Applications of hard-soft acid-base theory
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Applications of hard-soft acid-base theory
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[ID] => 560089
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[post_date] => 2025-01-14 05:13:36
[post_date_gmt] => 2025-01-14 10:13:36
[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
The HSAB (Hard Soft Acid Base) theory categorizes chemical species as acids or bases and as “hard”, “soft”, or “borderline”. It explains that soft acids or bases tend to be large and very polarizable, while hard acids or bases are small and non-polarizable. Since these categories are not absolute, there are species that are considered borderline, which lie in between hard and soft. Generally, we could characterize hard acids and bases as having:
- Small atomic radii
- Low polarizability
- High positive charge (3+ or higher)
- High electronegativity (base)
- High charge density
- High oxidation state
A hard acid or base may have some or all of these characteristics, but the hard-soft distinction is most directly linked to polarizability. Polarizability is the relative tendency of the electron cloud to be distorted from its normal shape.
Table 1. Table of representative hard, borderline, and soft acids and bases
The theory elaborates that hard acids prefer to bond with hard bases, and the resulting adduct tend to have more ionic character in its bonding. Correspondingly, soft acids prefer to bond with soft bases, and their adducts are more covalent in nature.
The concept has been used in chemistry to explain the stability of various compounds, pathways, or reaction mechanisms. In transition metal chemistry, it offers a qualitative approach to understanding metal-ligand interactions in complexation reactions. Here are the Keq values for two such ligand exchange reactions:
In toxicology, the HSAB theory has recently proved useful in predicting toxicant-target interactions, whereby most electrophiles are the toxicants and most nucleophiles are biological macromolecules, in particular, thiols and thiolates in cysteine and amines in lysine. Cytotoxicity occurs when toxicants like acrolein and vinyl chloride form adducts with biomacromolecules of similar hardness or softness, thus impair cellular function. One of the compounds under investigation is curcumin, a phenol found in turmeric, and its structure is below:
Figure 1. Structure of Curcumin
Source: Adapted from a paper by R. LoPachin, et al. Copyright 2011 by American Chemical Society.
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[question] => The HSAB theory can be used to predict solubility trends. Based on the determination of the hardness of the ions, what happens when AgF and LiI are placed together into solution?
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[answer] => 3
[description] => Reason for the Correct Answer:
The passage states that soft acids prefer to bind to soft bases, and hard acids prefer to bind to hard bases.
To determine the hardness for Ag+, look to the representative soft acids in Table 1, Cu+, Pd2+, and Tl+. Since Cu+ is considered soft, then Ag+ which is in the same group would be considered a soft acid.
Note that the softness is attributed to the fully filled d orbitals of those transition metals.
From the table, extract that I– is considered soft. AgI will bond together to precipitate out of solution since Ag+ and I– are both soft ions.
To determine the hardness for Li+, look to the representative hard acids, K+, Al3, and Sr2. If K+ is considered hard, then Li+ which is in the same group would be considered a hard acid.
Note that the hardness is mostly attributed to the small size of Li+.
From the table, extract that F– is considered hard. Since Li+ and F– are both hard ions,
LiF will precipitate out of solution.
)
[answers] => Array
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[0] => Array
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[each_answer] => A.Only AgI precipitates initially, but dissolves back into solution when Ag+ forms a complex ion with F– and H2O.
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[1] => Array
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[each_answer] => B.Only LiF precipitates.
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[2] => Array
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[each_answer] => C.Both LiF and Agl precipitate.
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[3] => Array
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[each_answer] => D.No precipitate is formed.
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => Which of the following trends has a negative correlation with hardness?
[value] => Array
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[answer] => 2
[description] => Reason for the Correct Answer:
The passage states that the hard-soft distinction is most directly related to polarizability and that polarizability is the tendency for the electron clouds to be distorted.
Electronegativity is the tendency to attract electrons within a bond. For electron clouds to be less easily distorted, electronegativity should be increasing.
Hardness has a positive correlation with electronegativity.
For less easily distorted electron clouds, charge density should be increasing. For positive charges,+4 has the highest charge density, and for negative charges, -1 has the highest charge density.
Hardness has a positive correlation with charge density.
An increase in oxidation state corresponds to an increase in charge density. Thus, hardness has a positive correlation with oxidation state.
Because polarizability has a positive correlation with softness, it has a negative correlation with hardness.
)
[answers] => Array
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[0] => Array
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[each_answer] => A.Oxidation state
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[1] => Array
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[each_answer] => B.Polarizability
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[2] => Array
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[each_answer] => C.Electronegativity
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[3] => Array
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[each_answer] => D.Charge density
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[2] => Array
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[quiz_unique_key] => 83407773
[question] => Recent clinical trials have suggested that 1,3-dicarbonyl enolates offer cytoprotective effects against toxicants of a similar softness or hardness. Your research team has identified methyl vinyl ketone (butenone) as a soft electrophile and 2,5-hexanedione as a hard electrophile. Against which of the following, if any, will curcumin offer the best protection?
[value] => Array
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[answer] => 3
[description] => Reason for the Correct Answer:
While curcumin may seem to have a structure of a diketone at the center, there are two adjacent α,β-unsaturated ketones.
Determine why the methyl vinyl ketone is soft and 2,5-hexanedione is hard. Methyl vinyl ketone is an α,β-unsaturated ketone, and 2,5-hexanedione is a diketone.
Essentially, there is conjugation in the soft electrophile.
Curcumin is most similar to methyl vinyl ketone since as an enol it would be unsaturated at the α and β position and it is conjugated. Curcumin would offer protection against soft toxicants.
)
[answers] => Array
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[each_answer] => A.2,5-hexanedione
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[each_answer] => B.Both
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[each_answer] => C.Methyl vinyl ketone
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[each_answer] => D.None
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[quiz_unique_key] => 2377279144
[question] => Acrolein or propenal is considered a soft electrophile and extremely toxic. Which of the following targets would acrolein have the greatest affinity for?
[value] => Array
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[answer] => 4
[description] => Reason for the Correct Answer:
The passage states that acrolein is a soft electrophile. It will bind most tightly with soft nucleophiles.
The guanidinium group of arginine is a N-functional group, and due to resonance all the nitrogens have a slightly positive charge. The amine group of histidine is also a N-functional group, which participates in resonance.
The deoxyribose in DNA has an oxygen in its ring structure, and the O has two lone pairs. The thiol group of cysteine is a S-functional group, and the S has two lone pairs.
Consider in terms of electronegativity. O is the most electronegative, making it least polarizable, while S is the least electronegative and most polarizable.
Consider in terms of size. S has electrons in a higher energy level than N or O along with two lone pairs, which makes it the most polarizable or softest of the three.
Another approach is to use the bases from Table 1 as representative of nucleophilicity since they are for the most part correlated. Soft bases include thiocyanate (SCN–), thiolate (RS–), and iodide (I–) ions, and hard bases include hydroxide (OH–), sulfate 
, and fluoride (F–) ions.
Both approaches indicate that the thiol group would be a soft nucleophile and would bind acrolein the best.
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[answers] => Array
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[0] => Array
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[each_answer] => A.Secondary amine of a histidine residue
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[1] => Array
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[each_answer] => B.Guanidinium group (RHNC(NH2)2) of an arginine residue
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[2] => Array
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[each_answer] => C.Ring oxygen of deoxyribofuranose moiety
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[each_answer] => D.Thiol group of a cysteine residue
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[quiz_unique_key] => 2261298308
[question] => How does the HSAB theory account for the difference in Keq values for the two ligand exchange reactions given in the passage?
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[answer] => 2
[description] => Reason for the Correct Answer:
A small Keq value (<<1) indicates the reverse reaction is favorable. The complex ion does not bind with F– in any considerable amount.
A large Keq value (>>1) indicates that the forward reaction is favored, and the second equation indicates that the complex ion prefers to bind with Br–. Mercury will swap H2O for Br–.
F– is a hard ion due to its compact size and high electronegativity. Br– is a soft ion due to its lower electronegativity and larger size.
For a complex ion, all the ligands surround Hg, so we must figure out the softness of Hg+.
Hg+ is in the same group as Cd+ and an additional shell of electrons, and Cd+ is a soft ion.
Hg+ also has fully-filled d orbitals, and therefore is considered a soft ion.
The passage states that soft ions prefer to bind with other soft ions and form a bond more covalent in nature.
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[answers] => Array
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[0] => Array
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[each_answer] => A.Hg+ and F– are hard ions and favorably bind together to form an ionic compound.
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[each_answer] => B.Hg+ and Br– do not match in terms of their hardness.
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[each_answer] => C.Hg+ and Br– are soft ions and prefer to bind forming a bond covalent in nature.
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[each_answer] => D.Hg+ and F– are soft ions and cannot form a covalent bond in solution.
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