We'll use as our first models the simple organic compounds ethane, We can see a clear trend in acidity as we move from left to right along the second row of. Acidity of Organic Compounds and Resonance. CALVIN A. VANDER WERF. University of Kansas, Lawrence, Kansas. NCE he has weighed all the many. Acidity of Organic Molecules. A. Bronsted-Lowry Acids and Bases. 1. BrØnsted-Lowry Definition. 2. pK a. Definition. 3. Predicting pK a. Values a. idenity of.


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Ch 1 : Acidity and Basicity

In the carboxylate ion, RCO2- the negative charge is delocalised across 2 electronegative oxygen atoms which makes it more stable than being localised on a specific atom as in the alkoxide, RO. General acidity trend of common organic acids this is a very useful sequence to remember and to be able to rationalise: Basicity A convenient way to look at basicity is based on electron pair availability The fluoride anion, F — is a tiny and vicious little beast, with the smallest ionic radius of any other ion bearing a single negative charge.

Its charge is therefore spread over a smaller volume than those of the larger halides, which is energetically unfavorable: Factor 3 acidity of organic compounds Resonance.

A huge stabilizing factor for a conjugate base is if the negative charge can be delocalized through resonance. Factor 4 — Inductive effects. Electronegative atoms can draw negative charge acidity of organic compounds themselves, which can lead to considerable stabilization of conjugate bases.

According to this, anything which stabilizes the conjugate base will increase the acidity.

Therefore pKa is also a measure of how stable the conjugate base is. Put another way, strong acids have weak conjugate bases, and vice versa.

Factor 1 — Charge. The acidity trends reflect this: Note that once a conjugate base Acidity of organic compounds is negative, a second deprotonation will make the dianion B 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.

5 Key Factors That Affect Acidity in Organic Chemistry — Master Organic Chemistry

The more electronegative an atom, the better able it is to bear a negative charge. Weaker bases have negative charges on more electronegative acidity of organic compounds stronger bases have negative charges on less electronegative atoms.

Thus, the methoxide anion is the most stable lowest energy, least acidity of organic compounds 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.

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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.

Recall the important general statement that we made a little earlier: 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 fator of between acidity of organic compounds Ka values for the two molecules!

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 minor but still important resonance contributor in which the nitrogen lone pair is part of a pi 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 so comfortable - it is not part of a delocalized pi system, and is acidity of organic compounds to form a bond with any acidic proton acidity of organic compounds might be nearby.


If an amide group is protonated, it will be at the oxygen rather than the nitrogen. Make a structural argument to account for its strength.


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