Chapter 7 Naming Organic Compounds（1）

Ideally, every organic compound should have a name which can be used to draw an unambiguous structural formula. However, for ordinary communication, to avoid a tedious description, the official IUPAC naming rules are not always followed in practice except when it is necessary to define a compound in a concise manner, or when the IUPAC name is simpler (viz. ethanol versus ethyl alcohol). Therefore a common or trivial name, which is often derived from the source of the compound, could be used.

Alkanes

Straight-chain alkanes are named using a suffix “-ane” and a prefix which indicates the number of carbon atoms in the chain, following standard rules. The first few are the following:

For example, the simplest alkane is CH4methane, the 8-carbon alkane CH3(CH2)6CH3is named octane, and the 12-carbon alkane is named dodecane. The first four alkanes have their names derived from methanol, ether, propionic acid, and butyric acid, respectively.

For cyclic alkanes, the names are simply prefixed with “cyclo-”. For example, C4H8is cyclobutane and C6Hi2is cyclohexane. Branched alkanes can be regarded as a straight-chain alkane with attached alkyl groups and named accordingly. A numeral is added ahead of the group to indicate the position of the carbon that the group is attached to, counting from the end of the alkane chain. For example, (CH3)2CHCH3, commonly known as isobutane, is given a systematic name 2-methylpropane by treating it as a propane chain with a methyl group bonded to the middle (2) carbon. Although 2-methylpropane could be used as the name, it is easier and more logical to simply call it methylpropane without using the number “2” since the methyl group could impossibly be attached to any of the other carbon atoms (that would lengthen the chain and lead to butane, not propane).

Depending on which end of the alkane chain is counted as “1”, there could be ambiguity in the position of the substituent. The principle for choosing the numbering is to ensure that the smallest number is used. For example, (CH3)2CHCH2CH3 (isopentane) should be named 2-methylbutane instead of 3-methylbutane.

If multiple side branches of the alkyl group with the same size are present, commas can be used to separate their positions and the group is prefixed with“di-”, “tri-”, “tetra-”, etc., according to the number of branches (e.g., C(CH3)4, 2,2-dimethylpropane). For different groups, they are added in alphabetical order and separated by commas or hyphens, for example, 3-ethyl-4-methylhexane. Use the longest possible chain as main alkane chain; therefore, use 3-ethyl-4-methylhexane instead of 2,3-diethylpentane, even though these two names describe the equivalent structure. Ignore the prefixes of “di-”, “tri-”, etc., when considering the alphabetical ordering of side chains (e.g., 3-ethy 1-2,4- dimethylpentane, not 2,4-dimethyl-3-ethylpentane).

Alkenes are named for their parent alkane chain with a suffix “-ene” and an infixed number indicating the position of the double-bonded carbon in the chain: CH2=CHCH2CH2CH3is pent-l-ene. Multiple double bonds take the form“-diene”, “-triene”, etc., with the size prefix of the chain taking an extra “a”: CH2=CHCH=CH2CH3 is penta-l,3-diene. Simple cis and trans isomers are indicated with a prefixed “trans-” or “cw-”: trans-pent-2-ene, c/s-pent-2-ene.

The same system is used to name alkynes, with the suffix “-yne” indicating a triple bond, e.g., ethyne (acetylene), propyne (methylacetylene), and 2-butyne (dimethylacetylene), as shown above.

Alcohols and Ethers

Alcohols (ROH) use the suffix “-ol” with an infix indicating the bonding position: CH3CH2CH2OH is propan-l-ol. The suffixes “-diol”, “-triol”, “-tetrad”, etc., are used for multiple –OH groups; for example, ethylene glycol CH2OHCH2OH is ethane-1,2-diol.

In the presence of higher precedence functional groups (see order of precedence, below), the prefix “hydroxy-” is used with the bonding position: CH3CH2CHOHCHO is 2-hydroxybutyraldehyde.

Ethers (ROR’) contain an oxygen atom with two carbon chains attached. The shorter of the two chains becomes the first part of the name with the “-ane” suffix changed to “-oxy”, and the longer alkane chain become the suffix of the name of the ether. Thus CH3OCH3is methoxymethane (dimethyl ether), and CH3OCH2CH3 is methoxyethane (ethyl methyl ether) (not ethoxymethane). If the oxygen is not attached to the end of the main alkane chain, then treat the whole shorter alkyl-plus-ether group as a side-chain and prefix with its bonding position on the main chain. Thus CH3OCH(CH3)2is 2-methoxypropane (or isopropyl methyl ether).

Halogens (Alkyl Halides)

Halogenated compounds are prefixed with the bonding position of the halogen functional groups, taking the form “fluoro-”, “chloro-”, “bromo-”, “iodo-”, etc. For multiple groups, prefixes such as “di-”, “tri-”, etc., are used, and different groups are ordered alphabetically as described earlier. Thus, CHC13(chloroform) is trichloromethane. The anesthetic halothane (CF3CHBr Cl) is l-bromo-l-chloro-2,2,2-trifluoroethane.

Ketones and Aldehydes

Ketones (RCOR’) generally take the suffix “-one” (pronounced own, not won) with an infix number indicative of the position: CH3CH2CH2COCH3is pentan-2-one. If a higher precedence suffix is used, the prefix “oxo-” can be used: CH3CH2CH2COCH2CHO is 3-oxohexanal.

Aldehydes (RCHO) are named with the suffix “-al”. In the presence of other functional groups, the chain is so numbered that the aldehyde carbon is in the “1” position.

Common names for ketones can be obtained by naming the two alkyl or aryl groups that are bonded to the carbonyl group as separate words followed by the word ketone.

• acetone • acetophenone

• benzophenone • ethyl isopropyl ketone

• diethyl ketone

The first three names shown above are still considered as acceptable IUPAC names.

The common name for an aldehyde is derived from the common name of the corresponding carboxylic acid by dropping the word acid and changing the suffix from “-ic” or “-oic” to “-aldehyde”.

• formaldehyde (from formic acid HCOOH) • acetaldehyde (from acetic acid CH3COOH)

For a required prefix form, “oxo-” is used (as for ketones), and the position number indicates the end of a chain: CHOCH2COOH is 3-oxopropanoic acid. If it is impossible to include the carbon in the carbonyl group in the attached chain (for instance, in the case of cyclic aldehydes), the prefix “formyl-” or the suffix“-carbaldehyde” is used: C5H9CHO is cyclopentanecarbaldehyde. If an aldehyde is attached to a benzene as the main functional group, the suffix becomes benzaldehyde. Carboxylic Acids

In general carboxylic acids are named with the suffix “-oic acid” (etymologically a back-formation from benzoic acid). Similar to aldehydes, they take the first position on the parent chain, so it is not necessary to indicate their position number. For example, CH3CH2CH2CH2COOH (valeric acid) is named pentanoic acid. For common carboxylic acids, some traditional names (e.g., acetic acid) are in widespread use so that they are considered as retained IUPAC names, although it is also acceptable to use “systematic” names such as ethanoic acid. Carboxylic acids attached to a benzene ring such as Ph COOH are named as benzoic acid or its derivatives.

If there are multiple carboxyl groups on the same parent chain, the suffix“-carboxylic acid” can be used (as “-dicarboxylic acid”, “-tricarboxylic acid”, etc.). In these cases, the carbon in the carboxyl group is not counted as being part of the main alkane chain. It is the same for the prefix form, “carboxyl-”. Citric acid serves as one example; it is named 2-hydroxypropane-l,2,3-tricarboxylic acid, not 2-carboxy, 2-hydroxypentanedioic acid.

Esters and Carboxylates

Esters (RCOOR1) can be named as alkyl derivatives of carboxylic acids with the alkyl (R’) group named first. The RCOO- part is then named as a separate word based on the carboxylic acid name, with the ending changed from “-oic acid” to“-oate”. For example, CH3CH2CH2CH2COOCH3is methyl pentanoate, and (CH3)2CHCH2COOCH2CH3is ethyl 3-methylbutanoate. For esters based on common acids such as ethyl acetate (CH3COOCH2CH3), ethyl formate (HCOOCH2CH3), or dimethyl phthalate, the IUPAC recommendation is to use these established names, known as retained names, thus, their “-oate” suffix changes to “-ate”. Some simple examples, named in either way, are shown in the figure below.

When the alkyl group is not attached at the end of the chain, the bond position to the ester group is infixed before “-yl”: CH3CH2CH(CH3)OOCCH2CH3may be named but-2-yl propanoate or but-2-yl propionate.

A carboxylate anion (RCO2-), is an ion with negative charge that contains the group –COO-. It is the conjugate base of a carboxylic acid. Examples are: adipates, acetate, acrylate, butyrate, citrate, dimethylsulfoniopropionate, formate, jasmonate, malonate, methacrylate, oxalate, glyceric acid, propionate, stearate, valerate, pyruvate, succinate. Accordingly, CH3COONa and C6H5COOK are called sodium acetate and potassium benzoate, respectively.

Amines and Amides

Amines (RNH2) are named based on the attached alkane chain with the suffix“-amine” (e.g., CH3NH2, methanamine). If necessary, an infix for the bonding position is used: CH3CH2CH2NH2, propan-1-amine; CH3CHNH2CH3, propan-2-amine. And the prefix form is “amino-”.

For secondary amines with the form of RNHR’, they are primarily named for the longest carbon chain attached to the nitrogen atom, and the other chain can be prefixed as an alkyl group with an italic N as the location prefix: CH3NHCH2CH3is v V-methylethanamine. Tertiary amines (RNR’R”) are named similarly: CH3CH2N(CH3) CH2CH2CH3is Af-ethyl-j V-methylpropanamine. Again, for the substituent groups, they can be ordered alphabetically.

Amides (RCONH2) take the suffix “-amide”. No prefix form or location number is required since they always terminate a carbon chain, e.g., CH3CONH2(acetamide) is named ethanamide.

The naming of secondary and tertiary amides is similar to the case of amines: treat the alkane chains bonded to the nitrogen atom as substituents and use the location prefix “TV”: HCON(CH3)2is AVV-dimethylmethanamide (commonly called N,N-dimethylformamide).

Cyclic Compounds

Cycloalkanes and aromatic compounds are treated as the main parent chain of the compound, and the position of substituents are numbered around the ring structure. For example, the three isomers of xylene CH3C6H4CH3, commonly the “ortho-”,“meta-”, and “para-” forms, are 1,2-dimethylbenzene, 1,3-dimethylbenzene, and 1,4-dimethylbenzene. It is also possible to treat the cyclic structures themselves as functional groups; in this case they can take the prefix “cycloalkyl-” (e.g.,“cyclohexyl-”) or for benzene, “phenyl-”.

For more complex cyclic structures, the IUPAC nomenclature rules have become rapidly more elaborate, and notations are available for compounds with conjoined rings. At the same time, many common names such as phenol, furan, indole, etc., are accepted as basis for naming their derivative compounds.