NAMING ISOMERS TABLE 12.1

Transcription

1 til12214_ch12_online.indd Page 1 12/10/10 8:04 M user-f463 Volume/207/MDQ243/tiL12214_disk1of1/ /tiL12214_pagefiles NMING ISOMERS n isomer naming system is needed because there are just too many isomers to keep track of. The system of naming the branched-chain alkanes is described by rules agreed upon by a professional organization, the International Union of Pure and pplied hemistry, or IUP. ere are the steps in naming the alkane isomers. Step 1: The longest continuous chain of carbon atoms determines the base name of the molecule. The longest continuous chain is not necessarily straight and can take any number of right-angle turns as long as the continuity is not broken. The base name corresponds to the number of carbon atoms in this chain as in Table For example, the structure has six carbon atoms in the longest chain, so the base name is hexane Step 2: The locations of other groups of atoms attached to the base chain are identified by counting carbon atoms from either the left or the right. The direction selected is the one that results in the smallest numbers for attachment locations. For example, the hexane chain TLE 12.1 The first 10 straight-chain alkanes Name Formula Structural Formula Name Formula Structural Formula Methane 4 exane 6 14 eptane 7 16 Ethane 2 6 Propane 3 8 Octane 8 18 utane 4 10 Nonane 9 20 Pentane 5 12 Decane PTER 12 Organic hemistry 1

2 til12214_ch12_online.indd Page 2 12/10/10 8:04 M user-f463 Volume/207/MDQ243/tiL12214_disk1of1/ /tiL12214_pagefiles has a 3 attached to the third or the fourth carbon atom, depending on which way you count. The third atom direction is chosen since it results in a smaller number. Step 3: The hydrocarbon groups attached to the base chain are named from the number of carbons in the group by changing the alkane suffix -ane to -yl. Thus, a hydrocarbon group attached to a base chain that has one carbon atom is called methyl. Note that the -yl hydrocarbon groups have one less hydrogen than the corre sponding alkane. Therefore, methane is 4, and a methyl group is 3. The first 10 alkanes and their corresponding hydrocarbon group names are listed in Table In the example, a methyl group is attached to the third carbon atom of the base hexane chain. The name and address of this hydrocarbon group is 3-methyl. The compound is named 3-methylhexane. Step 4: The prefixes di-, tri-, and so on are used to indicate if a particular hydrocarbon group appears on the main chain more than once. For example, (or) is 2,2-dimethylbutane and 3 3 TLE 12.2 lkane hydrocarbons and corresponding hydrocarbon groups lkane Name Formula ydrocarbon Group Formula Methane 4 Methyl 3 Ethane 2 6 Ethyl 2 5 Propane 3 8 Propyl 3 7 utane 4 10 utyl 4 9 (or) Pentane 5 12 myl 5 11 exane 6 14 exyl eptane 7 16 eptyl 7 15 Octane 8 18 Octyl 8 17 Nonane 9 20 Nonyl 9 19 Decane Decyl Note: 3 means where * denotes unattached. The attachment takes is 2,3-dimethylbutane. 3 place on a base chain or functional group. If hydrocarbon groups with different numbers of carbon atoms are on a main chain, they are listed in alphabetic order. For example, 2 PTER 12 Organic hemistry 12-2

3 til12214_ch12_online.indd Page 3 12/10/10 8:04 M user-f463 Volume/207/MDQ243/tiL12214_disk1of1/ /tiL12214_pagefiles EXMPLE 12.2 Write the structural formula for 2,2-dichloro-3-methyloctane. nswer: l (or) l is named 3-ethyl-2-methylpentane. Note how numbers are separated from names by hyphens. EXMPLE 12.1 What is the name of an alkane with the following formula? SOLUTION The longest continuous chain has seven carbon atoms, so the base name is heptane. The smallest numbers are obtained by counting from right to left and counting the carbons on this chain; there is a methyl group in carbon atom 2, a second methyl group on atom 4, and an ethyl group on atom 5. There are two methyl groups, so the prefix di- is needed, and the e of the ethyl group comes first in the alphabet, so ethyl is listed first. The name of the compound is 5-ethyl-2,4- dimethylheptane PTER 12 Organic hemistry 3

4 til12214_ch12_online.indd Page 4 12/10/10 8:04 M user-f463 Volume/207/MDQ243/tiL12214_disk1of1/ /tiL12214_pagefiles PRLLEL EXERISES Group 1. Draw the structural formulas for (a) n-pentane and (b) an isomer of pentane with the maximum possible branching. (c) Give the IUP name of this isomer. 2. Write structural formulas for all the hexane isomers you can identify. Write the IUP name for each isomer. 3. Write structural formulas for a. 3,3,4-trimethyloctane. b. 2-methyl-1-pentene. c. 5,5-dimethyl-3-heptyne. 4. Write the IUP name for each of the following: Group 1. Write structural formulas for (a) n-octane and (b) an isomer of octane with the maximum possible branching. (c) Give the IUP name of this isomer. 2. Write the structural formulas for all the heptane isomers you can identify. Write the IUP name for each isomer. 3. Write structural formulas for a. 2,3-dimethylpentane. b. 1-butene. c. 3-ethyl-2-methyl-3-hexene. 4. Write the IUP name for each of the following: r l D r 5. Which would have the higher octane rating, 2,2,3-trimethylbutane or 2,2-dimethylpentane? Explain with an illustration. 5. Which would have the higher octane rating, 2-methyl-butane or dimethylpropane? Explain with an illustration. 4 PTER 12 Organic hemistry 12-4

5 til12214_ch12_online.indd Page 5 12/10/10 8:04 M user-f463 Volume/207/MDQ243/tiL12214_disk1of1/ /tiL12214_pagefiles Group ontinued 6. Use the information in Table 12.5 to classify each of the following as an alcohol, ether, organic acid, ester, or amide. Group ontinued 6. lassify each of the following as an alcohol, ether, organic acid, ester, or amide. D D E E 12-5 PTER 12 Organic hemistry 5

6 til12214_ch12_online.indd Page 6 12/10/10 8:04 M user-f463 Volume/207/MDQ243/tiL12214_disk1of1/ /tiL12214_pagefiles SOLUTIONS TO DDITIONL GROUP & PRLLEL EXERISES FOR PTER 12 Group 1. (a) Group 1. (a) n-octane (b) (b) maximum possible branching (c) 2,2-dimethylpropane (c) 2,2,3,3 tetramethylbutane 2. n-hexane 2. The structures can be deduced from the names. The names are as follows: heptane; 2-methylhexane; 3-methylhexane; 2-propylbutane; 2,3-dimethylpentane. 3-methylpentane 2-methylpentane 2,2-dimethylbutane PTER 12 Organic hemistry 12-6

7 til12214_ch12_online.indd Page 7 12/10/10 8:04 M user-f463 Volume/207/MDQ243/tiL12214_disk1of1/ /tiL12214_pagefiles Group ontinued Group ontinued 3. (a) (a) 2,3-dimethylpentane (b) 3 (b) 1-butene (c) 3 (c) 2-methyl-3-ethyl-3-hexene (a) 2-chloro-4-methylpentane (b) 2-methyl-l-pentene (c) 3-ethyl-4-methyl-2-pentene 5. The 2,2,3-trimethylbutane is more highly branched, so it will have the higher octane rating. 2,2,3-trimethylbutane 4. (a) 2-bromo-3-methylbutane (b) 5-methyl-2-hexene (c) 2-chloro-2,4-hexadiene (d) 1-bromo-2-butene 5. Dimethylpropane would have a higher octane because it has more branches than 2-methyl-butane: dimethylpropane 3 2,2,-dimethylpentane 3 2-methyl-butane 3 6. (a) alcohol (b) amide (c) ether (d) ester (e) organic acid 6. (a) alcohol (b) organic acid (c) ether (d) amine (e) ester 12-7 PTER 12 Organic hemistry 7