有机化学教学课件：第三章 环烷烃

1、第三章 环烷烃(cycloalkane)一、环烷烃的命名和异构（Nomination and Isomerization)CycloalkanesCycloalkanes are alkanes that have carbon atoms that form a ring (called alicyclic compounds)Simple cycloalkanes rings of CH2 units, (CH2)n, or CnH2nStructure is shown as a regular polygon with the number of vertices equal to t

2、he number of Cs (a projection of the actual structure)Complex CycloalkanesNaturally occurring materials contain cycloalkane structuresExamples: chrysanthemic acid (cyclopropane), prostaglandins (cyclopentane), steroids (cyclohexanes and cyclopentane)菊酸前列腺素可的松Naming CycloalkanesCount the number of ca

3、rbon atoms in the ring and the number in the largest substituent chain. If the number of carbon atoms in the ring is equal to or greater than the number in the substituent, the compound is named as an alkyl-substituted cycloalkane For an alkyl- or halo-substituted cycloalkane, start at a point of at

4、tachment as C1 and number the substituents on the ring so that the second substituent has as low a number as possible.Number the substituents and write the nameCount the number of carbon atoms in the ring and the number in the largest substituent chain. If the number of carbon atoms in the ring is e

5、qual to or greater than the number in the substituent, the compound is named as an alkyl-substituted cycloalkane the second substituent has as low a number as possibleStereoisomersCompounds with atoms connected in the same order but which differ in three-dimensional orientation, are stereoisomersThe

6、 terms “cis” and “trans” should be used to specify stereoisomeric ring structuresRecall that constitutional isomers have atoms connected in different order Difference between the constitutional isomers and stereoisomersCis-Trans Isomerism in CycloalkanesRotation about C-C bonds in cycloalkanes is li

7、mited by the ring structureRings have two “faces” and substituents are labeled as to their relative facial positionstwo methyls on the same side (cis) of the ringthe two methyls on opposite sides (trans)ExamplePractice二、环烷烃的性质Properties of CycloalkanesMelting points are affected by the shapes and th

8、e way that crystals pack so they do not change uniformlyReactions of Cycloalkanes三、 环 烷 烃 的 构 象Stability of Cycloalkanes: The Baeyer Strain Theory Baeyer (1885): since carbon prefers to have bond angles of approximately 109, ring sizes other than five and six may be too strained to existRings from 3

9、 to 30 Cs do exist but are strained due to bond bending distortions and steric interactionsTypes of StrainAngle strain - expansion or compression of bond angles away from most stableTorsional strain - eclipsing of bonds on neighboring atomsSteric strain - repulsive interactions between nonbonded ato

10、ms in close proximityAngle strainConformations of Cyclopropane3-membered ring must have planar structureSymmetrical with CCC bond angles of 60Requires that sp3 based bonds are bent (and weakened)All C-H bonds are eclipsedConformations of CyclobutaneCyclobutane has less angle strain than cyclopropane

11、 but more torsional strain because of its larger number of ring hydrogensCyclobutane is slightly bent out of plane - one carbon atom is about 25 aboveConformations of CyclopentanePlanar cyclopentane would have no angle strain but very high torsional strainActual conformations of cyclopentane are non

12、planar, reducing torsional strainFour carbon atoms are in a planeThe fifth carbon atom is above or below the plane looks like an envelopeConformations of Cyclohexane Substituted cyclohexanes occur widely in natureThe cyclohexane ring is free of angle strain and torsional strainThe conformation is ha

13、s alternating atoms in a common plane and tetrahedral angles between all carbonsThis is called a chair conformationHow to Draw CyclohexaneAxial and Equatorial Bonds in Cyclohexane The chair conformation has two kinds of positions for substituents on the ring: axial positions and equatorial positions

14、Chair cyclohexane has six axial hydrogens perpendicular to the ring (parallel to the ring axis) and six equatorial hydrogens near the plane of the ring six axial bonds perpendicular to the ring6 个键是直立键，a键six equatorial bonds near the plane of the ring 6 个键是平伏键, e键Drawing the Axial and Equatorial Hyd

15、rogensConformational Mobility of Cyclohexane Chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions by a ring-flip Ring-flip of BromocyclohexaneWhen bromocyclohexane ring-flips the bromines position goes from equatorial to axial and so onAt room tempera

16、ture the ring-flip is very fast and the structure is seen as the weighted averageMonosubstituted CyclohexanesThe two conformers of a monosubstituted cyclohexane are not equal in energyThe equatorial conformer of methyl cyclohexane is more stable than the axial by 7.6 kJ/mol Methylcyclohexane5%95%尽管存

17、在椅式-椅式构象翻转，但是任意时刻甲基在平伏键上的分子都占 95%。甲基在直立键上比在平伏键上拥挤。CH3CH35%95%拥挤来源于同边直立键上氢的靠近 这种拥挤称为 “1,3-二直立键的排斥作用” ，是一种 van der Waals 应力Methylcyclohexane少于 0.01%大于 99.99%拥挤程度高的是大的基团, 如叔丁基。C(CH3)3C(CH3)3tert-butylcyclohexanevan der Waals应力导致强烈的1,3-二直立键排斥作用tert-butylcyclohexaneConformational Analysis of Disubstitute

18、d Cyclohexanes In disubstituted cyclohexanes the steric effects of both substituents must be taken into account in both conformationsThere are two isomers of 1,2-dimethylcyclohexane. cis and transIn the cis isomer, both methyl groups same face of the ring, and compound can exist in two chair conform

19、ationsConsider the sum of all interactionsIn cis-1,2, both conformations are equal in energyTrans-1,2-DimethylcyclohexaneMethyl groups are on opposite faces of the ringOne trans conformation has both methyl groups equatorial and only a gauche butane interaction between methyls (3.8 kJ/mol) and no 1,

20、3-diaxial interactionsThe ring-flipped conformation has both methyl groups axial with four 1,3-diaxial interactionsSteric strain of 4 3.8 kJ/mol = 15.2 kJ/mol makes the diaxial conformation 11.4 kJ/mol less favorable than the diequatorial conformationtrans-1,2-dimethylcyclohexane will exist almost e

21、xclusively (99%) in the diequatorial conformation 两个构象等价，每个都有一个甲基在直立键上，一个甲基在平伏键上。CH3CH3HH HCH3HCH3HCH3HCH3cis-1, 2-dimethylcyclohexane 两个构象不等价，稳定构象是两个甲基在平伏键上。CH3H3CHHHCH3HCH3HH3CHCH3trans-1, 2-dimethylcyclohexane顺式反式5212 kJ/mol5219 kJ/mol稳定性高稳定性低顺式异构体比反式稳定H3CCH3HHCH3H3CHH1, 3-dimethylcyclohexane两个构象

22、不等价，最稳定的是两个甲基都在平伏键上。H3CHHCH3CH3H3CHH HCH3HCH3cis-1, 3-dimethylcyclohexane 两个构象等价，每个都有一个甲基在直立键上，一个甲基在平伏键上。H3CHHCH3 HH3CHCH3H3CCH3HHtrans-1, 3-dimethylcyclohexane顺式反式CH35219 kJ/mol5212 kJ/mol稳定性低稳定性高反式异构体比顺式稳定H3CHHH3CCH3HH1, 4-dimethylcyclohexanecis-1, 4-dimethylcyclohexane 两个构象等价，每个都有一个甲基在直立键上，一个甲基在平

23、伏键上CH3H3CHHHCH3HCH3HH3CHCH3 两个构象不等价，最稳定的是两个甲基在平伏键上。CH3H3CHHHH3CHCH3HH3CHCH3trans-1, 4-dimethylcyclohexaneConformations of Polycyclic MoleculesDecalin consists of two cyclohexane rings joined to share two carbon atoms (the bridgehead carbons, C1 and C6) and a common bondTwo isomeric forms of decalin:

24、 trans fused or cis fusedIn cis-decalin hydrogen atoms at the bridgehead carbons are on the same face of the ringsIn trans-decalin, the bridgehead hydrogens are on opposite facesFlips and rotations do not interconvert cis and transBoat CyclohexaneCyclohexane can also be in a boat conformationLess st

25、able than chair cyclohexane due to steric and torsional strainC-2, 3, 5, 6 are in a planeH on C-1 and C-4 approach each other closely enough to produce considerable steric strainFour eclipsed H-pairs on C- 2, 3, 5, 6 produce torsional strain29 kJ/mol (7.0 kcal/mol) less stable than chairAdolf von Ba

26、eyer (19世纪)假设环烷烃是平面多边形当环烷烃的环小于或大于环戊烷时，键角会 从109.5 产生角应力Baeyer 将角应力作为不稳定因素对后来的理论 作出了贡献但是 Baeyer 的环烷烃的平面理论是错误的应力的类型扭应力由键重叠产生的应力van der Waals 应力 (空间应力)由原子拥挤产生的应力角应力由键角从正常值扭曲产生的应力 环烷烃的应力测定燃烧热通常用于比较异构体间的稳定性但是环丙烷、环丁烷等不是异构体燃烧热随着碳原子数的增加而增加因此将燃烧热除以碳原子数，比较“每个 CH2 基团” 的燃烧热环烷烃的燃烧热环烷烃kJ/mol每个CH2环丙烷2,091697环丁烷2,72

27、1681环戊烷3,291658环己烷3,920653环庚烷4,599657环辛烷5,267658环壬烷5,933659环癸烷6,587659环己烷的构象燃烧热表明在环己烷中角应力很小四面体的键角需要非平面的几何构型椅式是环己烷最稳定的构象所有键都是交叉式，碳上的键角接近四面体船式构象稳定性比椅式构象低所有的键都接近四面体，但是船头和船尾的两个氢太接近，由此船式构象中产生了 van der Waals 应力（ van der Waals 半径240pm)180 pm船式构象稳定性比椅式构象低船式构象中键的重叠产生扭应力扭船式构象比船式构象稳定性稍高一些在扭船式构象中， van der Waals

28、 应力和扭应力都要小些船式扭船式环己烷的椅式构象是最稳定的，在环己烷的衍生物中通常都是以椅式构象存在。环己烷的直立键和平伏键环上的 12 个化学键可以分为两类：每 6个一类6 个键是直立键，a键6 个键是平伏键, e键环己烷的构象翻转构象翻转椅式-椅式互转快过程 (活化能 = 45 kJ/mol)直立键变成平伏键，平伏键变成直立键半椅式半椅式扭船式半椅式扭船式半椅式扭船式45 kJ/mol45 kJ/mol23 kJ/mol单取代环己烷的构象分析 最稳定的构象是椅式 取代基在平伏键上最稳定甲基环己烷5%95%尽管存在椅式-椅式构象翻转，但是任意时刻甲基在平伏键上的分子都占 95%。甲基在直立键

29、上比在平伏键上拥挤。CH3CH3甲基环己烷5%95%拥挤来源于同边直立键上氢的靠近 这种拥挤称为 “1,3-二直立键的排斥作用” ，是一种 van der Waals 应力氟代环己烷40%60%拥挤程度最小的取代基是氟取代基的大小与其支链有关FF叔丁基环己烷少于 0.01%大于 99.99%拥挤程度高的是大的基团如叔丁基叔丁基有最大数量的取代基C(CH3)3C(CH3)3叔丁基环己烷van der Waals应力导致强烈的1,3-二直立键排斥作用二取代环己烷的构象分析1,4-二甲基环己烷立体异构体顺式反式CH35219 kJ/mol5212 kJ/mol稳定性低稳定性高反式异构体比顺式稳定H3CHHH3CCH3HH顺-1,4-二甲基环己烷的构象分析CH3 两个构象等价，每个都有一个甲基在直立键上，一个甲基在平伏键上H3CHHHCH3HCH3HH3CHCH3反-1,4-二甲基环己烷的构象分析 两个构象不等价，