Organic Nomenclature APchemistry.ncssm.edu/medchem/files/organic_nomenclature.pdf · Organic...

Organic Nomenclature 1

Organic Nomenclature

Organic compounds are compounds containing carbon bonded to other nonmetals such as hydrogen, nitrogen, oxygen, or the halogens. The term organic comes from the old idea that all carbon containing compounds had to be produced by a living organism. While living organisms produce a vast number of organic compounds, many of these compounds, as well as many compounds not found in nature, can be produced in the laboratory. Since carbon can form single, double, and triple bonds, and can form chains of carbon atoms bonded to each other, a wide variety of organic compounds can be formed. The name of the compound must be systematic in order to provide enough information to derive a chemical structure from the name or a name from the structure. Organic nomenclature is complicated by the fact that the same compound may have a standard IUPAC (International Union of Pure and Applied Chemistry) name and a common name that came from previous nomenclature systems or the original source of the compound. Often the common name is more frequently used than the standard name. While we will show some of the common names for reference, you will only be responsible for the standard name. Organic nomenclature is structured on the concept of different classes of compounds. Each class of compounds contains a group of atoms that defines that class of compound and controls their reactions. This group of atoms is called the functional group of the compound. The name of the compound is derived from the stem name for the longest continuous chain of carbon atoms in the molecule and a suffix for the functional group. The prefixes for the length of the carbon chain are shown in the table below.

Number of Carbon Atoms Stem Name Side Chain 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

Hydrocarbons

Hydrocarbons are compounds that contain only carbon and hydrogen atoms. There may be single or multiple bonds present.


Hydrocarbons Compound General Formula Example Name Alkane CnH2n+2 CH3CH2CH3 propane Alkene CnH2n CH3CH=CH2 propene Alkyne CnH2n-2 2-butyne Alkadienes CnC2n- 1,3-butadiene Cycloalkane CnH2n

cyclohexane

Arenes ArH

benzene

Alkanes Alkanes are sometimes referred to as saturated hydrocarbons since they have only single bonds. There are no additional electrons available for bonding so the molecule is “saturated” with atoms. The suffix for alkanes is ane. We will use alkanes as the model for several of the general rules of organic nomenclature. 1. Choose the longest continuous chain of carbon atoms. Use the stem name for the longest chain and add ane.

The longest continuous chain of carbon atoms may not always be written horizontally so you need to check the molecule carefully. In the structures below, the longest chain is six.

2. There may be hydrocarbon groups of atoms attached to the main carbon chain. These groups are called side chains. The name of the side chain is based on the number of carbons in the side chain and its name ends in -yl. Side chains are often called alkyl groups. The common unbranched side chain names are listed in the table above.

If there are side chains in the molecule, number the carbons from the end that gives the side chain the lowest number. Once you start to number from a specific end of the chain, continue numbering down the chain from that end. The side chain is assigned the number


of the carbon of the main chain to which it is attached. The compound may be drawn differently but will have the same name.

If the chain is short enough so that there is no doubt about the position of the side chain, a number is not used with the side chain.

3. If there are several side chains in the molecule, the number of the position of each side chain is included. If several of the same side chains are present, the prefixes di, tri, tetra, penta, hexa, etc. are attached to the name of the side chain. The side chains are listed in alphabetical order regardless of their number position or any prefixes attached to the name of the side chain. Halogen atoms are considered to be side chains and the names fluoro (F), chloro (Cl), bromo (Br), and iodo (I) are used.

(This is not 6-ethyl-7-ethylnonane. Start the numbering from the left so the side chains have lower numbers.)

(Start the numbering from the right.)


There are several branched side chains that have common names. The more important ones are shown below. The prefix iso refers to a side chain with a methyl group on the second carbon starting at the end of the chain opposite the point of attachment.

The standard name for these side chains chooses the longest continuous chain in carbon atoms in the side chain as the base name and then numbers and names the groups attached to it with the number one carbon being the point of attachment. The standard name and common name of some branched side chains are shown below.

Side Chain Standard Side Chain Name Common Side Chain Name

1-methylethyl Isopropyl

2-methylpropyl Isobutyl

1-methylpropyl Sec-butyl

1,1-dimethylethyl Tert-butyl

3-methylbutyl Isopentyl

Cycloalkanes


Carbon atoms can form rings as well as open chain molecules. The name is based on the number of carbons in the ring. The carbon atoms in the ring are numbered so the substituents have the lowest numbers.

Alkenes Alkenes contain a double bond and are referred to as unsaturated hydrocarbons. The π electrons could be available for bonding additional atoms. The suffix for alkenes is ene. Choose the longest continuous carbon chain that contains the double bond. Number the chain from the end that will give the double bond the lowest number. This sets the direction and order of the numbering in the whole molecule. The number of the double bond will be the number of the first carbon of the double bond starting from that end of the chain. The side chains are numbered and named as with alkanes. If the chain is short enough so that there is no doubt about the position of the double bond or the side chain, numbers are not used.


There are common names for some of the smaller alkenes using the side chain names for the numbers of carbons present and adding ene. These names are still used in industry and in naming the polymers made from these compounds. Polyethylene is the material in plastic food wrap and polypropylene is used in sports clothing.

Cycloalkenes The name is based on the number of carbons in the ring. The first carbon of the double bond is assumed to be the number one carbon. The second carbon of the double bond must be the number two carbon. The first double bond is not numbered. The carbon atoms in the ring are numbered in the direction so the substituents have the lowest numbers.

Alkadienes A compound may contain more than one double bond and are called dienes, trienes, etc. Choose the longest continuous carbon chain that contains the double bonds. Number the chain from the end that will give the double bonds the lowest numbers. This sets the direction and order of the numbering in the whole molecule.

Alkynes

Alkynes are unsaturated hydrocarbons containing a triple bond. The two sets of π electrons could be available for bonding additional atoms. The suffix for alkynes is yne.


Choose the longest continuous carbon chain that contains the triple bond. Number the chain from the end that will give the triple bond the lowest number. This sets the direction and order of the numbering in the whole molecule. The number of the triple bond will be the number of the first carbon of the triple bond starting from that end of the chain. The side chains are numbered and named as with alkanes. If the chain is short enough so that there is no doubt about the position of the triple bond, a number are not used for the triple bond.

The common name for ethyne is acetylene, a gas commonly used for welding.

Benzene compounds The formula of benzene is shown below.

While it appears to be a cyclohexatriene, it does not chemically behave like an alkene! The three π bonds simultaneously overlap to form a delocalized set of electrons that produces additional stability in the molecule and significantly different chemical properties. Benzene compounds are often referred to as aromatic compounds. The nomenclature of benzene and other aromatic compounds is complicated since many common names are used. We will consider only a few of the mono- and di-substituted compounds here. The prefixes ortho (the two groups are adjacent to each other on the ring), meta (the two groups are separated by one carbons), and para (the two groups are separated by two carbons and are on opposite sides of the ring) are used in the common names. The carbon atoms are numbered around the ring in the direction to give the substituents smaller numbers. In the structures below, the standard name is written first and the common name second.


(Note: If the benzene ring is viewed as a side chain, it is called a phenyl group.) The structures and names for some additional hydrocarbons are shown below. See if the names make sense using the rules discussed above.

Structure Standard Name (IUPAC) Common Name

butane

2,2 dimethylhexane

4-ethyl-3-methyloctane

2,2,3-trichlorobutane

6-methyl-2-butene

1-butyne

2-butyne


Functional Groups Containing Oxygen

Groups Containing Oxygen Compound General Formula Example Name Alcohol ethanol Ether methoxyethane

(methyl ethyl ether) Aldehyde

ethanal (acetaldehyde)

Ketone

propanone (dimethyl ketone)

Carboxylic acid

ethanoic acid (acetic acid)

Ester

ethyl ethanoate (ethyl acetate)

Alcohols

Alcohols are compounds that have an OH group (hydroxyl) bonded to hydrocarbon chains. (If the OH group is bonded to a carbon in an aromatic ring, the compound is a phenol and a different class of compound.) The OH group is covalently bonded to the carbon so alcohols do not ionize to produce hydroxide ions, OH-. The suffix for alcohols is ol. Choose the longest continuous carbon chain that contains the OH group. Number the chain from the end that will give the OH group the lowest number. This sets the direction and order of the numbering in the whole molecule. The number of the OH group will be the number of the carbon to which it is bonded. The side chains are numbered and named as with alkanes. If the chain is short enough so that there is no doubt about the position of the OH group, it is not given a number.

In the common naming system, the side chain name for that number of carbons and the word alcohol was used. Methanol was methyl alcohol, ethanol was ethyl alcohol, 1-propanol was propyl alcohol (or n-propyl alcohol where n meant normal – all the carbons bonded in a continuous chain), and 2-propanol was isopropyl alcohol (the alcohol in rubbing alcohol).


Ethers Ethers are compounds with two hydrocarbon (saturated, unsaturated, or aromatic) groups attached to an oxygen atom. The shorter chain group is given the suffix oxy and is combined with the hydrocarbon name of the other group. Numbers are added if the oxygen atom is not bonded to the end of each group.

The common naming system used the side chain name of each group (not necessarily in alphabetical order) and the word ether. In the structures below, the standard name is given first with the common name second.

Aldehydes

Aldehydes are the first member of the family of compounds that contain a carbon atom double bonded to an oxygen atom, C=O, which is called a carbonyl group. The aldehyde has one hydrocarbon group and one hydrogen atom attached to the

carbonyl group. The aldehyde group is The suffix for aldehydes is al. Choose the longest continuous carbon chain that contains the aldehyde group. The aldehyde group must be at the end of the chain or it will not have a hydrogen atom attached to the carbonyl group and will be a different functional group. The carbon of the aldehyde group is the number one carbon and is not given a number in the name. This sets the direction and order of the numbering in the whole molecule. The side chains are numbered and named as with alkanes. The common name of the aldehyde was based on the common name of the carboxylic acid to which it could be oxidized and the word aldehyde was added. There is no simple way to get from the standard name to the common name without knowing the common names of the acids. Formaldehyde and acetaldehyde are still commonly used. In the structures below, the standard name is given first with the common name second.


Ketones

Ketones have two hydrocarbon groups attached to the carbonyl group, C=O. The suffix for ketones is one. Choose the longest continuous carbon chain that contains the C=O group. Number the chain from the end that will give the C=O group the lowest number. This sets the direction and order of the numbering in the whole molecule. The number of the C=O group will be the number of the carbon of the carbonyl group. The side chains are numbered and named as with alkanes. If the chain is short enough so that there is no doubt about the position of the C=O group, it is not given a number. The common naming system used the side chain name of each group (not necessarily in alphabetical order) and the word ketone. In the structure below, the standard name is given first with the common name second. In the case of propanone, acetone is the most commonly used name for the compound and is derived from the fact that it can be oxidized to acetic acid. In the structures below, the standard name is given first with the common name second.

Carboxylic acids

Carboxylic acids have a hydrocarbon group attached to the carboxylic acid

group, . The suffix for carboxylic acid is oic acid. Choose the longest continuous carbon chain that contains the carboxylic acid group. The carboxylic acid group must be at the end of the chain. The carbon of the carboxylic acid group is the number one carbon and is not given a number in the name. This sets the direction and order of the numbering in the whole molecule. The side chains are numbered and named as with alkanes. Many carboxylic acids occur in plants and animals. The common name of the carboxylic acid was based on the natural source of the acid (often a derivative of a Latin


name). Formic acid is found in ants and acetic acid is found in vinegar. In the structures below, the standard name is given first with the common name second.

An NH2 group can be present as a substituent and is called an amino group. The combination of the two groups forms the important biological family of compounds, the amino acids.

Functional Derivatives of Carboxylic Acids Additional classes of compounds are formed by the reaction of carboxylic acids with other compounds. These are called functional derivatives of carboxylic acids. Esters Esters are formed by the reaction of a carboxylic acid and an alcohol. Esters account for the odors of many flowers and fruits.

The group that came from the alcohol and is attached to the oxygen in the ester is named first with the same name as a side chain and ends in yl. The rest of the molecule is named in the same manner as the carboxylic acid except the ending is changed from oic acid to


oate. The common name for the ester uses the common name for the acid. In the structures below, the standard name is listed first with the common name second.

Amides Amides are formed by the reaction of an acid chloride or acid anhydride and ammonia (NH3), a primary amine (RNH2), or a secondary amine (R2NH).

There can be two hydrogen atoms, one hydrocarbon group, or two hydrocarbon groups bonded to the nitrogen. If two hydrocarbon groups are bonded the nitrogen, they can be the same or different groups.

The groups that came from the amine and are attached to the nitrogen are named first with the same name as a side chain and end in yl. The rest of the molecule is named in the same manner as the carboxylic acid except the ending is changed from oic acid to amide. The common name for the amide uses the common name for the acid. In the structures below, the standard name is listed first with the common name second.

Acid Chloride Acid chlorides are formed by the reaction a carboxylic acid with thionyl chloride, SOCl2. Acid chlorides can be used to make esters and amides.


The compound is named in the same manner as the carboxylic acid except the ending is changed from oic acid to oyl chloride. The common name for the acid chloride uses the common name for the acid. In the structures below, the standard name is listed first with the common name second.

Acid Anhydrides Acid anhydrides are produced from carboxylic acids by a dehydration reaction. They are usually formed from two molecules of the same acid. Acid anhydrides can be used to make esters and amides.

The compound is named in the same manner as the carboxylic acid except the ending is changed from oic acid to oic anhydride. The common name for the acid anhydride uses the common name for the acid. In the structure below, the standard name is listed first with the common name second.

The structures and names for some additional oxygen containing are shown below. See if the names make sense using the rules discussed above.



1-butanol Butyl alcohol

2-butanol (sec-butyl alcohol)

4-methyl-2-pentanol

Ethoxyethane Diethylether

2-methoxybutane (methyl sec-butyl ether)

Methanal Formaldehyde

Ethanal Acetaldehyde

4-methylhexanal

6-methyl-3-heptanone

4-chloropentanoic acid

Benzoic acid

Methyl propanoate Methyl propionate

Methyl benzoate

Ethanamide Acetamide

Dimethyl methanamide Dimethyl formamide

Functional Groups Containing Nitrogen Amines

Amines are derivatives of ammonia (NH3) formed by replacing the hydrogen atoms with hydrocarbon groups. Amines are classified by how many hydrocarbon groups are present: primary amine (RNH2), secondary amine (R2NH), or


tertiary amine (R3N). The hydrocarbon groups can be the same or different. Choose the longest continuous carbon chain that contains the NH2 group. Number the chain from the end that will give the NH2 group the lowest number. This sets the direction and order of the numbering in the whole molecule. The number of the NH2 group will be the number of the carbon to which it is bonded. The side chains are numbered and named as with alkanes. If the chain is short enough so that there is no doubt about the position of the NH2 group, it is not given a number. The suffix for amines is amine. The prefix N- is used for each substituent. The common name uses the names of the side chains and the word amine. The hydrocarbon groups are named as a side chain and end in yl. The groups are not necessarily in alphabetical order. In the structures below, the standard name is given first with the common name second.

Nitro Compounds The nitro group, NO2, is named as a side chain and its position on the hydrocarbon chain is numbered if necessary.

Nitriles The nitrile group contains a carbon atom triply bonded to a nitrogen atom,

. Nitrile names are based on the number of carbon atoms present and the suffix nitrile is added. Nitriles can be reacted with water to produce carboxylic acids. The common name for nitriles is based on the common name of the carboxylic acid to which it can be hydrolyzed. In the structure below, the standard name is listed first with the common name second.


The structures and names for some additional oxygen containing are shown below. See if the names make sense using the rules discussed above.


Methanamine Methylamine

2-propanamine Isopropylamine

1,1-dimethylethanamine t-butylamine

2,4,N,N-tetramethyl-3-hexanamine

Propanenitrile propionitrile

1,4-dinitrobenzene p-dinitrobenzene

The material presented above should allow you to name many common organic compounds or to derive a structure from its name.

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