Lewis säure definition
Ausserdem ist die Stärke der Säuren und Basen - anders als nach der BRÖNSTED-Theorie - viel schwieriger quantifizierbar. Any species that can accept (react with) a proton, including but not limited to the –OH are bases.
Gilbert Newton Lewis (the same Lewis who is behind the electron-dot formulas) suggested an even more general way of classifying acids and bases.
A Lewis acid is an electron pair acceptor.
A Lewis base is an electron pair donor.
For example:
What is different compared with the Arrhenius and Brønsted approaches is that Lewis took the opposite view, and instead of focusing on the proton, he looked from the perspective of electrons.
For example, boron is a common Lewis acid since it lacks an octet and has a vacant p orbital, which it uses to accept a lone pair and thus serves as a Lewis acid:
So, any Lewis acid is an electron acceptor; therefore, any Brønsted acid is a Lewis acid. Think about this: any chemical reaction involves electrons flowing from one species to another.
Before doing that, let’s do a quick recap of acid-base definitions in general.
The first theory for classifying acids and bases was developed by Arrhenius in the 19th century. All Bronsted-Lowry bases (proton acceptors) are Lewis bases, but not all Lewis bases are proton acceptors. And here again, this reaction will be most often referred to as an addition reaction to an alkene, even though it still falls under the Lewis acid-base category.
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Practice
For each reaction, identify the Lewis acid and the base and draw the corresponding curved-arrow mechanism:
Answer
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Solution
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Definition of a Lewis Acid & Base
Core Concepts
So what is a Lewis acid and a Lewis base?
Lewis-Säure
Eine Lewis-Säure ist nach dem Säure-Base-Konzept von G. N. Lewis ein elektrophilerElektronenpaarakzeptor, kann also Elektronenpaare anlagern. A Lewis acid is defined as an electron acceptor, and a Lewis base is defined as a electron donor. Cr3+. Bei der Anlagerung von Liganden reagiert das Metallkation mit diesen zum Komplex.
Die Atome vieler Übergangsmetalle können als LEWIS-Säuren agieren, weil sie über leere d-Orbitale verfügen.
So werden ZIEGLER-NATTA-Katalysatoren, die aus Verbindungen von Übergangsmetallen bestehen, zur Polymerisation von Polyethylen genutzt. Nicht zuletzt kann auch an bestimmten Atomen, die in Molekülen an elektronegative Elemente wie Sauerstoff gebunden sind, die Elektronendichte soweit erniedrigt sein, dass sie als LEWIS-saures Zentrum agieren können.
And the Lewis bases are also called nucleophiles since they are electron-rich. Eine LEWIS-Säure muss ein unbesetztes Orbital haben, das mit dem Elektronenpaar der Base besetzt werden kann.
Aber auch die Reaktion eines Protons mit Basen wie ist lediglich ein Spezialfall einer LEWIS-Säure-Base-Reaktion.
Die LEWIS-Theorie ist zwar umfassender, aber schwerer handhabbar als die BRÖNSTED-Theorie.
It states that acids are proton donors and bases are proton acceptors.
For example:
Notice that the bases are not restricted to hydroxides only. Als Beispiel kann Aluminiumchlorid dienen, das in Wasser sauer reagiert, obwohl es keinen Wasserstoff enthält.
Mit dem Säure-Begriff nach Lewis ist es möglich, die sauren Eigenschaften von Stoffen zu erklären, die keine Protonendonatoren nach Brønsted sind.
Bei den Säuren umfasst die LEWIS-Definition jedoch eine wesentlich größere Klasse von Substanzen. The electron flow shows that the species giving the electrons are the Lewis bases, and the ones accepting them are the Lewis acids.
Let’s try to understand what this and the Brønsted acid-base theory have in common.
However, the Lewis acids are not restricted to protons only. He was found dead in his lab after working with hydrogen cyanide, probably of natural causes, not from the cyanide.
What is a Lewis Acid?
Lewis acid definition: A Lewis acid is any molecule or compound that contains an empty orbital that can accept a pair of non-bonding electrons; it can also be known as an electron pair acceptor or electrophile.
A Lewis acid is a substance that can accept a pair of electrons to form a new chemical bond.