Question

The enolic form of acetone contains (a) 9 sigma bonds, 1 pi bond and 2 lone pair of electrons (b) 8 sigma bonds, 2 pi bonds and 2 lone pair of electrons (c) 10 sigma bonds, 1 pi bond and 1 lone pair of electrons (d) 9 sigma bonds, 2 pi bonds and 1 lone pair of electrons

Short answer

Option (a): 9 sigma bonds, 1 pi bond, and 2 lone pairs of electrons.

Step-by-step solution

Understanding Enol Forms

Acetone is a ketone with the formula \( CH_3COCH_3 \). An enolic form involves tautomerism where a hydrogen atom and a double bond are rearranged, often resulting in an alcohol (-OH) group next to a carbon-carbon double bond. The enolic form of acetone, therefore, will convert one \( C=O \) to \( C=C \) and transform a \( CH_3 \) group into an alcohol group \( CH_2OH \).

Drawing the Enolic Structure

The enolic form derived from acetone is \( CH_2=C(OH)CH_3 \). In this structure: - The hydroxyl group (OH) attaches to the carbon, and the double bond existing in the keto form shifts next to this carbon. The connectivity becomes: \( CH_2=C(OH)-CH_3 \).

Counting Sigma Bonds

Sigma bonds are the single covalent bonds between atoms. In the enolic form \( CH_2=C(OH)CH_3 \), count the sigma bonds: - \( C-H \) in \( CH_2 \): 2 sigma bonds - \( C-C \) between \( CH_2 \) and \( C \): 1 sigma bond - \( C-O \) in \( OH \): 1 sigma bond - \( O-H \) in \( OH \): 1 sigma bond - \( C-H \) in \( CH_3 \): 3 sigma bonds - \( C-C \) between \( C \) and \( CH_3 \): 1 sigma bond Total sigma bonds = 9.

Counting Pi Bonds

Pi bonds are the additional bonds formed in double or triple bonds. In the enolic form, the \( C=C \) double bond contains:- 1 pi bond.Thus, there are 1 pi bond in \( CH_2=C(OH)CH_3 \).

Identifying Lone Pairs

Lone pairs are unshared pairs of valence electrons. The oxygen in the enolic form contributes the lone pairs: - Oxygen typically has 2 lone pairs when bonded with hydrogen in an alcohol group - Therefore, there are 2 lone pairs on the oxygen atom.

Comparing with Given Options

Comparing our findings with the options given: - 9 sigma bonds - 1 pi bond - 2 lone pairs Matches option (a): 9 sigma bonds, 1 pi bond, 2 lone pair of electrons.

Key concepts

Tautomerism

Tautomerism is an intriguing aspect of chemistry that involves the shifting of atoms and electrons within a molecule, creating structurally different forms. In simple terms, it refers to the equilibrium between two isomers capable of converting into each other through a chemical reaction. These isomers, known as tautomers, differ mainly in the position of certain atoms and in the presence and position of bonds. Often, the swap involves hydrogen atoms and a shift between a single bond and a double bond.

Considering the case of acetone, a well-known ketone, tautomerism results in the formation of its enolic form. Here, a hydrogen atom moves from an alpha carbon (next to the ketone group) to the oxygen atom, leading to the change from a C=O (carbonyl group) to a C=C bond, with the simultaneous formation of an alcohol (OH) group. This dynamic balance showcases the fascinating behavior of molecules able to adopt distinct functional forms under different conditions.

Chemical Bonding

Chemical bonding is the cornerstone of molecular structure and behavior, dictating how atoms connect to form molecules. It involves various types of interactions that hold the atoms together, primarily through the sharing or transferring of valence electrons.

• Covalent Bonds: Involve the sharing of electron pairs between atoms, creating a strong connection.
• Ionic Bonds: Electrons are transferred from one atom to another, leading to the attraction between oppositely charged ions.

In the enolic form of acetone, chemical bonding is exhibited through covalent interactions. This includes the singular connections known as sigma bonds, the supplementary pi bonds in double bonds, and the lone pairs on atoms like oxygen, which are non-bonding but play crucial roles in the molecule's properties and reactions.

Sigma bonds

Sigma bonds (σ-bonds) are the primary and most robust type of covalent bond, formed by the head-on overlap of atomic orbitals. In these connections, the electron density is concentrated along the bond axis, resulting in a cylindrically symmetrical distribution around the bond line.

Sigma bonds are fundamental as they form the backbone of most molecular structures, serving to connect atoms in a stable, single bond arrangement. In the enolic form of acetone, each individual sigma bond links the carbons of acetone with other atoms, such as hydrogen or oxygen. For instance,

• There are two sigma bonds connecting each carbon in the methyl group to hydrogen.
• The carbon-oxygen and oxygen-hydrogen bonds in the OH group are also sigma bonds.

Counting these gives us the total number of sigma bonds.

Pi bonds

Pi bonds (π-bonds) are a fascinating type of covalent bond formed when parallel overlap occurs between two adjacent atomic orbitals, typically p orbitals. This overlap creates an electron cloud above and below the plane of the atoms involved, allowing for electron delocalization.

Pi bonds provide additional stability and restrict rotation around the bond axis, thus influencing the geometry of the molecule. In double and triple bonds, pi bonds accompany a sigma bond to create stronger connections between specific atoms. For instance, in the enolic form of acetone, the C=C double bond incorporates a pi bond.

• The presence of the pi bond is crucial as it contributes to the planar structure of the molecule.
• It also affects the molecule's reactivity, making it more susceptible to certain chemical reactions compared to a single bond.

Lone pairs

Lone pairs consist of valence electrons that are not shared between atoms, thus not involved in any bonding. These unshared electron pairs are important as they influence the physical and chemical properties of the molecule, including its shape, polarity, and reactivity.

For instance, in the enolic form of acetone, oxygen possesses lone pairs. Oxygen, by nature of its electronegativity and electron configuration, carries two lone pairs in its outer shell.

• These lone pairs can participate in hydrogen bonding, affecting molecular interactions with other molecules.
• Their presence also impacts molecular geometry, as lone pairs repel bonding pairs, often leading to bent shapes around the atoms.

Understanding the role of lone pairs is essential in predicting and explaining a wide array of chemical phenomena and interactions.