Here are two typical reactions involving organic chemicals:
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What does a reaction scheme like either of those above tell you?
It tells you that the molecules or atoms to the left of the arrow will react together to give the molecule(s) on the right of the arrow when they are mixed together.
Simplifying:
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The arrow implies the reaction goes to completion – all of chemicals A and B are used up to form new chemicals C and D. Note – it tells you nothing about how quickly they react – some reactions are fast, some are slow. You can say that if you leave it long enough you will reach a point where you have a mixture of C and D, and no A and B will be left.
Some reactions do not go to completion – not all of A and B are converted to C and D, no matter how much time you give the mixture to react. These are equilibrium reactions and can be recognised from the two-way arrow:
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These reactions are reversible – when C and D have formed they can react together to re-form A and B. The precise amount of A + B and C + D that forms in an equilibrium reaction is governed by the equilibrium constant, Keq, for the reaction.
A and B are the chemicals you start off with. You will hear them described in several ways – the reactants (i.e., the things that are reacting together), the reagents (same thing), the starting materials (obvious), the precursors (fancy name for starting materials), the substrates (same again).
Classification of Organic reactions by Outcome
Now let us look at the two reactions at the start of page 1 again:
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The reaction at the top is an example of an addition reaction. One reagent (HCl) has added to the other reagent to give the product. Since each carbon can only have a maximum of four substituents, in order for an addition reaction to be possible the precursor must contain a multiple bond (a C=C ‘double bond’ in this case).
The second reaction is a substitution reaction. In a substitution reaction one or more atoms in one reagent are replaced by one or more atoms from the other reagent. In the example above the chlorine atom in chloromethane has been replaced by a hydroxyl group (-OH) to form methanol as the product.
Both of these descriptions involve classification according to outcome. The reaction classification tells you how the structure of the product has changed on comparing the product(s) with the precursor(s). Other types of reaction include elimination (one or more atoms are removed from the precursor to form the product) and rearrangement (the precursor and product have identical molecular formulae, but different molecular structures – see your notes from last week). We will come back to classification of reactions in this way in future lectures.
Classification of Organic reactions by mechanism
The reaction names above – substitution, elimination, etc, tell you what has happened in the reaction. They do not tell you how the change has occurred – the mechanism by which that particular type of reaction occurred. There are three common types of reaction mechanisms – nucleophilic, electrophilic and neutral (free radical). You will find out what these terms mean later – but when describing an organic reaction, both types of classification – outcome and mechanism – are usually run together. So a description of a reaction type will normally summarise what happens (the outcome) and how it happens (the mechanism).
Question: I have mentioned four reaction outcomes and three reaction mechanisms above – how many full classifications are possible? List them.
It should be obvious (I hope!) that there are twelve possible combinations. If this sounds a lot to memorise, remember that virtually all chemical reactions fall into one of these categories. In organic chemistry some categories are much more important than others. This topic will be developed next year in module CH5008 (Organic Chemistry).
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The reaction above is an example of a nucleophilic substitution reaction. This is the most common class of reaction in organic chemistry, so I will use this example for the rest of these notes.
Look again at this reaction scheme to make sure you fully appreciate the information that it is giving you. The scheme is stoichiometric in the precursors and products: The chloro-substituent in the precursor chloromethane has been replaced by a hydroxyl-substituent in the substitution product methanol. This can only happen by breaking the chemical bond between carbon and chlorine and then* creating a new bond between hydroxyl and carbon. (* Misleading, actually. The C-OH bond must form after the C-Cl bond has broken, or at the same time. It cannot form before the C-Cl bond breaks).
Question: Why?
Now look at the mechanistic reaction scheme below. This is exactly the same reaction as above, but is presented in a way to give you information about the way in which the reaction takes place. Again, you need to be able to extract as much information as possible:
[pic] 1. You will notice that I have left out the sodium atoms on either side of the equation arrow. This is because they have little influence in how the reaction works – they have to be present (since hydroxide anion and chloride anion cannot exist without a positive ion to balance charge), but it wouldn’t affect the mechanism much if Na+ were replaced by another metal, such as K+ or Rb+. So they are left out to make the illustration less cluttered. 2. There are two types of arrow in the scheme. The central black one has the same meaning as that on the last page – it tells you that the precursors (on the left) are completely converted to the products (on the right). The red curly arrows indicate electron movement. A double-headed arrow, as in the scheme above shows that two electrons are moving; a single-headed arrow (these are rare) show that only one electron is moving.
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Double-headed arrows are more common because a covalent bond contains two electrons. In the scheme above two electrons are moving from HO־ to carbon (forming a new C-OH covalent σ-bond) and, at the same time, two electrons are moving from the C-Cl bond to chlorine. This breaks the C-Cl σ-bond and chlorine breaks away as chloride anion, Cl־. 3. The two curly arrows are concerted, as noted in point 2: the electron-shifts occur at the same time. Each structure illustrated in a mechanistic sequence is a ‘snapshot’ of electron movement at a particular instant in time. If the electron-pair movements shown in the mechanism above had not been simultaneous, they would have been shown in separate illustrations. Here is a closely-related substitution:
[pic] In this mechanism the electron-pair shifts are not concerted, as they appear in different structures. The C-Cl bond breaks first, and at some later time a new bond forms between HO and C. The reaction has the same outcome, but a different mechanism.
