomness of a system? (A)entropy (B)enthalpy (C) free energy(D)halogenation

17. Which of the following statements correctly describes the contribution of ΔS° to ΔG° ? (A)The entropy term makes a greater contribution to ΔG° at low temperatures. (B)The entropy term makes a greater contribution to ΔG° at high temperatures. (C) The entropy term makes a greater contribution to ΔG° in exothermic reactions. (D) The entropy term makes a greater contribution to ΔG° in endothermic reactions.

18. Which of the following is true for the initiation step of a free radical chlorination reaction? (A)ΔH° ＞ 0 and ΔS° ＞ 0 (B)ΔH° ＞ 0 and ΔS° ＜ 0 (C) ΔH° ＜ 0 and ΔS° ＞ 0(D)ΔH° ＜ 0 and ΔS° ＜ 0

19. What term describes the structural relationship between cis-1,2-dimethylcyclopentane and trans-1,3-dimethylcyclopentane? (A)not isomers (B)constitutional isomers (C) enantiomers(D)diastereomers

21. How many asymmetric carbon atoms are present in the molecule shown?(A)0 (B)1 (C) 2(D)3

22. Which of the statements below correctly describes an achiral molecule? (A)The molecule has a nonsuperimposable mirror image. (B)The molecule exhibits optical activity when it interacts with plane-polarized light. (C) The molecule has an enantiomer. (D) The molecule might be a meso form.

23. Which of the following structures are achiral and meso?(A)1 (B)2 (C) 3(D)4

24. Which of the following best describes the carbon-chlorine bond of an alkyl chloride? (A)nonpolar； no dipole (B)polar；δ+ at carbon and δ- at chlorine (C) polar；δ- at carbon and δ+ at chlorine(D)ionic

25. Which of the following factors is favorable for nucleophilicity but not basicity? (A)highly polarizable (B)low electronegativity (C) negatively charged(D)sterically hindered

26. Which of the following compounds will undergo an SN2 reaction most readily? (A)(CH3)3CCH2I (B)(CH3)3CCl (C) (CH3)2CHI(D)(CH3)2CHCH2CH2CH2I