100 Nano-Stories: Steric Hindrance + Coordinate Covalent Bonds!
Episode #88: Substitutions & Donations (Organic Chemistry)!
Preface! ✨
It’s your favorite material science & nanotechnology enthusiast! Today, we will cover a bit more on some concepts of hydrolysis in the sol-gel processing of aerogels!
Here is the article that introduces Sol-Gel Chemistry + Hydrolysis for Aerogel Technologies:
TL;DR → Catalysts + Hydrolysis! 🔑
- Hydrolysis is the chemical breakdown of a compound/molecule due to a water reaction with the compound.
- A Catalyst is a chemical that speeds up a chemical reaction without being affected, and it lowers the activation energy of the reactants so the reaction between two compounds can occur and form the new compound.
- Let’s use TEOS (tetraethoxysilane) as our main precursor that will undergo hydrolysis. Our silica alkoxide will be Si(OC₂H₅)₄. When the molecule goes under hydrolysis, TEOS can allow up to 4 water molecules to replace the methyl groups with water.
- The more water molecules you add, the more methyl (CH3) groups are replaced with hydroxyl groups (OH).
- Precursors → Chemical compounds that participate in a chemical reaction to produce a new compound/molecule.
- But hydrolysis takes more time on other alkoxide precursors because there are methylene groups (CH₂) attached to the precursor.
- These Methylene groups make it harder for the Hydroxyl Groups to substitute the Methyl Group, (because of the length of the alkyl groups). This means that the process is also very slow, and gelation time increases as the alkyl groups increase (or the methylene groups increase).
Author’s Note:
This is an overview of the previous article. If you are still confused, I highly recommend reading the previous article mentioned above entirely before proceeding to this article! 🙏🏽
Now that we have a better understanding of what may hinder hydrolysis in aerogels, let’s discover the mechanisms behind this!
Steric Hindrance + Coordinate Covalent Bonding!💡
Steric Hindrance: Definitions! 🔑
As I mentioned in the TL;DR above, the longer the carbon chains, the harder it is for hydrolysis to occur, and it will take time.
The hydroxyl groups are going to be substitute with the methyl groups. But the more carbon and hydrogen atoms are in a compound, the time for the substitution of the nucleophile and the methyl group will get longer (mostly because of methylene groups).
This mechanism is called Steric Hindrance.
Steric Hindrance is when the incoming molecule (nucleophile) cannot react with the compound that wants the electrons of the incoming molecule (electrophile). They bump into each other but never react to make a molecule leave the molecule (leaving group).
Nucleophile → A chemical/reactant that wants to give a pair of electrons to create a covalent bond.
Electrophile → A chemical/reactant that wants the pair of electrons to create covalent bonds.
An example of Steric Hindrance is like you and a wall. Behind that wall is gold. But the wall is very high (20 meters high) and it’s blocking the gold, so it will be difficult to get to the other side and grab the gold. With chemicals, the nucleophile wants to react with the electrophile, but something is blocking the nucleophile’s path to react with the electrophile. That something is methyl and methylene groups.
Steric Hindrance: Mechanism! 🔑
In aerogel, the nucleophile is water. The electrophile is the precursor (TMOS, TEOS, MTES.) A leaving group is a methanol (an OH Group with a methyl group.)
Finally, in aerogel, the steric hindrance is an Sₙ2 Reaction.
Sₙ stands for Nucleophilic Substitution.
2 means that both the nucleophile and the leaving group play a role in the hydrolysis reaction.
So, TMOS has a fast Sₙ2 reaction because there are no additional methylene groups. But in TEOS, the Sₙ2 reaction is slower due to the presence of 4 methylene groups (the nucleophile has a harder time undergoing hydrolysis, and this is without a catalyst!).
Coordinate Covalent Bonding: Definitions! 🔑
Coordinate Covalent Bonding is when an atom donates a pair of (two) electrons to form a covalent bond. The pair of electrons being donated is also called a lone pair. There has to be a donation of two electrons from one atom, otherwise, it’s not a coordinate covalent bond.
Covalent Bonds occur when two atoms share their electrons equally.
An electron donor is called a Lewis Base.
An electron acceptor is called a Lewis Acid.
But in some cases, the electron donor will have a partial negative charge (δ-) because of electronegativity (especially if it is a molecule), and the electron acceptor has a partial positive charge (δ+). This is when the electron donor ATTACKS the electron acceptor to make the coordinate covalent bond.
Closing Thoughts! 💭
This introduces the mechanisms behind hydrolysis and potential coordinate covalent bonding in sol-gel chemistry (depending on the precursors and catalysts used when preparing the alcogel for drying and/or hydrophobicity)!
See you soon to explain how hydrophobicity works on the surface of the aerogel + mechanisms of surface chemistry! ✌🏽
Vocabulary! 📓
Precursors → Chemical compounds that participate in a chemical reaction to produce a new compound/molecule.
Catalyst → A chemical that speeds up a chemical reaction without being affected; and it lowers the activation energy of the reactants so the reaction between two compounds can occur and form the new compound.
Hydrolysis → The chemical breakdown of a compound/molecule due to a water reaction with the compound.
Nucleophile → A chemical/reactant that wants to give a pair of electrons to create a covalent bond.
Electrophile → A chemical/reactant that wants the pair of electrons to create covalent bonds.
Steric Hindrance → The magnitude of how slow the nucleophile reacts with the electrophile (magnitude can be fast, slow, or no reaction).
Sₙ2 → A Nucleophilic Substitution where the nucleophile and the leaving group play a role in the hydrolysis reaction.
Coordinate Covalent Bonding → An atom donates a pair of electrons to form a covalent bond (two electrons).
Covalent Bonds → Two atoms share their electrons equally.
Bonus Resources! 💻
Previous “100 Nano-Stories!”🔖
© 2021 by Carlos Manuel Jarquin Sanchez. All Rights Reserved.
