Arielle Cruz
When naming a compound with two alcohol groups, it is important to understand the rules of nomenclature. The International Union of Pure and Applied Chemistry (IUPAC) has established a set of guidelines for naming compounds with multiple functional groups. The first step is to identify the functional groups and their priorities, with the group of the highest priority being the parent structure and determining the parent name. The other groups are then considered substituents and are named using appropriate prefixes. The next step is to number the parent chain, starting from the end closest to the highest priority group, and adding the substituents alphabetically. The suffix is used to indicate the name of the parent structure, while the prefix is for the substituent. In the case of alcohol groups, the suffix -ol is added to the name of the parent alkane, along with a number to indicate the location of the hydroxyl group. For compounds with two alcohol groups, the term diol is added to the parent alkane name.
| Characteristics | Values |
|---|---|
| Classification of alcohols | Based on which carbon atom is bonded to the hydroxyl group |
| Primary alcohol | When the hydroxyl group is on a primary carbon atom (1°), bonded to only one other carbon atom |
| Secondary alcohol | The hydroxyl group is on a secondary carbon atom (2°), bonded to two other carbon atoms |
| Tertiary alcohol | The hydroxyl group is on a tertiary carbon atom (3°), bonded to three other carbon atoms |
| Allylic alcohol | When the hydroxyl group is bonded to an allylic carbon atom (adjacent to a C=C double bond) |
| Benzylic alcohol | When the hydroxyl group is bonded to a benzylic carbon atom (next to a benzene ring) |
| Phenol | When a hydroxyl group is attached directly to a benzene ring |
| IUPAC naming | The name for an alcohol uses the -ol suffix with the name of the parent alkane, together with a number to indicate the location of the hydroxyl group |
| Common name | The name of the alkyl group with the word alcohol; if the alkyl group is complex, the IUPAC name should be used |
| Priority of functional groups | The functional group with the highest priority is the "parent structure" and determines the "parent name"; the other groups are regarded as "substituents" |
| Acid vs. alcohol | Acid gets priority, so the parent chain takes the suffix "oic acid" and the alcohol takes the prefix "hydroxy" |
| Ketone | Treated as a substituent with the prefix "oxo" |
| Halogen and ether | Can only be substituents with corresponding prefixes |
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What You'll Learn
- The IUPAC system: use the -ol suffix with the name of the parent alkane
- Common practice: classify alcohols by the carbon atom attached to the hydroxyl group
- Phenol: when a hydroxyl group is attached to a benzene ring
- Prioritising functional groups: the group with the highest priority is the 'parent structure'
- Numbering the parent chain: start with the highest priority group and add substituents alphabetically
The IUPAC system: use the -ol suffix with the name of the parent alkane
When naming a compound with two alcohol groups, the IUPAC system follows a specific set of rules. Firstly, identify the longest continuous chain (LCC) of carbon atoms containing the OH group. This chain forms the basis of the name and is known as the parent compound or parent alkane. The number of carbon atoms in this chain determines the prefix of the name. For example, a chain of three carbon atoms would be propane, while a chain of ten carbon atoms would be decane.
Once the parent alkane is identified, the next step is to locate the position of the hydroxyl groups (-OH) on the chain. The chain is numbered from the end nearest the OH group, and the number indicating the position of the OH group is prefixed to the name of the parent hydrocarbon. For example, if there are two OH groups on the first and fifth carbon atoms, the name becomes 1,5-pentanediol.
The suffix -ol is then added to the name of the parent alkane to indicate the presence of an alcohol group. This involves replacing the ending -e of the parent alkane with -ol. For example, the parent alkane propane becomes propanol with the addition of the -ol suffix. If there is more than one OH group in the molecule (polyhydroxy alcohols), suffixes such as "-diol" or "-triol" are used. For instance, the presence of three hydroxyl groups results in the suffix -triol, as seen in 1,2,3-propanetriol.
Additionally, when a hydroxyl group is attached directly to a benzene ring, the compound is named a "phenol." The parent compound is phenol, and the carbon atom attached to the OH group is designated C1, although the number is not included in the name. This system of nomenclature provides a consistent and systematic approach to naming compounds with multiple alcohol groups, ensuring clear communication and understanding in the field of chemistry.
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Common practice: classify alcohols by the carbon atom attached to the hydroxyl group
It is common practice to classify alcohols based on the carbon atom attached to the hydroxyl group. This classification system is especially useful when studying the reactions of a particular group.
Alcohols can be grouped into three classes based on the number of carbon atoms attached to the carbon atom with the OH group. A primary (1°) alcohol has its OH group attached to a carbon atom connected to only one other carbon atom. Its general formula is RCH2OH. A secondary (2°) alcohol has its OH group attached to a carbon atom connected to two other carbon atoms. Its general formula is R2CHOH. A tertiary (3°) alcohol has its OH group attached to a carbon atom connected to three other carbon atoms. Its general formula is R3COH.
When naming alcohols, the IUPAC system is often used. This involves using the -ol suffix with the name of the parent alkane and adding a number to indicate the location of the hydroxyl group. The longest carbon chain containing the OH group is named, the -e ending of the alkane is replaced with -ol, and the chain is numbered from the end nearest the OH group. For example, 3-hexanol indicates that the OH group is on the third carbon atom of the parent chain, which has six carbon atoms.
In addition to the IUPAC nomenclature, common names for alcohols are formed by combining the name of the alkyl group with the word "alcohol." For example, ethyl alcohol (ethanol) is the active ingredient in alcoholic beverages. However, if the alkyl group is complex, the IUPAC name is preferred.
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Phenol: when a hydroxyl group is attached to a benzene ring
Phenol is an organic compound formed by connecting a benzene ring and a hydroxyl group (OH). When a hydroxyl group is attached to a benzene ring, the compound is called a "phenol". The parent compound is phenol, and the carbon attached to the OH group is C1. Phenol is the simplest phenolic compound and is also known as monohydroxybenzene (C6H5OH), benzenol, or carbolic acid. It is a highly reactive compound that is widely used in household products and as an industrial intermediate.
Phenol is a major environmental contaminant, found in industrial wastewater and contributing to water pollution. It is toxic, corrosive, and carcinogenic. Phenol compounds slowly degrade, forming various by-products, many of which are of environmental concern due to their toxicity.
The functional group present on phenol is an alcohol (OH). Phenols are similar to alcohols but form stronger hydrogen bonds, making them more soluble in water and giving them higher boiling points. Phenols with only one other substituent can be named using either appropriate numbers or the ortho (1,2), meta (1,3), and para (1,4) system. For example, the systematic name for thymol, a phenolic compound, is 5-methyl-2-isopropylphenol.
Some common phenolic compounds include vanillin, salicylic acid, pyrocatechol, resorcinol, cresol, hydroquinone, and eugenol. Hydroquinone, for instance, has two hydroxyl groups in a specific relationship that makes it highly susceptible to oxidation. This property is useful in developing photographic film.
Phenols are also important in commercial applications. For instance, phenol is treated with sodium hydroxide to form phenoxide ions, which are used to make salicylic acid, a precursor to aspirin and methyl salicylate. Additionally, phenolic resins account for a large portion of phenol production, with phenol-formaldehyde resin being one of the earliest plastics.
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Prioritising functional groups: the group with the highest priority is the 'parent structure'
When a molecule contains multiple functional groups, there can be confusion about which group to name it after. To resolve this, a priority sequence or hierarchy of functional groups has been established by the international chemistry community (IUPAC, the International Union of Pure and Applied Chemistry). The rules for seniority can be found in the IUPAC Blue Book.
The first step is to find the parent chain or parent structure. This is based on the longest continuous carbon chain. If there are multiple chains of equal length, the one with the most branch points is selected as the parent. The parent chain gets the suffix. For example, if the parent chain is an alkane and the compound contains two alcohol groups, the parent name of the alkane is incorporated, and the term "diol" is added. The compound becomes 1,2-Ethanediol.
The next step is to number the parent chain, starting from the end nearer the first branch point. If there is branching an equal distance from both ends, then consider the next branch point. The functional group with the highest priority is given the lowest number. For example, between the acid and the alcohol, the former gets priority, so the alcohol takes a prefix (“hydroxy”).
Some functional groups have been deemed unworthy of ever getting their own suffixes. For nomenclature purposes, they are always subservient to the -ane, -ene, or -yne ending of the parent hydrocarbon. These groups include the halides, ethers, azide, and nitro functional groups.
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Numbering the parent chain: start with the highest priority group and add substituents alphabetically
When numbering the parent chain, the first step is to identify the functional group with the highest priority. This group will determine the parent name of the compound. Once the highest priority group is identified, the other groups are named as substituents using appropriate prefixes. The substituents are then added to the parent chain in alphabetical order.
It is important to note that some groups, such as halides, ethers, and nitro groups, are always considered substituents and are given prefixes. In the final name of the compound, these groups are placed in alphabetical order. For example, when numbering 2-bromo 3-nitro butane, bromine comes higher in the alphabet than nitro, so it is assigned the lower number.
When dealing with cyclic and acyclic compounds, prefixes are assigned in increasing numerical order. The substituent cited first as a prefix in the name is given the lowest locant. For example, in the compound "2,2-dimethyl-3-(3-isopropylphenyl)cyclopropanol", the benzene ring with an isopropyl group is called "3-isopropylphenyl", with "3-" indicating the position of the isopropyl group on the benzene ring.
Additionally, when an alkene and an alkyne are present in the same molecule, the ending will always be "-yne" as "-ene" comes before "-yne" alphabetically. However, when determining the lowest locant, the alkene takes priority if there is a tie between the two.
When numbering the parent chain, it is crucial to refer to the priority list of functional groups. This list determines the order of priority for the groups, with the highest priority group taking precedence. For example, in a compound containing both an alcohol and an amine group, the alcohol group has higher priority and is designated with a suffix, receiving the lower number.
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Frequently asked questions
Identify the longest carbon chain that contains the carbon atom bearing the OH group.
Drop the final -e from the alkane name and add the suffix -ol.
Number the longest carbon chain starting at the end nearest the OH group and use the appropriate number to indicate the position of the OH group.
Name the substituents and give their numbers as for an alkane or alkene.
