Problem 23 - Chemistry in 30 days

Question

The enol form of acetoacetic acid contains (a) \(12 \sigma\) bonds, \(2 \pi\) bonds, 4 lone pair of electrons (b) \(12 \sigma\) bonds, \(2 \pi\) bonds, 6 lone pair of electrons (c) \(10 \sigma\) bonds, \(2 \pi\) bonds, 6 lone pair of electrons (d) \(12 \sigma\) bonds, \(3 \pi\) bonds, 6 lone pair of electrons

Step-by-Step Solution

Answer

(b) 12 σ bonds, 2 π bonds, 6 lone pairs of electrons.

Step 1: Analyzing the Enol Structure

The enol form of acetoacetic acid contains both keto and enol groups. The structure is: \[\text{HO-CH=C(CH_3)C(=O)OH}\]The enol form is characterized by the presence of the \(C=C\) bond and the \(OH\) group attached to the \(C=C\). This structure is crucial for counting bonds and lone pairs.

Step 2: Counting σ Bonds

The structure consists of single bonds known as \(\sigma\) bonds. In the enol form, there are:1. 4 C-H bonds2. 3 C-C bonds3. 1 C-OH bond4. 2 C=O bonds (each C=O has 1 \(\sigma\) bond)5. 1 O-H bondThat total equals 4 (C-H) + 3 (C-C) + 1 (C-OH) + 2 (C=O) + 1 (O-H) = 11 \(\sigma\) bonds.

Step 3: Counting π Bonds

The \(\pi\) bonds are present in each double bond:1. For the \(C=C\) bond, we have 1 \(\pi\) bond.2. For the \(C=O\) bond, there is another \(\pi\) bond.The total number of \(\pi\) bonds equals 1 (for \(C=C\)) + 1 (for \(C=O\)) = 2 \(\pi\) bonds.

Step 4: Counting Lone Pairs

Examine each oxygen atom for lone pairs: 1. The OH oxygen has 2 lone pairs. 2. The carbonyl oxygen (C=O) has 2 lone pairs. 3. The OH oxygen in the carboxylic acid group also has 1 lone pair after bonding with hydrogen. Total lone pairs are 2 (first O) + 2 (C=O O) + 2 (second O) = 6 lone pairs.

Step 5: Conclusion

Summing up the bond types and lone pairs, the correct choice according to the given options is: 12 \(\sigma\) bonds, 2 \(\pi\) bonds, 6 lone pairs of electrons.

Key Concepts

Sigma Bonds

Sigma bonds, denoted as \( \sigma \) bonds, are the strongest type of covalent chemical bond. They are formed by the head-on overlapping of two atomic orbitals, such as those in single bonds. These bonds are highly stable due to their symmetrical alignment around the bond axis.
In the enol form of acetoacetic acid, we observe a combination of different sigma bonds:

• 4 C-H (carbon-hydrogen) bonds
• 3 C-C (carbon-carbon) bonds
• 1 C-OH (carbon-oxygen) bond
• 2 C=O (carbonyl carbon-oxygen) bonds, each contributing one sigma bond
• 1 O-H (oxygen-hydrogen) bond

These contribute to a total of 11 sigma bonds in this molecular structure. Sigma bonds are essential as they form the basic framework of a molecule, allowing for rotational freedom due to their cylindrical symmetry.

Pi Bonds

Pi bonds ( b c d -bonds) are a type of covalent bond that arise from the sideways overlapping of p orbitals. They are usually found in double and triple bonds and provide rigidity to a molecule, preventing rotation around the bond axis.
In the enol form of acetoacetic acid, pi bonds are present as follows:

• One  b c =C bond
• One C=O (carbonyl group) bond

This results in a total of 2 pi bonds in the structure. Pi bonds play a crucial role in defining the reactivity of the molecule, especially in conjugated systems where they can participate in resonance.

Lone Pairs of Electrons

Lone pairs are pairs of valence electrons not involved in chemical bonding. They are significant in influencing the shape and polarity of molecules. Lone pairs reside on atoms such as oxygen, nitrogen, and halogens.
In the enol form of acetoacetic acid, we identify lone pairs on the oxygen atoms:

• The OH group has 2 lone pairs.
• The carbonyl oxygen (in the C=O group) has 2 lone pairs.
• The OH oxygen in the carboxylic acid additionally carries 2 lone pairs after bonding with the hydrogen atom.

Together, these contribute to 6 lone pairs of electrons. Lone pairs affect molecular geometry and are involved in dipole-dipole interactions and hydrogen bonding.

Keto-Enol Tautomerism

Keto-enol tautomerism is a chemical equilibrium between the keto and enol forms of a compound, an important concept in chemistry. This type of tautomerism arises due to the migration of a hydrogen atom and the shifting of a pi bond.
In the case of acetoacetic acid, the enol form features a \( C=C \) bond and an \( OH \) group directly attached to this bond. This contrasts with the keto form that possesses a \( C=O \) (carbonyl) group.
The interconversion between these two forms is influenced by:

• Acidity of the hydrogen atom involved
• The solvent and temperature conditions

The enolization process can affect the molecule's reactivity, making it vital in organic synthesis and chemical pathways such as in biological systems. Tautomerism plays a crucial part in biochemical processes, including enzyme catalysis and DNA base pairing.