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Introduction to Reaction Schemes for Organic Chemistry

Key Concepts

  • A reaction scheme is a visual aid to show how one compound can be converted into another.

  • A reaction scheme is a flow chart of chemical reactions.

  • Note that not all products may be present in the reaction scheme.

  • A reaction scheme can help you identify unknown reactants and/or products in a series of chemical reactions.

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, so we could draw a flow chart, or reaction scheme, using this information:

alkane hydrogenation
alkene

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:

    dihaloalkane
    halogenation
alkane hydrogenation
alkene
halogenation (uv)    
haloalkane

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

    dihaloalkane
    halogenation
alkane hydrogenation
alkene
halogenation (uv)   hydrohalogenation
haloalkane

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

    dihaloalkane polyalkene
 
 
 
alkanoic acid
CO2
strong
oxidation

+ HCOOH
   
    halogenation addition polymerisation strong oxidation    
alkane hydrogenation
alk-1-ene
(1-alkene)
mild oxidation
alkanediol
  hydrohalogenation elimination   hydration   dehydration    
halogenation
haloalkane substitution
alkanol active metal
alkanolate
    +NH3
+ carboxylate
  esterification
→ →
→ →    
    alkanaminium   oxidation   oxidation  
        alkanal oxidation
alkanoic acid esterifcation
→ →
ester
      → → → → → → → →

If, for example, I wanted to produce an ester, the 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

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

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!

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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:

    1,2-dibromoethane
  H
|
  H
|
 
Br-C-C-Br
  |
H
  |
H
 
polythene
(polyethylene)

-[-CH2-CH2-]n-

2CO2
hot conc. KMnO4
2HCOOH
   
    Br2(aq) addition polymerisation hot conc. KMnO4    
ethane
  H
|
  H
|
 
H-C-C-H
  |
H
  |
H
 
H2

Ni 500°C
ethene
(ethylene)

H      H
  \   /  
 C=C 
  /   \  
H      H
cold alk. KMnO4
ethane-1,2-diol
(ethylene glycol)

  H
|
  H
|
 
HO-C-C-OH
  |
H
  |
H
 
  HCl / AlCl3 NaOH (non-aqueous)   H2O / H+   hot conc. H2SO4    
Cl2 / UV
chloroethane
  H
|
  H
|
 
Cl-C-C-H
  |
H
  |
H
 
NaOH(aq)
ethanol
  H
|
  H
|
 
H-C-C-OH
  |
H
  |
H
 
Na(s)
sodium ethanolate
  H
|
  H
|
 
H-C-C-O-Na+
  |
H
  |
H
 
    +ammonia
(NH3)

+ sodium ethanoate
  + CH3COOH
→ →
→ →    
    ethanaminium chloride
  H
|
  H
|
 
Cl-+H3N-C-C-H
  |
H
  |
H
 
CH3COO-Na+   H+ / Cr2O72-   H+ / Cr2O72-  
        acetaldehyde
(ethanal)

  H     O
  |   //  
H-C-C 
  |   \  
  H     H
H+ / Cr2O72-
acetic acid
(ethanoic acid)

  H     O
  |   //  
H-C-C 
  |   \  
  H     O-H
+ C2H5OH
→ →
ethyl acetate
(ethyl ethanoate)

  H
|
  O
||
  H
|
 H
|
 
H-C-C-O-C-C-H
  |
H
     |
H
 |
H
 
      → → → → → → → →

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 necessary isolation and purification steps which are not shown on the reaction scheme):

  1. ethene (ethylene) + acidified water → ethanol

  2. ethanol + sodium metal → sodium ethanolate

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Worked Example

Refer to the following incomplete reaction scheme:

A Cl2 light
B KOH(aq)
ethanol
CH3CH2OH
MnO4-

H+(aq)
D
    HCl/AlCl3   H2O / H3PO4, 300°C   CH3CH2OH / H2SO4
    C   E

Give the IUPAC name and condensed (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) condensed (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

A

ethane
CH3CH3

Cl2 light
B

chloroethane
CH3CH2Cl

KOH(aq)
ethanol
CH3CH2OH
MnO4-

H+(aq)
D

acetic acid (ethanoic acid)
CH3COOH

    HCl/Al3   H2O / H3PO4, 300°C   CH3CH2OH / H2SO4
    C

ethylene (ethene)
CH2=CH2

  E

ethyl acetate (ethyl ethanoate)
CH3CH2-O-CO-CH3

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

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