Organic Chemistry Structural isomers Naming Branched Alkanes

Isomers Compounds with the same molecular formula but different structural arrangement are known as structural isomers. The more C atoms there are in the formula, the more structural isomers there will be. 4C (1) 5C(3) 6C(5) 8C(18) C20(>303,000)

Structural Isomers C5H12 – Yes, they Are isomers In order to determine if something is an isomer determine the molecular formula To the right we have pentane (Top) 2 – methyl butane What is the mol. Formula for each? Are they structural isomers? C5H12 – Yes, they Are isomers

Isomers Isomers have different structures and different chemical and physical properties. Butane 2-methylpropane Formula C4H10 C4H10 B.P. 0 C -12 C M.P. -138 C -159 C Density 0.622 0.604 Sol. In 100ml Alcohol 1813 ml 1320 ml

Drawing simple alkanes Recall that the members of the group of alkanes Forms a homologous series and each member of This series differs from the last by 1 –CH2- unit When we draw the structural formulas of the 1st Three members of this group there is only one Way each can be drawn. CH4 CH3-CH3 CH3-CH2-CH3

Branched-chain alkanes Beginning with butane, C4H10, there is more than 1 way to arrange the atoms besides one carbon after another. CH3-CH2-CH2-CH3 & CH3 | CH3-CH-CH3

Branched-chain alkanes In both butane structures we have the same numbers and kind of atoms namely, C4H10 The general formula for each is also CnH2n+2 so each represents the alkane “Butane” Yet there is a difference. The difference lies in what atoms are joined to what atoms CH3-CH2-CH2-CH3 & CH3 * | CH3-CH-CH3 * Nope Are there any other ways to draw 4 carbon butane?

Note that what we have drawn for butane represents Real structural changes and not just apparent changes As a result of the rotation of a C-C single bond. Let’s Look a pentane to see what we mean by this. Rotation of this bond Leads to this configuration Of pentane And rotation of this Bond leads to this All of these structures however represent the SAME molecule!

Pentane isomers To find new structural isomers of the straight chain Alkanes we need to move the point of attachment of The various carbon atoms. So how many structural Isomers does pentane Have? Straight chain isomer Remember, carbon Must have 4 bonds. No more, No less!

Naming branched-chain alkanes Find the longest continuous chain or backbone of C atoms. c-c-c-c-c-c-c-c-c-c c c What’s the longest chain? c The base name is derived from the number of C’s in the longest chain. 10 carbons would be decane

Naming branched-chain alkanes Branches are added as a prefix and are named by counting the number of C atoms. The “branch” alkane name ends in “yl.” Methyl CH3— Ethyl CH3CH2 – Propyl CH3CH2CH2 – Butyl CH3CH2CH2CH2 – c-c-c-c-c-c-c-c-c-c c c c

Naming branched-chain alkanes The location of the branch (or substituent group) is shown by assigning numbers to the C’s in the backbone. Number from the end that gives the lowest number for the first branch. Substituent groups (branches) are listed alphabetically c-c-c-c-c-c-c-c-c-c c c c 1 2 3 4 5 6 7 8 9 10 7-ethyl-3-methyldecane There may be more than 1 of the same type of branch. Use di, tri, tetra etc. for 2, 3, and 4 Number the locations and separate the # by a comma & separate the last # from the name by a dash

H H–C–H H H H–C–C–C–H Example: H-C-H | Longest continuous chain has 3 carbon atoms – propane. 2 Branches each have 1 carbon – dimethyl. Branches have to be at C-2. WHY?? (Only give the number if necessary.) Dimethylpropane or C5H12 or CH3C(CH3)2CH3

Name this compound CH3 | H3C-C-CH2-CH3 Longest continuous chain has 4 carbon atoms – butane. 2 Branches each have 1 carbon – dimethyl. Branches are at C-2. WHY?? 2,2-Dimethylbutane or C6H14 or CH3C(CH3)2CH2CH3 Name this compound CH3 | H3C-C-CH2-CH3

Name this compound: Longest continuous chain has H 6 carbon atoms: hexane Branch is 1 carbon long: methyl Branch is located at C-3 3-methylhexane or C7H16 or CH3CH2CH2CH(CH3)CH2CH3 or CH3(CH2)2CH(CH3)CH2CH3 H H – C – H H H H H – C – C – C – C – H H H H–C–H H–C–H H H

More Naming Problems

Problems: Draw the following: 2 – methylpropane 3 – ethyl – 4 - methylnonane 2,2 – dimethylpropane 2,3,4 – trimethyldecane What name does this compound have?