In comparison to CH 3 - in Assessment 2.86, the related molecule BH 4 - is itself not a Lewis base at boron. Why?

All right. Hi, everyone. So for this question, let's go ahead and provide a brief explanation as to why SI H three negative is a Lewis base at silicon. But the molecule A LH four negative is not a Lewis base at aluminum. Here we have four different answer choices labeled A through D proposing different reasons as to why SI H three negative can either donate or potentially accept electrons whereas aluminum cannot. So depending on the definition of a Lewis base, we can actually eliminate two of these answer choices. Recall that a Luis acid base reaction describes the transfer of an electron pair as opposed to a proton, a low base by definition is the species that donates the electron pair. Whereas the Lewis acid is the species that accepts the electron pair. So with this in mind, we can see that the explanations proposed in options B and D describe silicone accepting electrons despite being a low base. So because that's not correct, we can go ahead and eliminate those entirely because we know that silicone does in fact donate an electron pair considering SI H three negative is the lowest base. Now, a Lewis base must contain lone pairs of electrons, which means that compounds possessing a negative charge can also act as lowest basis. So let's consider the structures of si H three negative and al LH four negative starting off with silicone at the center of the first compound silicone is a compound in group four a of the periodic table, meaning it has four valence electrons. Now, three of those valence electrons are found in each of the three bonds to all three hydrogen atoms. But the negative charge on silicone is because there's an extra loan pair of electrons on silicon specifically. So not only does silicone have a loan pair of electrons, it also has a negative charge which explains why it behaves as a Lewis base. Now, on the other hand, aluminum in a LH four negative is a metal found in group three a of the periodic table. So three valence electrons even though aluminum can form covalent bonds. But the negative charge on aluminum exempt from the fact that aluminum contains four bonds to four hydrogen atoms. Aluminum is an exception to the octet rule because aluminum like boron is neutral with only six electrons surrounding it. That's why the presence of eight gives aluminum a negative to. Now, it's worth mentioning that the negative charge on aluminum does imply that there are extra electrons. So a LH four negative could behave as a low base, but aluminum is not specifically the lowest base instead a hydrogen atom with electrons attached could be donated instead. So that's the main distinction here, aluminum itself is not the lowest base here. Instead, hydride or hydrogen with a loan pair of electrons would be. So with this in mind, if I scroll up here, once more, our answer is going to be option C in the multiple choice si H three negative is a Lewis base because it has an extra long pair that can be donated, but a LH four negative does not. So with that being said, thank you so very much for watching and I hope you found this helpful.

Table of contents

Skip topic navigation

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

3. Acids and Bases

Acids and Bases

Organic Chemistry

3. Acids and Bases

Acids and Bases

Previous problemNext problem

Problem 87

Textbook Question

In comparison to CH₃^- in Assessment 2.86, the related molecule BH₄^- is itself not a Lewis base at boron. Why?

Verified step by step guidance

Understand the concept of a Lewis base: A Lewis base is a species that donates a pair of electrons to form a covalent bond. This requires the presence of a lone pair of electrons on the atom in question.

Analyze the structure of CH₃⁻: In CH₃⁻, the carbon atom has a lone pair of electrons, making it capable of donating this pair and acting as a Lewis base.

Examine the structure of BH₄⁻: In BH₄⁻, the boron atom is surrounded by four hydrogen atoms, forming a tetrahedral structure. Boron in this case has a full octet due to the bonding with the hydrogens.

Determine the availability of lone pairs on boron in BH₄⁻: Unlike CH₃⁻, boron in BH₄⁻ does not have a lone pair of electrons. All of its valence electrons are involved in bonding with the hydrogen atoms.

Conclude why BH₄⁻ is not a Lewis base: Since boron in BH₄⁻ lacks a lone pair of electrons to donate, it cannot act as a Lewis base, even though the molecule carries a negative charge. This is a key distinction from CH₃⁻.

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above

Play a video:

Was this helpful?

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Lewis Acids and Bases

Lewis acids are electron pair acceptors, while Lewis bases are electron pair donors. In the context of boron and carbon compounds, understanding these definitions is crucial for determining the reactivity and classification of molecules. For instance, CH₃⁻ is a Lewis base because it can donate an electron pair, whereas BH₄⁻ does not behave as a Lewis base due to the electron deficiency of boron.

Electron Deficiency in Boron

Boron is an electron-deficient element, typically forming three covalent bonds and having an incomplete octet. This characteristic makes boron less likely to donate electron pairs, which is a key trait of Lewis bases. In BH₄⁻, the boron atom is surrounded by four hydrogens, stabilizing its electron configuration but not allowing it to act as a base.

Resonance and Stability

Resonance refers to the delocalization of electrons across multiple structures, which can stabilize a molecule. In the case of BH₄⁻, the resonance structures do not allow for the boron atom to effectively donate an electron pair, as the overall stability of the molecule is maintained without it acting as a Lewis base. This contrasts with CH₃⁻, where the carbon can easily donate its lone pair.

Watch next

Master The Lewis definition of acids and bases. with a bite sized video explanation from Johnny

Start learning

In comparison to CH3- in Assessment 2.86, the related molecule BH4- is itself not a Lewis base at boron. Why?