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Ch. 20 - Enolates: Carbonyl Addition and Substitution
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
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Tutti i libri di testoMullins 1st EditionCh. 20 - Enolates: Carbonyl Addition and SubstitutionProblema 18c
Capitolo 19, Problema 18c

Specify which in each pair is the harder Lewis acid. Justify your choice beyond just looking at Table 20.2.
(c) Mg2+ vs. Na+

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Step 1: Recall the definition of a Lewis acid. A Lewis acid is a species that can accept a pair of electrons. Hard Lewis acids are typically small, highly charged, and have low polarizability.
Step 2: Analyze the chemical properties of Mg2+ and Na+. Mg2+ is a divalent cation (charge of +2), while Na+ is a monovalent cation (charge of +1). Higher charge density generally makes a species a harder Lewis acid.
Step 3: Consider the size of the ions. Mg2+ is smaller in size compared to Na+, which increases its charge density and makes it a harder Lewis acid.
Step 4: Refer to the table provided. Mg2+ is explicitly listed under 'Hard acids,' while Na+ is also listed under 'Hard acids.' However, Mg2+ is harder due to its higher charge density and smaller ionic radius.
Step 5: Justify the choice beyond the table. The hardness of Mg2+ compared to Na+ can be explained by its stronger electrostatic attraction to electron pairs due to its higher charge and smaller size, which aligns with the principles of hard and soft acid-base theory.

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Lewis Acids and Bases

Lewis acids are species that can accept an electron pair, while Lewis bases are those that can donate an electron pair. This concept is fundamental in understanding acid-base reactions in organic chemistry, as it expands the definition beyond protons (Brønsted acids) to include a wider range of chemical interactions. The strength of a Lewis acid is often determined by its ability to stabilize the negative charge of the electron pair it accepts.
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The Lewis definition of acids and bases.

Hard and Soft Acids and Bases (HSAB) Theory

The HSAB theory categorizes acids and bases as 'hard' or 'soft' based on their charge density and polarizability. Hard acids, like Mg²⁺, have high charge density and prefer to interact with hard bases, while soft acids, like Na⁺, have lower charge density and interact better with soft bases. This classification helps predict the stability of complexes and the outcome of chemical reactions, making it essential for comparing the strength of Lewis acids.
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02:49
The Lewis definition of acids and bases.

Comparative Strength of Lewis Acids

When comparing Lewis acids, factors such as charge, size, and electronegativity play crucial roles in determining their strength. For instance, Mg²⁺ is a harder Lewis acid than Na⁺ due to its higher charge and smaller ionic radius, which leads to stronger interactions with electron pairs. Understanding these differences is vital for predicting reactivity and selectivity in organic reactions, especially when analyzing pairs of acids.
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Comparing hydrocarbon acidity
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