Question

Describe the alternative system for naming ketones. Give two examples of ketones, along with their names in both systems.

Short answer

The alternative system for naming ketones is the common system, which names ketones by identifying the two alkyl groups bonded to the carbonyl carbon, arranging them alphabetically, and adding "ketone" at the end. The IUPAC system names ketones by identifying the longest continuous carbon chain containing the carbonyl group, numbering the carbons, and replacing the "-e" in the alkane name with "-one." Example 1: Propanone (IUPAC) or Dimethyl Ketone (Common) Example 2: 2-Butanone (IUPAC) or Ethyl Methyl Ketone (Common)

Step-by-step solution

IUPAC System for Ketones

In the IUPAC system, ketones are named by identifying the longest continuous carbon chain containing the carbonyl group (C=O). The ketone is then numbered to give the carbonyl group the lowest possible number. Next, drop the "-e" from the alkane chain's name and add "-one," indicating that it's a ketone.

Common System for Ketones

In the common system, ketones are named by identifying the two alkyl groups that are bonded to the carbonyl carbon and writing them in alphabetical order. The word "ketone" is then placed at the end of the name to indicate the presence of a carbonyl group.

Example 1: Propanone

In the IUPAC system: Step 1: Identify the longest continuous carbon chain: Propane Step 2: Number the carbons: 1, 2, 3. Step 3: Drop "-e" and add "-one": Propanone. In the common system: Step 1: Identify the two alkyl groups bonded to the carbonyl carbon: Methyl (CH3) - two of them Step 2: Arrange the alkyl groups in alphabetical order: Dimethyl Step 3: Add "ketone" at the end: Dimethyl Ketone.

Example 2: 2-Butanone

In the IUPAC system: Step 1: Identify the longest continuous carbon chain: Butane Step 2: Number the carbons: 1, 2, 3, 4. Step 3: Drop "-e" and add "-one" with the number of carbonyl group: 2-Butanone. In the common system: Step 1: Identify the two alkyl groups bonded to the carbonyl carbon: Methyl (CH3) and Ethyl (C2H5) Step 2: Arrange the alkyl groups in alphabetical order: Ethyl Methyl Step 3: Add "ketone" at the end: Ethyl Methyl Ketone.

Key concepts

IUPAC System for Ketones

When naming ketones using the International Union of Pure and Applied Chemistry (IUPAC) guidelines, one must first locate the longest carbon chain that includes the carbonyl group. The carbonyl group is critical in this naming process due to its significance in the molecular structure of ketones. Once the chain is identified, the carbons are numbered such that the carbonyl carbon receives the lowest possible number. The name of the compound is derived by replacing the terminal '-e' of the corresponding alkane name with '-one' for the ketone. For example, if the longest chain is four carbons long and the carbonyl group is on the second carbon, the ketone is named 2-butanone. It's important to note that if there are other substituents or groups attached, they must be included in the name with their appropriate locant numbers and names.

For clarity, let's review a simple ketone: propanone. In this scenario, the longest continuous carbon chain is 'propane', and since the carbonyl group is found on the middle carbon, it’s implicit that the numbering starts from the end nearest to this group. Next, 'propane' becomes 'propanone', clearly indicating this compound is a ketone.

Common System for Ketones

Alternatively, the common or traditional naming system for ketones involves referencing the two alkyl groups bound to the carbonyl carbon. The names of these alkyl groups are listed in alphabetical order, followed by the word 'ketone'. This method is grounded in the molecular structure of ketones, having the carbonyl group bonded to two hydrocarbon side chains. For example, if both sides of the carbonyl group are attached to methyl groups, the common name would be 'dimethyl ketone'. Whereas, if different groups are attached, such as a methyl and an ethyl group, the common name becomes 'ethyl methyl ketone'.

While the IUPAC system is more systematic and widely used in academic settings, the common system remains prevalent in many industrial and commercial contexts due to its straightforward and descriptive approach.

Carbonyl Group

Central to the identification and naming of ketones is understanding the carbonyl group. The carbonyl group, given by the formula \( C=O \), is a functional group consisting of a carbon atom double-bonded to an oxygen atom. It is this group that characterizes aldehydes, ketones, carboxylic acids, esters, and amides. In ketones, the carbonyl carbon is bonded to two other carbon atoms which means it appears within the carbon skeleton rather than at the end as it would in aldehydes. The chemical reactivity and properties of ketones are largely determined by this carbonyl group because the double bond is polar, making ketones susceptible to nucleophilic addition reactions.

The presence of the carbonyl group also affects boiling points and solubility: ketones generally have higher boiling points than alkanes but lower than alcohols of similar molecular weight, and they are soluble in organic solvents and water.

Molecular Structure of Ketones

Ketones are organic compounds characterized by the presence of the aforementioned carbonyl group (\( C=O \) between two carbon atoms within the carbon skeleton. This distinguishing trait impacts their physical and chemical behavior. When discussing the molecular structure, it's noted that the geometry around the carbonyl is trigonal planar, with bond angles of approximately 120 degrees. The R and R' groups attached to the carbonyl can be the same or different, and each brings its own influence on the overall molecule’s properties.

For visualization, take the case of propanone: it is composed of a three-carbon chain with the central carbon bonded to the oxygen of the carbonyl group. The simplicity or complexity of the ketone's molecular structure can affect the solubility, boiling point, and reactivity. For instance, bulky alkyl groups on either side of the carbonyl can impede solubility in water but could also lower reactivity to nucleophilic attack. Understanding these molecular structures is essential for students to predict reactivity and physical properties, as well as for correctly applying naming conventions.