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ORGANIC LETTERS The Î±-Effect in Gas-Phase%2 Reactions Revisited 2006 Vol. 8, No.I 119-121 : Yi Ren and Hiroshi Yamatakau College of Chemistry, Sichuon University, Chengdu 610064, P. R. China, and Department of Chemistry. Riklyo University, Nishi-kebukuro 3-34-1, Tothima-ku Tokyo 171-8301, Japan yiren$7@ hotmail.com [email protected] jp Received November 6, 2005 50 G21+1 basicity (kJimo This papenetamines gasphase sa reactions at saturated carbon for model reactionshu, cyc-cvu + cr our 'O., Meo-. NH,, HS-. CI, Br,IF, HO0-, Me00", HSS, and NHNH1 using the G2+) theory. The calculated results show that the a effect does exist in the gas-phase S,2 reaction at the sp carbon, contrary to the currently sccepted notion of the absence of the a-effect In the gas phase A class of nucleophiles, which have an electronegative atom differential ground state (GS) destabilization, transition state containing one or more unshared lone pair electrons adjacent (TS) stabilization, and solvent effect.S Among these, the o the nucleophilic atom, is called a-nucleophiles and shows interpretation in terms of the GS destabilization cannot be enhanced reactivity compared to that expected from a the origin, since the GS destabilization not only enhances the reactivity but also makes the equilibrium favorable, as pointed out explicitly in the literature. The a-effect should solution. Several inserpretations have been given including arise from a factor that stabilizes specifically the TS but no the product state. It has widely been considered that the a-effect does not exist in the gas phase and that solvent Brensted-type correlation (log k vs pKa).2 The a-effect has been reported in many different types of reactions in Sichaan Uaiversity Rakkyo Usiversity (1) Edwards. J. O: Pearson, R.G.JAm Chem Soc. 1962, 84, 16 4) () Hot, S Bancel, E. Tatrahedron Let 1984, 25, 3411. (b) Buncel Hot, S; Buncel, E. lur. J. Chem 1983, 26, 313 E: Hor, S. Terrahedron Leit 1983, 24, 4777 CJ) (a) Buscel,E:Wilson, H: Chaaqui, C. Am Ohm Soc. 1982 (5) (a) DePuy, C. H; Della, E W; Filey, J.: Grabewski, J1.1: Bierbaum, V.M.J.Aå¤CMm Soc. 1983. 10.5. 2481,(b) wolfes:sehela D 1: 04, 4896 (b) Fountain, K. RiHatchinson, L. K: Mulhearn. D. C ; Xu, Y. B J. Ora aen. 199. 58. 7183, (c) UnãLFL; Park. Y M.; Bunced. Lenx. 2004 50, 7801 10 1021lpl0528830 CCC: $33 50 2006 American Chenical Soclety Schlegel, H B.; Minot, C.; Eisenstein, 0. Telahedran Lat 1982, 23, 61s (c) Evatieck, J D: Blake, J. F; Jorgensen. w.L. ro9. 2349 (d) Hot. S.J. Org. arm. Conu, 2000, 1917. (d) Bancel, E.-Un. I, H. Ternshedro, Am Cham. Soc. t987 1982, 47, 3545 Publined on web 12-1200S

effects play an important role of that factor is not clear yet. However, the real picture in our calculations. Relative energies correspond to enthalpy changes at 298.15 K in kJ/mol. All optimized Experimental evidence against the a-effect in the gas phase structures involved in the reactions are given in Supporting was reported in 1985 by Depuy and co-workers who Information. The key energetic quantities are presented in showed that the branching ratios among the proton transfer. Table 1. The data in Table 1 indicate that the calculated carbonyl addition, and substination channels for the reaction of methyl acetate with HO and HOO did not change with the base (nucleophile). However, the conclasion based on Table 1. Cakculated Basicities of Nucleophiles and Overall the direct comparison of the branching ratios is not exclusiveBarriers, Afe (ml, for Equation 1 since the base (nocleophile) dependence conld be different for the different reaction channels. Furthermore, the basic assamption behind the argument, aamely, the absence of the a-effect for the proton-transfer reaction, is also questionable. The most cited computational evidence against the a-effect was given in 1987 by Jorgensen et al, who reported that, for the gas-phase Sw2 reaction of CH Cl, the TS with HOO was less stable than that with H0. However, the level of calculations was not high, and the TS was not thoroughly Me00 examined in this pioncering study Ä°STsa Â± 2.1-56.6 1487.8 +042 52.2 14680Â± 120 1448 0 t 13o 15960 + 4.050.4 15673 + 3.3 45.8 kS- 1353.5 Â± O 42 In the present study, we have cakculated a series of gas phase anionic Sw2 reactions (eq 1) of CHCi with seven normal (HO, MeO, NH:, HS, Cl, Br, and 1) and four a-sacleophiles (H00, Me00, HSS, and NH,NH and found that the level of computation is important to obeain reliable results. Furthermoee, HOO is conformationaly more flexible than HO. and we could obtain a new and mone stable TS not considered previously. All of these results require modification of the conclusion based on the previous calculations as well as on the gas-phase experiment and clearly show that the a-effect exists in the gas phase Su2 reactions at an saturated carbon (sp hybridization). G2l(+) basicities are in good agreement with the experimental Figare 1 shows that a good correlation (R 0993) exists between the overall barriers (AH) and the basicities of CI. Br.I,HOO", McOO, HSS, and NH,NH) A modified form of G2 theory, G2(+.introduced by Radom et al. and successfully applied in the stady of Sk2 reactices at saturated carbon, nitrogen, and oxygen, was (t) (a) Glakbevsev, M. N.; Pros, A: Radom, LIAM hem Soe 195. 117. 2024 (b) Ghshovtev, M. N: Post, A; Radom. L. J Am H B; Bach, R. D: Radom, L. A. Am Chem Soe 1996, 118, 11258.( 1200 1300 1 150 600700 1800 (7) Frisch, M.14 Trucka, O.W M A Cheeseman JR. Zkrski, V. G: Monigomery, J A: Suatmann Figere 1. Plots of the G2(+) overall barriers ((Ho) the basicities of mormal nucleophiles () and a-nucleophiles (O) for K. N.: Szain, M.C Farkas, O: Tomasi, J:Barone, V.: Cossi, M: Cammi, Priersion, G A.: Ayala P. Y. Cui, Q:Mokuma, K Malick. D. K. v.: Baboul. A. Stefanov. B 11,Lis. G ; Liashenko, A; Paker, P. eq 1 at 298 15 K Kom-ne t Go pens R Main R L. Fo. Ket), T AI Laham, six normal nucleophiles CHO.. Meo", HS, Cr. Br, and D C Y. Nanayakkars, A. Gonaaler, C.Chalacombe, M.1. The observed correlation reinforces the generally ac- L. A. Gearsian 95, Revisicn cepted view that the macleophilic reactivity is basically controlled by the basicity of nucleophile if the substrate is 02fixed. It is worth noticing that the points for three a-nucleo- (10) NIST Sandard Refenence Database Number 69, July 2003 Rekase Org Lent, Vol & No. 1, 2006 A., Guastian, Inc Bc HH Ch 5191. (c) Ren, Y: Zhu, H. J. J. Am Soc Mass Spectron 2004, 15, 673 9) Res, Y: Wolk JL. Hoa, S. Jnter, J. Mass Spectron 2082, 220, 1

philes out of the four examined deviate downward from the (by 8.7 k/mol) than the trans one probably due to the correlation line, displaying detectable a-effect in the S2 favorable electrostatic interaction betaween the a-atom (0) raction. The point for NH&NH- is slightly above the line, in HO0 and the positively charged methyl moiety at the but this nocleophile is more reactive than NH3" despite its TS. Such interaction is weaker in the trans conformer smaller basicity, as can be seen in Figure 1:It should also NH Ys NH,NH-. In these two cases the attacking atoms be noted that both MeO" and HO" behave normally, whereas are nitrogen. The calculated gas-phase basicity of NH NH HOO-deviates downward from the correlation line. The is smaller than N, by 20.4 klimol, whereas the results indicate that the deviation of HOO-is not due to the size effect, namely, an extra stability due to a polarizabiliry effect, often observed in the gas-phase calculations for a large species. The four pairs of the normal and corresponding be higher for NH NH than for NH a-nucleophiles are compared in detail below for the reaction of NH,NH' + CH,Cis lower than that for NH +CH/CI by 3.3 kJ/mol. This leads to the expectation that the reaction efficiency for the gas-phase reaction should HS vs HSS. The attacking atoms are sulfur in these HO- vs HO0. The reaction of HO +CH,CI has been two nucleophiles. The calculated G2(+) AH values in extensively studied experimentally" and computatonallyn Table l show that he ÎÎ s 6.3 kJ mol for the reaction The reaction of HOO +CHCI is less studied theoretically of a-nucleophile HSS with CH,CI, slightly lower than that and only calculated at a lower level. The caleulations by for HS with CH,CI by 0.7 kJ/mol, even though HSS is Jorgensen et al. at the MPn 2 and 3WHF6-31+G less basic than HS- by 34.3 kJ/mol. Thus, the a-effect is level of theory suggested that the Alf'o. for HO+CHCI clearly observed. is lower than that for HOO CH,CI by more than 10 MeO vs Me00. The calculated overal kl/mol. However, the present study revealed that the TSs for reactions of HO+CH,Cl and HO0+CHCl are 55.5 and 56.6 kJ/mol below the reactants at the G20+) level, barrier for Meo-t, Ð¡ÐÐ is-506 klrnel, which is higher than that for HO+CH CI by 4.9 kJ/mol, consistent with the smaller basicity of MeO" in the gas phase. Although the Alf for respectively, thas indicating that H00- is more reactive than a-nucleophile MeOO" is slightly higher than that for Meo HO, even though HOO. is a much weaker base than HO.. by 48 k1ml due to its smaller basicity and stenc effects. The different results can be attributed to the different TS there is still a obvious downward deviation for Me00 in es for the reaction of HOO; we found a new TS Figure 1. This result again confirms the a-effect in the gas- with a skew conformer (Figure 2, C-O-0-H 108.5Â°), phase Sw2 reaction. The TS prefers the skew conformation (C-0-0-C 103.8Â°) due to the favorable interaction between the a-atom with the methyl moiety as in the case In sammary, the high level G2(+) calculations show that three out of the four a-nucleophiles examined exhibit downward deviation from the plot of overall barrier vs basicity and accelerate the reaction through the stabilizatiorn of the transition stractures.The other a-nucleophile (NH,NH) clearly shows an enhanced reactivity compared to that of the corresponding normal nacleophile (NH2"). The a-effect does exist in the gas-phase Sx2 reaction at saturated carbon, contrary to the currently accepled notion of the absence of the a -effect in the gas phase. skew-conformer (0.0 kWmol Acknowledgment. Y.R. thanks Japan Society for the Promotion of Science for the invication Fellowship Programs for Research in Japan (no. L05546). H.Y. thanks Rikkyo Umiversity Fromtier Project "Life's Adaptation Strategies to Environmental Changes Figure 2. Two different TSs in the reaction of HOO" +CH.CI All atomic distances are in angstroms and angles are in degrees. The values in parentheses are relative enthalpies at 298.15 Supporting Information Available: Optimized geom- etries and G2(+) enthalpies at 298.15 K of all species in eq I. This material is available free of charge via the Intermet at http:llpubs.acs.org. addition to the trans conformer (C-0-0-H 180.0) ed previously N The skew conformation is more stable OL0S26930 (12) (a) Evanseck, J. D; Blake, 1. F:Jorgense, W.L. A An Chen Soe. 1987, 109, 2349 (b) Re. S:Morokuma, K. J. Phyt. Chem. A 2001, 1) (a) Henchman, M; Paulson, J. F Hiert P.M. J Am Chen. Soe 2002, 106, 10977 A. A: Paciso, J. F.: Clary, D. C.J Am. Chem. Soc. 1986, 408, 3142 Lat, Vol 8,â1" 2006

Show transcribed image text ORGANIC LETTERS The Î±-Effect in Gas-Phase%2 Reactions Revisited 2006 Vol. 8, No.I 119-121 : Yi Ren and Hiroshi Yamatakau College of Chemistry, Sichuon University, Chengdu 610064, P. R. China, and Department of Chemistry. Riklyo University, Nishi-kebukuro 3-34-1, Tothima-ku Tokyo 171-8301, Japan yiren$7@ hotmail.com [email protected] jp Received November 6, 2005 50 G21+1 basicity (kJimo This papenetamines gasphase sa reactions at saturated carbon for model reactionshu, cyc-cvu + cr our 'O., Meo-. NH,, HS-. CI, Br,IF, HO0-, Me00", HSS, and NHNH1 using the G2+) theory. The calculated results show that the a effect does exist in the gas-phase S,2 reaction at the sp carbon, contrary to the currently sccepted notion of the absence of the a-effect In the gas phase A class of nucleophiles, which have an electronegative atom differential ground state (GS) destabilization, transition state containing one or more unshared lone pair electrons adjacent (TS) stabilization, and solvent effect.S Among these, the o the nucleophilic atom, is called a-nucleophiles and shows interpretation in terms of the GS destabilization cannot be enhanced reactivity compared to that expected from a the origin, since the GS destabilization not only enhances the reactivity but also makes the equilibrium favorable, as pointed out explicitly in the literature. The a-effect should solution. Several inserpretations have been given including arise from a factor that stabilizes specifically the TS but no the product state. It has widely been considered that the a-effect does not exist in the gas phase and that solvent Brensted-type correlation (log k vs pKa).2 The a-effect has been reported in many different types of reactions in Sichaan Uaiversity Rakkyo Usiversity (1) Edwards. J. O: Pearson, R.G.JAm Chem Soc. 1962, 84, 16 4) () Hot, S Bancel, E. Tatrahedron Let 1984, 25, 3411. (b) Buncel Hot, S; Buncel, E. lur. J. Chem 1983, 26, 313 E: Hor, S. Terrahedron Leit 1983, 24, 4777 CJ) (a) Buscel,E:Wilson, H: Chaaqui, C. Am Ohm Soc. 1982 (5) (a) DePuy, C. H; Della, E W; Filey, J.: Grabewski, J1.1: Bierbaum, V.M.J.Aå¤CMm Soc. 1983. 10.5. 2481,(b) wolfes:sehela D 1: 04, 4896 (b) Fountain, K. RiHatchinson, L. K: Mulhearn. D. C ; Xu, Y. B J. Ora aen. 199. 58. 7183, (c) UnãLFL; Park. Y M.; Bunced. Lenx. 2004 50, 7801 10 1021lpl0528830 CCC: $33 50 2006 American Chenical Soclety Schlegel, H B.; Minot, C.; Eisenstein, 0. Telahedran Lat 1982, 23, 61s (c) Evatieck, J D: Blake, J. F; Jorgensen. w.L. ro9. 2349 (d) Hot. S.J. Org. arm. Conu, 2000, 1917. (d) Bancel, E.-Un. I, H. Ternshedro, Am Cham. Soc. t987 1982, 47, 3545 Publined on web 12-1200S
effects play an important role of that factor is not clear yet. However, the real picture in our calculations. Relative energies correspond to enthalpy changes at 298.15 K in kJ/mol. All optimized Experimental evidence against the a-effect in the gas phase structures involved in the reactions are given in Supporting was reported in 1985 by Depuy and co-workers who Information. The key energetic quantities are presented in showed that the branching ratios among the proton transfer. Table 1. The data in Table 1 indicate that the calculated carbonyl addition, and substination channels for the reaction of methyl acetate with HO and HOO did not change with the base (nucleophile). However, the conclasion based on Table 1. Cakculated Basicities of Nucleophiles and Overall the direct comparison of the branching ratios is not exclusiveBarriers, Afe (ml, for Equation 1 since the base (nocleophile) dependence conld be different for the different reaction channels. Furthermore, the basic assamption behind the argument, aamely, the absence of the a-effect for the proton-transfer reaction, is also questionable. The most cited computational evidence against the a-effect was given in 1987 by Jorgensen et al, who reported that, for the gas-phase Sw2 reaction of CH Cl, the TS with HOO was less stable than that with H0. However, the level of calculations was not high, and the TS was not thoroughly Me00 examined in this pioncering study Ä°STsa Â± 2.1-56.6 1487.8 +042 52.2 14680Â± 120 1448 0 t 13o 15960 + 4.050.4 15673 + 3.3 45.8 kS- 1353.5 Â± O 42 In the present study, we have cakculated a series of gas phase anionic Sw2 reactions (eq 1) of CHCi with seven normal (HO, MeO, NH:, HS, Cl, Br, and 1) and four a-sacleophiles (H00, Me00, HSS, and NH,NH and found that the level of computation is important to obeain reliable results. Furthermoee, HOO is conformationaly more flexible than HO. and we could obtain a new and mone stable TS not considered previously. All of these results require modification of the conclusion based on the previous calculations as well as on the gas-phase experiment and clearly show that the a-effect exists in the gas phase Su2 reactions at an saturated carbon (sp hybridization). G2l(+) basicities are in good agreement with the experimental Figare 1 shows that a good correlation (R 0993) exists between the overall barriers (AH) and the basicities of CI. Br.I,HOO", McOO, HSS, and NH,NH) A modified form of G2 theory, G2(+.introduced by Radom et al. and successfully applied in the stady of Sk2 reactices at saturated carbon, nitrogen, and oxygen, was (t) (a) Glakbevsev, M. N.; Pros, A: Radom, LIAM hem Soe 195. 117. 2024 (b) Ghshovtev, M. N: Post, A; Radom. L. J Am H B; Bach, R. D: Radom, L. A. Am Chem Soe 1996, 118, 11258.( 1200 1300 1 150 600700 1800 (7) Frisch, M.14 Trucka, O.W M A Cheeseman JR. Zkrski, V. G: Monigomery, J A: Suatmann Figere 1. Plots of the G2(+) overall barriers ((Ho) the basicities of mormal nucleophiles () and a-nucleophiles (O) for K. N.: Szain, M.C Farkas, O: Tomasi, J:Barone, V.: Cossi, M: Cammi, Priersion, G A.: Ayala P. Y. Cui, Q:Mokuma, K Malick. D. K. v.: Baboul. A. Stefanov. B 11,Lis. G ; Liashenko, A; Paker, P. eq 1 at 298 15 K Kom-ne t Go pens R Main R L. Fo. Ket), T AI Laham, six normal nucleophiles CHO.. Meo", HS, Cr. Br, and D C Y. Nanayakkars, A. Gonaaler, C.Chalacombe, M.1. The observed correlation reinforces the generally ac- L. A. Gearsian 95, Revisicn cepted view that the macleophilic reactivity is basically controlled by the basicity of nucleophile if the substrate is 02fixed. It is worth noticing that the points for three a-nucleo- (10) NIST Sandard Refenence Database Number 69, July 2003 Rekase Org Lent, Vol & No. 1, 2006 A., Guastian, Inc Bc HH Ch 5191. (c) Ren, Y: Zhu, H. J. J. Am Soc Mass Spectron 2004, 15, 673 9) Res, Y: Wolk JL. Hoa, S. Jnter, J. Mass Spectron 2082, 220, 1
philes out of the four examined deviate downward from the (by 8.7 k/mol) than the trans one probably due to the correlation line, displaying detectable a-effect in the S2 favorable electrostatic interaction betaween the a-atom (0) raction. The point for NH&NH- is slightly above the line, in HO0 and the positively charged methyl moiety at the but this nocleophile is more reactive than NH3" despite its TS. Such interaction is weaker in the trans conformer smaller basicity, as can be seen in Figure 1:It should also NH Ys NH,NH-. In these two cases the attacking atoms be noted that both MeO" and HO" behave normally, whereas are nitrogen. The calculated gas-phase basicity of NH NH HOO-deviates downward from the correlation line. The is smaller than N, by 20.4 klimol, whereas the results indicate that the deviation of HOO-is not due to the size effect, namely, an extra stability due to a polarizabiliry effect, often observed in the gas-phase calculations for a large species. The four pairs of the normal and corresponding be higher for NH NH than for NH a-nucleophiles are compared in detail below for the reaction of NH,NH' + CH,Cis lower than that for NH +CH/CI by 3.3 kJ/mol. This leads to the expectation that the reaction efficiency for the gas-phase reaction should HS vs HSS. The attacking atoms are sulfur in these HO- vs HO0. The reaction of HO +CH,CI has been two nucleophiles. The calculated G2(+) AH values in extensively studied experimentally" and computatonallyn Table l show that he ÎÎ s 6.3 kJ mol for the reaction The reaction of HOO +CHCI is less studied theoretically of a-nucleophile HSS with CH,CI, slightly lower than that and only calculated at a lower level. The caleulations by for HS with CH,CI by 0.7 kJ/mol, even though HSS is Jorgensen et al. at the MPn 2 and 3WHF6-31+G less basic than HS- by 34.3 kJ/mol. Thus, the a-effect is level of theory suggested that the Alf'o. for HO+CHCI clearly observed. is lower than that for HOO CH,CI by more than 10 MeO vs Me00. The calculated overal kl/mol. However, the present study revealed that the TSs for reactions of HO+CH,Cl and HO0+CHCl are 55.5 and 56.6 kJ/mol below the reactants at the G20+) level, barrier for Meo-t, Ð¡ÐÐ is-506 klrnel, which is higher than that for HO+CH CI by 4.9 kJ/mol, consistent with the smaller basicity of MeO" in the gas phase. Although the Alf for respectively, thas indicating that H00- is more reactive than a-nucleophile MeOO" is slightly higher than that for Meo HO, even though HOO. is a much weaker base than HO.. by 48 k1ml due to its smaller basicity and stenc effects. The different results can be attributed to the different TS there is still a obvious downward deviation for Me00 in es for the reaction of HOO; we found a new TS Figure 1. This result again confirms the a-effect in the gas- with a skew conformer (Figure 2, C-O-0-H 108.5Â°), phase Sw2 reaction. The TS prefers the skew conformation (C-0-0-C 103.8Â°) due to the favorable interaction between the a-atom with the methyl moiety as in the case In sammary, the high level G2(+) calculations show that three out of the four a-nucleophiles examined exhibit downward deviation from the plot of overall barrier vs basicity and accelerate the reaction through the stabilizatiorn of the transition stractures.The other a-nucleophile (NH,NH) clearly shows an enhanced reactivity compared to that of the corresponding normal nacleophile (NH2"). The a-effect does exist in the gas-phase Sx2 reaction at saturated carbon, contrary to the currently accepled notion of the absence of the a -effect in the gas phase. skew-conformer (0.0 kWmol Acknowledgment. Y.R. thanks Japan Society for the Promotion of Science for the invication Fellowship Programs for Research in Japan (no. L05546). H.Y. thanks Rikkyo Umiversity Fromtier Project "Life's Adaptation Strategies to Environmental Changes Figure 2. Two different TSs in the reaction of HOO" +CH.CI All atomic distances are in angstroms and angles are in degrees. The values in parentheses are relative enthalpies at 298.15 Supporting Information Available: Optimized geom- etries and G2(+) enthalpies at 298.15 K of all species in eq I. This material is available free of charge via the Intermet at http:llpubs.acs.org. addition to the trans conformer (C-0-0-H 180.0) ed previously N The skew conformation is more stable OL0S26930 (12) (a) Evanseck, J. D; Blake, 1. F:Jorgense, W.L. A An Chen Soe. 1987, 109, 2349 (b) Re. S:Morokuma, K. J. Phyt. Chem. A 2001, 1) (a) Henchman, M; Paulson, J. F Hiert P.M. J Am Chen. Soe 2002, 106, 10977 A. A: Paciso, J. F.: Clary, D. C.J Am. Chem. Soc. 1986, 408, 3142 Lat, Vol 8,â1" 2006
Read the paper titled "The a-effect in gas-phase SN2 reaction revisited" (Ren, Y, Yamataka, H. Org. Lett., 2006, 8, 119-121) You will answer the following questions. 1. What is the a-effect? 2. In the first paragraph, they talk about the comparison between reactivity and acid-base: log k vs pKa. Why is that? 3. What is the central thesis of the paper?

Hi! I am working on a lab for Organic Chemistry and I want to make sure my answers are correct before submitting it!

Thank you in advance!

Stereoisomerism: A Model Exercise

In this experiment you will construct models with your molecular model set that illustrate the concepts of chirality, chiral center (stereogenic center, asymmetric carbon atom), enantiomers, diastereomers, and meso forms. You will also learn about two conventions, R-S and Fischer, for designating the configurations of chiral molecules.

You will be asked several questions related to each construction exercise. Record the answers and then when you have finished all of the exercises, take the assessment listed in this folder. The questions will be repeated so that you can submit your answers electronically for grading.

Chiral centers

Procedure: Construct a model* in which a tetrahedral (sp 3) carbon atom (black) has four different model atoms attached to it. Use the light blue ball, red, blue and, green polyhedrons to represent four different atoms or groups attached to the central atom (black).

1. Does the model have a plane of symmetry?

a. Yes

b. No

A carbon atom that has four different groups attached to it is a chiral center, or an asymmetric carbon atom. The carbon marked with a # in 3-methylhexane is a chiral center.

2. What groups are attached to the chiral center in 3-methylhexane?

a. Hydrogen

b. Methyl

c. Ethyl

d. n-propyl

e. n-butyl

Replace the green atom (or group) in your model with a second red atom. Now two of the groups attached to the carbon atom are identical:

3. Does the model now have a plane of symmetry?

a. Yes

b. No

4. ï¿½ï¿½ Describe it.

a. Plane through the light blue ball, black polyhedron and blue polyhedron

b. Plane through a red polyhedron, black polyhedron and blue polyhedron

c. Plane through the light blue ball, black polyhedron and red polyhedron

d. Plane through a red polyhedron, black polyhedron and red polyhedron

Draw structural formulas for the following compounds, and mark any chiral centers (asymmetric carbons) with an asterisk:

l-bromobutane, 2- bromobutane, 1,2-dibromobutane,

1,3-dibromobutane, l, 4-dibromobutane, 2,3-dibromobutane.

5. Which of the compounds contain chiral centers?

a. 1-bromobutane

b. 2-bromobutane

c. 1,2-dibromobutane

d. 1,3-dibromobutane

e. 1,4-dibromobutane

f. 2,3-dibromobutane

Chirality and Enantiomers

A center of chirality (from the Greek cheir, hand) imparts the property of handedness to a molecule. In this part of the experiment, the left- or right- handedness of molecules with a chiral center will be illustrated with models.

A molecule is said to be chiral (that is, to have the property of handedness) if its mirror image is not superimposable. The mirror image of a left hand, for example, is a right hand. A molecule that is achiral has a mirror image that is superimposible. We shall see that any molecule with a plane of symmetry is achiral.

Procedure: Reconstruct the original model (carbon (black) with light blue ball, red, blue and, green polyhedrons attached). Set the model on the desktop so that the substituent light blue ball atom points toward the ceiling.

Looking down on the model and proceeding clockwise from the green atom, record the colors of the three atoms that rest on the desktop.

Now construct a second model that is the mirror image of the first, and place it on the desktop with the light blue ball atom up:

6. In which direction, clockwise or counterclockwise, must you proceed in order to list the same sequence of colors of the three atoms resting on the desk's surface?

a. Clockwise

b. Counterclockwise

Try to superimpose the two models.

7. The models are

a. Superimposable

b. Not superimposable

The two models that you have just constructed represent chiral molecules- they lack a plane of symmetry and have mirror images that are notsuperimposable. Two substances, the molecular structures of which are related as an object and its nonsuperimposable mirror image are called enantiomers. They differ from each other only in properties that have a direction or "handedness," such as, for example, the direction(clockwise or counterclockwise) in which they rotate a beam of plane-polarized light. Because of this latter property, such substances are sometimes called optical isomers. They are optically active.

Now we will examine the consequence of having at least two identical atoms or groups attached to a tetrahedral carbon atom.

Replace the green atom in each model with a red atom, so that each model has two identical groups attached to the central carbon atom:

8. Are the models still mirror images?

a. Yes

b. No

9. Does either of the models have a plane of symmetry?

a. Yes

b. No

10. Are the models superimposable?

a. Yes

b. No

11. Do the models represent identical molecules or different molecules?

a. Identical

b. Different

Place each model on the desk so that the light blue ball substituent points up.

12. To define the same sequence of colors for the three atoms resting on the desk top, must you proceed

a. Clockwise

b. Counterclockwise

c. Either way

13. Are the models chiral (handed)?

a. Yes

b. No

The models you have just studied represent achiral molecules. Their mirror images are identical. They are optically inactive. Any molecule that has a plane of symmetry is achiral.

Diastereomers and Meso Forms

For any molecule that has two or more chiral centers, it is possible to have stereoisomers that are not mirror images. Stereoisomers that are not related as enantiomers are diastereomers. Diastereomers differ in all properties, chiral and achiral.

Procedure: Construct a model with four different groups (light blue ball, blue, red, and green) attached to a central carbon atom (black). Construct another model identical to the first. (Be sure they are identical by making sure the models are superimposable.) Now remove the green substituent from each model and connect the two carbon atoms with a bond. Use this model with the two stereogenic centers for the next series of questions (14 -26).

14. How many chiral centers (asymmetric carbons) does this model have?

a. 1

b. 2

c. 3

d. 4

e. 5

Note that there are four different groups attached to each chiral center and that each chiral center has the same four groups attached.

15. Does the model have a plane of symmetry in any of its conformations?

a. Yes

b. No

Construct the mirror image of the first model.

16. Is the mirror image identical to or different from the first model?

a. Identical

b. Different

17. What term describes the two models?

a. Enantiomer

b. Diastereomer

18. Is each model chiral or achiral?

a. Chiral

b. Achiral

Now interchange a red and blue atom on the same carbon in one of the models.

19. Are the models identical or different now?

a. Identical

b. Different

20. Are they mirror images (enantiomers)?

a. Yes

b. No

21. Are they stereoisomers?

a. Yes

b. No

22. What term describes the two models?

a. Enantiomer

b. Diastereomer

Carefully examine the conformations of the model in which you interchanged the red and blue atoms.

23. Does the model have a conformation with a plane of symmetry?

a. Yes

b. No

24. Would the mirror image of this model be identical (superimposable) or different from the model itself?

a. Identical

b. Different

Verify your prediction by constructing the mirror-image model.

25. This model is

a. Chiral

b. Achiral

26. Would a molecule corresponding to this model be optically active?

a. Yes

b. No

The last model studied here represents a meso form. The model possesses two chiral centers, but they are of equal and opposite chirality. This situation arises when a molecule has two identical chiral centers. Because the molecule has a readily accessible conformation with a plane of symmetry, it is achiral and optically inactive.

Tartaric acid is a molecule that corresponds to the models constructed in this section of the experiment.

Tartaric Acid

It exists in three forms; two are optically active enantiomers, and the third is an optically inactive meso form that is a diastereomer of the optically active forms. ,

27. Draw Newman projection formulas (looking at the bond between C-2 and C-3) for the three tartaric acids. Label pairs of enantiomers and diastereomers, as well as the meso form.

When a molecule has two different chiral centers, it may exist in four optically active forms (two pairs of enantiomers). To illustrate this with the models you just used, replace one of the colored atoms on one of the carbon atoms with a black atom. There are now four distinct ways of constructing the models: two pairs of enantiomers. Construct the two pairs of enantiomers.

28. What name is given to a pair of molecules consisting of one molecule from each of the two pairs of enantiomers you just constructed?

a. Enantiomers

b. Diastereomers

c. Meso forms

The R-S Convention

The letter R (from rectus, right) or S (from sinister, left) is used to designate the configuration at a chiral center. The four atoms or groups attached to the chiral center are arranged in a priority order according to atomic number: the higher the atomic number, the higher the priority. If two atoms have the same atomic number, we move to the next atoms out from the chiral center, or even further, until we observe a difference in atomic number. We then view the molecule from the side opposite the group with the lowest priority. If the remaining three groups in order from highest to lowest priority form a clockwise array, the configuration is R; if they form a counterclockwise array, the configuration is S.

Procedure: Construct a model of 2-chlorobutane.

29. Which carbon in the chain is a chiral center?

a. 1

b. 2

c. 3

d. 4

There are four groups attached to this chiral center

.

30. Which group has the highest priority?

a. Cl

b. Methyl

c. Ethyl

d. H

31. Which group has the lowest priority?

a. Cl

b. Methyl

c. Ethyl

d. H

32. What is the priority order of the other two groups?

a. Ethyl > Methyl

b. Methyl> Ethyl

Set the model on the desktop so that it can be viewed from the side opposite the hydrogen. Put the chlorine atom at the top.

33. When viewing the model, the three remaining groups in priority- order sequence form a

a. Clockwise array and the model has a R configuration.

b. Counterclockwise array and the model has a R configuration.

c. Clockwise array and the model has a S configuration.

d. Counterclockwise array and the model has a S configuration.

Interchange any two groups attached to the chiral center.

34. What configuration does the model have now?

a. The same as before.

b. The opposite configuration.

Note that to change configuration we must disconnect and remake bonds, whereas we can change conformation by rotating groups around single bonds.

Fischer Projection Formulas

It is sometimes convenient to have a two-dimensional representation for three-dimensional molecules, particularly when we are studying stereoisomerism. One common convention, devised by the German organic chemist Emil Fischer and named after him, is described in this section

Procedure: Construct a model of an asymmetric carbon atom [with a black, a blue, a green, and a red ball attached] that corresponds to the following three-dimensional drawing:

Set the model on the desk as shown. With your left hand, tip the model to the left so that only the red and green balls rest on the table. Viewed from the top, the black and blue balls project toward you (left and right, respectively).

The convention for Fischer projection formulas is as follows: The asymmetric carbon lies in the plane of the page or paper, horizontal groups come out of the plane of the paper toward the viewer, and vertical groups recede behind the paper away from the viewer.

Save the model you have just constructed for comparison with other models that you will construct.

Let us first consider the effect of interchanging any two groups in a Fischer projection formula. Your model corresponds to formula A below:

Consider formula A', with groups blue and green interchanged. Construct a model corresponding to A'.

35. Is A' identical to A, or is it the enantiomer of A?

a. Identical

b. Enantiomer

Now construct a model corresponding to A" (like A, but with green and red interchanged).

36. Is A" identical to A, or is it the enantiomer of A?

a. Identical

b. Enantiomer

37. What generalization can you make about the interchange of any two groups in a Fischer projection formula?

a. The new molecule is an enantiomer of the first.

b. The molecule is identical to the first.

38. If you were to make an even number of group interchanges in a Fischer projection formula, would you obtain the original molecule or its enantiomer?

a. Original molecule

b. Enantiomer of the original model

39. Do the following Fischer projections represent the same molecule or enantiomers?

a. Same molecule

b. Enantiomers

Rotation of a Fischer projection formula in the plane of the paper also affects the structure it represents.

.

Consider formula B, which corresponds to the Fischer projection formula of A rotated 90Â° clockwise in the plane of the paper. Build a model of formula B. (Remember Fischer Projection Formulas are two dimensional diagrams written on paper of three dimensional molecules.)

40. What minimum number of group interchanges in the model are necessary to convert formula B back to formula A?

a. Zero

b. One

c. Two

d. Three

41. Is model B identical to model A, or is it the enantiomer of model A?

a. Identical

b. Enantiomer

Formula C results from a 90Â° counterclockwise rotation in the plane of the paper of the Fischer projection formula of A. Build a model of formula C.

42. Is model C identical to model A, or is it the enantiomer of A?

a. Identical

b. Enantiomer