Bimolecular Nucleophilic Substitution

The following video clip is an updated version of the bimolecular nucleophilic substitution reaction. It makes use of a generic nucleophile which is an anion reacting with a generic alkyl halide.



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^{Article written by Kwok YL 2011.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Ionic Equilibrium - use of solids as indicators

Indicators are used for you to help us know that a particular reactant is completely used up for a reaction. Through acid-base titration, we have seen that indicators inform us that a reactant is completed used through  a change in colour of the solution.  These indicators first create a colour in the original solution and when the end-point is reached, they will show a change in colour.

However, a colour change  in the solution is not the only way to indicate that the reaction is completed. Another way is to form a coloured solid while doing a titration. An example would be to determine the concentration of Cl^- ions present in a solution by titrating the solution against a standard solution of AgNO₃.

(A) Scenario: How it is used.

When you add Ag⁺ ions to a solution containing Cl^- ions, a white precipitate is seen. This happens because the K_sp value of AgCl is very small and hence the product of the concentrations of Ag⁺ ions and Cl^- ions (also known as ionic product) can easily be larger than the K_sp. This results in precipitation of the AgCl solid.  Chromate ions (CrO₄^2-) are added so that when a red precipitate of Ag₂CrO₄ is formed, you would that all the AgCl has been precipitated out.  This enables us to determine how much Ag⁺ ions was added into the reaction mixture and as a result allow us to know the amount of Cl^- ions present initially.

(B) Explanation:  How this works.

The numerical values of the K_sp of AgCl and Ag₂CrO₄ are 1.77 x 10^-10 and 1.12 x 10^-12. Although the K_sp value of Ag₂CrO₄ is smaller, only a small concentration of CrO₄^2- ions is added while a larger concentration of Cl^- ions is present. Hence, it is easier for the ionic product of AgCl to exceeds its K_sp than that of Ag₂CrO₄.

Furthermore, when the red precipitate of Ag₂CrO₄ is formed, we can be quite certain that all of the Cl^- ions are precipitated out. This is because of the following:

• Although K_sp of the Ag₂CrO₄ is of a smaller value, the K_sp expression of silver chromate is essentially a concentration³ term. This is in comparison to silver chloride whose K_sp expression is a concentration². Hence, the solubility of silver chromate is greater than that of AgCl. This implies that the former is more soluble than the latter and it is quite fair to say it is more difficult to precipitate out too.

• In addition, for the same small concentration of Ag⁺ ions present (since both solids are insoluble you do not expect a large concentration of Ag⁺ ions), a larger concentration of CrO₄^2- ions is present in the solution than Cl^- ions. This would mean that a greater percentage of the Cl^- ions is precipitated out.

(C) The steps taken: The mathematics behind solving for the percentage of Cl^- ion precipitated.

When Ag₂CrO₄ is first formed, we have obtained a saturated solution of Ag₂CrO₄. Hence, by solving the K_sp of Ag₂CrO₄, we can obtain the concentration of Ag⁺ ions left in the solution. As both the equilibria which show the dissociation of AgCl and Ag₂CrO₄ exist in the same system, the concentration of Ag⁺ ions is the same for both equilibrium. It is not possible to obtain two distinct concentration of Ag⁺ ions (one for each insoluble solid doing slight dissociation) when both solids exist in the same solution. Therefore, the concentration of Ag⁺ ions is common for both ionic solids.

Hence, with this piece of information, when we can solve for the concentration of Cl^- ions in the solution using the K_sp expression of AgCl. This turns out to be a very small number which suggest almost all of the Cl^- ions has been precipitated out of the solution, therefore giving us an endpoint.

(D) Conclusion: Final thoughts.

Curiously, is it possible to have absolutely no Cl^- ions present in this context? The answer is no . As the solid AgCl remains in the reaction mixture, some would definitely dissolve giving rise to a concentration of Cl^- ions in the solution.

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^{Article written by Kwok YL 2011.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Patterns in Chemistry

I must confess that I was never fascinated by Chemistry; at least not when I was a 15 years old. I was more intrigued by numbers and I was fascinated by how the lung and the heart works.  However, it was during my years in junior college that made me fond of the subject as I saw that beneath the massive number of equations and facts, lies interesting patterns for me to identify and be acquainted with.  This was a challenge that I enjoyed:  It grew to become a fervent interest and ultimately a career. 

As a 15 years old student, I remembered that the colours and the smells were something that made this subject memorable.  But as much as there are many things that I could rely on my senses to help me appreciate the subject, there were even more that were beyond the capacity of my senses to reach out to.  This made it hard for me to relate with the abstract facts.   However, I discovered that learning Chemistry often require one to be able to spot that pattern; for most of the time, we are not faced with questions on the factual information but rather questions that required us to apply the knowledge.  Hence, recognising patterns is important.

I could remember the difficulties that my peers faced in studying Organic Chemistry.  It was a similar fear that some of my students experienced.  I empathise their difficulty as this topic has numerous equations, conditions and reagents to put into memory.  Simply, this topic was a string of roman letters which form words that seemingly look English but is not English; yet it has its own meaning.  Essentially, Organic Chemistry could be seen as a language on its own.  

However, the success in learning this “language” is simply being able to identify the patterns that are involved and relate them to other patterns or principles that one has prior knowledge of.  There are only polar and non-polar covalent bonds.  The polar bonds are broken during “nucleophilic substitution” and “nucleophilic addition” reactions.  While, the non-polar ones are broken in either “free radical substitution”, or “electrophilic substitution” or “electrophilic addition” reactions.  With the aid of the “dot and cross” diagram and the definitions of “nucleophile”, “electrophile” and “free radical”, I was able to identify that the first has a lone pair of electrons available for donation, the second has an empty and energetically accessible orbital to accept a pair of electrons, and the last has an unpaired electron.  Lastly, if a stronger covalent bond is broken, we would expect more severe conditions needed for the reactions to occur.

1. When I am given an unknown equation, instead of trying to dig out when and where I memorized this equation, I chose to ask
2.  What is the polarity of the bond that is most likely to be broken?
3. Could the other species be either a nucleophile, or an electrophile or a free radical (It can’t be all three and if it were, you will be stuck.)?
4.  Is the covalent bond broken relatively strong or weak to those that I know?

Hence, the subject becomes less frustrating and more student-friendly.

However the beauty of this subject lies in the twists that it provides.  I guess this was ultimately the reason that drew me toward its.  I was fascinated by (instead of frustrated by) the weakness of the patterns which allows me to predict all the cases but yet seemingly insufficient to include absolutely everything within it.  That motivated me to improve the definition of my pattern to make it more concise (maybe even precise) and hopefully more accurate until the next factual information that weakens this pattern, and the cycle repeats.