Question

Di-t-butylsilanediol is stable to heat and acid. It will not lose water, even with dehydrating agents above \(400^{\circ} \mathrm{C}\). Offer an explanation.

Short answer

Di-t-butylsilanediol's stability to heat and acid is due to its molecular structure, where the central silicon atom is surrounded by two hydroxyl groups and two large tert-butyl groups that cause steric hindrance. This steric hindrance prevents other molecules, including dehydrating agents, from approaching and reacting with the hydroxyl groups. Additionally, the strength of the Si-O bonds contributes to the overall stability, making it resistant to losing water even at high temperatures.

Step-by-step solution

Understand the Structure of Di-t-butylsilanediol

In di-t-butylsilanediol, the silicon atom is the central atom, being bonded to two hydroxyl groups (OH) and two tert-butyl groups. The structure is Si(OH)_2C(CH_3)_3. The t-butyl groups provide steric hindrance or bulk, due to their size. This effectively reduces the reactivity of the molecule.

Consider Steric Hindrance of the t-butyl groups

Steric hindrance refers to the reduction in reactivity due to large groups surrounding the reacting center, effectively blocking the approach of the reactive species. In the case of di-t-butylsilanediol, the two large tert-butyl groups around the central silicon atom impede the approach of other molecules, dehydrating agents in this case.

Understand the Stability Against Acids

Strong acids do not react with di-t-butylsilanediol because the steric hindrance of the t-butyl groups prevents the protons from acids reacting with the hydroxyl groups, as they cannot easily approach the central silicon atom.

Stability to heat and resistance to loss of water

The stability of di-t-butylsilanediol to heat can be attributed to the strength of the Si-O bond, which has a bond energy of around 400 kJ/mol. The steric hindrance of the t-butyl groups also contributes to the stability, as it prevents nearby atoms from interacting with one another. The resistance to losing water can be explained by the inability of dehydrating agents to approach the hydroxyl groups due to steric hindrance. Dehydrating agents cannot form strong enough interactions to remove water and form Si-O-Si bonds, due to the restricted space around the central silicon atom. In conclusion, the stability of di-t-butylsilanediol to heat and acid can be attributed to steric hindrance provided by the tert-butyl groups and the strength of the Si-O bonds. The significant steric bulk of this molecule prevents dehydrating agents from efficiently removing water even at high temperatures.

Key concepts

Steric Hindrance

When considering the chemical behavior of di-t-butylsilanediol, understanding the concept of steric hindrance is crucial. Steric hindrance occurs when the size of groups within a molecule prevents other atoms or molecules from getting close to a reactive site. In this case, the tert-butyl groups attached to the silicon atom are quite bulky, significantly limiting the space around the reactive hydroxyl groups.

This spatial limitation makes it difficult for other substances, such as dehydrating agents or protons from acids, to interact with the hydroxyl groups. The tert-butyl groups effectively 'shield' the silicon center from potential reactants. This hindrance is a key factor contributing to the molecule's stability against heat and acid, and its resistance to dehydrating agents, even at temperatures above 400°C.

Si-O Bond Strength

Analyzing di-t-butylsilanediol's stability requires a deep dive into the bond between silicon and oxygen. The Si-O bond is notable for its considerable bond strength, with bond energy typically around 400 kJ/mol. This formidable bond energy means that a significant amount of energy would be required to break these bonds.

The robust Si-O bond contributes to the molecule's thermal stability. Even under high-temperature conditions, where many compounds might decompose or engage in chemical reactions, the Si-O bonds in di-t-butylsilanediol remain intact. Thus, the molecule does not lose water readily because the hydroxyl groups holding on to the silicon atom are not easily removed.

Resistance to Dehydrating Agents

Dehydrating agents are substances used to remove water (H2O) from other compounds. In most chemical reactions involving dehydration, these agents facilitate the removal of water molecules, often resulting in the formation of a double bond or a new bond between atoms within a compound. However, di-t-butylsilanediol presents an exceptional case.

Due to the steric hindrance discussed earlier, dehydrating agents find it challenging to access the hydroxyl groups in di-t-butylsilanediol. Moreover, the Si-O bond strength means that any interaction attempting to break these bonds must overcome considerable energy barriers. The combined effect of these two features renders the molecule unusually resistant to dehydration, even when faced with potent dehydrating agents and elevated temperatures, supporting its observed stability under these conditions.