有机化学：第5章 立体化学

1、Chapter 5,Stereochemistry 第五章 立体化学,Organic Chemistry A (1,Content,5.1 Introduction 5.2 Molecule with one asymmetric carbon 5.3 Molecule with two asymmetric carbon 5.4 Molecule with more than two asymmetric carbon 5.5 Cyclic compound 5.6 Other chiral molecules 5.7 Measure of polarity and reslution of

2、 racemic compounds,5.1.1 Chirality,5.1 Introduction,Plane-polarized light 平面偏振光（偏振光,Nicol,Light source,Non-chiral compounds, like water,该化合物“无旋光性” “无光学活性,该化合物具有“旋光性” 为“旋光物质”或“光活性物质,偏转角 旋光度,物质的旋光性：物质使偏振光振动平面发生旋转的性质,Specific rotation 比旋光度,= 100/(cl,D25 = 100/(cl,D: the D line of a sodium lamp (=589.6n

3、m,The Polarimeter,旋光度 c： 样品的浓度（100mL溶液中样品的克数） l： 盛液管的长度（单位：cm,5.1.2 Asymmetric carbon atom,In the 19th century, several asymmetric crystals were discovered. Also, the solutions obtained from the asymmetric crystals were asymmetric,Louis Pasteur,什么样的化合物具有旋光性,not superposable,Chirality 手性,晶体结构中缺少某种对称因

4、素，导致实物与镜像互不重合,从而推测具有手性的分子，其结构中也因缺少某种对称因素，导致实物与镜像互不重合,判据,实物与镜像不能重合,分子具有手性,充分且必要,1874 Newzealand Vant Hoff,The carbon atom linking four different groups or atoms is called an “asymmetric carbon,Asymmetric carbon,不对称碳原子：与四个互不相同的一价基团相连接的碳原子。 加“ * ”表示,If an symmetric carbon can be substituted and becomes

5、 asymmetric, it is called a “prochiral carbon,Prochiral carbon,Any molecule with such a carbon molecule, which is linked to four different groups or atoms, shows some chirality,Stereocenter,Vant Hoff 认为含有不对称碳的分子具有旋光性,优点：使用方便 缺点：很多例外 有些分子中存在不对称碳，但却无手性： 有些分子中没有不对称碳，但却有手性,5.1.3 Symmetric element,How to

6、 judge chirality of a molecule,A molecule is not tally with its enantiomorphism. But molecules with two asymmetric carbons may be symmetric,A molecule without asymmetric carbon may be chiral,If a molecule lacks some specialized symmetric element, the molecule shows chirality. 结构中缺少某种对称因素是该分子具有旋光性的原因

7、,Asymmetric axis,C2,C3,C3,分子中的对称因素,对称轴 Cn n：轴的阶,Cn表示该结构具有n重轴。当分子绕该轴转动3600/n的角度后，得到的构象与原分子完全一致。 当分子绕该轴转动一周，可得n个与原始构型无法区别的等价构型,C2,C3,Planes of symmetric (mirror plane,对称面,假如有一个平面可以将分子分割成两部分，而其中一部分正好是另一部分的镜象，这个平面就是分子的对称面,symmetric plane: the paper plane,an infinitude of symmetric planes,Symmetric centr

8、e,对称中心 i,i：若分子中有一点i，通过i点画直线，如果在离i等距离的直线两端有相同的原子或原子团，则点i称为分子的对称中心,Symmetric element and chirality,has symmetric plane has no symmetric centre,has no symmetric plane has no symmetric centre,no chirality no optical activity,chirality,does not mach its enantiomorphous form,optical activity,对称因素与对称性,如果一个

9、分子既没有对称面，又没有对称中心，那么该分子具有手性,有对称面 无手性,既无对称面，又无对称中心 有手性,5.2.1 R/S system and Fischer projection formulas 5.2.2 Enantiomer 5.2.3 Racemate,5.2 Molecule with one asymmetric carbon,5.2.1 R/S system and Fischer projection formulas,1, D/L system,Defined as L configuration,Defined as D configuration,以“乳酸为基准，衍

10、生化的方法,采用衍生化方法而推出 与旋光性无关 衍生过程中可能会产生歧义 目前主要应用于氨基酸、糖等分子中,2, R/S system,lowest priority,Highest priority,Third priority,Second priority,R(rectus,S(sinister,基团大小,顺序规则,特点：R/S与构型一一对应，不必找基准物进行衍生,顺时针,逆时针,观察方向,观察方向,L,D,S,R,Try to decide the R/S configuration in these molecules,3, Fischer projection formula,画法

11、,指向纸内,指向纸外,利用Fischer投影式判断R/S构型,1,2,3,4,最小的基团远离观察者， 与R/S构型判断的要求一致,逆时针 S构型,1,2,3,4,最小的基团指向观察者， 与R/S构型判断的要求不一致,逆时针 R构型,两个规定,规定之一：Fischer投影式不可在纸面上翻转1800,翻转1800,S,R,相当于发生一次基团交换 结果：构型改变,规定之二：Fischer投影式可以在纸面上旋转1800,旋转1800,S,S,相当于发生两次基团交换 结果：构型不变,结论：基团交换偶数次 构型不变 基团交换奇数次 构型改变 应用：复杂情况下的R/S构型判断,最小的基团指向观察者， 与R/

12、S构型判断的要求不一致,5.2.2 Enantiomer 含一个不对称碳的分子：对映体及其性质,含一个不对称碳原子的分子，其分子内既无对称面，又无对称中心，因此具有手性。 必定存在一个与该分子互为实物与镜像关系的分子，二者均有手性，二者互为对映异构体,互为实物与镜像的关系，因此旋光度大小相等，符号相反。 分子中任何两个原子之间的距离及相对位置都相同，因此分子的内能相同。 在非手性环境下无区别（除旋光性,对映体的性质,In symmetric environment,In asymmetric environment,The enantiomer shows no difference. Mel

13、ting point, boiling point, even reaction rate in asymmetric solution are the same. 在手性环境中二者性质不同,When in asymmetric solution, reacting with asymmetric reagent or reacting with asymmetric catalysts, the reaction rates are different,生命体形成手性环境 可识别不同的手性分子,5.2.3 Racemate 外消旋体,1 mol 左旋体 1 mol 右旋体,外消旋体,混合物

14、旋光度为零 性质与相应的左、右旋体不同,外消旋化：旋光化合物在物理或化学因素作用下变成两个对映体的平衡混合物，因而失去旋光性的过程,A equimolar mixture of two enantiomers,Using reactants and solvents with no chirality, racement is always obtained. 利用非手性原料，在非手性条件下只能得到外消旋体 Racement show different properties, e.g. melting point, to pure enantiomers. Racemization is s

15、uch a process in which an chiral compound is converted into two enantiomers, and thus loses its optical reactivity,5.3.1 Molecule with two same asymmetric carbons 5.3.2 Optical activity and conformation 5.3.3 Molecule with two different asymmetric carbons,5.3 Molecule with two asymmetric carbon,当一个分

16、子含有n个不对称碳原子，其异构体数最多为2n个。 含有多个不对称碳原子的Fischer投影式的画法,5.3.1 Molecule with two same asymmetric carbons,Enantiomer 对映异构体,Diastereomers 非对映异构体,共有3个异构体,meso compound 内消旋体,非对映异构体的性质不同,meso compound 内消旋体,单一化合物 旋光度为零 性质与那一对对映异构体均不同,symmetric, but unstable,stable, still symmetric,问题 ,全重叠式，不优势,5.3.2 Optical acti

17、vity and conformation 旋光性与构象,1, meso compound,利用Fischer投影式模型分析，分子具有对称面，无手性,平面分析,利用立体分析，分子应有手性,立体分析,Conformation analysis,2, enantiomers,5.3.3 Molecule with two different asymmetric carbons,Stereoisomer,Diastereomer,5.4.1 Epimer 5.4.2 Pseudoasymmetric carbon,5.4 Molecule with more than two asymmetric

18、 carbon,5.4.1 Epimer 差向异构体,Epimer,如果两个光学异构体仅有一个不对称碳原子的构型相反，其它不对称碳原子的构型都相同，这两个光活异构体则互为差向异构体,5.4.2 Pseudoasymmetric carbon 假不对称碳原子,有几个不对称碳原子,是否不对称碳 取决于2C和4C的构型,meso,meso,The third carbons of these four isomers are asymmetric,共四个异构体,R,R,S,S,R,S,R,S,3,3,3,3,3C 为非不对称碳,均有旋光性 互为对映异构体,3C 为不对称碳,均无旋光性 为内消旋体 互

19、为非对映异构体,假不对称碳,Optical Stereomer C3 is symmetric,Not optical Meso C3 is asymmetric,5.5.1 Cyclopropane, cyclobutane, and cyclopentane 5.5.2 derivatives of Cyclohexane,5.5 Cyclic compound,关键：平面分析 构象分析,5.5.1 Cyclopropane, cyclobutane, and cyclopentane,1, Cyclopropane,A,A,A,A,A,A,A,Trans,Cis,A,B,B,A,A,B,

20、B,A,Cis,Trans,A,A,A,A,A,A,2, Cyclobutane,3, Cyclopentane,Learn on your own,5.5.2 Derivatives of Cyclohexane,顺-1,2-二取代,平面分析,构象分析,Rotation,Conformation transition,ii)aa,ii)ee,iii)ee,Conformation transition,iii)aa,iv,iv,结论：就环己烷而言，平面分析与构象分析的结论相同,5.6.1 Allene type compounds 5.6.2 Diphenyl compounds 5.6.3

21、 Spiro compounds 5.6.2 Compounds with stereocenters other than carbon,5.6 Other chiral molecules,5.6.1 Allene type compounds,如果任何一端或两端的碳原子上连有相同取代基，则分子中存在对称面，该分子无手性,反之，则该分子有手性,5.6.2 Diphenyl compounds,If on 2,2,6,6 sites there are large groups which block the rotation of the benzene rings, the two ph

22、enol rings are not on the same plane,5.6.3 Spiro compounds,5.6.2 Compounds with stereocenters other than carbon,Also, molecules with N, S, Si, As show this property,5.7.1 Measure of polarity (Learn on your own) 5.7.2 Reslution of racemic compounds 5.7.3 Enantiomer excess percentage 5.7.4 Configuration and bio-activities,5.7 Measure of polarity and reslution of racemic compounds,5.7.1 Measure of polarity (Learn on your own,D25 = 100/(cl,5.7.2 Re