How To Tell Which Molecule Is More Acidic

Five main factors affecting acidityToday we’re going to talk about what’s behind the trends in the acidity of different molecules and discuss the most important factors that determine these values. the most important measures you can learn in organic chemistry, and not knowing some basic pKa values ​​before an exam is like stepping on the poker table without knowing the value of the hand: you will lost shirt. basics of acidity and basicity. Here is the condensed version:

Bronsted acid is a proton donor, Lewis acid is an electron pair acceptor. Converse: Bronsted base = proton acceptor, Lewis base = electron pair donor.

The conjugate base is what you get when you remove a proton (H+) from a compound. For example, HO- is the conjugate base of water. O2- is the conjugate base of HO-. In contrast, a conjugate acid is what you get when you add a proton to a compound. The conjugate acid of water is H3O+.

Quick question: is pH 1 an acid or a base? pKa is similar to pH in that a low (and even negative) value indicates a strong acid. That’s because pKa is based on equilibrium:

According to this, anything that stabilizes the conjugate base will increase the acidity. Hence pKa is also a measure of the strength of the conjugate base. In other words, strong acids have weak conjugate bases and vice versa.

With that, let’s get started.Table of contents

Element #1 – Fees.

Factor 2 – The role of the atom

Factor #3 – Resonance

Factor #4 – Inductive Effect

Element #5 – Orbit

1. Factor No. 1 – Fees.

The removal of a proton, H+, reduces the formal charge on an atom or molecule by one. Of course, this is easiest to do when an atom carries a +1 charge in the first place, and it becomes increasingly difficult as the overall charge becomes negative. The acid trends reflect this: Note that once the conjugate base (B-) is negative, a second deprotonation produces a dianion (B 2-). Although this is unlikely, the formation of dianions can be difficult due to the negative charge accumulation and corresponding electron repulsion.

2. Factor No. 2 – The role of the atom

This point causes a lot of confusion due to the presence of two seemingly opposing trends. Here’s the first point: acidity increases as we go through a row in the periodic table. This makes sense, doesn’t it? It means that HF ​​is more electronegative than H2O, NH3 and CH4 due to the greater electronegativity of fluorine compared to oxygen, nitrogen and carbon. A negatively charged fluorine is a happy fluorine Read more: how to clean outerwear HF itself is not a “strong” acid, at least not in the sense that it ionizes completely in water. HF is a weaker acid than HCl, HBr and HI. What’s going on here?You can make two arguments for why this is so. The first reason has to do with the shorter (and stronger) HF bonds compared to the larger hydrogen halides. The second concerns the stability of the conjugate base. The fluoride anion, F(-) is a tiny and ferocious monster, with the smallest ionic radius of any other ion carrying a single negative charge. Its charge is thus spread over a smaller volume than that of the larger halides, which is not energetically beneficial: for one thing, F(-) begs for dissolution. , which will result in a lower entropy term in G. Note that this trend is also true for H2O and H2S, with H2S being about 10 million times more acidic.

3. Factor #3 – Resonance

A huge stabilizing factor for a conjugate base is if the negative charge can be split via resonance. Classic examples are with phenol (C6H5OH) which is a million times more acidic than water and with acetic acid (pKa is ~5).Be careful though – it’s not enough for a system to simply be adjacent to a proton – the conjugate base electrons must be in an orbital that allows for effective overlap (for a conundrum in this vein that often offends Harvard premeds, see here.)

4. Factor #4 – Inductive Effect

Negatively charged atoms can attract negative charges towards themselves, which can lead to considerable stabilization of conjugate bases. Check out the following examples:Predictably, this effect would be related to two main factors: 1) the electronegativity of the element (the more electronegative, the more acidic) and the distance between the electronegative and negatively charged element.

5. Element No. 5 – Orbit

Read more: how to scroll through photos on windows 10 Also, acidity is nicely related to conjugate base stability. And the stability of the conjugate base depends on how well it can hold the newly found electron pair. In an effect similar to electronegativity, the more s character in the orbital, the closer the electrons will be to the nucleus, and the lower energy they will be (= stable!).Let’s see the difference between pKa of acetylene and alkane – 25! It is 10 to the power of 25, as in, “100 times greater than Avogadro’s number”. Just to give you an idea of ​​the scale. That’s the amazing thing about chemistry – the sheer range of powers of different phenomena is amazing. This is credited to Dr. Christine Pruis, Senior Lecturer at Arizona Tempe State University.OLDhargeOnetomCHEAPpowerEASYipole InductionOrbitals = FIRE FIRE.Read the mnemonic skills carefully, but you will understand. Read more: black desert online how to get a wagon

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