Organic Chemistry/Naming

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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)[1] 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[edit]

Basic Nomenclature[edit]

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[edit]

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: CnH2n+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:

N-Pentan.png

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[edit]

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: CnH2n

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[edit]

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: CnH2n-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[edit]

General Alcohol Formula: CnH2n+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 - 2-Propanol (IUPAC)

Example: CH3CHOHCH3 - Isopropyl Alcohol (Common)

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

Ketones[edit]

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: CnH(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[edit]

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.

Esters[edit]

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[edit]

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: CnH2n+1CHO

Functional Group: CHO

Examples of aldehydes:

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

CH3CH2CHO Propanal

CH3CH2CH2CHOButanal

Carboxylic Acids[edit]

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: CnH2n-1COOH

Functional Group: COOH

Examples of carboxylic acids:

CH3COOH Ethanoic Acid (usually called Acetic acid)

CH3CH2COOH Propanoic Acid

Benzenes[edit]

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 Huckle 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[edit]

Phenyls[edit]

C6H5

Allyls[edit]

Allyl as a functional 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[2]

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[edit]

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).

See Also[edit]

References[edit]

  1. http://www.iupac.org/
  2. Organic Chemistry 5th ed Paula Bruice