立体化学
McMurry Organic Chemistry 6th edition Chapter 1 (c) 2003
Chapter 4
Organic Compounds: Cycloalkanes 环烷类 and Their Stereochemistry 立体化学
Cyclics
Chrysanthemic acid 除虫菊酸
Occurs naturally as esters
Active insecticidal constituents of chrysanthemum flowers
Prostaglandins 前列腺素
Potent hormones
Control physiological functions in humans
Steroids 固醇类
Tetracyclic lipids
Naturally occurring hormones in plants and animals
Most organic compounds contain rings of carbon atoms
4.1 Naming Cycloalkanes 命名
Cycloalkanes 环烷类 (alicyclic compounds)
Saturated 饱和的cyclic hydrocarbons containing rings of carbon atoms
(CH2)n or CnH2n
Naming Cycloalkanes
Alkyl-substituted cycloalkane 环的碳数较多时
If the number of carbon atoms in the ring is equal to or greater than the number in the substituent
Cycloalkyl-substituted alkane 直链的碳数较多时
If the number of carbon atoms in the largest substituent is greater than the number in the ring
Rule 1 0 Find the parent by counting carbon atoms
Naming Cycloalkanes
Start at attachment point and number so all substituents have as low a number as possible. 让取代基有最小的数字
Rule 2 0 Number the substituents, and write the name
Naming Cycloalkanes
When two or more substituents could potentially receive the same numbers, number by alphabetical priority: 必要时,以字母顺序决定
Naming Cycloalkanes
If halogens 卤素 are present, treat them just like 如同 alkyl groups 烷基:
Additional examples:按字母顺序列出
Problem 4.1 (p. 114)
Problem 4.2 (p. 114)
4.2 Cis-Trans Isomerism 顺反异构化
in Cycloalkanes
Cycloalkanes vs. Open-chain Alkanes
Similarities 相似0 nonpolar; fairly inert
Differences 不同0 cycloalkanes are less flexible 较无弹性 than open-chain alkanes
Cis-Trans Isomerism in
Cycloalkanes
Two faces (as viewed edge on):
Top face 平面上
Bottom face 平面下
Isomerism possible in substituted cycloalkanes顺反
Cis-Trans Isomerism in Cycloalkanes
Constitutional Isomers 组成异构物
Isomers that have their atoms connected in a different order 不同顺序
Stereoisomers 立体异构物
Isomers that have their atoms connected in the same order but differ in three-dimensional geometry三度空间不同
cis isomer
trans isomer
Cis-Trans Isomerism in Cycloalkanes
cis-trans isomers 顺反异构物
Stereoisomers that differ in their stereochemistry about a double bond or ring 双键或环
cis- 顺 (Latin, "on the same side")
trans- 反 (Latin, "across")
Worked Example 4.1
Naming Cycloalkanes
Name the following substances, including cis- or trans- prefix
Worked Example 4.1
Naming Cycloalkanes
Strategy
In these views, the ring is roughly in the plane of the page, a wedged bond 粗线protrudes up 上 above the page, and a dashed bond 虚线recedes below下 the page. Two substituents are cis 同边 if they are both above or below the page, they are trans反边 if one is above and one is below
Worked Example 4.1
Naming Cycloalkanes
Solution
trans-1,3-Dimethylcyclopentane
cis-1,2-Dichlorocyclohexane
Problem 4.4 (p. 117)
Problem 4.5 (p. 117)
4.3 Stability of Cycloalkanes 环烷类的
稳定度: Ring Strain 环张力
Angle strain 角张力 (与理想键角的差异)
The strain induced in a molecule when bond angles are forced to deviate from the ideal 109 tetrahedral value (Adolf von Baeyer 0 1885) 由混成轨域决定
Stability of Cycloalkanes:
Ring Strain
Angle strain 角张力
Experimental data show that Baeyer's theory is only partially correct
Baeyer assumed all cycloalkanes to be flat
Angle strain occurs only in small rings that have little flexibility 不易弯曲
Stability of Cycloalkanes:
Ring Strain 环张力
The three kinds of strain that contribute to the overall energy of a cycloalkane:
Angle strain 角张力0 the strain due to expansion or compression of bond angles 键角
Torsional strain 扭转张力0 the strain due to eclipsing of bonds on neighboring atoms 键的重叠
Steric strain 立体张力0 the strain due to repulsive interactions when atoms approach each other too closely 原子太接近 (凡得瓦力)
Problem 4.8 (p. 119)
4.4 Conformations of Cycloalkanes
Cyclopropane 环丙烷
Most strained of all the rings 张力最大 (最不稳定)
Angle strain 0 caused by 60 C-C-C bond angles
Torsional strain 0 caused by the eclipsed C-H bonds on neighboring carbon atoms
Conformations of Cycloalkanes
Bent 弯曲的 C-C bonds
Orbitals can't point directly toward each other
Orbitals overlap at a slight angle
Bonds are weaker and more reactive than typical alkane bonds 较弱的键, 活性较高
C-C bond:
255 kJ/mol (61 kcal/mol) for cyclopropane
355 kJ/mol (85 kcal/mol) for open-chain propane
Conformations of Cycloalkanes
Cyclobutane 环丁烷
Total strain is nearly the same as cyclopropane 与环丙烷差不多
Angle strain 0 less than cyclopropane
Torsional strain 0 more than cyclopropane because of larger number of ring hydrogens
Not planar (puckered) 弯曲的
One carbon atom lies 25 above the plane of the other three
Conformations of Cycloalkanes
Cyclopentane 环戊烷
Less strain than cyclopropane or cyclobutane 张力较小
Planar cyclopentane: (假设情形)
Angle strain 0 very minimal
Torsional strain 0 large amount (共有10对)
Twists to a nonplanar (puckered) conformation (真实的情形)
Balance between increased angle strain 增加角张力 and a decreased torsional strain 减少扭转张力
4.5 Conformations of Cyclohexane
Substituted cyclohexanes 环己烷
Most common cycloalkanes 最常见
Occur widely in nature
Steroids 固醇类
Pharmaceutical agents
药物
Conformations of Cyclohexane
Cyclohexane
Adopts chair conformation 椅式构形
No angle strain 没有角张力
All C-C-C bonds near 109
No torsional strain 没有扭转张力
C-H bonds staggered 交错的
Conformations of Cyclohexane
Drawing chair conformation of cyclohexane
Step 1 0 draw parallel lines平行线, slanted向下倾斜 downward and slightly offset from each other
Step 2 0 place topmost carbon atom above and to the right of the plane of the other four, and connect the bonds
Step 3 0 place bottommost carbon atom below and to the left of the plane of the middle four, and connect the bonds
Note: Bonds to the bottommost carbon atom are parallel to the bonds to the topmost carbon. 各线互相平行
Conformations of Cyclohexane
Chair conformation 椅式构形
Angle strain 0 none
Torsional strain 0 none
Twist-boat conformation 扭曲船式 (较不稳定)
Molecules adopt this geometry only under special circumstances
Angle strain 0 minimal
Torsional strain 0 large amount
Steric strain 0 large amount
4.6 Axial 轴 and Equatorial 赤道
Bonds 键 in Cyclohexane
Chair conformation of cyclohexane
Chemical behavior of many substituted cyclohexanes is influenced by conformation 构形影响活性
Simple carbohydrates (glucose) adopt a cyclohexane chair conformation directly affecting their chemistry
Axial 轴 and Equatorial 赤道
Bonds in Cyclohexane
Chair conformation of cyclohexane
There are two kinds of positions for substituents on the cyclohexane ring
Axial positions 0 6 axial positions perpendicular to ring and parallel to ring axis 垂直分子平面
Equatorial positions 0 6 equatorial positions are in rough plane of the ring around the equator
axial positions
equatorial positions
Axial and Equatorial Bonds in Cyclohexane
Chair conformation of cyclohexane
Each carbon atom in cyclohexane has one axial and one equatorial hydrogen 每一个碳有一个轴和一个赤道键
Each face of the ring has three axial and three equatorial hydrogens in an alternating 交替 pattern
Axial and Equatorial Bonds in Cyclohexane
Chair conformation of cyclohexane
Drawing axial and equatorial positions
Axial and Equatorial Bonds in Cyclohexane
Chair conformation of cyclohexane
There is only one form of a monosubstituted 单取代cyclohexane 没有顺反异构物
Cyclohexane rings are conformationally mobile
Different chair conformations readily interconvert exchanging axial and equatorial positions - called a ring-flip 环翻转
Axial and Equatorial Bonds in Cyclohexane
Chair conformation of cyclohexane
Ring-flip occurs by keeping the middle four carbon atoms in place while folding the two end carbons in opposite directions
Axial substituent becomes an equatorial substituent after ring-flip and vice versa 翻转后, 轴与赤道取代基互相转换
Worked Example 4.2
Drawing the Chair Conformation of a Substituted Cyclohexane
Draw 1,1-dimethylcyclohexane in a chair
conformation, indicating which methyl group in
your drawing is axial and which is equatorial
Worked Example 4.2
Drawing the Chair Conformation of a Substituted Cyclohexane
Strategy
1,1-dimethylcyclohexane
Draw a chair cyclohexane ring, and then put two
methyl groups on the same carbon. The methyl
group in the rough plane of the ring is equatorial,
and the other (directly above or below the ring) is
axial.
Worked Example 4.2
Drawing the Chair Conformation of a Substituted Cyclohexane
Solution
1,1-dimethylcyclohexane
Problem 4.12 (p. 126)
Problem 4.13 (p. 126)
4.7 Conformations of Monosubstituted Cyclohexanes
The two conformers of a monosubstituted 单取代 cyclohexane are not equally stable 二种构形异构物, 能量不同
Substituent is almost always more stable in equatorial position
Energy difference between axial and equatorial conformations is due to steric strain caused by 1,3-diaxial interactions 双轴互斥
Conformations of Monosubstituted Cyclohexanes
Steric strain
The steric strain between an axial methyl group and an axial hydrogen atom in methylcyclohexane is identical to the steric strain in gauche butane
Equatorial methylcyclohexane has no such interactions and is more stable
Conformations of Monosubstituted Cyclohexanes
1,3-Diaxial steric strain
Depends on the nature and size of the substituent
Steric strain increases paralleling increase of bulk of alkyl group
H3C- < CH3CH2- < (CH3)2CH-
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