CHE 230 Exam 1 _Fall 2010 - Department of Chemistry - Illinois ...

ILLINOIS STATE UNIVERSITYDepartment of Chemistry, Fall 2010Organic Chemistry I, CHE 230, Exam 1Page 1Page 2Page 3Page 4Page 5Page 6_____/120 pts_____/180 pts_____/200 pts_____/200 pts_____/200 pts_____/100 ptsBonus______/30 ptsTotal_____/1000 ptsFirst letter of last nameName (Print):____________________This document should only be used as a study guide.© 2010 Prof. S. R. Hitchcock, Department of Chemistry, Illinois State University, Normal, IL 61790-41600

1. Write out the ground state electron configuration for carbon. (5 pts) 1s 2 2s 2 2p 22. How many valence electrons does carbon have available? (5 pts) 43. What is the hybridization of carbon in methane, CH 4 ? (5 pts) sp 34. Would methane be considered to be a polar molecule (Y/N)? (5 pts) N5. Which combination of properties below best describes why carbon forms covalent bonds and NOT ionicbonds? (20 pts)a) High ionization potential and high electron affinityb) Low ionization potential and low electron affinityc) High ionization potential and low electron affinityd) Low ionization potential and high electron affinityC6. Write out the electron configuration for sodium ( 11 Na) and provide a brief explanation for why sodium canreadily lose an electron. (20 pts)11Na [1s 2 ] [2s 2 2p 5 ] [3s 1 ] - e -→ [1s 2 ] [2s 2 2p 6 ]closed shell configuration!!7. Write out the electron configuration for fluorine ( 9 F) and provide a brief explanation for why fluorine canreadily gain an electron. (20 pts)9F [1s 2 ] [2s 2 2p 5 ] + e -→ [1s 2 ] [2s 2 2p 6 ] closed shell configuration!!8. Consider the compound sodium fluoride (boiling point = +993 o C) and methane (boiling pt = -161 o C).Why do these molecules have such different boiling points? (40 pts)The difference in boiling point is attributed to the intermolecular forces in sodium fluorideversus methane. Sodium fluoride is a very polarized molecule. This polarization gives riseto a network of interaction that holds the molecules together. Methane is not polar and doeshave not the same forces in play.9. Draw reasonable Lewis structures for the following molecules (80 pts):C 2 H 2 N 3-N N NH 2 CS SO 2© 2010 Prof. S. R. Hitchcock, Department of Chemistry, Illinois State University, Normal, IL 61790-41601

10. Determine the formal charge on the BOLD atoms of the molecules that are listed below. (20 pts total)-1 +1 -1 011. Consider the cationic molecules methyl cation, CH 3 + and the ammonium cation, NH 4 + . Explain themeaning of the positive charge in each case. (30 pts)The methyl cation is a true electrophile at carbon as it is missing electron density and the carbon only hassix electrons associated with it. The nitrogen in the ammonium cation does not possesss the same reactivityas its charge is a result of the sharing of electron density. There is no vacancy of electron density at thenitrogen.12. Identify the following molecules as being either electrophiles (E) or nucleophiles (N) or unreactive (U) atthe central atom. Note: Lone pair electrons are not drawn. If you are concerned about the presence of lone pairelectrons, then you should draw the Lewis structure of the molecule. (30 pts)SiH 3+BF 4-NH 4+NH 2-H 3 O +AlCl 4-E U U N U U13. Show a reaction between an electrophile and nucleophile from the above molecules in problem 12. (20 pts)© 2010 Prof. S. R. Hitchcock, Department of Chemistry, Illinois State University, Normal, IL 61790-41602

14. Using molecular orbital theory, provide a rationale for why H 2 is stable and He 2 is not stable. (40 pts)Diatomic hydrogen has an overall stabilizing interaction between the overlapping atomic orbitals of theindividual hydrogens to give rise to a new more stable molecular orbital. In the case of diatomic heliumthere is a stabilizing and destabilizing interaction that is the source of the problem.15. Hybridization is often used in chemistry to explain observed bonding interactions in molecules. Startingwith ground state carbon ( 6 C), explain the rationalization used for invoking sp 3 hybridization of the centralcarbon in methane. (40 pts)2p2p2s1sground state2s1sexcited statesp 31shybridized stateThe ground state has two slots for bonding. The excited state has 4 slots but they areunequal. Methane has 4 equal bonds. The hybridized version has the four equivalent slots.16. Draw a molecular orbital picture for the molecule ethylene, C 2 H 4 (H 2 C=CH 2 ). Show the sigma (σ) bondingorbitals as well as the pi (π) bonding orbitals. (40 pts)courtesy of the University of California, Davis17. Determine the hybridization level of the atoms that are in BOLD. (40 pts)© 2010 Prof. S. R. Hitchcock, Department of Chemistry, Illinois State University, Normal, IL 61790-41603

18. Complete the following. (40 pts)geometrybond anglessp 3 hybridization tetrahedral 109.5sp 2 hybridization trigonal planar 12019. Consider the molecules methane (CH 4 ), ammonia (NH 3 ), and water (H 2 O). What is the bond angleassociated with methane? What is the trend in the bond angles of the other two compounds as compared tomethane? Are the bond angles larger or smaller? Provide a rationale for your answer. (20 pts)Methane bond angle 109.5Ammonia 107Water 104.5The bond angle is decreasing because there is an increasing number of lone pair electrons on the centralatom. The lone pair electrons occupy a greater volume as they are more diffuse than a standard bond.20. Provide a reasonable resonance structure for each of the molecules illustrated below. State if the structuresare equivalent or if one structure is a greater contributor and why. (20 pts each/80 pts total)© 2010 Prof. S. R. Hitchcock, Department of Chemistry, Illinois State University, Normal, IL 61790-41604

21. Draw the resonance structure for the following molecule. (10 pts) Which structure is real? (10 pts) Drawthe resonance hybrid. (10 pts) What experimental evidence supports the resonance hybrid? (10 pts) (40 pts)Neither resonance form is real. Only the resonance hybrid is real. X-ray crystallography suggestthat the two carbon oxygen bonds are equivalent in length. This would further suggest that twocarbon oxygen bonds are of the same strength.22. Which one of the following molecules has a molecular dipole moment associated with? (20 pts)a) BH 3 , boraneb) AlCl 3 , aluminum trichloride (Electronegativity of Cl ~3.2)c) NH 3 , ammoniad) BBr 3 , boron tribromide (Electronegativity of Br ~3.0)C23. Consider the molecules CO 2 and SO 2 . Using Lewis structures determine which molecule has a dipolemoment and which molecule does not. Use dipole arrow to support your argument. (40 pts)24. Determine the number of primary, secondary, tertiary, and quaternary carbons contained in the moleculeillustrated below. (40 pts)methyl methylene methine quaternary5 2 3 125. Consider the expression ∆G o = ∆H o - T∆S o . What is the meaning of the ∆H o term and the T∆S o . (40 pts)© 2010 Prof. S. R. Hitchcock, Department of Chemistry, Illinois State University, Normal, IL 61790-41605

∆H oT∆S oEnthalpy-This is associated with the bond strength of the starting materials and ending products.Entropy-This is associated with randomness that is in a system. It involves the degrees of freedomthat molecules can take26. Which term is a better measure of how fast a reaction is, ∆G o or E a ? Show a schematic to make your case.(40 pts)E a is because it involves the energy of activation to overcome the transition state. ∆G onlydetermines if one side of the equation is better than the other.27. Write out the dissociation expression for hydrogen bromide (HBr) in water (H 2 O) and show the K aexpression. What is the numerical K a value if the pK a is equal to a value of -9? What does this value reallymean? Would you consider hydrogen bromide to be a strong acid once placed in water? (80 pts)The bond in HBr is considerably weaker than that of HF. This is due to the poor overlap ofthe bromine (5p) orbital with the hydrogen 1s orbital. In contrast, the HF bond involves theoverlap between a (2p) orbital on fluorine and a 1s orbital on hydrogen.28. Provide a solid rationale for why acetic acid (pK a = 5) is more acidic than ethanol (CH 3 CH 2 OH, pK a = 18).Draw out the relevant structures to make your point. Is it not sufficient to suggest a difference in the pK anumber or that ionization occurs more readily in one substrate. (40 pts)OH 3 C O H HH OKa =+ +H 3 COHO H OOHH O H H 3 C OHOH 3 C O HKa =110 5The conjugate base of ethoxide has a localized charge on oxygen whereas the negative charge onthe acetate anion is resonance delocalized through two oxygens. The more atoms that can sharethe charge, the more stable the system is.29. Provide a strong rationale why HI is more acidic than HF. (20 pts)© 2010 Prof. S. R. Hitchcock, Department of Chemistry, Illinois State University, Normal, IL 61790-41606

The bond in HI is considerably weaker than that of HF. This is due to the poor overlap ofthe iodine (5p) orbital with the hydrogen 1s orbital. In contrast, the HF bond involves theoverlap between a (2p) orbital on fluorine and a 1s orbital on hydrogen.30. Consider the reaction below. Determine the direction that this reaction will proceed in. [(pK a ≈ 5 foracetic acid)] and [pK a ≈ 16, water (H 2 O)]. State which acid is the weaker acid and which is the strongeracid. In addition, determine the exponential magnitude of the reaction (10 x ). (40 pts total)OH 3 CCOHacetic acidpka = 5+NaOHH 3 Creaction is driven to the right side due to thedifference in pKa values. The right side is favoredby a factor of 10 11 .OCONa+ H 2 OpKa =16© 2010 Prof. S. R. Hitchcock, Department of Chemistry, Illinois State University, Normal, IL 61790-41607