Nomenclature of Organic Molecules Dr. Carl Hoeger

Transcription

1 Nomenclature of rganic Molecules Dr. Carl oeger Introduction Nomenclature, as you might expect, is an simple yet important topic in chemistry. Without a standardized system of nomenclature scientists would not be able to communicate with one another regarding their discoveries! rganic chemistry, because of the vast numbers of arrangements possible for carbon-based backbone, requires that this system be simple, flexible yet rugged enough to deal with this vast number of compounds. We will discuss the nomenclature scheme as developed by the International Union of Pure and Applied Chemistry (IUPAC). IUPAC Basics The nomenclature scheme as developed by the IUPAC is what is known as a substitutive scheme; this means that all nomenclature will ideally be based on substitutions of some base set of structures. Since the saturated carbon skeleton of the alkanes is a feature that is found is all organic molecules, it was decided that this would be the base set we will use. Table 1 gives the names and structures of the first twelve simple alkanes and the first eight cycloalkanes. Table 1. Parent ydrocarbons for IUPAC Nomenclature Name # of C s in chain Chemical Structure (different representation types shown) Methane 1 C 4 Name as substituent Methyl Ethane 2 C 3 C 3 Ethyl Propane 3 C 3 C 2 C 3 Propyl Butane 4 C 3 C 2 C 2 C 3 Butyl Pentane 5 Pentyl exane 6 exyl eptane 7 * ctane 8 C 3 (C 2 ) 6 C 3 * Nonane 9 C 3 (C 2 ) 7 C 3 * Decane 10 C 3 (C 2 ) 8 C 3 * Undecane 11 * Dodecane 12 * Cyclopropane 3 Cyclopropyl Cyclobutane 4 Cyclobutyl Cyclopentane 5 Cyclopentyl Cyclohexane 6 Cyclohexyl Cycloheptane 7 * Cyclooctane 8 * *: Rarely seen as a substituent

2 : name for the straight chain substitutent (e.g. butyl = C 3 C 2 C 2 C 2 -); name will change if not straight.t chain In the IUPAC scheme, all organic molecules will be named as if they were derivatives of alkanes; therefore, if we know how to name alkanes we should be able to name anything. Therefore, we will need to be able to do three steps to be able to name any organic molecule: 1. Identify the longest contiguous* chain of carbon atoms and the alkane to which it corresponds; 2. Identify the substitutent groups present on the longest chain; and 3. Determine the correct indexing (or numbering) direction. *-contiguous: Neighboring; adjacent; connecting without a break (NTE: we will modify step 1 when we get to naming molecules that contain functional groups). We will now begin by applying these rules to the simplest organic family: the Alkanes. Alkanes Alkanes are the simplest organic family and form the basis for all nomenclature. Procedures learned here will be implemented over and over again as you go through the other functional group families. Lets start by using the three step program to name the following molecule: Step 1. The first thing that must be done is to find the longest chain. You must always remember that the longest chain may NT be the straightest chain of carbon atoms! Below I have shown you four possible atom counts (indicated by the bold lines): 8 C's 8 C's 7 C's 9 C's

3 Notice that the one that gives the longest contiguous chain of carbon atoms seems to veer off from the straight path and it is this count that we must use. The alkane that has a nine carbon chain is nonane and therefore this molecule will also have as its base name nonane. What if there is more than one longest chain? This does not happen often but when it does the rule is to (in order of importance) a) first pick the chain that gives the fewest number of substituents; b) then pick the chain with the least complicated groups on it; and c) then pick the chain with the lowest set of index numbers. Step 2. Now that we know what the longest chain is, we need to name any and all substituents that are bound to the longest chain. In the example here, those will be the carbon groups coming off the emboldened chain shown. Carbon substituents (see Table 1) are named based on the number of carbons in the group; you remove the -ane from the alkane name and replace it with -yl to indicate it is a substitutent. For the molecule we are looking at here the names of these groups are shown below. methyl methyl ethyl methyl ethyl When substituents have three or more carbons, there exists the possibility to have isomers of the side chain. Consider the pentyl substituent as an example (the wavy line indicates the point of attachment to the main chain). ne can have attachments the group attached to the longest chain through C1, C2 or C3; we name these in IUPAC fashion as follows: pentyl- R n-pentyl- (2-pentyl)- (3-pentyl)- For the straight chain alkyl group (attachment through C1) the number 1 is understood and therefore dropped. Another way that this attachment is represented is by using a lowercase n ; this represents the word normal and always means attachment through C1. When attached via carbons other than 1 the attachment point is represented with a number and the alkyl group is set off using parentheses. While most substituents follow this simple naming scheme, some, for historical reasons, have unique names. These are shown below in Table 2.

4 Table 2. Special Substituent names in IUPAC Nomenclature Substituent Name Structure isopropyl sec-butyl tert-butyl (or t-butyl) phenyl benzyl ( this is often simply abbreviated as Ph-) C 2 ( or Bz-) Step 3. We must now number the longest chain in such a way that the sum of the index numbers will be the lowest possible; this is easily achieved by numbering the chain so that the first substituent has the lowest numerical value possible. If there is a tie, go to the next substituent. For our current molecule there are two different directions: ccw (shown here with bold italics) and cw (regular typeface). For both directions the first substituent is on carbon 4 but in the cw direction there are TW substituents on C4, so that is the correct direction to choose. All that is left is to put it all toegether methyl ethyl 6 methyl methyl ccw ethyl 2 1 cw 4,6-diethyl-4,5,5-trimethylnonane Finally, notice how multiple groups with the same name are handled: the index number for each occurrence is recorded (twice if on the same carbon) and the number of times it occurs is represented with a greek prefix (di = two, tri = three, tetra = four, etc.). Cycloalkanes Cycloalkanes are relatively straightforward to name as there is no longest chain to worry about, only the largest ring. Numbering is done so that the lowest set of index numbers is obtained. Carbon 1 is picked in such a way so that this obligation is met. If there are more than one possibility carbon 1 is chosen based on alphabetical order. Some examples are shown below:

5 3 C C 2 C 3 1-chloro-4-ethyl-2-methylcyclopentane 1-bromo-3-isopropyl-5-methylcyclohexane Finally, when is a cycloakane treated as the main chain and when is it treated as a substituent? This applies only to monosubstituted cycloalkanes on monsubstituted alkane chains. Consider the examples below: sec-butylcyclopentane 2-cyclopentylhexane The rule we will apply is this: whichever has the larger number of carbons in it will be the compound upon which the name will be based; the other will be the substituent. Let s now look at other functional families aloalkanes or Alkyl alides These are not really a functional group family, but since they are more reactive than alkanes they are given a special place in the organic hierarchy. There are no special nomenclature rules for them except that in a longest chain tie you choose the direction that gives the halogen the lowest index number. Two examples are shown below: F 4-bromo-4,6-diethyl-5,5,6-trimethylnonane NT 6-bromo-4,6-diethyl-4,5,5-trimethylnonane 3-ethyl-5-fluoroheptane Alternative nomenclature (also known as Trivial nomenclature). While the IUPAC nomenclature is considered absolute, many compounds still have trivial or common names that are still in everyday usage. For each functional group this alternative nomenclature will be addressed when important. For haloalkanes that nomenclature alternative is the alkyl halide scheme; this is where the compound is named by giving the name of the alkyl group attached to the halide followed by the name of the halide. This is only used for very simple compounds, usually having less that six carbons. Some examples are shown below:

6 I F IUPAC: Trivial: 2-iodopropane 1-chloropropane fluorocyclohexane isopropyl iodide n-propyl chloride cyclohexyl fluoride Alkenes Functional groups: first principles. When functional groups are present the rules for nomenclature change, actually for the simpler. The steps in naming a compound now are rewritten as such: 1. Identify the longest contiguous chain of carbon atoms that CNTAIN ALL the carbons in the functional group; determine which alkane has the same number of carbons; 2. Identify the substitutent groups present on the longest chain (this rule is the same); and 3. Number the chain so that the carbons of the functional group have the lowest numbers possible (notice that the index numbers of the substituents are no longer as important as before). In IUPAC nomenclature, functional groups are denoted as suffixes with a number indicating where the functional group is. Let s start with the alkenes. The first real functional group we run across is the alkene. Alkenes are hydrocarbons that contain double bonds. The presence of a double bond is communicated using the suffix ene. The longest chain is determined, the name of the alkane upon which it is derived is determined, the ane at the end is dropped and replaced with ene. The location of the double bond is indicated by putting the index number of the first carbon of the double bond in front of the ene. If the double bond starts at C1 the number is understood (but can be reported if desired). Some examples are shown below: 6-bromo-2-chloro-3-methylhept-3-ene butene C 2 C 3 5-chloro-3-ethylcyclohexene An alternative (and completely correct) way to do this is to put the index number in front of the parent chain s name instead of in front of the suffix; thus, 6-bromo-2-chloro-3-methylhept-3-ene and 6- bromo-2-chloro-3-methyl-3-heptene are both correct. If more than one double bond is present the number of double bonds is represented using greek prefixes; in this case the 1 is not ignored. Some examples: I penta-1,3-diene 3-chloro-3-iodo-4-methyl-5-cyclopentylocta-1,4,6-triene R 3-chloro-3-iodo-4-methyl-5-cyclopentyl-1,4,6-octatriene

7 Alternative nomenclature. None Alkynes The next functional group we run across is the alkyne, hydrocarbons that contain triple bonds. The presence of a triple bond is communicated using the suffix yne. The nomenclature is identical to that of alkenes. Examples are: C 3 C 2 C C C 2 C 2 C 3 hept-3-yne C C 2-bromo-5-chlorohex-3-yne Alternative nomenclature. The only notable exception is for the simplest alkyne -C C-, which is more commonly known as acetylene. ther than that there are no exceptions of note. Alcohols The next functional group we run across is the alcohol, the first of the functional groups that contain atoms other than C, or halogen. The presence of an is communicated using the suffix ol. The nomenclature is identical to that discussed earlier. For alcohols, the carbon on which the is located must be part of the longest chain. Examples are: C3 3-bromo-2-ethyl-2-me thylbutan-1-ol C( C 3 ) 2 3-isopropyl-1-methylcyclohexanol notice the "1" must be shown for the methyl 1-chloropentane-2,3-diol Alternative nomenclature. For alcohols the alternative is the alkyl alcohol scheme; this is where the compound is named by giving the name of the alkyl group attached to the alcohol followed by the name of the halide. This is only used for very simple compounds, usually having less that six carbons. Some examples are shown below: cyclopentanol R cyclopentyl alcohol C 3 C 2 ethyl alcohol isopropyl alcohol Ethers Just as the alcohol is an organic derivative of water (R-), so are ethers, which can be thought of as a water molecule that has had both s replaced with carbon groups (R--R ). Ethers are a group of simple molecules with a rather complicated nomenclature scheme. Since we deal almost exclusively

8 with simple ethers, rather than spend time learning this obtuse nomenclature scheme we will almost exclusively use the alternative scheme for ethers, which is to name then as alkyl alkyl ethers. This simple scheme will allow us to name almost all of the ethers we will run across in this course. Some examples are as follows: C 3 diethyl ether methyl t-butyl ether cyclobutyl isopropyl ether Aldehydes The next functional group we run across is the aldehyde (R-C), the first of the functional groups that contain a carbon-oxygen double bond (a carbonyl group). Since an aldehyde function must by definition always be at C1, no index number for it is required; and the fact that it is an aldehyde is communicated by dropping the e off of the name of the alkane from which it is derived and appending the suffix al. The nomenclature is identical to that discussed earlier. For aldehydes, the carbonyl carbon must be the first carbon of the longest chain. Examples are: C C 3-bromo-2-ethyl-2-methylbutanal 2,3-dimethylbutanal 2-cyclopropylbutanal The only glitch in this nomenclature scheme comes when you have NLY a cycloalkyl group attached to the C function. ere, the nomenclature scheme adopts a unique way to name these. The C group is called carbaldehyde and the cycloalkyl group is name as if it were simply a cycloalkane, leading to the compound being named cycloalkanecarbaldehyde. Below are some examples: C C C 2 C 3 3-c hl oro-4-e thylc yc lohe xa ne carba ldehyde c yc lopropa ne carba ldehyde 2-methylcycl opentanec arba lde hyde Alternative nomenclature. For aldehydes the alternative is based on the trivial name of the corresponding carboxylic acid (see carboxylic acid section below). This nomenclature scheme is not important with the exception of a very few compounds; it is easy to tell it is an aldehyde because it will have the word aldehyde in its name. A few it is important to know are:

9 formaldehyde C 3 acetaldehyde benzaldehyde Ketones While the aldehyde has a carbonyl group with one other carbon group attached to it (R-C), the ketone functional group has alkyl groups attached to BT sides of the carbonyl (R-C-R ); the fact that it is an ketone is communicated by dropping the e off of the name of the alkane from which it is derived and appending the suffix one. The nomenclature is identical to that discussed earlier: longest chain must include the carbon of the carbonyl, index number must be the lowest possible for that carbon, and numerical position of carbonyl must be provided. If more than one ketone is present the number of them in the molecule is given using greek prefixes. Examples are: C 3,4-dimethylpentan-2-one 4-cyclopropylhexan-3-one 5-isopropylcyclohexane-1,3-dione Alternative nomenclature. For ketones an alternative scheme is based on fact that a ketone is an alkyl, alkyl carbonyl compound. Thus, if the ketone is a simple ketone (that is, one that has relatively simple alkyl groups on the carbonyl group) it can be named using this scheme. Examples using this scheme are: ethyl methyl ketone phenyl isopropyl ketone propyl cyclopentyl ketone Amines Just as the alcohols are organic derivatives of water, amines are organic derivatives of ammonia. They are the bases of organic chemistry. Amines follow two nomenclature schemes; while one is IUPACbased and the other is a common nomenclature scheme both are widely used to about the same extent. Amines can be classified as to the number of alkyl groups directly attached to the nitrogen: R 1 N R 1 N R 1 N 2 R 1 amine N-R 2 -R 1 amine R 2 R 2 R 3 N-R 3 -N-R 2 -R 1 amine 1 o amine 2 o amine 3 o amine

10 In the IUPAC scheme, the R x group with the longest and most complex chain is considered to be the longest chain; the alkyl groups left on the nitrogen are designated by an italicized N followed by the name of the alkyl group. Examples are: Ph N 2 N 2 1-phenylpropan-1-a mi ne N N-ethyl-N-methylethanamine c yc lohe xa na mine N N-isopropylpropan-1-amine The alternative scheme that is also used a lot is to simply name the amine as an alkyl alkyl alkyl amine (without the N s). So, from the previous set of examples, N-ethyl-N-methylethanamine would also be known as methyl diethyl amine. This nomenclature is used extensively for simple secondary and tertiary amines. Carboxylic Acids The next functional group is the most highly oxidized of the functional groups, the carboxylic acid (commonly referred to as acids, they abbreviated as C or C 2 ). These are the acids of organic chemistry (evident from their name!). The carboxylic acids (like aldehydes and ketones) also contain a carbonyl group; however the carbonyl is also attached to an oxygen leading to what is known as a carboxyl group: carbonyl X =, N, halogen R X carboxyl (Note: in fact, a carbonyl group attached to an oxygen, nitrogen R a halogen is, in general, called a carboxyl group, as they are all derivatives of carboxylic acids). Since an carboxylic acid function must (by definition) always be at C1 and it must be the first carbon of the longest chain, no index number for it is required; and the fact that it is an carboxylic acid is communicated by dropping the e off of the name of the alkane from which it is derived and appending the suffix oic acid. The nomenclature is identical to that discussed earlier. Examples are: B r buta noic a cid C 2-bromopropanoic acid C 2 2-chloro-3-methylcyclohexaneca rboxylic a cid C 2 C 2 2-(1-methylcyclopentyl )ethanoic a cid 3,4-dimethylpentanoi c acid ne thing to note here is how cycloalkyl carboxylic acids are named.

11 Alternative nomenclature. rganic acids are some of the oldest compounds known. For acids the alternative is based on the trivial name of the corresponding carboxylic acid. This archaic nomenclature scheme is sometimes used to the exclusion of the IUPAC system, especially for simple carboxylic acids (4 carbons or less). A few it is important to know are, along with their use are shown below: C 2 C 3 C 2 C 3 C 2 C 2 C 3 C 2 C 2 C 2 C formic acid acetic acid propionic acid butyric acid benzoic acid Esters, Amides and Aromatics Polyfunctional Compounds