Vivian Marlowe
Carboxylic acids are organic compounds that are widely found in nature, often combined with functional groups such as alcohols, aldehydes, or ketones. The process of naming carboxylic acids involves two primary systems: the IUPAC system and the common naming system. The IUPAC system, which stands for International Union of Pure and Applied Chemistry, assigns a characteristic suffix to these compounds by replacing the 'e' ending of the parent chain with 'anoic acid'. On the other hand, the common naming system is rooted in historical usage, with names that do not always follow IUPAC conventions. For instance, ethanoic acid is the IUPAC name for acetic acid, its more common moniker. This paragraph serves as an introduction to the topic of how to name a carboxylic acid with an alcohol, providing an overview of the nomenclature systems and the role of carboxylic acids in organic compounds.
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What You'll Learn
The IUPAC naming system for carboxylic acids
Carboxylic acids can be named using the IUPAC system or common names. The IUPAC naming system for carboxylic acids involves modifying the root name of the alkane by replacing the suffix '-e' with '-oic acid'. For example, while ethanoic acid is the IUPAC name, it is more commonly known as acetic acid. The common naming system is rooted in historical usage, as carboxylic acids were among the first organic compounds studied.
In the IUPAC system, the carboxylic group is given the highest nomenclature priority. This means that the carboxyl group is given the lowest possible location number and the appropriate nomenclature suffix is included. The –e ending is removed from the name of the parent chain and is replaced with '-anoic acid'. A carboxylic acid group must always lie at the end of a carbon chain, so it is always given the #1 location position in numbering and it is not necessary to include it in the name.
In the case of molecules containing carboxylic acid and alcohol functional groups, the OH is named as a hydroxyl substituent. However, the 'l' in hydroxyl is generally removed. In molecules containing a carboxylic acid and aldehydes and/or ketones functional groups, the carbonyl is named as an "oxo" substituent. When a carboxyl group is added to a ring, the suffix '-carboxylic acid' is added to the name of the cyclic compound. The ring carbon attached to the carboxyl group is given the #1 location number.
For dicarboxylic acids, the location numbers for both carboxyl groups are omitted because both functional groups are expected to occupy the ends of the parent chain. The ending '-dioic acid' is added to the end of the parent chain. When carboxylic acids are included with an alkene, the following order is followed: (Location number of the alkene)-(Prefix name for the longest carbon chain minus the -ane ending)-(an –enoic acid ending to indicate the presence of an alkene and carboxylic acid).
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Common names for carboxylic acids
Carboxylic acids are organic acids characterized by a carboxyl (-COOH) functional group. The naming of these compounds is governed by IUPAC nomenclature, which ensures systematic and consistent naming of chemicals. However, many carboxylic acids are also known by their common names, often derived from Latin and Greek words that describe their source. Here are some examples of common names for carboxylic acids:
Formic Acid
The simplest carboxylic acid, formic acid (HCOOH), was first obtained by the distillation of ants (from the Latin "formica," meaning "ant"). It is also found in the bites of some ants, as well as in the stings of wasps and bees.
Acetic Acid
Pure acetic acid, sometimes called concentrated acetic acid, is also known as glacial acetic acid. It is a common component of vinegar.
Propionic Acid
Propionic acid (CH3CH2COOH) is rarely encountered in everyday life. It has three carbon atoms: C–C–COOH.
Butyric Acid
Butyric acid (CH3CH2CH2COOH) is known for its extremely foul smell. It is found in rancid butter and is one of the components of body odor.
Valeric Acid
Valeric acid, also known as valeraldehyde, is the common name for the straight-chain carboxylic acid with five carbon atoms.
Benzoic Acid
Benzoic acid (C6H5COOH) is the acid with the carboxyl group attached directly to a benzene ring.
Lactic Acid
Lactic acid is the common and preferred name for what systematic rules call 2-hydroxypropanoic acid.
Other Common Names
Other carboxylic acids with common names include citric acid, found in lemons, and oxalic acid, found in spinach.
IUPAC Naming
It is worth noting that the IUPAC naming system for carboxylic acids involves replacing the alkane suffix with "oic acid." For example, the root name for a two-carbon alkane (ethane) becomes ethanoic acid, which is the IUPAC name for acetic acid.
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How to name carboxylic acids with an alkene
Carboxylic acids are given the highest nomenclature priority by the IUPAC system. This means that the carboxyl group is given the lowest possible location number and the appropriate nomenclature suffix is included. The general rule for naming carboxylic acids is to identify the longest carbon chain and determine the appropriate suffix based on functional groups. The –e ending is removed from the name of the parent chain and is replaced with -anoic acid.
When carboxylic acids are included with an alkene, the following order is followed: (Location number of the alkene)-(Prefix name for the longest carbon chain minus the -ane ending)-(an –enoic acid ending to indicate the presence of an alkene and carboxylic acid). Remember that the carboxylic acid has priority, so it should get the lowest possible location number. Also, remember that cis/trans or E/Z nomenclature for the alkene needs to be included if necessary.
For example, let's consider a molecule with an alkene at C3 and a carboxylic acid at C1 of a 4-carbon chain. The location number of the alkene is 3, the prefix name for the longest carbon chain minus the -ane ending is "but," and the ending to indicate the presence of an alkene and carboxylic acid is -enoic acid. Therefore, the IUPAC name for this molecule is 3-butenoic acid.
Another example is a molecule with an alkene at C2 and a carboxylic acid at C1 of a 5-carbon chain. In this case, the location number of the alkene is 2, the prefix name for the longest carbon chain minus the -ane ending is "pent," and the ending to indicate the presence of an alkene and carboxylic acid is -enoic acid. So, the IUPAC name for this molecule is 2-pentenoic acid.
It is important to note that the common naming system for carboxylic acids is also used, which is rooted in historical usage. This system often uses names that do not follow IUPAC conventions. For instance, while the IUPAC name for acetic acid is ethanoic acid, it is more commonly known as acetic acid. Therefore, it is beneficial to memorize the first five carboxylic acids based on the number of carbon atoms and their common names: formic (1 carbon), acetic (2 carbons), propionic (3 carbons), butyric (4 carbons), and valeric acid (5 carbons).
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The carboxyl group's location number
The carboxyl group location number is a crucial aspect of naming carboxylic acids. In the IUPAC nomenclature system, the carboxyl group is assigned the lowest possible location number, typically #1, as it must always be at the end of a carbon chain. This location number is omitted from the name since it is understood that the carboxyl group occupies the #1 position.
The priority of the carboxyl group in nomenclature means that it defines the parent chain and gives the corresponding suffix. For example, if the parent chain is heptane, the presence of a carboxyl group results in "heptanoic acid." The suffix "-anoic acid" is added to the root name of the alkane, with the "-e" ending of the alkane name being replaced.
In the common naming system, the location of the carboxyl group is indicated using Greek letters. The carbon adjacent to the carbonyl carbon is designated as alpha (α), the next carbon is beta (β), followed by gamma (γ), delta (δ), and so on. This approach is unique to common names and differs from the systematic IUPAC method, which uses numerical locants.
When a carboxyl group is added to a ring structure, the suffix "-carboxylic acid" is appended to the name of the cyclic compound. The carbon atom attached to the carboxyl group is designated as #1, and the numbering continues around the ring to minimise the numbering of substituents.
Additionally, when dealing with carboxylic acids that have substituents, the numbering starts from the carbon attached to the carboxyl group and progresses in a direction that minimises the locant numbers of the substituents. This ensures clarity and consistency in naming organic compounds.
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Greek letters indicating the position of substituents
In organic chemistry, Greek letters are commonly used to denote the position of substituents in carboxylic acids. The carbon atom adjacent to the carbonyl group is labelled using the Greek letter "alpha" or "α". Moving further away from the carbonyl group, the next carbon atom is labelled "beta" or "β", and so on. This nomenclature system is particularly useful for depicting different kinds of substituted carbonyl groups. For example, a ketone with an OH group on the beta carbon would be called a "β-hydroxy ketone". If the OH group were one carbon further down, it would be a "γ-hydroxy ketone".
The use of Greek letters is especially relevant when discussing the alpha carbon in carbonyl compounds. The alpha carbon is significant because it can participate in various reactions, such as aldol and Claisen condensations, and is involved in the unique reactions of the alpha carbon, such as the special nature of the alpha carbon in biochemistry.
In the context of carboxylic acids, the Greek letter alpha is commonly used to refer to the carbon atom adjacent to the carboxyl group. This carbon atom is crucial because it forms a double bond with an oxygen (O) atom and a single bond with a hydroxyl group (OH). The carbonyl group (C=O) and the hydroxyl group together give the carboxyl group (COOH) its name.
It is worth noting that the IUPAC system of nomenclature assigns a characteristic suffix to carboxylic acids. The -e ending of the parent chain is replaced with -anoic acid. Additionally, the carboxyl group is given the highest priority, so it is assigned the lowest possible location number, typically #1, and its position is indicated by the number of the carbon atom to which it is attached.
When naming molecules containing carboxylic acid and alcohol functional groups, the OH group is named as a hydroxyl substituent, with the l in hydroxyl typically omitted. This system of nomenclature helps to standardize the naming of carboxylic acids and ensures that the position of substituents is clearly indicated, whether through numerical carbon numbering or the use of Greek letters.
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Frequently asked questions
The IUPAC system of nomenclature assigns a characteristic suffix to these classes. The –e ending is removed from the name of the parent alkane chain and is replaced with -anoic acid. For example, ethane becomes ethanoic acid.
The common naming system is rooted in historical usage. This system often uses names that do not follow IUPAC conventions. For instance, while ethanoic acid is the IUPAC name, it is more commonly known as acetic acid.
Some common names for carboxylic acids are formic (1 carbon), acetic (2), propionic (3), butyric (4), and valeric acid (5).
In molecules containing carboxylic acid and alcohol functional groups, the OH is named as a hydroxyl substituent. However, the l in hydroxyl is generally removed.
