Problem 48 - Chemistry: The Central Science in SI Units, Global Edition

Question

Aldehydes and ketones can be named in a systematic way by counting the number of carbon atoms (including the carbonyl carbon) that they contain. The name of the aldehyde or ketone is based on the hydrocarbon with the same number of carbon atoms. The ending -al for aldehyde or-one for ketone is added as appropriate. Draw the structural formulas for the following aldehydes or ketones: (a) 3-ethyl-2-pentanone, (b) 2,2 -diethylbutanal, (c) pentanal, (d) 1-bromo-3-heptanone.

Step-by-Step Solution

Answer

The structural formulas for the given aldehydes and ketones are as follows: (a) 3-ethyl-2-pentanone: \( \text{CH}_3\text{C}(=O)\text{CH}_2\text{CH}(\text{C}_2\text{H}_5)\text{CH}_3\). (b) 2,2-diethylbutanal: \( \text{CH}_3\text{CH}(\text{C}_2\text{H}_5)_2\text{CH}_2\text{C}(=O)\text{H}\). (c) pentanal: \( \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{C}(=O)\text{H}\). (d) 1-bromo-3-heptanone: \( \text{CH}_2\text{Br}\text{CH}_2\text{C}(=O)\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_3\).

Step 1: (a) 3-ethyl-2-pentanone

: 1. Identify the parent chain: In this case, it is a pentanone (a ketone with a 5-carbon chain). 2. Write the structure of the parent chain: \( \text{CH}_3\text{C}(=O)\text{CH}_2\text{CH}_2\text{CH}_3\). 3. Identify the substituent: It is an ethyl group at the 3rd carbon. 4. Add the ethyl group to the parent chain: \( \text{CH}_3\text{C}(=O)\text{CH}_2\text{CH}(\text{C}_2\text{H}_5)\text{CH}_3\). The structural formula for 3-ethyl-2-pentanone is: \( \text{CH}_3\text{C}(=O)\text{CH}_2\text{CH}(\text{C}_2\text{H}_5)\text{CH}_3\).

Step 2: (b) 2,2 -diethylbutanal

: 1. Identify the parent chain: In this case, it is butanal (an aldehyde with a 4-carbon chain). 2. Write the structure of the parent chain: \( \text{CH}_3\text{CH}_2\text{CH}_2\text{C}(=O)\text{H}\). 3. Identify the substituents: Two ethyl groups at the 2nd carbon. 4. Add the ethyl groups to the parent chain: \( \text{CH}_3\text{CH}(\text{C}_2\text{H}_5)_2\text{CH}_2\text{C}(=O)\text{H}\). The structural formula for 2,2-diethylbutanal is: \( \text{CH}_3\text{CH}(\text{C}_2\text{H}_5)_2\text{CH}_2\text{C}(=O)\text{H}\).

Step 3: (c) pentanal

: 1. Identify the parent chain: In this case, it is pentanal (an aldehyde with a 5-carbon chain). 2. Write the structure of the parent chain: \( \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{C}(=O)\text{H}\). The structural formula for pentanal is: \( \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{C}(=O)\text{H}\).

Step 4: (d) 1-bromo-3-heptanone

: 1. Identify the parent chain: In this case, it is a heptanone (a ketone with a 7-carbon chain). 2. Write the structure of the parent chain: \( \text{CH}_3\text{CH}_2\text{C}(=O)\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_3\). 3. Identify the substituent: A bromo group at the 1st carbon. 4. Add the bromo group to the parent chain: \( \text{CH}_2\text{Br}\text{CH}_2\text{C}(=O)\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_3\). The structural formula for 1-bromo-3-heptanone is: \( \text{CH}_2\text{Br}\text{CH}_2\text{C}(=O)\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_3\).

Key Concepts

Naming Conventions in Organic Chemistry

Chemistry has its own language, and when it comes to organic molecules, naming conventions are like grammar rules. They ensure everyone understands the molecule being referenced. In organic chemistry, substances are often named based on their structure. This is crucial because it provides insight into the number of carbon atoms, their arrangement, and other groups attached to the molecule.

For ketones, the suffix "-one" is added. They are usually named by identifying the longest carbon chain containing the carbonyl group (\(C=O\)). For aldehydes, the suffix "-al" lets you know there's a carbonyl group at the end of a carbon chain.

• Start by identifying the parent chain, the longest continuous chain of carbon atoms.
• Look for functional groups; these are often added as prefixes or suffixes.
• Substituents, like branches off the main chain, are noted as prefixes, specifying their position on the chain. For example, "ethyl" indicates a 2-carbon substituent.

Understanding these basics helps in effectively communicating the structure and nature of various organic compounds.

Aldehydes

Aldehydes are a class of organic compounds characterized by the presence of a carbonyl group (\(C=O\)) attached to at least one hydrogen atom. You can spot an aldehyde through its molecular formula, which usually ends with "-al."

Here are some key points about aldehydes:

• The carbonyl group in an aldehyde is always located at the end of the carbon chain.
• The simplest aldehyde is formaldehyde (\( ext{HCHO}\)).
• Aldehydes are often more reactive compared to ketones because of the hydrogen atom linked to the carbonyl group.
• When naming an aldehyde, locate the carbon chain, count the number of carbons, and add '-al' as the ending.

Familiar aldehydes include acetaldehyde and benzaldehyde, both important in industrial applications and in the formation of fragrances and flavors.

Ketones

Ketones are another class of organic compounds featuring the carbonyl group. Unlike aldehydes, this carbonyl group is located between two carbon atoms. They are generally less reactive than aldehydes due to this structural arrangement.

To identify and name ketones, follow these guidelines:

• Find the longest carbon chain that includes the carbonyl group.
• The suffix "-one" denotes a ketone in a compound's name.
• Number the chain so that the carbonyl carbon gets the lowest number possible.
• Identify any substituents, adding them as prefixes with the appropriate locants.

Molecules like propanone (commonly known as acetone) and butanone are typical examples of ketones, well-known for applications in solvents and chemical manufacturing processes.

Structural Formulas of Organic Compounds

Structural formulas are graphical representations of organic molecules that show how atoms are arranged and bonded. Knowing how to interpret and draw them is a fundamental skill in organic chemistry.

Here are critical aspects of structural formulas:

• Each line represents a bond, with angles and line lengths often reflecting bond types.
• Imagine the 3D structure for more complex molecules, as linear structural formulas can become unwieldy.
• Arrange carbon atoms in a chain first, then add substituents and functional groups like hydroxyl (\(-OH\)) and carbonyl (\(C=O\)) groups.
• Lastly, verify that you've respected the valency of atoms by ensuring each forms the correct number of bonds.

By mastering structural formulas, you build the ability to understand and predict the structure, properties, and reactivity of organic compounds. They are foundational in organic chemistry and crucial for communicating complex molecular information efficiently.