Question: In some nucleophilic substitutions under SN1 conditions, complete racemization does not occur, and a small excess of one enantiomer is present. For example, treatment of optically pure 1-bromo-1-phenylpropane with water forms 1-phenylpropan-1-ol. a. Calculate how much of each enantiomer is present using the given optical rotation data. b. Which product predominates—the product of inversion or the product of retention of configuration? c. Suggest an explanation for this phenomenon.
Answer
a. S enantiomer is 55.21%, and R enantiomer is 44.79%.
b. Product of retention
c. The phenomenon is a unimolecular substitution reaction.
The racemization term comes into the concept when the racemic mixture is formed in any reaction. In the racemic mixture, the amount of each enantiomer (R and S) is equal.
The formula to calculate the ee is as follows:
ee =specific rotation of solutionspecific rotation of pure enontiomer×100{"x":[[5,20,30,32,26,12,4,4,12,25,31],[42,55,68,67,60,48,41,40,45,56,64,68],[114,140],[114,141],[160,816,1471],[369,367,357,345,340,344,355,365,370,368,356,343,339],[380,378,385,395,408,410,405,394,379],[416,429,441,443,431,423,415,416,422,435,443],[479,474,460,451,454,469,481],[489,493],[492,491,496],[524,524,504,504,503],[504,522],[532,536],[535,534,539],[574,569,555,546,549,564,576],[621,620,621,620,620,632,644,649],[672,662,658,660,669,681,687,688,684,672],[726,725,718,708,700,697,696],[697,715],[762,760,753,742,735,738,749,758,760,762,762,762,766],[807,805,799,791,782,778,777],[779,796],[818,820],[819,816,823],[839,831,831,835,846,856,861,860,854,838],[868,867,867,868,876,889,897,895],[943,934,933,932,940,956,961,962,956,943],[991,991,971,971,970],[971,989],[1060,1058,1046,1034,1031,1038,1048,1059,1061,1057,1047,1035,1030],[1083,1073,1069,1071,1080,1092,1098,1099,1095,1083],[1110,1107,1116,1125],[1134,1133,1137,1147,1159,1164,1161],[1202,1200,1188,1179,1173,1172],[1173,1192],[1211,1210,1217],[1211,1213],[1239,1229,1225,1227,1236,1248,1254,1255,1251,1239],[1264,1263,1263,1264,1271,1286,1293,1290],[231,228,218,207,202,203,214,225,231,231,225,212,203,200],[241,238,247,255,265,270,267,255,240],[279,292,305,304,297,285,278,277,282,293,301,305],[343,338,324,315,318,333,345],[354,353,360],[356,353],[390,390,370,370,369],[370,388],[398,402],[401,400,405],[440,433,413,412,425,438,442],[482,481,482,481,481,493,505,510],[533,522,519,519,526,539,547,549,547,533],[585,584,577,567,559,556,555],[556,574],[621,619,607,594,594,602,613,620,621,621,620,625],[663,661,649,640,634,633],[634,653],[671,675],[674,673,678],[699,689,685,687,696,708,714,715,711,699],[724,723,723,724,732,743,753,751],[807,797,793,795,804,816,822,823,819,807],[852,852,832,832,831],[832,850],[893,891,898,909,921,923,918,907,892],[932,931,935,945,957,962,959],[971,970,971,970,970,982,994,999],[1009,1022,1034,1036,1024,1016,1008,1009,1015,1028,1036],[1075,1090,1100,1102,1096,1084,1075,1074,1077,1086,1096,1102],[1111,1110,1110,1111,1119,1132,1140,1138],[1162,1151,1148,1148,1155,1168,1176,1178,1176,1162],[1185,1184,1184,1185,1193,1206,1214,1212],[1252,1249,1238,1227,1223,1222],[1223,1243],[1262,1264],[1263,1260,1267],[1283,1275,1275,1279,1290,1300,1305,1304,1298,1282],[1312,1311,1312,1312,1318,1328,1332,1331,1332,1332,1337,1347,1354,1352],[1365,1378,1391,1390,1383,1371,1364,1363,1368,1379,1387,1391],[1402,1401,1402,1402,1402,1414,1426,1430],[1490,1511,1518],[1489,1498,1517],[1532,1547,1548,1548],[1570,1559,1555,1554,1558,1575,1582,1583,1582,1580,1569],[1607,1596,1592,1591,1595,1612,1619,1620,1619,1617,1606],[1637.9555555555555,1641.3756613756614],[1675.5767195767196,1675.5767195767196,1675.5767195767196,1675.5767195767196,1675.5767195767196,1675.5767195767196,1672.1566137566138,1672.1566137566138,1672.1566137566138,1668.736507936508,1668.736507936508,1665.316402116402,1665.316402116402,1665.316402116402,1665.316402116402,1661.8962962962962,1658.4761904761906,1655.0560846560847,1655.0560846560847,1655.0560846560847,1651.635978835979,1651.635978835979,1648.215873015873,1644.7957671957672,1644.7957671957672,1644.7957671957672,1644.7957671957672],[1675.5767195767196,1678.9968253968254,1678.9968253968254],[1679.8830671033845]],"y":[[179,179,174,156,143,142,157,189,206,208,197],[179,179,170,151,142,142,155,172,201,209,204,196],[165,165],[184,184],[184,184,184],[64,55,50,53,66,78,85,89,100,112,117,115,99],[169,56,51,51,61,92,107,113,113],[87,88,80,60,51,52,69,100,114,117,109],[62,50,52,73,108,118,110],[30,28],[51,110,116],[23,9,8,28,117],[51,51],[30,28],[51,110,116],[62,50,52,73,108,118,110],[51,116,116,62,62,56,50,58],[51,56,75,104,118,116,103,75,56,51],[102,110,115,117,112,101,9],[51,51],[82,60,49,54,75,109,117,112,94,52,51,107,115],[102,112,116,117,114,103,8],[51,51],[31,27],[51,110,115],[115,106,75,57,51,56,75,102,117,115],[52,117,116,61,52,51,62,117],[52,52,78,97,115,116,95,75,53,51],[23,9,8,28,117],[51,51],[63,53,49,54,70,78,81,88,100,112,117,112,99],[51,56,75,104,118,116,103,75,56,51],[15,110,119,112],[51,103,113,117,113,104,51],[103,112,117,115,106,9],[51,51],[51,115,115],[36,33],[51,56,75,104,118,116,103,75,56,51],[51,116,116,63,52,51,63,116],[257,247,243,244,255,267,275,279,287,301,308,309,302,292],[360,248,243,243,250,269,298,307,307],[280,280,271,252,243,243,256,273,302,310,305,297],[254,242,244,265,300,310,302],[243,302,309],[223,228],[215,201,200,220,309],[243,243],[222,220],[243,302,308],[255,239,251,293,309,305,294],[243,308,308,254,254,248,242,250],[243,247,266,293,309,309,294,273,248,242],[294,302,307,309,304,293,201],[243,243],[266,250,241,255,292,307,308,289,243,242,298,307],[295,304,309,307,298,201],[243,243],[222,220],[243,302,308],[243,248,267,296,310,308,295,267,248,243],[243,307,308,254,245,243,253,307],[243,248,267,296,310,308,295,267,248,243],[215,201,200,220,309],[243,243],[360,246,243,245,260,284,301,306,306],[243,295,305,309,305,296,243],[243,308,308,254,254,248,242,250],[279,280,272,252,243,244,261,292,306,309,301],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a. The specific rotation is calculated from the formula of ee in the following manner:
ee=+5-48ee=-10.42
The calculated ee is for S enantiomer.
Therefore, 10.42% excess of the S enantiomer, 89.58% of racemic R and S (44.79% of S and 44.79% of R) is present.
Hence, S enantiomer is 55.21%, and R enantiomer is 44.79%.
b. The product of retention occurs because the initial substrate in the molecule has the S configuration. The product also has the S configuration because the amount of S enantiomer is 55.21%.
c. The phenomenon is a unimolecular substitution reaction in which the reaction proceeds by the formation of benzyl carbocation intermediate. Further in the reaction, the water molecule attacks at the carbocation with the same stereochemistry.
