Chapter 3
Chem 243
Reactions are changes in chemical structures. This occurs by the breaking and forming of chemical bonds. We can write equations showing the starting materials (reactants), followed by some kind of an arrow pointing to the product molecules of the reaction. We will often depict reactions like this, and often not even bother balancing the reactions.
Thinking about reactions in this way, suggests that all molecular transformations take place in a single magical step. Just wave a magic wand (or more likely a stirring rod) and you convert reactants to products instantly. Well, this ain't alchemy or magic. In many cases there are more than one step in a chemical reaction. When we write out all the steps in a chemical reaction, we call it a mechanism. Each step shows a particular molecular transformation and the steps are listed in the order in which we believe they occur. There are several types of mechanisms we will study and many reactions can follow each kind of mechanism. So once the mechanism is generally known, it can describe a large number of specific reactions. This helps to organize and simplify organic chemistry.
Yet another type of symbolism is introduced here. (Maybe this really is alchemy!) These are the dreaded curved arrows. Acurved arrow indicates the movement of an electron pair from one location to another. If the arrow has only one barb, sort of like a harpoon, then this indicates the movement of one electron. We will see the latter after we introduce radical reactions.
The curved arrows can be included in the reactions of the mechanism to give a complete description of how the reaction proceeds.
We can believe these mechanisms as long as they agree with experimental observations. As soon as our observations contradict the mechanism, we have to devise a new mechanism. In any case, the mechanisms are only what we think is happening and only symbolic. Reality (whatever that is) might be quite different, but we try our best.
Why do we care about mechanisms? Well if we can determine what the reaction intermediates are, (the products of each intermediate step of the reaction mechanism) then we might be able to influence the path or rate of the reaction. This means that if a set of reactants can give several products,each obtained by a different mechanism or reaction path, then we might be able to adjust our conditions to favor one path over the others and produce one molecule in greater abundance. This is important in industrial processes. It would mean less waste of material and energy .
Acid/base chemistry is important in the study of organic reactions. Besides the common acids and bases with which you are familiar, many of the species, neutral or ionic, can be described as Lewis acids (electron deficient) or Lewis bases (electron pair donors). Keeping in mind your knowledge of basic physics, the interaction of positive and negative charges guides most of the reactions we will study. Also, remember that neutral species can also be electron deficient, if sharing a pair of electrons with another species can lead to a complete octet of valence electrons in the deficient species, it can be described as a Lewis acid. (Note the various compounds of boron and aluminum.)
Understand the structural features of a molecule to determine whether it will be a stgrong or weak acid or base. This involves application of electronegativity (inductive effect) and resonance theory.
There are some physical constants that you will need to understand, equilibrium constants, pK, free energy, etc. Know the relationship of these to acidity, basicity and the direction a reaction will take. Entropy is also another important consideration in many reactions.
There are two ways of breaking bonds, heterolysis and homolysis, and these should be understood in terms of the reactions you will be studying. Does the lysis lead to anions, cations or neutral species?
Solvents also have an effect on the acidity and basicity of compounds. This will also have a large effect on reaction rates and the direction of equilibria.
Suggested Problems for Chapter 3
3.1 Throughout the course you will have to be able to write correct chemical reaction equations. Though we will not always be concerned with balancing the equations, they must be correct in their reactants and products. Practice this extensively now and you will have it easier later.
3.2 Acids and bases are very important concepts, so become very confortable with this.
3.3 You should know how to use symbols like the various arrows used in organic chemistry. It’s just another language.
3.4, 5, 6, 7 Be able to do this just for your understanding of the concepts.
3.8 Knowing entropy changes for reactions will be important, know this.
3.9 Mostly we will have to estimate free energy changes, but calculate it here. It won’t hurt you!
3.10 Apply previous concepts to current question.
3.11 This material really is cumulative.
3.12 Not difficult.
3.13 Reactions are important.
3.14 This type of question is fundamental and important.
3.15 Basic concept.
3.16 Again, this is important.
3.16, 17 More of the same, but still important.
3.19 Acid base concepts are very important.
3.20 Do this just so that you can understand how the electrons shift. It’s more symbolic language.
3.21 Obvious when you think about it.
3.22 Think about it.
3.24 Fundamental concept.
3.26 This is simple synthesis, don’t be intimidated. It gets worse later!
3.27, 28, 29 Acids and bases
3.33 Acidity and basicity from structures.
3.34 Straightforward.
3.40 Do this systematically and it will seem straightforward.