有機(jī)化學(xué)教學(xué)課件：第三章 環(huán)烷烴

1、第三章 環(huán)烷烴(cycloalkane)一、環(huán)烷烴的命名和異構(gòu)（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、環(huán)烷烴的性質(zhì)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、 環(huán) 烷 烴 的 構(gòu) 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 個(gè)鍵是直立鍵，a鍵six equatorial bonds near the plane of the ring 6 個(gè)鍵是平伏鍵, e鍵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%盡管存

17、在椅式-椅式構(gòu)象翻轉(zhuǎn)，但是任意時(shí)刻甲基在平伏鍵上的分子都占 95%。甲基在直立鍵上比在平伏鍵上擁擠。CH3CH35%95%擁擠來源于同邊直立鍵上氫的靠近 這種擁擠稱為 “1,3-二直立鍵的排斥作用” ，是一種 van der Waals 應(yīng)力Methylcyclohexane少于 0.01%大于 99.99%擁擠程度高的是大的基團(tuán), 如叔丁基。C(CH3)3C(CH3)3tert-butylcyclohexanevan der Waals應(yīng)力導(dǎo)致強(qiáng)烈的1,3-二直立鍵排斥作用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 兩個(gè)構(gòu)象等價(jià)，每個(gè)都有一個(gè)甲基在直立鍵上，一個(gè)甲基在平伏鍵上。CH3CH3HH HCH3HCH3HCH3HCH3cis-1, 2-dimethylcyclohexane 兩個(gè)構(gòu)象不等價(jià)，穩(wěn)定構(gòu)象是兩個(gè)甲基在平伏鍵上。CH3H3CHHHCH3HCH3HH3CHCH3trans-1, 2-dimethylcyclohexane順式反式5212 kJ/mol5219 kJ/mol穩(wěn)定性高穩(wěn)定性低順式異構(gòu)體比反式穩(wěn)定H3CCH3HHCH3H3CHH1, 3-dimethylcyclohexane兩個(gè)構(gòu)象

22、不等價(jià)，最穩(wěn)定的是兩個(gè)甲基都在平伏鍵上。H3CHHCH3CH3H3CHH HCH3HCH3cis-1, 3-dimethylcyclohexane 兩個(gè)構(gòu)象等價(jià)，每個(gè)都有一個(gè)甲基在直立鍵上，一個(gè)甲基在平伏鍵上。H3CHHCH3 HH3CHCH3H3CCH3HHtrans-1, 3-dimethylcyclohexane順式反式CH35219 kJ/mol5212 kJ/mol穩(wěn)定性低穩(wěn)定性高反式異構(gòu)體比順式穩(wěn)定H3CHHH3CCH3HH1, 4-dimethylcyclohexanecis-1, 4-dimethylcyclohexane 兩個(gè)構(gòu)象等價(jià)，每個(gè)都有一個(gè)甲基在直立鍵上，一個(gè)甲基在平

23、伏鍵上CH3H3CHHHCH3HCH3HH3CHCH3 兩個(gè)構(gòu)象不等價(jià)，最穩(wěn)定的是兩個(gè)甲基在平伏鍵上。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世紀(jì))假設(shè)環(huán)烷烴是平面多邊形當(dāng)環(huán)烷烴的環(huán)小于或大于環(huán)戊烷時(shí)，鍵角會 從109.5 產(chǎn)生角應(yīng)力Baeyer 將角應(yīng)力作為不穩(wěn)定因素對后來的理論 作出了貢獻(xiàn)但是 Baeyer 的環(huán)烷烴的平面理論是錯(cuò)誤的應(yīng)力的類型扭應(yīng)力由鍵重疊產(chǎn)生的應(yīng)力van der Waals 應(yīng)力 (空間應(yīng)力)由原子擁擠產(chǎn)生的應(yīng)力角應(yīng)力由鍵角從正常值扭曲產(chǎn)生的應(yīng)力 環(huán)烷烴的應(yīng)力測定燃燒熱通常用于比較異構(gòu)體間的穩(wěn)定性但是環(huán)丙烷、環(huán)丁烷等不是異構(gòu)體燃燒熱隨著碳原子數(shù)的增加而增加因此將燃燒熱除以碳原子數(shù)，比較“每個(gè) CH2 基團(tuán)” 的燃燒熱環(huán)烷烴的燃燒熱環(huán)烷烴kJ/mol每個(gè)CH2環(huán)丙烷2,091697環(huán)丁烷2,72

27、1681環(huán)戊烷3,291658環(huán)己烷3,920653環(huán)庚烷4,599657環(huán)辛烷5,267658環(huán)壬烷5,933659環(huán)癸烷6,587659環(huán)己烷的構(gòu)象燃燒熱表明在環(huán)己烷中角應(yīng)力很小四面體的鍵角需要非平面的幾何構(gòu)型椅式是環(huán)己烷最穩(wěn)定的構(gòu)象所有鍵都是交叉式，碳上的鍵角接近四面體船式構(gòu)象穩(wěn)定性比椅式構(gòu)象低所有的鍵都接近四面體，但是船頭和船尾的兩個(gè)氫太接近，由此船式構(gòu)象中產(chǎn)生了 van der Waals 應(yīng)力（ van der Waals 半徑240pm)180 pm船式構(gòu)象穩(wěn)定性比椅式構(gòu)象低船式構(gòu)象中鍵的重疊產(chǎn)生扭應(yīng)力扭船式構(gòu)象比船式構(gòu)象穩(wěn)定性稍高一些在扭船式構(gòu)象中， van der Waals

28、 應(yīng)力和扭應(yīng)力都要小些船式扭船式環(huán)己烷的椅式構(gòu)象是最穩(wěn)定的，在環(huán)己烷的衍生物中通常都是以椅式構(gòu)象存在。環(huán)己烷的直立鍵和平伏鍵環(huán)上的 12 個(gè)化學(xué)鍵可以分為兩類：每 6個(gè)一類6 個(gè)鍵是直立鍵，a鍵6 個(gè)鍵是平伏鍵, e鍵環(huán)己烷的構(gòu)象翻轉(zhuǎn)構(gòu)象翻轉(zhuǎn)椅式-椅式互轉(zhuǎn)快過程 (活化能 = 45 kJ/mol)直立鍵變成平伏鍵，平伏鍵變成直立鍵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單取代環(huán)己烷的構(gòu)象分析 最穩(wěn)定的構(gòu)象是椅式 取代基在平伏鍵上最穩(wěn)定甲基環(huán)己烷5%95%盡管存在椅式-椅式構(gòu)象翻轉(zhuǎn)，但是任意時(shí)刻甲基在平伏鍵上的分子都占 95%。甲基在直立鍵

29、上比在平伏鍵上擁擠。CH3CH3甲基環(huán)己烷5%95%擁擠來源于同邊直立鍵上氫的靠近 這種擁擠稱為 “1,3-二直立鍵的排斥作用” ，是一種 van der Waals 應(yīng)力氟代環(huán)己烷40%60%擁擠程度最小的取代基是氟取代基的大小與其支鏈有關(guān)FF叔丁基環(huán)己烷少于 0.01%大于 99.99%擁擠程度高的是大的基團(tuán)如叔丁基叔丁基有最大數(shù)量的取代基C(CH3)3C(CH3)3叔丁基環(huán)己烷van der Waals應(yīng)力導(dǎo)致強(qiáng)烈的1,3-二直立鍵排斥作用二取代環(huán)己烷的構(gòu)象分析1,4-二甲基環(huán)己烷立體異構(gòu)體順式反式CH35219 kJ/mol5212 kJ/mol穩(wěn)定性低穩(wěn)定性高反式異構(gòu)體比順式穩(wěn)定H3CHHH3CCH3HH順-1,4-二甲基環(huán)己烷的構(gòu)象分析CH3 兩個(gè)構(gòu)象等價(jià)，每個(gè)都有一個(gè)甲基在直立鍵上，一個(gè)甲基在平伏鍵上H3CHHHCH3HCH3HH3CHCH3反-1,4-二甲基環(huán)己烷的構(gòu)象分析 兩個(gè)構(gòu)象不等價(jià)，