Nomenclature and Structure of Aldehydes and Ketones

We know that the solution used to preserve biological species is formaldehyde. But do we know that formaldehyde is scientifically known as methanal? Similarly, the active ingredient in nail polish removers is acetone, which is scientifically known as \(2-\)propanone or dimethyl ketone. But why is formaldehyde called methanal and acetone as \(2-\)propanone or dimethyl ketone?

Both methanal and acetone are carbonyl compounds and consists of the aldehyde \((-\rm{CHO})\) and ketone \(({\rm{R}}_2 {\rm{C = O}})\) functional group. These compounds have their trivial or common names mostly derived from their source of isolation. However, their scientific names are based on a set of rules defined by IUPAC. In this article, let’s learn everything about the Nomenclature & Structure of Aldehydes and Ketones in detail.

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Aldehyde

Aldehydes are the organic compounds that consist of the aldehydic \((-\rm{CHO})\) functional group. The aldehyde functional groups comprise a carbon atom attached to an oxygen atom through a double bond and to a hydrogen atom through a single bond. The general structural formula of aldehydes is \(\rm{R-CHO}\), where \(\rm{R}\) refers to the alkyl or aryl group.

An aldehyde group always lies at the end of a carbon chain. The general molecular formula of the homologous aldehyde series is \({{\text{C}}_{\text{n}}}{{\text{H}}_{{2{\text{n}} + 1}}}{\text{CHO}}\), where \(\rm{n} = 1,\,2,\,3…\)

The carbonyl carbon atom attached to an oxygen atom through a double bond and to a hydrogen atom through a single bond is \(\rm{sp}^2\) hybridised.

Hybridisation of the aldehydic carbon atom \(=\) Number of atoms attached \(+\) Lone pairs

\(= 3 + 0 = 3\left( {{\rm{s}}{{\rm{p}}^2}} \right)\)

The \(\rm{sp}^2\) hybridised carbon forms \(3\) sigma bonds (one \(\rm{C-H}\) sigma bond, one \(\rm{C-R}\) sigma bond, and one \(\rm{C-O}\) sigma bond). The formation of \(3\) sigma bonds gives aldehydes a basic trigonal shape with bond angles of \(120\) degrees. Only two out of three \(\rm{p}\) orbitals of carbon participate in hybridisation. One \(\rm{p}\) orbital is unhybridized, which forms a pi bond with the unhybridised \(\rm{p}\) orbital of the oxygen atom. This \(\rm{p}\) orbital is directed above and below the plane of the paper.

The Naming of Aldehydes

Let’s learn the naming of aldehyde in detail:

Common Names of Aldehydes

Aldehydes derive their common names from their corresponding carboxylic acids. The ‘–ic acid’ ending of carboxylic is replaced with -aldehyde. These names also reflect the Latin or Greek term for the original source of the acid or aldehyde.

The common names of substituted aldehydes are often designated with Greek letters. The carbon next to the \(-\rm{CHO}\) group is called the \(\alpha\) carbon, followed by \(\beta\) (beta), \(\gamma\) (gamma), \(\delta\) (delta), etc.
For example:

IUPAC System of Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) system assigns a characteristic suffix to the aldehydic compounds. The -ane suffix of the parent hydrocarbon chain is changed to “\(\rm{al}\)” dropping the “\(\rm{e}\)” of the parent alkane name. Some basic IUPAC rules for naming aldehydes are:

1. The longest carbon chain of carbon atoms containing the \(-\rm{CHO}\) group is chosen as the parent compound.
2. The aldehyde \((-\rm{CHO})\) functional group is always present at the end of the hydrocarbon chain; hence it occupies the #1 locant in numbering, and it is not necessary to include it in the name.

3. The aldehyde functional group has a higher priority over alkyl groups, halogen substituents, as well as double and triple bonds in the numbering of the parent chain.
\( – {\text{COOH}}, – {\text{S}}{{\text{O}}_3}{\text{H}}, – {\text{COOR}}\left( {{\text{R}} = {\text{alkyl}}\;{\text{group}}} \right),{\text{COCl}}, – {\text{CON}}{{\text{H}}_2}, – {\text{CN}}, – {\text{HC}}\)
\(= {\text{O}}, > {\text{C}} = {\text{O}}, -{\text{OH}}, -{\text{N}}{{\text{H}}_2}, > {\text{C}} = {\text{C}} < ,- {\text{C}} \equiv {\text{C}} – \)
4. The ‘\(-\rm{e}\)’ suffix of the parent hydrocarbon alkane chain is replaced by ‘\(-\rm{al},\)’ and the number indicating the \(-\rm{CHO}\) group’s position is not mentioned in the name of the parent hydrocarbon.

The naming of aldehyde groups in the presence of other functional groups

1. For substituents or functional groups ranking below aldehydes, the parent chain name is defined by the suffix -anal. The groups standing below in the functional group priority table are added as a prefix.
   (substituent) Prefix – PARENT CHAIN – anal (suffix)
   [Lower priority]                                [highest priority]

For substituents present in the aldehyde parent chain, the numbering of the chain is done to give the substituents the lowest number at the first point of difference.

1. In molecules containing aldehyde and alcohol functional groups, the \(-\rm{OH}\) group acts as a substituent (low priority than \(-\rm{CHO})\). It is named as a hydroxy substituent, as shown in the above table

2. In molecules containing a carboxylic acid and aldehyde functional group, carboxylic acid ranks higher than the aldehyde group; hence the parent chain is defined by the suffix -oic acid.
Two cases arise while naming molecules containing a carboxylic acid and aldehyde functional group:
a. If \(-\rm{CHO}\) group is not a part of the parent chain, then it is indicated by “formyl”.
b. If \(-\rm{CHO}\) group is a part of the parent chain, then it is indicated by “oxo”.

3. In molecules containing an aldehyde and amine functional group, the amine is named as an “amino” substituent.

4. In the case of molecules containing an aldehyde linked to an alkene, the following order is followed:

(Location number of the alkene)-(Prefix name for the longest carbon chain minus the -ane ending)-(add -enal ending to indicate the presence of an alkene and an aldehyde group)

The aldehyde group has a higher priority over alkene (double bond), so it should get the lowest possible locant number. In the case of alkenes, cis/trans or E/Z nomenclature should be included if required. It is unnecessary to specify the carboxyl group’s location because it will automatically be carbon #1.

Naming aldehydes added to a ring

6. When the aldehyde group is added to a ring, the suffix -carbaldehyde is added to the name of the cyclic compound. The ring carbon attached to the carboxyl group is given the #1 location number.

7. If substituents are also present, the numbering starts from the carbon connected to the \(\rm{CHO}\) group. It goes either in a clockwise or anticlockwise direction that minimizes the numbering of the substituents:

8. The compound is named benzaldehyde if the aldehyde group is directly attached to an aromatic ring.

a. In a substituted aromatic aldehydic compound, the name of the compound is derived from benzaldehyde. The aromatic ring carbon that carries the aldehyde group is marked by the position number “1”. The numbering starts from the carbon connected to the \(\rm{CHO}\) group. It goes either in a clockwise or anticlockwise direction that minimizes the numbering of the substituents

b. If the \(-\rm{CHO}\) group is not directly attached to the benzene ring, the aldehyde is named as an aryl derivative of the corresponding aldehyde.

Naming dialdehydes

9. For dialdehydes, the location numbers for both aldehyde groups are omitted because both functional groups are expected to occupy the ends of the parent chain. The ending –dial is added to the end of the parent chain.

Ketones

Ketones are the organic compounds that consist of the ketonic \(({\rm{R}}_2 {\rm{C}} = 0)\) functional group. The ketonic functional groups constitute a carbon atom known as the carbonyl carbon, which is attached to an oxygen atom through a double bond. The remaining two bonds of the carbonyl atom are bonded to other carbon atoms or hydrocarbons. The general structural formula of ketones is \(\rm{R-CO-R}\), where \(\rm{R}\) and \(\rm{R}’\) refer to the alkyl or aryl group.

The ketonic functional group always lies within the carbon chain. Hence, the simplest compound with the ketonic functional group is acetone. The structure is shown below.

The carbonyl carbon atom attached to the oxygen atom through a double bond and to alkyl groups on either side is \(\rm{sp}^2\) hybridized.

Hybridisation of the ketonic carbon atom \(=\) Number of atoms attached \(+\) Lone pairs

\(= 3 + 0 = 3(\rm{sp}^2)\)

The \(\rm{sp}^2\) hybridised carbon forms \(3\) sigma bonds (two \(\rm{C-C}\) sigma bonds and one \(\rm{C-O}\) sigma bond). The formation of \(3\) sigma bonds gives ketones a basic trigonal shape with bond angles of \(120\) degrees. Only two out of three p orbitals of carbon participate in hybridisation. One \(\rm{p}\) orbital is unhybridized, which forms a pi bond with the unhybridised \(\rm{p}\) orbital of the oxygen atom. This \(\rm{p}\) orbital is directed above and below the plane of the paper.

The Naming of Ketones

Common Names of Ketones

The common names for ketones are similar to ethers and are derived by naming each alkyl or aryl group bonded to the carbonyl atom as a separate word followed by the word “ketone”. The attached alkyl groups are arranged in the name alphabetically.

The common names for substituted ketones are derived by using Greek letters, \({\rm{\alpha \alpha ‘}}\,{\rm{,\beta \beta ‘}}\) and so on beginning with the carbon atoms next to the carbonyl group, indicated as \(αα′\).

IUPAC System of Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) system assigns a characteristic suffix to the ketonic compounds. The -ane suffix of the parent hydrocarbon chain is changed to “one.” Some basic IUPAC rules for naming ketones are:

1. The ketone functional group is always present within the parent chain. Hence, the longest carbon chain of carbon atoms containing the ketonic group is chosen as the parent compound.
2. The -ane suffix of the parent hydrocarbon alkane chain is replaced by -one, and the number indicating the ketone group’s position is necessary to mention in the name of the parent hydrocarbon.

3. The ketone functional group is always present within the hydrocarbon chain; hence the numbering is done to give the ketonic functional group the lowest locant.
4. The locant indicating the position of the carbonyl group should be mentioned and can be placed before the parent or before the suffix “one.”  Both names are acceptable according to the IUPAC recommendations. i.e. butan\(-2-\)one or \(2-\)butanone would be suitable names.

5. The ketonic functional group has a priority lower than aldehydes. However, if it happens to be that the ketone is the highest priority in the molecule, only then the suffix changes to “one”.
\( – {\text{COOH}}, – {\text{S}}{{\text{O}}_3}{\text{H}}, – {\text{COOR}}\left( {{\text{R}} = {\text{alkyl}}\;{\text{group}}} \right),{\text{COCl}}, – {\text{CON}}{{\text{H}}_2}, – {\text{CN}}, – {\text{HC}}\)
\(= {\text{O}}, > {\text{C}} = {\text{O}}, – {\text{OH}}, – {\text{N}}{{\text{H}}_2}, > {\text{C}} = {\text{C}} < , – {\text{C}} \equiv {\text{C}} – \)

Naming of ketone groups in the presence of other functional groups

6. For substituents or functional groups ranking below ketones, the parent chain name is defined by the suffix -one. The groups standing below in the functional group priority table are added as a prefix.
(substituent) Prefix – PARENT CHAIN – one (suffix)
[Lower priority]                                [highest priority]

In substituted ketones, the numbering of the parent chain is done to give the substituents the lowest number at the first point of difference.

i. In molecules containing ketone and alcohol functional groups, the \(-\rm{OH}\) group acts as a substituent (low priority than ketones). It is named as a hydroxy substituent, as shown in the above table.

• ii. In molecules containing a carboxylic acid and ketone functional group, carboxylic acid ranks higher than ketone group, hence the parent chain is defined by the suffix ‘-oic acid’ and the ketone group acts as a substituent with ‘oxo’ prefix. The numbering of the parent chain is done to give the carboxylic group lowest locant.

iii. In the case of molecules containing an ketone linked to an alkene, the following order is followed:

(Location number of the alkene)-(Prefix name for the longest carbon chain replacing -ane with -en indicating the presence of an alkene)-(locant number)-(add -one ending to indicate the presence of ketone functional group)

The ketone functional group has a higher priority over alkene (double bond), so it should get the lowest possible locant number. In the case of alkenes, cis/trans or E/Z nomenclature should be included if required. It is necessary to specify the carboxyl group’s position because ketones are present within the molecule.

Naming ketones added to a ring

7. When naming a cyclic ketone, the ring carbon atom attached to the carbonyl group is given the #1 location number.

8. If substituents are also present, the numbering starts from the carbon connected to the ketone group. It goes either in a clockwise or anticlockwise direction that minimizes the numbering of the substituents:

Naming diketones

9. For diketones, the location numbers for both ketone groups are necessary to mention. The ending –dione is added to the end of the parent chain.

Solved Example on Ketone

Name the Ketone:

The parent chain consists of six carbon atoms and is hexane. The \(-\rm{CHO}\) group forms a part of the parent chain and hence, its base name is hexanal. The ketone ranks lower to aldehyde and hence is considered as substituents and get a prefix. For ketones, the prefix “oxo” is used along with its position.

Hence, the IUPAC name of the compound is:

Summary

Aldehydes and ketones are an important class of compounds that has a wide range of applications. The cinnamon flavour of cinnamon roll and vanilla flavour of ice creams is due to cinnamaldehyde and vanillin, respectively. Vanillin is a phenolic aldehyde. The jasmine fragrance is due to \({\rm{Z}}\)-jasmone, a ketone. Due to its widespread usage, it is important that we learn the methods and rules of naming these aldehydes and ketones. In this article, we learned the IUPAC nomenclature as well as common names of some aldehydes and ketones.

FAQs on Nomenclature and Structure of Aldehydes and Ketones

Q.1. What is the general formula of ketone?
Ans: The general formula of ketones is \({{\text{C}}_{\text{n}}}{{\text{H}}_{{\text{2n}}}}{\text{O}}\), where \(\rm{n} = 1,\,2,\,3…\). The simplest ketone is of \(3\) carbon atoms, so its formula will be, \({{\text{C}}_{\text{3}}}{{\text{H}}_{\text{6}}}{\text{O}}\) also known as propanone. It can also be written as \({\text{C}}{{\text{H}}_3}{\text{COC}}{{\text{H}}_3}\) the carbon of the ketone is counted with the carbon of the hydrocarbon.

Q.2. How do you name a ketone?
Ans: Ketones are named by finding the carbonyl group with its location number. If the substituents rank lower than ketone, then the ‘-one’ is added to the parent name with its locant number. However, with functional groups that rank higher than ketone, the prefix ‘-oxo’ is added along with its locant number.

Q.3. How can you tell the difference between aldehydes and ketones?
Ans: Both aldehyde and ketones possess a carbonyl group, which is a carbon double bonded to oxygen. An aldehyde has at least one hydrogen connected to the carbonyl carbon. The second group is either a hydrogen or a carbon-based group. In contrast, a ketone has two carbon-based groups connected to the carbonyl carbon.

Q.4. Which is more polar aldehyde or ketone?
Ans: Ketones are more polar than aldehydes. The polarity of ketones is due to the electron-donating inductive effect of two alkyl groups. Since aldehydes have one less alkyl group, polarity is lowered.

Q.5. Which test is used to identify ketones?
Ans: Tollens’ reagent test, also known as the silver-mirror test, is a qualitative laboratory test used to distinguish between an aldehyde and a ketone.

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