Problem 96 - A Complete Resource Book in Chemistry for JEE Main

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

Step-by-Step Solution

Answer

Option (b): 8 sigma bonds, 2 pi bonds, and 2 lone pairs of electrons.

Step 1: Understanding the Structure of Acetone

Acetone has the chemical formula C_3H_6O . Its structure can be drawn as: CH_3-CO-CH_3 with a carbonyl group (C=O) in the middle. In the keto form, acetone has 9 sigma bonds, 1 pi bond (in the C=O), and no lone pairs.

Step 2: Identifying the Enol Form

The enol form of acetone involves a tautomeric shift from the keto form, where a hydrogen from one of the methyl groups transfers to the oxygen of the carbonyl group. This results in a double bond between the alpha carbon and the oxygen, and a hydroxyl ( -OH ) group forms.

Step 3: Counting the Sigma and Pi Bonds

In the enol form, there are 6 sigma bonds from C-H, 2 sigma bonds from C-C, and 1 sigma bond from C-O, making a total of 9 sigma bonds. The C=C and C-O double bonds add 2 pi bonds to the structure.

Step 4: Counting Lone Pairs

The hydroxyl ( -OH ) oxygen has 2 lone pairs of electrons, similar to other alcohols, even after the tautomerization.

Step 5: Conclusion

Considering the enol form, acetone has 9 sigma bonds, 2 pi bonds, and 2 lone pairs of electrons. Thus, the answer is option (b).

Key Concepts

Enol-Keto Tautomerism

Enol-keto tautomerism is an interesting and important concept in organic chemistry. It involves the chemical equilibrium between two structural forms: enols and ketones (or aldehydes). A keto form is typically more stable due to the presence of a carbonyl group (14) which is highly favored. However, under certain conditions, it can transform into its less stable enol form.

This process is called tautomerism. In the enol form, a carbonyl (14) group shifts slightly via the movement of a hydrogen atom from one of the carbons to the oxygen atom. This forms a hydroxyl group (068) and introduces a double bond between carbon atoms.

Understanding how acetone transforms between these forms helps in many scientific applications, particularly in understanding reaction mechanisms and the behavior of different chemical species under varying conditions.

• Keto Form: Carbonyl group, more stable.
• Enol Form: Hydroxyl group, introduces double bond, less stable.

Sigma and Pi Bonds

Sigma and pi bonds are fundamental concepts in the study of chemical bonding. They are types of covalent bonds that involve the sharing of electrons between atoms. Sigma (6) bonds are the first bonds formed between two atoms. They are characterized by head-on overlapping of atomic orbitals and are generally considered strong and stable. In the case of acetone, we count these bonds by identifying how many atoms are connected through single bonds, including C-H, C-C, and C-O bonds.
Pi (7) bonds result from the side-on overlapping of p-orbitals and are generally weaker than sigma bonds. They add to the connection between atoms that already have a sigma bond. In the enol form, of acetone, we observe pi bonds in the C=C and C-O double bonds, confirming the presence of these intriguing interactions.

• Sigma Bond Example: C-H, C-C
• Pi Bond Example: Double bonds like C=C and C-O

Lone Pairs of Electrons

Lone pairs of electrons play a crucial role in molecular structure and reactivity. They are a set of valence electrons that are not shared in any bonds but instead belong to a single atom. In the structure of molecules like acetone in its enol form, understanding the presence and influence of these lone pairs is vital.

For instance, in the hydroxyl (068) group of the enol form, the oxygen atom carries two lone pairs of electrons. These lone pairs significantly affect the molecule's polarity, reactivity, and geometry, often influencing how other molecules interact with it.

Lone pairs can be responsible for hydrogen bonding and other types of molecular interactions, making them essential in the discussion of chemical behavior and substance formation.

• Role in Polarity: Affects molecular geometry and dipole moment.
• Role in Reactivity: Participates in hydrogen bonding and other interactions.