A Summary of the Factors that Determine Acid Strength - Chemistry LibreTexts
Your organic teachers are quite likely to ask you questions like identify the most acidic Remember that acidity and basicity are the based on the same chemical atoms within the same row of the periodic table, the more electronegative the The size of the group also weakens the bond H-X (note this trend should be. Optical basicity; acidity/basicity scale of solid oxides; ionization .. 3- CORRELATION BETWEEN IONIZATION ENERGY AND OPTICAL. In this section we will analyze how structure affects acidity and basicity. Acids and The acidity constant essentially expresses the relative ratio of the ionized.
As before, we begin by considering the conjugate bases. In both species, the negative charge on the conjugate base is held by an oxygen, so periodic trends cannot be invoked.
For acetic acid, however, there is a key difference: What this means, you may recall, is that the negative charge on the acetate ion is not located on one oxygen or the other: The delocalization of charge by resonance has a very powerful effect on the reactivity of organic molecules, enough to account for the difference of over 12 pKa units between ethanol and acetic acid and remember, pKa is a log expression, so we are talking about a difference of over between the acidity constants for the two molecules.
The acetate ion is that much more stable than the ethoxide ion, all due to the effects of resonance delocalization. The resonance effect also nicely explains why a nitrogen atom is basic when it is in an amine, but not basic when it is part of an amide group. Recall that in an amide, there is significant double-bond character to the carbon-nitrogen bond, due to a second resonance contributor in which the nitrogen lone pair is part of a p bond.
Notice that in this case, we are extending our central statement to say that electron density — in the form of a lone pair — is stabilized by resonance delocalization, even though there is not a negative charge involved. The lone pair on an amine nitrogen, by contrast, is not part of a delocalized p system, and is very ready to form a bond with any acidic proton that might be nearby. Often it requires some careful thought to predict the most acidic proton on a molecule. Ascorbic acid, also known as Vitamin C, has a pKa of 4.
There are four hydroxyl groups on this molecule — which one is the most acidic? If we consider all four possible conjugate bases, we find that there is only one for which we can delocalized the negative charge over two oxygen atoms. Rank the compounds below from most acidic to least acidic, and explain your reasoning.
Acidity and Basicity - UBC Wiki
The inductive effect Compare the pKa values of acetic acid and its mono- di- and tri-chlorinated derivatives: Factor 1 — Charge. The acidity trends reflect this: Note that once a conjugate base B- is negative, a second deprotonation will make the dianion B While far from impossible, forming the dianion can be difficult due to the buildup of negative charge and the corresponding electronic repulsions that result.
Factor 2 — The Role of the Atom This point causes a lot of confusion due to the presence of two seemingly conflicting trends. This makes sense, right? It makes sense that HF is more electronegative than H2O, NH3, and CH4 due to the greater electronegativity of fluorine versus oxygen, nitrogen, and carbon.
- 2.5: Factors That Determine Acid Strength
- Inductive effect
- Five Key Factors That Influence Acidity
A fluorine bearing a negative charge is a happy fluorine. You could make two arguments for why this is.
The first reason has to do with the shorter and stronger H-F bond as compared to the larger hydrogen halides. In the conjugate base of ethane, the negative charge is borne by a carbon atom, while on the conjugate base of methylamine and ethanol the negative charge is located on a nitrogen and an oxygen, respectively.
Remember that electronegativity also increases as we move from left to right along a row of the periodic tablemeaning that oxygen is the most electronegative of the three atoms, and carbon the least. The more electronegative an atom, the better able it is to bear a negative charge. Weaker bases have negative charges on more electronegative atoms; stronger bases have negative charges on less electronegative atoms.
Thus, the methoxide anion is the most stable lowest energy, least basic of the three conjugate bases, and the ethyl carbanion anion is the least stable highest energy, most basic.
Conversely, ethanol is the strongest acid, and ethane the weakest acid. When moving vertically within a given column of the periodic table, we again observe a clear periodic trend in acidity.
This is best illustrated with the haloacids and halides: Vertical periodic trend in acidity and basicity Conversely, acidity in the haloacids increases as we move down the column.
In order to make sense of this trend, we will once again consider the stability of the conjugate bases. Because fluorine is the most electronegative halogen element, we might expect fluoride to also be the least basic halogen ion.
But in fact, it is the least stable, and the most basic!
1.24 A Summary of the Factors that Determine Acid Strength
It turns out that when moving vertically in the periodic table, the size of the atom trumps its electronegativity with regard to basicity. This illustrates a fundamental concept in organic chemistry: We will see this idea expressed again and again throughout our study of organic reactivity, in many different contexts. For now, we are applying the concept only to the influence of atomic radius on base strength.