Problem 1 - Basic Principles of Organic Chemistry

Question

Draw the Lewis electron-pair structure of 2 -propanone (acetone) clearly showing the bonding and nonbonding electron pairs in the valence shell of each atom. Draw structural formulas for other compounds having the composition \(\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O}\) that possess a. an aldehyde function. b. an ether function in a ring. c. an alcohol function and a double bond. d. an alcohol function and a ring.

Step-by-Step Solution

Answer

Acetone has a C=O. a) CH₃CH₂CHO. b) Ring ether. c) CH₂=CHCH₂OH. d) Cyclopropanol.

Step 1: Identify the molecular formula of acetone

Acetone (2-propanone) has the molecular formula \( \text{C}_3\text{H}_6\text{O} \). It consists of 3 carbon atoms, 6 hydrogen atoms, and 1 oxygen atom.

Step 2: Draw the Lewis structure of acetone

In acetone, the central carbon atom is bonded to two other carbons and one oxygen atom. The oxygen is double-bonded to form a carbonyl group (C=O). Each carbon atom forms four bonds to satisfy the octet rule. The Lewis structure is: \( \begin{array}{cccc} \text{H}_3\text{C} & - & \text{C} & = \text{O} \ & & | & \ & & \text{CH}_3 & \\end{array} \) All valence electrons are accounted for, with the oxygen having two lone pairs and each hydrogen forming a single bond.

Step 3a: Structure with an aldehyde function

For an aldehyde function, one end of the molecule must contain \( \text{CHO} \). A possible structure is: \( \text{CH}_3\text{CH}_2\text{CHO} \) This structure shows an aldehyde group (\( \text{CHO} \)) at the end of the carbon chain.

Step 3b: Structure with an ether function in a ring

An ether has an \( \text{O} \) atom connected to two carbon atoms. In a ring structure, it could be: \( \begin{array}{c} \text{C} \, \vphantom{{}_|} \/ \, \, \text{O} \; \text{C} \ \backslash \vphantom{{}_|} \\text{C} \, \vphantom{{}_|} \end{array} \) This structure represents an ether functional group (oxygen in the ring) with a three-carbon ring.

Step 3c: Structure with an alcohol function and a double bond

An alcohol has a hydroxyl group (\( \text{OH} \)). Combining it with a double bond could lead to: \( \text{CH}_2\text{=CH}\text{CH}_2\text{OH} \) This structure contains a double bond between the first and second carbon atoms and an \( \text{OH} \) group on the third carbon atom.

Step 3d: Structure with an alcohol function and a ring

For a ring structure with an \( \text{OH} \) group, it can be cyclopropanol: \( \begin{array}{c} \text{C} \, \vphantom{{}_|} \/ \, \, \text{C} \ \backslash \vphantom{{}_|} \\text{C} \, - \text{OH} \end{array} \)This structure shows a three-membered carbon ring with a hydroxyl group attached.

Key Concepts

Acetone

Acetone, also known as 2-propanone, is one of the simplest ketones. It has the molecular formula \( \text{C}_3\text{H}_6\text{O} \). Acetone is a common solvent and is widely used in the chemical industry and various household products. Understanding its structure is key to comprehending its chemical properties.
In acetone's Lewis structure, the central carbon atom is bonded to two other carbon atoms and one oxygen atom. This forms a carbonyl group \((\text{C}=\text{O})\), which is a defining feature of ketones. The oxygen atom in this group has a double bond with the central carbon, and it also carries two lone pairs of electrons. Each of the three carbon atoms follows the octet rule, ensuring they each have eight electrons in their valence shell through bonding and sharing electrons.
The Lewis structure can be represented as \( \begin{array}{cccc} \text{H}_3\text{C} & - & \text{C} & = \text{O} \ & & | & \ & & \text{CH}_3 & \end{array} \). This depiction shows all the valence electrons around the individual atoms and emphasizes the carbonyl group's significance in acetone's chemistry.

Aldehyde Function

An aldehyde is characterized by the presence of a carbonyl group \((\text{C}=\text{O})\) bonded to at least one hydrogen atom. It's generally located at the end of a carbon chain, distinguishing it from ketones. Aldehydes are prevalent in nature, forming flavors, fragrances, and several biologically significant molecules.
For a molecule with the formula \( \text{C}_3\text{H}_6\text{O} \) that possesses an aldehyde function, we need to modify acetone's structure. In this setup, a possible structure with an aldehyde group is \( \text{CH}_3\text{CH}_2\text{CHO} \). Here, the aldehyde group \((\text{CHO})\) appears with the carbonyl group at one end of the molecule.
The existence of the hydrogen atom directly bonded to the carbonyl carbon is what defines it as an aldehyde, distinguishing it from ketones where the carbonyl group is flanked by two carbon atoms. This subtle change in structure leads to significant differences in chemical reactivity and properties, often making aldehydes more reactive than ketones in various chemical reactions.

Ether Function

Ethers contain an oxygen atom connected to two alkyl or aryl groups, which can make them versatile intermediates in organic synthesis. In simple terms, an ether consists of an oxygen atom bridging two different carbon sites. Ethers are used widely as solvents due to their generally low reactivity.
A ring structure with an ether function involves an oxygen atom interspersed within the carbon chain forming a cyclic compound. With the formula \( \text{C}_3\text{H}_6\text{O} \), an ether often takes the form of a small ring with the oxygen incorporated.
An example structure might look like this:
\( \begin{array}{c} \ \ \ \text{C} \vphantom{{}_|} \ \/ \ \text{O} \ \text{C} \ & \backslash \vphantom{{}_|} \ \ \ \text{C} \vphantom{{}_|} \end{array} \). This showcases a three-membered carbon ring with an oxygen atom in between. This type of structure is not only significant in simple organic chemistry but also serves as a functional group in more complex organic molecules.

Alcohol Function

Alcohols are defined by the presence of at least one hydroxyl group \((\text{OH})\) attached to a carbon atom. This feature imparts alcohols with their unique properties, like their solubility in water and ability to act as weak acids. Structurally, they offer versatility for chemical reactions, closing the versatility gap between alkanes and more reactive groups like carboxylic acids.
Within the molecular constraints of \( \text{C}_3\text{H}_6\text{O} \), there are several ways to arrange the atoms to feature an alcohol group. One structure could include both a double bond and an alcohol function: \( \text{CH}_2=\text{CH}\text{CH}_2\text{OH} \). Here, you'll find a hydroxyl group attached to the third carbon, with a double bond bridging the first and second carbon atoms.
Another arrangement can accommodate an alcohol group in a cyclic structure, known as cyclopropanol: \( \begin{array}{c} \text{C} \vphantom{{}_|} \ \/ \ \text{C} \ \backslash \vphantom{{}_|} \ \text{C} - \text{OH} \end{array} \). This ring structure, with an OH group attached, adds intrigue to the diversity of the alcohol functional group, highlighting how positioning within the carbon framework dramatically alters chemical properties and reactivity.