Chapter 4 Alkanes 1.For alkanes beyond butane, add -ane to the Greek root for the number of carbons. C-C-C-C-C-C = hexane 2.Alkyl substituents: drop the -ane and add -yl. -C 2 H 5 is ethyl Rules for Naming Alkanes

● identify the longest chain of carbon atoms. In the exampl e shown (Fig. 2a), the longest chain consists of five carbon atoms and a pentane chain; ● number the longest chain of carbons, starting from the end nearest the branch point (Fig. 2b); ● identify the carbon with the branching group (number 2 in Fig. 2b); ● identify and name the branching group. CH 3 is called methyl and not methane. ● name the structure by first identifying the substituent and its position in the chain, then naming the longest chain. 2-methylpentane

√ x = = 13

Pentanes => 构造异构体

IUPAC Names Find the longest continuous carbon chain. Number the carbons, starting closest to the first branch. Name the groups attached to the chain, using the carbon number as the locator. Alphabetize substituents. Use di-, tri-, etc., for multiples of same substituent. 

Longest Chain The number of carbons in the longest chain determines the base name: ethane, hexane. If there are two possible chains with the same number of carbons, use the chain with the most substituents. =>

Number the Carbons Start at the end closest to the first attached group. If two substituents are equidistant, look for the next closest group =>

Name Alkyl Groups CH 3 -, methyl CH 3 CH 2 -, ethyl CH 3 CH 2 CH 2 -, n-propyl CH 3 CH 2 CH 2 CH 2 -, n-butyl =>

Propyl Groups H n-propylisopropyl H A primary carbon A secondary carbon =>

Butyl Groups n-butylsec-butyl H H A primary carbon A secondary carbon =>

Isobutyl Groups H H A primary carbon A tertiary carbon => isobutyltert-butyl

Alphabetize Alphabetize substituents by name. Ignore di-, tri-, etc. for alphabetizing. 3-ethyl-2,6-dimethylheptane =>

Higher Alkanes Anti conformation is lowest in energy. “Straight chain” actually is zigzag. =>

Constitutional isomers （ 构造异构体 ） are compounds which have the same molecular formula but have the atoms joined together in a different way. Constitutional isomers have different physical and chemical properties.

Physical Properties Solubility: hydrophobic Density: less than 1 g/mL Boiling points increase with increasing carbons (little less for branched chains). Melting points increase with increasing carbons (less for odd- number of carbons).

Hydrocarbons mp -135 o C bp-5 o C mp -145 o C bp-10 o C mp -130 o C bp+30 o C mp -160 o C bp+28 o C mp -20 o C bp+9 o C 丁烷异构体 戊烷异构体

Branched Alkanes Lower b.p. with increased branching Higher m.p. with increased branching Examples: H CH 3 CH CH 3 CH 2 CH 2 CH 3 bp 60°C mp -154°C CH 3 CH CH 3 CH CH 3 CH 3 bp 58°C mp -135°C => bp 50°C mp -98°C CH 3 C C 3 CH 3 CH 2 CH 3

Major Uses of Alkanes C 1 -C 2 : gases (natural gas) C 3 -C 4 : liquified petroleum (LPG) C 5 -C 8 : gasoline C 9 -C 16 : diesel, kerosene, jet fuel C 17 -up: lubricating oils, heating oil Origin: petroleum refining =>

Conformational isomers

Conformational Analysis Torsional strain: resistance to rotation. For ethane, only 3.0 kcal/mol =>

Cyclic Alkanes Carbon atoms can form rings containing only carbon-carbon single bonds. C 3 H 6, C 4 H 8, C 6 H 12

Cyclopropane Large ring strain due to angle compression Very reactive, weak bonds =>

Chair Conformer =>

Axial and Equatorial Positions =>

Reactions of Alkanes Combustion => Cracking and hydrocracking (industrial) Halogenation

Reactions of Alkanes Radicals

Petroleum is a complex mixture of alkanes and cycloalkanes that can be separated by distillation 柴油 油脂 沥青

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自由基反应机理 Initiation 链引发 Propagation （ 链增长） Termination （链终止） CH 3. + CH 3.  CH 3 CH 3

Reaction coordinate