Organic chemistry/Naming

Subject classification: this is a chemistry resource.

There are thousands of known organic compounds and millions of others yet to be discovered. Because of the wide variety of compounds a naming system was developed by International Union of Pure and Applied Chemistry (IUPAC) to give a unique descriptive name to each compound. In this system the name of the compound directly shows its structure through an elaborate system of suffixes and prefixes. Common names are used for compounds that were either around before IUPAC or who have a more widely known name than the IUPAC. For example, trichloromethane is IUPAC name for chloroform, though it is almost universally referred to by students and scientists alike as chloroform.

Naming parent chains

Basic Nomenclature

The IUPAC nomenclature of organic molecules is based on the longest carbon chain in the molecule. For alliphatics, which are hydrocarbons containing chains of C, the following prefixes are used:

# of C	Stem name	Alkyl group name
1	meth	methyl
2	eth	ethyl
3	prop	propyl
4	but	butyl
5	pent	pentyl
6	hex	hexyl
7	hept	heptyl
8	oct	octyl
9	non	nonyl
10	dec	decyl
11	undec	undecyl
12	dodec	dodecyl

Alkanes

Chemical structure of methane, the simplest alkane

Alkanes are a compound in which the carbon backbone contains only single bonds. They are named according to the number of carbons they have. Alkanes combust to produce water and carbon dioxide in air. Some familiar alkanes are methane (natural gas), propane (gas used in homes), and butane (blue-flamed cigarette lighters). Alkanes containing four carbons or more can form different chain isomers; that is, they can become "branched," forming, for example, iso-butane.

General Alkane Formula: C_nH_2n+2

Suffix: "ane"

Example: CH₃CH₂CH₃ - Propane

To name branched molecules, find the longest continuous chain first. This chain forms the base name. Number the carbon atoms, starting at the end closest to the branching. Locate the branch by the number of the C stem to which it is attached on the main chain. Name the branch, which is called an alkyl group, by using the chart above. If more than one of the same alkyl groups are present as branches, the number of these branches is indicated by prefix multipliers (di-, tri-, etc.). The location of each of these alkyl groups is indicated by a number, using the lowest combination possible.

Example: CH₃CH(CH₃)CH₃

Name As: 2-methyl propane or common name is iso-butane

Example: CH₃CH(CH₃)CH₂CH(CH₃)CH₃

Names As: 2,4-dimethyl pentane, as we start naming from the end closest to a branch (in this case, the back end)

For IUPAC names, the following table of prefixes is important. Each prefix corresponds to a different number of carbons.

Number of Carbons	Base Name	Number of Carbons	Base Name
1	Meth	11	Undec
2	Eth	12	Dodec
3	Prop	13	Tridec
4	But	14	Tetradec
5	Pent	15	Pentadec
6	Hex	16	Hexadec
7	Hept	17	Heptadec
8	Oct	18	Octadec
9	Non	19	Nonadec
10	Dec	20	Icos

The simplest systematic name is given to straight chain alkanes. An alkane is a compound containing solely hydrogen and carbon, held together only by single bonds. Straight chain implies that there are no branches or rings in the structure. To name simple alkanes, you need to know the appropriate base part for the number of carbons in a continuous chain. Say for example, you had the following molecule:

This molecule contains a continuous chain of five carbons. By referencing the chart, we see that five carbons merits the prefix "pent-". Because this molecule contains only carbon and hydrogen held together by single bonds, it is an alkane, which merits the suffix "-ane". Therefore the name of this molecule is pentane.

Alkenes

Alkenes, like Alkanes, have carbon backbones but contain at least one carbon-carbon double bond. Double bonds are reactive and can be reduced to single bonds using hydrogen gas, pressure, and a catalyst, such as Pd or Pt. Alternatively, Alkenes can be reduced to Alkanes by reacting with Hydrogen gas over a Nickel catalyst. Nickel acts as a heterogeneous surface catalyst, weakening the H-H bonds in hydrogen, thus speeding up the reaction. This also leads to syn addition, where both of the hydrogens are added to the same side of the double bond.

General Alkene Formula: C_nH_2n

Suffix: "ene"

Example: CH₂CH₂ - Ethene

See the alkane naming scheme for branched molecules, as the same principal applies to alkenes we simply retain the "ene" rather than the "ane" ending. However, the placement of the double bond matters when naming.

Example: CH₃(CH)₂CH₃ - 2-butene

Example: CH₂CHCH₂CH₃ - 1-butene

IUPAC rules actually specify that alkenes should be named as but-1-ene, but-2-ene, and so on; but the practice of putting the number before the name remains widespread in the US.

To figure out which is which, number the carbon backbone starting with end closest to the double bond. Whichever number carbon the double bond starts on is the prefix which you add to the name.

Another problem in naming is when there is more than one double bond.

Example: CH₂(CH)₂CH₂ - 1,3-butadiene

Example: CH₂CCCH₂ - 1,2,3-butatriene

To name these types of compounds, again, number the carbons starting from the end closest to a double bond. The number prefixes will correspond to the carbons at which each double bond starts, and an additional suffix (di, tri, tetra, etc.) will be added in front of the 'ene' suffix.

Alkynes

Alkynes have carbon-carbon triple bonds. Like alkenes, Alkynes are reactive and can be brought down to either double or single bonds. If excess hydrogen is used, and they are allowed to react, they will become single bonds. If a poison catalyst (such as Pd/CO3, palladium with carbonate) is used instead of a regular catalyst, the alkynes will be selectively reduced to alkenes.

General Alkyne Formula: C_nH_2n-2

Suffix: "yne"

Example: CHCH - Ethyne

See the alkane naming scheme for branched molecules, as the same principal applies to alkynes we simply retain the "yne" rather than the "ane" ending.

Alcohols

General Alcohol Formula: C_nH_2n+1OH

Suffix: "ol"

Example: CH₃CH₂OH - Ethanol (IUPAC)

Example: CH₃CH₂OH - Ethyl Alcohol (Common Name)

Example: CH₃CH₂CH₂OH - 1-Propanol (IUPAC)

Example: CH₃CH₂CH₂OH - Propyl Alcohol (Common)

Example: CH₃CHOHCH₃ - Propan-2-ol (IUPAC)

Example: CH₃CHOHCH₃ - Isopropyl Alcohol (Common)

The numbered prefix is indicative of an isomer, not the location of a double bond.

Ketones

Ketones are carbonyl compounds. Thus they contain the C=O group. This group is responsible for the typical peak present in the mass spectrometer profiles for carbonyl compounds. The suffix "-one" is added to the alkane name to indicate a ketone. Ketones, if formed from alcohols, must be formed from secondary alcohols as you need to have two groups either side of the "carbonyl" carbon.

General Ketone Formula: C_nH_(2(n-1)+2)O

Suffix: "one"

Examples of ketones include:

CH₃COCH₃ Name: propanone. This is the lowest member of the series.

CH₃COCH₂CH₃ Name: butanone.

CH₃CH₂COCH₂CH₃ Name: pentan-3-one. The "-3-" indicates the position of the C=O (carbonyl) group. It shows that the carbonyl group is present on the third carbon atom in the chain. Such naming is essential in ketones (except propanone and butanone). For example, an isomer of pentan-3-one is pentan-2-one, which has a different structural formula.

Ethers

In Ethers the functional group is -O-. The General Formula of ethers is R-O-R or R-O-R'. R-O-R are simple ethers. Whereas R-O-R' represents mixed ethers. R and R' are different alkyl groups. According to IUPAC they are called Alkoxy Alkanes (O-R is alkoxy group).

For Example,

CH₃OCH₃ is a simple ether whose common name is Dimethyl Ether and the IUPAC name is Methoxy Methane.

C₂H₅OCH₃ is a mixed ether whose common name is Ethyl Methyl ether and the IUPAC name is Methoxy Ethane. You should name with the lowest group first: methyl, then ethyl, etc... which explains the IUPAC difference in naming, as the methyl come before the ethyl.

Esters

Carbon Compounds consisting of a carbonyl carbon adjacent to an ether (-COO-). Esters are often characterized by a unique smell or taste. Esters are used by manufacturers due to their unique smells and tastes. Suffix is -oate.

Aldehydes

Aldehydes are carbonyl compounds with a C=O and a H bonded to the same carbon. The suffix attached to an aldehyde is "al". The quickest way to form an aldehyde is to oxidise a primary alcohol using potassium dichromate.

Suffix: "al"

General formula: C_nH_2n+1CHO

Functional Group: CHO

Examples of aldehydes:

CH₂O Formaldehyde (common name) or Methanal (IUPAC), is the simplest aldehyde.

CH₃CH₂CHO Propanal

CH₃CH₂CH₂CHOButanal

Carboxylic Acids

Carboxylic acids are carbonyl compounds containing a C=O and an -OH group bonded to the same carbon. They can be obtained by oxidising a aldehyde or a primary alcohol using potassium dichromate (K₂Cr₂O₇) with concentrated sulphuric acid.

Suffix: "oic acid"

General formula: C_nH_2n-1COOH

Functional Group: COOH

Examples of carboxylic acids:

CH₃COOH Ethanoic Acid (usually called Acetic acid)

CH₃CH₂COOH Propanoic Acid

Benzenes

Benzenes, part of the aromatic group, are all based on the benzene ring (C₆H₆). In it, there are three double bonds that create a resonance structure. Resonance theory states that we can draw two different structures for benzene where the double bonds are in different places, but neither structure adequately represent the molecule, so instead we think of the molecule as a resonance hybrid, a combination of the two. Benzene is incredibly stable, and is also planar. Although the term aromatic should refer to a smell, is really refering to a Huckel number of pi electrons.

General formula: C₆H₆

Derivatives of benzene are named by numbering the C atoms in a clockwise manner, starting at one for the first group. Alkyl monosubstituted benzenes are names as alkyl benzenes.

Example: C₆H₅CH₃ - Methylbenzene, a common name for methylbenzene is Toluene.

If the alkyl branch is attached to the benzene ring by a carbon other than an end carbon, the benzene is then considered a branch of the carbon chain, and is called phenyl.

Naming as substituents

Phenyls

C₆H₅

Allyls

Allyl as a group is represented as -CH₂-CH=CH₂. Allyl is considered to be a common name. The term allyl deals with a carbon attached to a carbon in a double bond (that is sp2 hybridized). This is not to be confused with allylic as allylic refers to just a carbon attached to the carbon in the double bond, not the entire group of three carbons. An example of Allyl groups with its corresponding systematic names is

H₂C=CHCH₂Br allyl bromide or 3-bromopropene.

Note it is allyl because the bromine is attached to the CH₂ carbon which is sp² hybridized, double bonded.

Vinyls

The vinyl group is similar to the allyl group. However, vinyl refers to a group attached directly to an sp² hybridized (double bonded) carbon. The vinyl group is represented as -CH=CH₂. This is not to be confused with vinyllic carbons which are only the carbons participating in the double bond, not the entire group. An example of naming a vinyl compound with its systematic name is shown below

H₂C=CHCl vinyl chloride or chloroethene

To relate vinylic and allylic (not vinyl and allyl) consider the following part of a carbon chain:

R-CH₂-CH=CH-CH₂-R

The first and last carbons shown are allylic (CH₂), the middle two carbons participating in the double bond are vinylic (CH).

Further reading

•  IUPAC nomenclature of organic chemistry