Isomers of alkanes: C C C C C

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1 Isomers of alkanes: Alkanes are expressed by the simplest formula for organic molecules that do not contain a functional group. All alkanes have a general formula of C n H 2n + 2. Isomers have different structures with the same molecular formula. As the formula becomes larger with more carbons, the number of isomers increases dramatically. Deriving isomeric structures from a molecular formula We will work on these structures using the carbon skeleton. It is important to keep in mind that we are using a systematic approach to drawing the isomers. After we draw the isomers, we will identify any structures that are repeated and eliminate them from our list. Keep in mind that we are always looking for the longest carbon chain that contains the functional group AND the most branching. C 6 H 14 First isomer: Hexane Start with the longest carbon chain you can have: in this case 6 C long. 5 6 This is the first isomer. Add the appropriate number of hydrogens CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 Second isomer: 2-methylpentane Remove one carbon from the chain. Now your longest chain is 5, or pentane. The carbon removed will become a methyl group that can be added along the chain. Where you will add the alkyl group depends on the number of carbons in the alkyl group and the length of the chain. The rule of thumb is the number of carbons in the alkyl group cannot be the same or less than the position on the chain. For example, we want to add a methyl group to this pentane chain. If we put the methyl group, which has one carbon on the number one carbon in the chain, we get back hexane. It has to go on the number 2 position in the chain. Put the methyl group on the number 2 carbon in the chain below. C 5 Third isomer: 3-methylpentane Again with the pentane chain, move the methyl group one more down the chain, to the number 3 position on the chain. Notice that it does not have the same structure as 2- methylpentane. C 5 ws 02-alkane isomers.doc Page 1 of 5 9/2/14

2 Fourth & fifth isomer: 2,2-dimethylbutane Shorten the chain one more time. Now your longest chain is 4, or butane. Now we have two carbons that can be added as alkyl groups. We can either add two methyl groups to the chain or one ethyl group to the chain. Start with the ethyl group. By our rule of thumb, we cannot add the ethyl group to positions 1 or 2. Draw an ethyl group on position 3 on the chain below. Notice that when you count the longest chain of this structure, it is no longer butane; it is a pentane. You have just remade the third isomer. This often happens when drawing isomers. It is not a valid structure because we get a structure already drawn. Does this mean we should discount other alkyl groups when doing isomers? NO! But it does mean we need to be careful that we do not repeat structures. We will work with the two methyl groups next. Again, our longest carbon chain is butane. We can either put the two methyl groups on the same carbon, or on two different carbons. Warning! They cannot go on terminal carbons. These are all the isomers that one can draw for C 6 H 14. The larger the number of carbons (and also hydrogens) the more structures of isomers can be drawn. We are going to look at C 8 H 18. Starting with the longest carbon chain, octane is the first isomer that we can draw with this formula. C C Remove one carbon from the octane chain. The heptane chain is the longest chain for the second, third, and fourth isomers. Only one alkyl group can be made from the leftover carbon methyl. C C C ws 02-alkane isomers.doc Page 2 of 5 9/2/14

3 Move the methyl group down the chain starting in the number 2 position. Each move represents an isomer. There are three different positions on the chain. The next move gets back a structure already drawn. Remove two carbons from the octane chain. The hexane chain is the longest chain for fifth, sixth, seventh, eighth, ninth, tenth, and eleventh isomers Two alkyl groups can be made from the leftover carbon methyl and ethyl. 5 6 Start with the two methyl groups placed on the same position on the chain. Move the methyl groups down the chain starting in the number 2 position for both of them. Again, each move represents an isomer. There are two different positions on the chain. The next move gets back a structure already drawn Now put the methyl groups on different positions on the chain. Keep one position constant as the second methyl moves down the chain. There are three structures that can be drawn with a methyl stationary at position 2, while the second methyl is placed at other positions on the chain Move the methyl stationary on position 2 to position 3 and again, move the second position down the chain. You will have one structure that can be drawn for this configuration. That takes care of the methyl substituted hexanes, but we still have an ethyl hexane to draw. By the rule of thumb, we have to put the ethyl group on the number 3 position. This ends up being the only position we can place the ethyl group without returning to an already drawn structure. Place the ethyl group on the chain below. 5 6 Remove three carbons from the octane chain. The pentane chain is the longest chain for the twelfth, thirteenth, fourteenth, fifteenth, and sixteenth isomers. Four alkyl groups can be made from the leftover carbon methyl, ethyl, propyl, and isopropyl. C 5 ws 02-alkane isomers.doc Page 3 of 5 9/2/14

4 Start with the two methyl groups placed on the same position on the chain. Move the remaining methyl group down the chain starting in the number 3 position. Again, each move represents an isomer. There are two different positions on the chain. The next move gets back a structure already drawn. C C 5 5 Now put the two methyl groups on carbon number 3 in the chain and the remaining methyl group on the number 2 carbon in the chain. The remaining isomer uses 3 methyl groups on different carbons on the chain. None are on the same carbon as before. See if you can draw the two isomers described in this part on the chains below. C C 5 5 The pentane chain based isomers have 2 more isomers. Both use an ethyl and a methyl alkyl group as substituents. Based on what we have done so far, can you devise a systematic method to place the ethyl and the methyl groups on these chains? C C 5 5 Notice that we did not used the propyl group or the isopropyl group. The pentane chain is not long enough to support the propyl or isopropyl group and still have different structures. Our last isomer is based on a butane chain. Remove four carbons from the octane chain. The butane chain is the longest chain for the last, seventeenth isomers. Eight alkyl groups can be made from the leftover carbon methyl, ethyl, propyl, isopropyl, butyl, sec-butyl, tert-butyl, and isobutyl. Of these groups, only methyl can be used without duplicating an isomer. Start with the two methyl groups placed on the same position on the chain. Place the other two methyl groups on the chain in such a way that the longest chain is butane. Now that you feel like an expert on drawing isomers, try C 9 H 20. I was able to draw 34 isomers. Here is what I got. Nonane, 3 different methyl octanes, 3 dimethyl heptanes (on same position), 6 dimethyl heptanes (on different positions), 2 ethyl heptanes, 8 trimethyl hexanes, 4 ethyl methyl hexanes, 4 tetra methyl pentanes, 1 diethyl pentane, and 2 ethyl dimethyl pentane. Are there more? I hope not. Your assignment if you choose to accept it: Draw all of the isomers of C 9 H 20. After you draw all of the isomers, name the following types of isomers: one of the dimethyl ws 02-alkane isomers.doc Page 4 of 5 9/2/14

5 heptane (same carbon), two of the dimethyl heptanes (different carbons), one of the ethyl heptane, one of the trimethyl hexane, one of the ethyl methyl hexane, one of the tetra methyl pentane, and one of the ethyl dimethyl pentane ws 02-alkane isomers.doc Page 5 of 5 9/2/14