Question

Draw a structure for a compound that meets each of the following descriptions: (a) A 5 -carbon cyclic ketone. What is the approximate \(\mathrm{C}-\mathrm{O}\) bond angle in this ketone? (b) An 8-carbon ketone with six carbons as its longest chain (c) A beta-ketoaldehyde, \(\mathrm{C}_{6} \mathrm{H}_{10} \mathrm{O}_{2}\) (d) A cyclic alpha-hydroxyketone, \(\mathrm{C}_{5} \mathrm{H}_{8} \mathrm{O}_{2}\)

Short answer

(a) Cyclopentanone; C-O bond angle ≈ 120°. (b) 2-Octanone. (c) 3-Ketohexanal. (d) 1-Hydroxycyclopentanone.

Step-by-step solution

Identifying a 5-Carbon Cyclic Ketone

To draw a 5-carbon cyclic ketone, consider a 5-membered ring structure like cyclopentanone. Cyclopentanone contains a ketone group (C=O) within a 5-carbon ring. Label the carbons in a pentagon shape, and position the carbonyl group between any two carbon atoms.

Determining the C-O Bond Angle

In a typical ketone, the oxygen is sp² hybridized. The bond angle around the carbonyl carbon, including the C-O bond, is approximately 120° due to the trigonal planar arrangement of bonds around the carbon atom.

Drawing an 8-Carbon Ketone

For part b, draw an 8-carbon compound where a ketone group is included in the longest chain, which in this case contains six carbon atoms. You could position the ketone group at the second position of the chain, making it 2-octanone. The structure will then be CH₃-CH₂-CH₂-CO-CH₂-CH₂-CH₃.

Constructing a Beta-Ketoaldehyde

A beta-ketoaldehyde has a keto group and an aldehyde group with one carbon separating them. For C₆H₁₀O₂, draw the following structure: O=C-CH-CH₂-CH₂-CH₂-CHO. This chain has a ketone group at the beta position to the aldehyde group.

Building a Cyclic Alpha-Hydroxyketone

For a cyclic alpha-hydroxyketone with the formula C₅H₈O₂, consider a hydroxyl group and a ketone group in the cyclic compound. One possibility is to draw a 5-membered ring like cyclopentanone, with a hydroxyl group (OH) on the carbon adjacent to the carbonyl carbon. This results in a structure like: HO-C-CH₃-CH₂-CO-C.

Key concepts

Cyclic Ketones

Cyclic ketones are a fascinating group of organic compounds characterized by a carbon ring structure featuring a ketone group. They play a crucial role in both organic chemistry and industrial applications. A common example of a cyclic ketone is cyclopentanone, which consists of a five-membered ring with a carbonyl (\( \text{C}=\text{O} \)) group. This type of ketone is interesting because the cyclic nature restricts the rotation of the carbon atoms. This results in unique chemical properties.

• Cyclic structure promotes stability.
• Distinct reactivity patterns emerge.
• Used in the synthesis of polymers and perfumes.

In cyclopentanone, the presence of the cyclic structure offers an interactive platform for studying reactions like ring-opening.
Exploring these reactions helps in understanding how cycles affect chemical properties.

Beta-Ketoaldehyde

A beta-ketoaldehyde is an organic compound that contains both a ketone and an aldehyde functional group. These groups are separated by one carbon atom, labeling the position as 'beta'. This kind of compound often sparks interest due to its versatility in synthesis. The term "beta" specifies its position relative to the aldehyde group. In the given exercise example, the formula for a typical beta-ketoaldehyde is \( \text{C}_6\text{H}_{10}\text{O}_2 \), showcasing the structure: \[ \text{O=C-CH-CH}_2\text{-CH}_2\text{-CH}_2\text{-CHO} \].

Some interesting factors to consider include:

• The ketone and aldehyde's closeness influences the compound's reactivity.
• Useful for applications in organic synthesis, notably in the formation of heterocycles.
• Offers insight into reactions such as aldol condensations.

The presence of both functional groups in such proximity allows for the possibility of intramolecular reactions, leading to an exciting study of its chemical behavior.

Alpha-Hydroxyketone

An alpha-hydroxyketone is a functional compound where a hydroxyl group (\( \text{OH} \)) sits adjacent to a ketone group. In organic chemistry, these compounds are key intermediaries in various synthetic pathways involving oxidation and reduction reactions. Cyclopentanone with an added hydroxyl group gives us a cyclic alpha-hydroxyketone.

The presence of an alpha-hydroxyl modifies its chemical characteristics:

• Increases reactivity due to adjacent functional groups.
• Plays a crucial role in the synthesis of drugs and agrochemicals.
• Participates in important transformations like α-hydroxy ketone rearrangement.

For the molecule \( \text{C}_5\text{H}_8\text{O}_2 \) mentioned in the exercise, the structure would typically involve a five-membered ring with a hydroxyl group attached to the alpha carbon next to the keto group. This composition helps capitalize on the ketone’s inherent reactivity.

Carbonyl Bond Angle

The bond angle in a carbonyl compound is a critical factor that influences reactivity. In ketones, this angle contributes significantly to its chemical properties and interaction with other molecules. The carbonyl group features a carbon atom double-bonded to an oxygen atom, denoted \( \text{C}=\text{O} \), and this carbon typically shows \( \approx 120^\circ \) bond angles.

Important considerations for these angles include:

• Trigonally planar geometry due to \( \text{sp}^2 \) hybridization.
• Influences the molecular packing in solid-state structures.
• Plays a significant role in electrophilic reactions.

Understanding the exact bond angle helps predict molecular geometry and reactivity.
For example, in the ketone structure cyclopentanone, the \( \text{C}=\text{O} \) bond angle determines how acids, bases, or nucleophiles interact with it.