有機化學教學課件：第二章 烷烴

1、第 二 章 烷 烴(Alkane)一、烷烴的命名和異構（Nomination and Isomerization)AlkanesAlkanes: Compounds with C-C single bonds and C-H bonds only (no functional groups)Connecting carbons can lead to large or small moleculesThe formula for an alkane with no rings in it must be CnH2n+2 where the number of Cs is nAlkanes ar

2、e saturated with no more hydrogen can be addedThey are also called aliphatic compoundsAlkane IsomersCH4 = methane, C2H6 = ethane, C3H8= propaneThe molecular formula of an alkane with more than three carbons can give more than one structureC4 (butane) = butane and isobutaneC5 (pentane) = pentane, 2-m

3、ethylbutane, and 2,2-dimethylpropaneAlkanes with Cs connected to no more than 2 other Cs are straight-chain or normal alkanesAlkanes with one or more Cs connected to 3 or 4 Cs are branched-chain alkanes Constitutional IsomersIsomers that differ in how their atoms are arranged in chains are called co

4、nstitutional isomersCompounds other than alkanes can be constitutional isomers of one anotherThey must have the same molecular formula to be isomersAlkyl GroupsAlkyl group remove one H from an alkane (a part of a structure)General abbreviation “R” (for Radical, an incomplete species or the “rest” of

5、 the molecule)Name: replace -ane ending of alkane with -yl ending CH3 is “methyl” (from methane)CH2CH3 is “ethyl” from ethaneSome Complex Alkyl GroupsTypes of Alkyl groupsClassified by the connection site a carbon at the end of a chain (primary alkyl group)a carbon in the middle of a chain (secondar

6、y alkyl group)a carbon with three carbons attached to it (tertiary alkyl group)Naming AlkanesCompounds are given systematic names by a process that usesPrefix-Parent-SuffixFollows specific rulesNamed as longest possible chainCarbons in that chain are numbered in sequencesubstituents are numbered at

7、their point of attachmentCompound name is one word (German style)Complex substituents are named as compounds would beStep 1: Choose the longest carbon chain with maximum substituents as main chainlongest carbon chain with maximum substituentsStep 2: Number the main chain starting from the end nearer

8、 the substituentIf there is branching an equal distance away from both ends, begin numbering at the end nearer the second substituentStep 3: Identify and number the substituentStep 4: Write the name as a single wordName a complex substituent as though it were itself a compound構象：通過旋轉單鍵，一個分子 產(chǎn)生不同的空間排

9、列二、烷烴的構象（Conformation）Conformations of EthaneConformers interconvert rapidly and a structure is an average of conformersMolecular models are three dimensional objects that enable us to visualize conformers Representing three dimensional conformers in two dimensions is done with standard types of dra

10、wings Representing ConformationsSawhorse representations show molecules at an angle, showing a molecular modelC-C bonds are at an angle to the edge of the page and all C-H bonds are shownNewman projections show how the C-C bond would project end-on onto the paperBonds to front carbon are lines going

11、 to the centerBonds to rear carbon are lines going to the edge of the circleInterconversion EnergyThere barrier to rotation between conformations is small (12 kJ/mol; 2.9 kcal/mol) The most stable conformation of ethane has all six CH bonds away from each other (staggered)The least stable conformati

12、on has all six CH bonds as close as possible (eclipsed) in a Newman projection energy due to torsional strainStaggered conformersConformations of PropanePropane (C3H8) torsional barrier around the carboncarbon bonds 14 kJ/molEclipsed conformer of propane has two ethane-type HH interactions and an in

13、teraction between CH and CC bondConformations of Butaneanti conformation has two methyl groups 180 away from each otherRotation around the C2C3 gives eclipsed conformationStaggered conformation with methyl groups 60 apart is gauche conformation直鏈烷烴最穩(wěn)定的構象是碳碳之間都是反交叉型己烷重 疊 式乙烷的構象分析重疊式乙烷交叉式乙烷交叉式乙烷乙烷交叉式構

14、象的投影式Newman式鋸架式HHHHHHHHHHHH反交叉型HHHHHHHHHHHH當兩個鍵間的夾角為180，稱為反交叉型180鄰交叉型HHHHHHHHHHHH當兩個鍵間的夾角為60，稱為鄰交叉型600 60 120 180 24030036012 kJ/mol乙烷的重疊構象比交叉構象能量高出 12 kJ/mol。重疊構象由于扭應力而不穩(wěn)定。扭應力的不穩(wěn)定是由鍵的重疊產(chǎn)生的。扭應力丁烷的構象分析: C2-C3 旋轉0 60 120 180 2403003603 kJ/mol14 kJ/mol丁烷的 鄰交叉型 構象比反交叉型構象能量 高出 3 kJ/mol.鄰交叉型 構象的不穩(wěn)定是由于van

15、der Waals 應力 (也稱為空間應力) ，是由于原子過于靠近 產(chǎn)生的。van der Waals 應力鄰交叉型反交叉型兩個甲基重疊的構象是丁烷所有構象中最不穩(wěn)定的這種不穩(wěn)定源于扭應力(重疊鍵)和van der Waals 應力兩者van der Waals 應力重疊式直鏈烷烴最穩(wěn)定的構象是碳碳之間都是反交叉型己烷復雜烷烴的構象分析三、烷烴的物理性質Physical PropertiesBoiling points and melting points increase as size of alkane increasesForces between molecules (tempora

16、ry dipoles, dispersion) are weak烷烴的物理性質沸點: 同數(shù)碳原子的烷烴異構體中，直鏈異構體沸點最高熔點: 兩條熔點曲線密度： 1溶解性: 不溶于水，溶于有機溶劑，特別是非極性溶劑戊烷bp 98C辛烷bp 125C壬烷bp 150C烷烴的沸點1） 直鏈烷烴沸點隨著碳原子數(shù)的增加而增加。辛烷: bp 125C2-甲基庚烷: bp 118C2,2,3,3-四甲基丁烷: bp 107C2 ）含同數(shù)碳原子的烷烴異構體中，直鏈異構體沸點最高， 支鏈越多，沸點越低。烷烴的沸點烷烴是非極性的，分子間的吸引力只有誘導偶極-誘導偶極作用力。誘導偶極-誘導偶極吸引力+兩個非極性分子其

17、正負電荷的中心重合在一起+電子的運動在一個分子中產(chǎn)生瞬間的偶極 (左邊)+一個分子暫時的偶極 (左邊) 誘導另一個分子產(chǎn)生互補的偶極 (右邊)，其結果兩個分子間就產(chǎn)生微弱的吸引力四、烷烴的反應Properties of AlkanesCalled paraffins (low affinity compounds) because they do not react as most chemicalsThey will burn in a flame, producing carbon dioxide, water, and heatThey react with Cl2 in the pre

18、sence of light to replace Hs with Cls (not controlled)烷烴的反應1、 烷烴的燃燒2、 烷烴的熱解3、 烷烴的鹵化CnH2n+2 + (3n+1)/2 O2 n CO2 + （n+1） H2O烷烴的燃燒燃燒熱：- Hc ， 為正值4817 kJ/mol5471 kJ/mol6125 kJ/mol654 kJ/mol654 kJ/mol庚烷辛烷壬烷（1）燃燒熱隨著碳原子數(shù)的增加而增加8CO2 + 9H2O5452 kJ/mol5458 kJ/mol5471 kJ/mol5466 kJ/molO2+252O2+252O2+252O2+252（2）碳原子相同時，直鏈烷烴的燃燒熱最大，支鏈越多，燃燒熱越小。支鏈分子比其直鏈異構體位能更低，更加穩(wěn)定烷烴的氯化和溴化烷烴的鹵化烷烴與 Cl2 或 Br2反應影響產(chǎn)物分布的因素 總反應的決速步是氫的提取?？紤]烷基游離基的相對穩(wěn)定性：穩(wěn)定的游離基生成速度快，所以2-氯丁烷的生成速度快。pg 341 middle在確