Reaction Schemes for Organic Chemistry Tutorial

Key Concepts

• A reaction scheme is a visual aid to show how one compound can be converted into another.
• A reaction scheme is also known as a reaction pathway because it shows you the chemical path taken to convert one compound in another.
• A reaction scheme, or reaction pathway, is a flow chart of chemical reactions.
• A reaction scheme, or reaction pathway, is a summary of the all the chemical reactions required to convert reactants into products.
• Note that not all products may be present in the reaction scheme (reaction pathway).
• A reaction scheme (reaction pathway) can help you identify unknown reactants and/or products in a series of chemical reactions.

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Generalised Reaction Scheme for Alk-1-enes (1-alkenes)

Most of the chemical reactions involving non-biological aliphatic organic compounds that you are likely to encounter in school can be summarised as a reaction scheme for the alk-1-enes (1-alkenes).

For example, you know that you can react an alkene with hydrogen to produce the corresponding alkane in a hydrogenation reaction as shown in the balanced chemical equation below:

but if I wanted a quick way to represent this, and make it easier to add more chemical reactions which use either the alkene or the alkane, then I would simplify this and draw a flow chart, or reaction scheme, using this information:

You could react both the alkane and the alkene with a halogen such as chlorine gas or bromine water, in which case the alkene would readily react but the alkane would only react slowly and in the presence of ultraviolet light.

We can add this information to our reaction scheme:

We also know that we can use a hydrogen halide to add accross the double bond of the alkene to produce the haloalkane (alkyl halide) in a hydrohalogenation reaction, so we can add this information to our reaction scheme:

If we continue adding more and more information to our reaction scheme we would end up with something that looks like the one below:

I can use this reaction scheme to decide how to synthesis a particular organic compound.

If, for example, I wanted to produce an ester, this reaction scheme shows me two possible methods:

• react an alkanol with an alkanoic acid in a direct (or Fischer) esterification reaction
• react a haloalkane with a carboxylate ion

But this reaction scheme does more than that!

This reaction scheme even shows me how to produce an alkanol, an alkanoic acid and a haloalkane using an alk-1-ene (1-alkene), or even an alkane, as the starting reagent!

For example, I could use the reaction scheme to produce a series of steps that could be used to produce an ester from alkane:

Step 1: halogenation of alkane (produces haloalkane)

Step 2: reaction of haloalkane with carboxylate ion (produces ester)

Drawing a generalised reaction scheme for the reactions you have studied in class is a great way to revise, and help you remember, those reactions!

Reaction Scheme for Ethene (ethylene)

Reaction schemes are most powerful when they relate to the specific reactions of a specific molecule.

This is because we can specify the reagents and reaction conditions required for each reaction on the reaction scheme.

The reaction scheme below shows the reactions for ethene (ethylene) based on the generalised reaction scheme above:

If, for example, I were asked to produce sodium ethanolate using ethene (ethylene) as the starting reagent, I can follow the reaction scheme above to produce it in 2 steps (not including the necessary isolation and purification steps which are not shown on the reaction scheme):

Step 1: ethene (ethylene) + acidified water → ethanol

Step 2: ethanol + sodium metal → sodium ethanolate

Worked Example Using the Reaction Scheme for Ethene (ethylene)

Refer to the following incomplete reaction scheme:

Give the IUPAC name and condensed or semi-structural formula for the molecules labelled A, B, C, D and E in the reaction scheme above.

1. Start with what you have been given!

   We have been given the location of ethanol, CH₃CH₂OH, in the reaction scheme, so start looking to the right, left, up and down from there.

2. To the right of ethanol, we see a strong oxidation of ethanol using acidified potassium permanganate. This will produce acetic acid (ethanoic acid), so D is :

   (i) name: acetic acid (ethanoic acid)

   (ii) semi-structural formula: CH₃COOH

3. Below D, we see a reaction with ethanol in the presence of sulfuric acid, that is, an esterfication reaction. Acetic acid (ethanoic acid) will react with ethanol, CH₃CH₂OH, in the presence of sulfuric acid to produce the ester ethyl acetate (ethyl ethanoate), so E is:

   (i) name: ethyl acetate (ethyl ethanoate)

   (ii) condensed (semi-structural) formula: CH₃CH₂-O-CO-CH₃

4. Now we will work backwards from ethanol to B, that is, what compound could react with KOH_(aq) to produce ethanol?

   Most likely there is an atom on the molecule B that is substituted with the OH group from the KOH, and, halogen atoms (Br, Cl, I) make very good leaving groups!

5. Move one more step to the left, and we see that A reacts with Cl₂ in the presence of light to produce B, so B is most likely to contain the halogen chlorine, Cl
6. B contains 2 carbon atoms (in order to form ethanol with 2 carbon atoms) and is a saturated compound (single bonds between carbon atoms), and contains a chlorine atom:

   (i) name: chloroethane

   (ii) condensed (semi-structural) formula: CH₃CH₂Cl

7. A must be a saturated hydrocarbon (it needs light to start the halogenation reaction) containing just 2 carbon atoms:

   (i) name: ethane

   (ii) condensed (semi-structural) formula: CH₃CH₃

8. C reacts with HCl in the presence of AlCl₃ to produce CH₃CH₂Cl (B)
   and C reacts with water in the presence of acid to produce ethanol, CH₃CH₂OH
   C is most likely to be ethylene (ethene) because this molecule undergoes addition reactions with water to produce ethanol, and, undergoes hydrohalogenation with HCl so that H and Cl add across the double bond:

   (i) name: ethylene (ethene)

   (ii) condensed (semi-structural) formula: CH₂=CH₂

Our complete reaction scheme now looks like the one below:

So we have identified all the unknown compounds in the reaction scheme:

A: ethane, CH₃CH₃

B: chloroethane, CH₃CH₂Cl

C: ethylene (ethene), CH₂=CH₂

D: acetic acid (ethanoic acid), CH₃COOH

E: ethyl acetate (ethyl ethanoate), CH₃CH₂-O-CO-CH₃