Atomic size trend anion11/29/2023 The \(pK_a\) of the thiol group on the cysteine side chain, for example, is approximately 8.3, while the \(pK_a\) for the alcohol group on the serine side chain is on the order of 17. More importantly to the study of biological organic chemistry, this trend tells us that thiols are more acidic than alcohols. HI, with a \(pK_a\) of about -9, is almost as strong as sulfuric acid. Because fluoride is the least stable (most basic) of the halide conjugate bases, HF is the least acidic of the haloacids, only slightly stronger than a carboxylic acid. For now, we are applying the concept only to the influence of atomic radius on base strength. We will see this idea expressed again and again throughout our study of organic reactivity, in many different contexts. This illustrates a fundamental concept in organic chemistry:Įlectrostatic charges, whether positive or negative, are more stable when they are ‘spread out’ over a larger area. The atomic radius of iodine is approximately twice that of fluorine, so in an iodide ion, the negative charge is spread out over a significantly larger volume: But in fact, it is the least stable, and the most basic! It turns out that when moving vertically in the periodic table, the size of the atom trumps its electronegativity with regard to basicity. Because fluorine is the most electronegative halogen element, we might expect fluoride to also be the least basic halogen ion. In order to make sense of this trend, we will once again consider the stability of the conjugate bases. Vertical periodic trend in acidity and basicity:Ĭonversely, acidity in the haloacids increases as we move down the column. This is best illustrated with the haloacids and halides: basicity, like electronegativity, increases as we move up the column. When moving vertically within a given column of the periodic table, we again observe a clear periodic trend in acidity. Conversely, ethanol is the strongest acid, and ethane the weakest acid. 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). Weaker bases have negative charges on more electronegative atoms stronger bases have negative charges on less electronegative atoms. The more electronegative an atom, the better able it is to bear a negative charge. Remember from section 2.4A that electronegativity also increases as we move from left to right along a row of the periodic table, meaning that oxygen is the most electronegative of the three atoms, and carbon the least. 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. Look at where the negative charge ends up in each conjugate base. The key to understanding this trend is to consider the hypothetical conjugate base in each case: the more stable (weaker) the conjugate base, the stronger the acid. We can see a clear trend in acidity as we move from left to right along the second row of the periodic table from carbon to nitrogen to oxygen. Horizontal periodic trend in acidity and basicity: We’ll use as our first models the simple organic compounds ethane, methylamine, and ethanol, but the concepts apply equally to more complex biomolecules with the same functionalities, for example the side chains of the amino acids alanine (alkane), lysine (amine), and serine (alcohol). \)įirst, we will focus on individual atoms, and think about trends associated with the position of an element on the periodic table.
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