1 -Structural isomers -Naming Branched Alkanes Mr. Shields Regents Chemistry U16 L02

2 Isomers Compounds with the same molecular formula but different structural arrangement are known as structural 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. The more C atoms there are in the formula, the more structural isomers there will be. 4C (1)5C (3)6C (5)8C (18) 20 C (>303,000)

3 Structural Isomers In order to determine if 2 molecules are isomers of one another first determine the molecular formula’s of both In order to determine if 2 molecules are isomers of one another first determine the molecular formula’s of both To the right we have To the right we have pentane (Top) pentane (Top) 2 – methyl butane (bottom) 2 – methyl butane (bottom) What is the mol. Formula for each? Are they What is the mol. Formula for each? Are they structural isomers? structural isomers? C 5 H 12 – Yes, they Are isomers

4 Isomers Isomers have different structures and different chemical and physical properties. Butane2-methylpropane FormulaC 4 H 10 C 4 H 10 B.P.0 C-12 C M.P.-138 C-159 C Density Sol. In 100ml Alcohol1813 ml1320 ml CH 3 CH 2 OH and CH 3 OCH 3 are also isomers of one another NOTE!

5 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 –CH 2 - unit When we draw the structural formulas of the 1 st Three members of this group there is only one Way each can be drawn. CH 4 CH 3 -CH 3 CH 3 -CH 2 -CH 3 Drawing simple alkanes

6 Branched-chain alkanes Beginning with butane, C 4 H 10, there is more than 1 way to arrange the atoms besides one carbon after another (i.e. straight chain). Beginning with butane, C 4 H 10, there is more than 1 way to arrange the atoms besides one carbon after another (i.e. straight chain). Can you figure out how many ways it can be drawn? Can you figure out how many ways it can be drawn? CH 3 -CH 2 -CH 2 -CH 3 & CH 3 | CH 3 -CH-CH 3

7 Branched-chain alkanes In both butane structures we have the same numbers and kind of atoms namely, C 4 H 10 In both butane structures we have the same numbers and kind of atoms namely, C 4 H 10 The general formula for each is also C n H 2n+2 so each represents the alkane “Butane” The general formula for each is also C n H 2n+2 so each represents the alkane “Butane” Yet there is a difference. The difference lies in what atoms are joined to what atoms Yet there is a difference. The difference lies in what atoms are joined to what atoms CH 3 -CH 2 -CH 2 -CH 3  CH 3 *| CH 3 -CH-CH 3 * C 4 H 10 C 4 H 10 Note that these are totally different compounds having there own unique Chemical & Physical Properties!

8 Note that what we have drawn for butane represents Real structural changes and not just apparent changes Resulting from 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 represent the SAME molecule!

9 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. Remember, carbon Must have 4 bonds. No more, No less! So how many structural Isomers does pentane Have? Straight chain isomer

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

11 Naming branched-chain alkanes Branches are added as a prefix and are named by counting the number of C atoms. Branches are added as a prefix and are named by counting the number of C atoms. The “branch” alkane name ends in “yl.” The “branch” alkane name ends in “yl.” Methyl CH 3 — Ethyl CH 3 CH 2 – Propyl CH 3 CH 2 CH 2 – Butyl CH 3 CH 2 CH 2 C H 2 – c-c-c-c-c-c-c-c-c-c c c c c c

12 Naming branched-chain alkanes The location of the branch (or substituent group) is shown by assigning numbers to the C’s in the backbone. 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. Number from the end that gives the lowest number for the first branch. Substituent groups (branches) are listed alphabetically Substituent groups (branches) are listed alphabetically There may be more than 1 of the same type of branch. There may be more than 1 of the same type of branch. Use di, tri, tetra etc. for 2, 3, and 4 Use di, tri, tetra etc. for 2, 3, and 4 Number the locations and separate the nos. by a comma & Number the locations and separate the nos. by a comma & separate the last no. from the name by a dash 7-ethyl-3-methyldecane c-c-c-c-c-c-c-c-c-c c c c c c

13 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 C 6 H 14 or CH 3 C(CH 3 ) 2 CH 2 CH 3 CH 3 | H 3 C-C-CH 2 -CH 3 | CH 3 Name this compound

14 H H–C–H H H–C–C–C–H H H-C-H | H Longest continuous chain has 3 carbon atoms – propane. 2 Branches each have 1 carbon – dimethyl. And … both Branches have to be at C-2. WHY?? So …. Only specify the branch number if necessary. Ex: this is Dimethylpropane & not 2,2-dimethylpropane Example:

15 H H – C – H H H H H – C – C – C – C – H H H H–C–H H–C–H H H Longest continuous chain has 6 carbon atoms: hexane Branch is 1 carbon long: methyl Branch is located at C-3 3-methylhexane or C 7 H 16 or CH 3 CH 2 CH 2 CH(CH 3 )CH 2 CH 3 Name this compound:

16 More Naming Problems Methylbutane 2,3,7-Trimethyloctane Dimethylpropane 3-ethylpentane CH3CH(CH3)CH(CH3)CH2CH2CH2CH(CH3)CH3

17 More Naming Problems C C C | | | C-C-C-C-C-C-C | C | C What is the name Of this compound ? 4-ethyl-2,5-dimethyloctane WOW !

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

19 Name this compound C-C-C | C-C-C-C-C-C-C-C | | | C C C | C 6,6-diethyl-3,5- dimethylnonane 1 9