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As We Proceed Through Organic Chemistry We Will Often Focus Our 1 Lecture 8 As we proceed through organic chemistry we will often focus our interest on a limited portion of a structure when the remainder of the structure is not important to the topic of discussion. Symbolic representations for generic portions of a structure are given below and are commonly used. We will often use these representations in this and the coming chapters. R = any general carbon group (occasionally, it could also represent hydrogen) Ar = any general aromatic group, (when more specificity than just R is desired) I. Nomenclature Rules For Alkanes and Cycloalkanes The following list provides the names for carbon chains of various lengths. They must be memorized (through C12 for problems in this book). a. CH4 methane (C1) k. CH3(CH2)9CH3 undecane (C11) u. CH3(CH2)19CH3 henicosane (C21) b. CH3CH3 ethane (C2) l. CH3(CH2)10CH3 dodecane (C12) v. CH3(CH2)20CH3 doicosane (C22) c. CH3CH2CH3 propane (C3) m. CH3(CH2)11CH3 tridecane (C13) w. CH3(CH2)21CH3 triicosane (C23) d. CH3CH2CH2CH3 butane (C4) n. CH3(CH2)12CH3 tetradecane (C14) x. CH3(CH2)22CH3 tetraicosane (C24) e. CH3(CH2)3CH3 pentane (C5) o. CH3(CH2)13CH3 pentadecane (C15) y. CH3(CH2)23CH3 pentaicosane (C25) f. CH3(CH2)4CH3 hexane (C6) p. CH3(CH2)14CH3 hexadecane (C16) z. CH3(CH2)24CH3 hexaicosane (C26) g. CH3(CH2)5CH3 heptane (C7) q. CH3(CH2)15CH3 heptadecane (C17) aa. CH3(CH2)25CH3 heptaicosane (C27) h. CH3(CH2)6CH3 octane (C8) r. CH3(CH2)16CH3 octadecane (C18) bb. CH3(CH2)26CH3 octaicosane (C28) i. CH3(CH2)7CH3 nonane (C9) s. CH3(CH2)17CH3 nonadecane (C19) cc. CH3(CH2)27CH3 nonaicosane (C29) j. CH3(CH2)8CH3 decane (C10) t. CH3(CH2)18CH3 icosane (C20) dd. CH3(CH2)28CH3 triacontane (C30) Steps to Name an Alkane 1. Locate the longest carbon chain present. This becomes the parent name. Make sure to check at each branch point for the longest chain path. (Unless it is obvious, count at each branch point through all possible paths.) Where is the longest chain? (R = substituent branch) 2 incorrect 1 3 R 2 4 6 8 10 R 4 6 8 10 12 1 7 9 5 7 9 11 3 5 This chain is longer with This chain is 10 Cs long. 12 Cs so we pick this one. 2 Lecture 8 2. If there are several branches radiating out from a central carbon, you can count how long those branches are and use the longest two plus the central carbon and add them all together. 4 4 3 = a central carbon position 4 2 These two branches are 5 3 1 3 equivalent in length, but 6 the lower one is chosen 3 1 5 4 2 2 4 These two branches are equivalent because it has more 1 3 5 2 and because they are longest must branches on it. It must 4 4 3 6 be part of the longest chain. Either also be part of the longest 3 chain. 4 one can be used. 4 4 = part of the longest chain longest chain = 5 + 6 + 1 = 12 parent name = dodecane Number the longest chain from the end nearest a branch point or first point of difference. (The lowest first number decides which end of the chain you number from.) What end do you number from? incorrect 2 11 12 R on C 1 3 R on C4 10 9 R 9 7 3 1 R 4 6 8 10 12 6 5 4 5 7 9 11 8 2 A lower number for the substituent branch, R, is preferred. C4 is better than C9 so number from the left end. The number will be used to specify the position of the alkyl branch. If additional chains are present, the lowest number of the first chain determines the numbering direction on the longest chain. C3 substitution is lower than C4 substitution 11 12 10 The direction of the numbering is reversed because the new branch would get R 9 7 5 3 1 a lower number, C3, at the first point of differnece, than C4 if the numbering 8 6 4 2 occurred from the opposite direction. 3. When an alkane portion is present as a substituent/branch (i.e. it is not part of the longest carbon chain) one drops the -ane suffix of a similar length alkane and adds the suffix -yl. Alkane becomes alkyl when it is a substituent; (ethane Æ eth + -yl Æ ethyl.) These substituent names are placed in front of the parent name, as prefixes, with their designating numbers immediately in front of them. Use the numbers obtained from rule 2 to show the location(s) of any substituent(s) or branch(es). Each substituent gets a number, even if it is identical to another substituent and on the same carbon. Hyphens are used to separate the numbers from the letters. Separate substituent position numbers from one another with commas (if the numbers are adjacent). The substituents are listed in 3 Lecture 8 alphabetical order. The numerical prefixes (see rule 4) do not count in deciding the alphabetical order (unless they are inside parentheses). 11 12 substituents 10 3-methyl There are two one carbon branches on C3 and C9 carbons 8 8-ethyl 9 7 5 3 1 and two two carbon branches, both on the C8 carbon. The 6 4 2 8-ethyl 9-methyl first branch at C3 determines the direction of numbering because it generates the lowest possible number at the first point of difference. parent name = dodecane 1 C branch = methyl 7 C branch = heptyl 2 C branch = ethyl 8 C branch = octyl 3 C branch = propyl 9 C branch = nonyl 4 C branch = butyl 10 C branch = decyl 5 C branch = pentyl 11 C branch = undecyl 6 C branch = hexyl 12 C branch = dodecyl 4. For identical substituents, use the prefixes di-, tri-, tetra-, penta-, hexa-, etc. to indicate 2, 3, 4, 5, 6, etc. of these substituents. These prefixes are not considered in deciding the alphabetical order of each substituent (unless inside parentheses). 11 12 number of identical numerical 10 substituents prefix 2 di- 8 9 1 3 tri- 7 5 4 3 6 2 4 tetra- 5 prnta- 6 hexa- 7 hepta- 8,8-diethyl-3,9-dimethyldodecane etc. 5. With two or more possible longest chains of identical length, choose as the parent name the one with the greater number of substituents. This will produce simpler substituent names. 7-butyl Number from the right because 9-propyl the first branch appears at C . These two branches are 2 5-methyl equivalent in length, but 2-methyl the lower one is chosen 10 because it has more 8 2 9 7 5 3 These two branches are equivalent. branches on it. 12 11 4 6 Either one can be used. 1 10-methyl 8,8-diethyl The substituent alphabetical orders are: b > e > m > p (don't count "d" of di or "t:" of tri-). If you do not write these in 7-butyl-8,8-diethyl-2,5,10-trimethyl-9-propyldodecane the correct order, your structure will still be drawn correctly from the given name. 4 Lecture 8 6. For complex substituents (substituents that have substituents on themselves), follow the above rules for alkanes except: i. The -ane suffix of the subparent name is changed to -yl (see rule 3 above) ii. The longest chain of the complex branch always uses the carbon directly attached to the parent chain as C1. Starting at this position one would count the longest substituent chain possible, as shown below. iii. Parentheses are used to separate the entire complex substituent name, its numbers, its branches, and its subparent name, from the principle parent name. A number and a hyphen precede the entire complex substituent name in parentheses to indicate its location on the parent chain. If the complex branch has a common name, this can be used and no parentheses are necessary, but you have to memorize these. iv. Prefixes do count in alphabetizing the branch names when part of a complex substituent name and inside parentheses. This is not true for simple substituents on the parent chain. 2,10-dimethyl 4' =part of the longest chain 2'' 3' 5-(2-methylpropyl) 3' 1' = point of attachment to longest chain 2'' 1'' 2' 3' 1' 7-(1-ethylbutyl) 2' 1'' 1'' = point of attachment to longest branch chain 1' 1' 10 8 2 1' Outside the parentheses the numerical prefix does not 8-(1,2-dimethylpropyl) 9 5 3 1211 7 6 4 count for alphabetical order, but inside the parentheses 9-propyl 2' the numerical prefix does count for alphabetical order. 1' 1 If you do not write these prefix names in the correct order, 1'' 1' 3' parent chain = dodecane your structure will still be drawn correctly from the given 1'' name. 8-(1,2-dimethylpropyl)-7-(1-ethylbutyl)-2,10-dimethyl-5-(2-methylpropyl)-9-propyldodecane v. If identical complex substituent names are present, a different set of prefixes is used. These are listed below. The appropriate prefix is placed before the parentheses containing the complex substituent name. Prefix Number of Identical Complex Branches bis- 2 tris- 3 tetrakis- 4 pentakis- 5 hexakis- 6 etc. 1' 2' 2 1 5 7 3 1' 2 4 6 10 2' 1 8 3 9 6 4 5 4,4,5,7-tetrakis(1-methylethyl)decane 1,4-bis(1,1-dimethylethyl)cyclohexane 5 Lecture 8 Examples 2 1 CC CC CC Rule 1 - The longest chain is six carbons. 3 1 2 C C CCC C C CCC C C CCC Rule 2 - Number from the end closer to the 3 3 21 two two carbon branches, as C3 positions C C C C C C C C C instead of C positions.
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