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Why do silicon and carbon behave so differently chemically? Uncover the secret of silicon alpha effect!
In the world of chemistry, the contrast between silicon (Si) and carbon (C) is often thought-provoking. Although the two elements are in the same family, they exhibit completely different chemical behaviors. Especially in organosilicon compounds, researchers have discovered the so-called "silicon alpha effect", which adds additional complexity to many reactions and affects the reaction platform and rate. Why does this phenomenon happen, and what is the reason behind it?
The wonderful chemical behavior of silicon
In organosilicon compounds, the "negative hyperconjugation effect" is a theoretically proposed phenomenon that can explain the stability of certain positive charges under certain circumstances. This hyperconjugation can destabilize or stabilize certain accumulations of positive charges. The electronic effects of silicon are closely related to its neighboring carbon atoms, which affects the stereochemistry of the molecule and the rate of hydrolysis.
The elements in the second row are generally effective at stabilizing adjacent negative charges, as opposed to their counterparts in the first row.
When we consider the electronegativities of silicon and carbon, we find that silicon has a lower electronegativity than carbon, which results in a polarization of the electron density toward carbon. This silicon-centered stability is called the "silica alpha and beta effect." The alpha effect describes how chemical reactions are facilitated near silicon atoms, while the beta effect involves the stabilization of positive charges on carbon atoms.
Experimental evidence of silicon effect
In 1946, Leo Sommer and Frank C. Whitmore conducted an important experiment by chemically chlorinating liquid ethyltrichlorosilane to obtain an isomer with an unexpected hydrolysis reaction. They found that all the chlorine attached to the silicon hydrolyzed, but not the chlorine adjacent to the carbon, leading to a series of interesting behaviors.
They concluded that silicon suppressed electron emission activity on alpha carbon.
With the deepening of research, different teams have successively discovered the silicon effect in the properties of some compounds. For example, trimethylsilyl methylamine is significantly more basic than neopentylamine, while trimethylsilyl methylamine is significantly more basic than neopentylamine. Acetic acid is less acidic than trimethylacetic acid. These observations seem to illustrate the uniqueness of silicon.
The operation of electron orbitals
The alpha and beta effects of silicon arise from the fact that heteroatoms of the third period can stabilize adjacent negative charges through (negative) hyperconjugation. In the alpha effect, when combined with aluminum or a metal such as a metal, a negatively charged substance appears, the reaction rate of which is accelerated by the presence of silicon. This is because the C-Si σ orbital partially overlaps the C-X (leaving group) σ* antibonding orbital, thereby reducing the flexibility of the carboxylic acid bond and favoring the formation of carbocation.
Silicon effect in silicon ethers
The silicon alpha effect we mentioned is particularly obvious in the field of carbon. However, the silicon alpha effect that is more important in industrial practice appears in silicon ethers. In a hydrolytic environment, certain alpha-silica-terminated prepolymers cross-link at rates 10-1,000 times faster than traditional Cγ-functional trialkoxypropylsilane and durable dialkoxymethylpropane.
How does this shocking speed occur? What chemical secrets does it hide?
Study on structural mechanism
However, exploring the mechanism of the silicon effect is not easy. Reinhold and colleagues studied the hydrolysis kinetics and mechanism of this class of compounds through systematic experiments. They found that under basic conditions, the hydrolysis rate is primarily driven by the electroaffinity of the silicon center, whereas under acidic conditions it is more complexly affected by the reactivity of the molecule and the concentration of the reactant species. In fact, the hydrolysis rate in acidic environment is also affected by the degree of protonation of silicon or functional group X.
In the face of the exploration of these chemical reactions, we perhaps should reflect on: Can the reaction behavior of silicon inspire us to gain new insights into the interactions between other elements?
