Problem 21 - Chemistry in 30 days

Question

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

Step-by-Step Solution

Answer

Correct answer: (c) 10 sigma bonds, 1 pi bond, and 1 lone pair.

Step 1: Identify the Structure of Acetone

Acetone is a simple ketone with the chemical formula \(C_3H_6O\) and structural formula \((CH_3)_2CO\). It normally has 9 sigma bonds (one double bond, meaning one pi-bond, and the rest are single sigma bonds) and no lone pairs on carbon atoms.

Step 2: Draw the Enolic Form

The enolic form involves converting one of the alpha hydrogen atoms into an OH group, thus forming an enol. The structure becomes HO-\(CH_2=C(CH_3)\), adding an OH group (enol form) adjacent to a double bond.

Step 3: Count the Sigma Bonds

Enolic form of acetone consists of 8 sigma bonds from the three C-H single bonds, one C=C double bond (containing one sigma bond), one C-C single bond, and one O-H bond.

Step 4: Count the Pi Bonds

The enolic form includes one C=C bond, which contains 1 pi bond.

Step 5: Identify Lone Pairs

The oxygen atom in the O-H group has one lone pair of electrons, consistent with the typical lone pair configuration on oxygen in such molecules.

Key Concepts

Understanding Sigma Bonds

In the world of chemistry, sigma bonds (\( \sigma \) bonds) are fundamental concepts that form the backbone of molecule structures. A sigma bond is a type of covalent bond that arises when two atomic orbitals overlap linearly between the atoms. This overlap occurs directly between the nuclei of the two atoms, making sigma bonds particularly strong and stable.

When considering molecules like acetone or its enolic form, each carbon (C) to hydrogen (H) single bond, and carbon to carbon single bond, is a sigma bond. Essentially, these connections form the basic "skeleton" of the molecule:

• In the original structure of acetone, there are 9 sigma bonds.
• When acetone transitions into its enolic form, it has 8 sigma bonds.

The reduction in sigma bonds occurs because a double bond is introduced in the enolic form, altering the structure.

Diving into Pi Bonds

Pi bonds (\( \pi \) bonds) are the partners to sigma bonds within double and triple bonds. Pi bonds occur when atomic orbitals overlap sideways. This type of bond is weaker compared to sigma bonds because the overlap is not directly in line with the nuclei of the bonding atoms.

In enolization, the creation of an enolic form introduces a double bond, which consists of one sigma and one pi bond:

• The change from acetone to its enolic form involves the formation of a C=C double bond.
• This C=C bond includes one pi bond.

The double bond provides rigidity to the molecule's structure and distinguishes the enolic form from the standard ketone form of acetone.

The Role of Lone Pairs

Lone pairs of electrons are non-bonding pairs. They are valence electrons that do not participate in bonding within a molecule. Instead, they are localized around a single atom, often having an influence on molecular geometry and reactivity.

Oxygen, in particular, commonly carries lone pairs when it forms covalent bonds with other atoms:

• In the enolic form of acetone, the oxygen attached to the hydroxyl group (OH) has one lone pair of electrons.
• This lone pair can affect how the molecule interacts with other substances and can be involved in further chemical reactions.

Lone pairs also contribute to the overall polarity of molecules, influencing how they interact with their environment.