FREE RADICAL REACTION
      

A free radical is formed by a homolytic cleavage of the CC σ-bond. That means each carbon gets back its own contribution of valence electron when the bond breaks up. If this is not so then the carbon will become positively or negatively charged.

This class of reaction follows a standard mechanism:

  • Step 1 − Initiation,
  • Step 2 − Propagation, and
  • Step 3 −Termination.

There are also side reactions that often accompanied this chain reaction, which varies from reaction to reaction.

The two most important free radical reactions in hydrocarbon chemistry are:

  • the halogenation of alkanes;   X2   +   RH   �   RX + HX
    where X2 = F2, Cl2 or Br2
  • polymerisation of vinyl compounds;   n CH2=CHZ   �   −(CH2−CHZ)n−
    where Z = a substituent group.

MECHANISM

INITIATION STEP

Hydrocarbons when heated to high temperature (like 400�C) will break up to give free radicals. But such high temperature is detrimental for chemical reactions since we want to avoid breaking up the hydrocarbon chains.

For useful free radical reaction we need some compounds or systems that can give a free radicals at low temperature not higher than 100�C; the lower the better. There are three possibilities:

  • UV IRRADIATION

    UV radiation has the appropriate energy to excite certain compounds to bring about homolysis. Examples are chlorine and bromine.

    X2 UV
    �    		 2   X�

  • THERMAL DECOMPOSITION

    Peroxides are compounds with −O−O− bond, and can be homolysied at as low as 60�C.
    −O−O−heat
    �    		2   −O�








    Persulphate






    Benzoyl peroxide

    A well known, although not the most useful for free radical reaction, is hydrogen peroxide, HO−OH. Hydrogen peroxide is not thermally stable. The more useful peroxides for aqueous system are the (sodium, potassium or ammonium) persulphates.

    For non-polar solvents we commonly use benzoyl peroxide.

  • REDOX REACTION

    We can also find reactions that can produce free radicals at room temperature. Some examples are:

      (1) H2O2   +   Fe+� � HO‾   +   HO�   +   Fe+�
      (2) ‾O3S−O−O−SO3‾   +   Fe+� � SO4‾�   +   SO4‾�   +   Fe+�
      (3) R−CH2−OH   +   Ce+4 � R−CH(�)−OH   +   e+�

    • The free radical produced above can be used to attack a compound to give a propagating radical. This step is known as the initiation step. For hydrocarbons the propagating free radical can be obtained by;

    PROPAGATION STEP

    This will be the attack by the propagating free radical to generate another propagating free radical. For RH the free radical would abstract the proton to generate another alkyl free radical, while for the vinyl compounds the free radical would add onto the CC π-bond to also give an alkyl free radical.

    Free radical prefer to attack a CC π-bond as compared to abstracting a proton. The next preference is for the formation of alkyl radicals according to its stability:

    allylic free radical > tertiary free radical > secondary free radical > primary free radical > methyl free radical.

    Free radical do not attack the aromatic hydrocarbons because of its resonance stability.

    For a conjugated allylic radical R−CH=CH−CH(�)−R' it can rearrange itself so that the radical moves to two carbons away R−CH(�)−CH=CH−R'.

    The amount of initiator used must be kept very low, like about 0.1%, as the objectve is for it to start the reaction. The reaction should then be self-sustaining. This is very important since the initiator is very reactive towards free radical. Remember it is important to avoid by-products in a reaction. It cost us to remove them when we isolate the product we want. So once the free radical is generated we must ensure that every time it collides with another compound it is the reactant. Example if we want to prepare methyl chloride initiated by a peroxide;

    CH4   +   Cl2   �   CH3Cl   +   HCl

    We want to make sure that every CH3� produced will knock into a chlorine molecule all the time. If it knocked into a peroxide initiator we will get RO−CH3 and RO�. This will kill a chain and also waste a RO� species. Effectively it kill two reaction chains, consequently slowing down the reaction.

    Of course each reaction will present its own problem. For example for the reaction;

    CH3CH2CH3   +   Cl2

    We can get either two products CH3CH2CH2Cl and CH3CHClCH3 at least. There is most likely to be more. For hydrocarbons with different possible carbon centres the choice of attack by a free radical would be according to Arrhenius' law;

    • the stability of the radical that would be formed (activation energy);
    • the statistical probability of collision (collision frequency); and
    • the reactivity of the attacking free radical (activation energy).

    The attack on the proton of the middle carbon for propane will give a more stable secondary radical, but the statistical probability of collision with the terminal protons as versus the middle protons is 6:2. So we will always get a mixture of the two chlorides no matter what we do. No chemical process engineering is going to solve this problem.

    Tutorial 6.1

    If we wish to prepare 2-chloropropane what is the best reaction to use? Hint   Answer

    Sometimes we will opt for bromination instead of chlorination; if the bromide product is acceptable for the application. Bromine free radical is less reactive. Because of this it will not react at the first collision, but will do so if it can form a more stable free radical. So the reaction is more skewed towards the stability of the free radical formed.








    α−chloroethylbenzene






    β−chloroethylbenzene

    One good illustration of this is the free radical chlorination of ethylbenzene. The products are 56% of α−chloroethylbenzene and 44% β−chloroethylbenzene. But with bromination the product is just α−bromoethylbenzene.

    TERMINATION

    Free radicals are present in very, very low amount. However after thousands of propagation reactions the free radical finally ran out of luck. It knocked into another free radical and got killed. This is the termination reaction.

    Being active species there is no activation energy barrier here. The only controlling factor is the statistical probability of collision. The fact that they are such active species means they do not exist in the system for too long. Actually it is not possible to isolate them for analysis. They are like "ghost", we cannot physically see, but our chemistry makes sense by assuming their existence. Of course its presence can now be proved by sophisticated techniques we have developed.

    POLYMERISATION

    When there is a π-bond present the free radical would prefer to attack the π-bond first before it goes for the proton in the alkyl portion. The free radical reaction of alkene must be one of the most important reactions in the chemical industry. If we take ethylene, a primary product in the petrochemical industry, add ca 0.1% (by mole) of peroxide to it and then heat it slightly, a sequence of reaction will follow.

    (RO)2 � 2 RO� (1)
    RO�   +   CH2=CH2 � ClCH2C(�)H2 (2)
    RO−CH2C(�)H2   +   CH2=CH2 � RO−CH2CH2CH2C(�)H2 (3)

    Since the amount of peroxide is very low the chances for an alkyl free radical to collide with it is rather limited. So reaction (3) will repeat itself hundreds of times before the alkyl free radical collides with a peroxide molecule.

    ------CH2C(�)H2   +   (RO)2 � ------CH2CH2−OR   +   RO� (4)
    RO�   +   CH2=CH2 � RO−CH2C(�)H2 (2)

    The other possibility would be;

    ------CH2C(�)H2   +   �CH2CH2-------- � ------CH2CH2CH2CH2-------- (4)

    The product we get can be represented by − [CH2CH2]n−, where n can be over 1000. We named it polyethylene (poly in Greek means "many". Many units of ethylene). Polyethylene (or PE) is used to make supermarket plastic bag.

    This free radical chain reaction is then known as a Free Radical Polymerisation.

    If instead of ethylene we carry out the reaction with styrene, CH2=C(C6H5)H, another primary product in the petrochemical industry, we would get −[CH2−C(C6H5)H]n−, polystyrene. Polystyrene is the white foam plastic container when you buy takeout or a cup of hot drink.

    Tutorial 6.2

    For the free radical attack on styrene why do we get
      Cl�   +   CH2=C(C6H5)H � ClCH2−C(�)(C6H5)H
    Instead of Cl�   +   C(C6H5)H=CH2 � ClC(C6H5)H−C(�)H2    Answer

    This chemistry was so important that in 1953 Hermann Staudinger was awarded the Nobel Prize in Chemistry for his discoveries in the field of macromolecular (big molecule) chemistry. He is generally acknowledged as father of Polymer Chemistry; the chemistry which led to the development of commercially important plastics and elastomers.

    Starting from ethylene, propylene, and acetylene, the products of the cracking of petroleum, a high volume of alkene products are prepared for the manufacture of commercial plastics and elastomers.

    Tutorial 6.3

    Vinyl chloride, CH2=CHCl, and vinyl acetate, CH2=C (OC(O)CH3)H, can also be polymerised with the help of an initiator. An initiator is a compound that will decompose easily to give free radicals at a low temperature of between 60�C to 100�C to. Representing the initiator by In2 present the reaction mechanism for the polymerisation of these two alkenes. Name an application for each of the product?    Answer

    PRECAUTIONS

    Free radical reaction is not sensitive to water so it is one of the easier reactions to execute. However it is sensitive to oxygen. So it is important to conduct the reaction in an oxygen-free environment. One technique is to bubble nitrogen gas gently into the reaction mixture, aqueous or organic, for about 2 minutes or more and conduct the reaction in an enclosed flask.

    YOU MAY WANT TO KNOW

    However free radical reaction is not confined to the laboratory. It is happening around us all the time. For example if you leave articles made from plastics outdoor, very often you will find that it eventually becomes brittle and breaks up. A good example is clothes pegs or polypropylene ropes. This is because the free radical, generated by the UV radiation of the sun on the oxygen in the atmosphere, is able to abstract a proton from the polypropylene, −[CH2−CH(CH3)]n−, or the polystyrene, −[CH2−CH(Ph)]n−, to form stable free radical.





    		 














    The plastics being solid could not execute a chain reaction. Instead the tertiary free radical can only react with the oxygen (or any other suitable molecules in the atmosphere) to form peroxides and so on. This eventually resulted in the breaking of the bond to give aldehydes, ketones, or some other compounds.

    Medical science speculated that free radicals also exist in our bodies, most likely as a result of a redox reaction. Natural products do have hydroperoxide groups in them and we do generate reducing chemicals like bisulphite in our digestive system. The free radicals are considered harmful chemicals, which can destroy cells in our body, or as an agent for cancerous growth. So when you are advised to eat colour food as a health measure, what it means is you must take in food containing CC π-bond. This would neutralise the free radicals into harmless compound.
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