Classroom

In this section of Resonance, we invite readers to pose questions likely to be raised in a classroom situation. We may suggest strategies for dealing with them, or invite responses, or both. "Classroom" is equally a forum for raising broader issues and sharing personal experiences and viewpoints on matters related to teaching and learning science.

G Nagendrappa

Department of Studies in

Chemistry

Central College Campus

Bangalore University

Dr Ambedkar Veedi

Bangalore 560 001 ,lndia.

Email :nagendrappa@vsnl .net

Part 1. Introduction, Reso­

nance, Vo1.10, No.2, pp .72-78,

2005 .

Keywords

Free radical polymerisation,

radical oxidation, radical nitro­

sation, polyethylene, PVC, neo­

prene, rubber, phenol, acetone,

nylon-6 .

An Appreciation of Free Radical Chemistry

2. Free Radical Reactions in Industry

The Initiation of the Polymer Industry

Until the early 1930's, free radical chemistry was essentially confined to laboratory research. Its defining moment came

when an accidental discovery was made at the Imperial Chemical Company in 1932. Ethylene was being subjected to high pressure

(1400 atm) at about 170°C. Among fifty attempts, one experiment in which benzaldehyde was I?resent gave a white waxy solid. A few months later, another accidental discovery was made. Ethylene in the reaction vessel had leaked out. To compensate

the loss, an additional quantity of ethylene was pumped in. This time, polymer formed readily. The cause for this was found to be oxygen present in the right amount (-3-4%) as an impurity in

ethylene. Several other such accidental discoveries were made, which helped in developing the polymer industry.

WW II provides a Big Thrust

When the chemistry behind such polymerisation processes was

recognised as free radical in nature, other systems and experimental conditions were tried out. By the time World War

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II broke out (1939), free radical polymer chemistry had made considerable progress. The occupation of Indonesia and South­East Asia by Japan cut off the supply of natural rubber to the allied powers. This gave further impetus to develop polymers as a substitute for rubber. Neoprene rubber (polychloroprene or poly-2-chloro-l,3-butadiene), which had properties very similar to natural rubber (polyisoprene orpoly-2-methyl-l,3-butadiene), was adopted for use in automotive tyres and many other purposes.

~H2C~ '-CH,k Natural rubber (Polyisoprene)

Neoprene (Polychloroprene)

At about the same time, polyvinylchloride (PVC), butyl rubber (a copolymer of 2-methylpropene with 2-5% isoprene), polystyrene, and butadiene-styrene copolymers were also created by employing free radical processes.

The Polymerisation Process

Polymers can be prepared by either free radical chain processes or by ionic (both cationic and anionic) chain reactions. However, the free radical polymerisation methods are the most popular. About three fourths of all polymers manufactured make use of free radical routes. The reasons are free radical polymerisation is easy to perform, requires minimal purification of monomers, can be performed under a wide variety of conditions and is economical.

In the early years of polymer technology, the formation of dead polymers all through the course of polyrnerisation was common. This led to poor control over molecular weights, molecular architecture or crystalline character, which meant variations in the properties of the polymers produced even under the same experimental conditions. However, the situation has vastly changed, and it has been possible to prepare dormant polymers, which can be revived as and when needed to continue the

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R-Q-O-R peroxide

heat or

hv

CLASSROOM

2 R-O Alkoxy radical

-

(Initiator, In-) (1)

--...., .. ~ In-H2C-CH2 (2)

H2C=CH2 ~ In-H2C-CH2+CH2-CH2 i:-n Polyethylene

In-[H2C]mCH2 ~ In-H2C-CH2+CH2-CH2 tin m+n

-

(3)

(4)

In-[H2C]nCH2 + In-[H2C]mCH2 ----. In-[H2C]nCH3 + In-[H2C]mCH=CH2 (5)

eq (2): Initiation

eq (3): Propagation eq (4): Termination by coupling

eq (5): Termination by disproportionation

polymerisation further. The result is that polymerisation can be Scheme 1.

manipulated in such a way as to get polymers of desired molecular weights, crystallinity and other properties.

Mechanism of Polymer Formation

Like other free radical chain reactions, the free radical olefin

polymerisation is a chain process, comprising the initiation step, the propagation step and the termination step, as discussed in Part 1.

The basic mechanism of ethylene polymerisatlon to polyethylene

involving these features is shown in Scheme 1, and it holds good equally well for other olefins.

The polymer chain that has an electron and can grow further by propagation is called a 'living polymer'. However, the polymer growth terminates (resulting in dead polymers) by coupling of

radical chains (4), disproportionation (5), chain transfer, or in presence of radical inhibitor.

The Ordered Growth of the Polymer

Polar effects lead to regiospecific growth of polymers, e.g., polyvinyl chloride, polystyrene, etc., if a substituent is present

-RE-S-O-N-A-N-C-E--I -M-a-rc-h--2-0-0-5------------~------------------------------7-3

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Polyvinyl chloride (PVC)

CI-CH=CH 2 + In In

(

CI

R= In~. or inhibitor)

Polystyrene

Ph-CH=CH2 + In In

(

Ph

R= In~. or i nhi bitor )

Styrene-maleic anhydride copolymer

In + V' _ ~In 01:)..0 maleic

anhydride (M) styrene (S)

Structure 1.

nucleophilic radical

o

electriphilic radical

.... In-S-M-S-M-S-M-Ordered copolym"er

on the double bond (Structure 1). Note that the substitutes are located at regular intervals in the polymer chains.

In the 1980s, inhibitors were discovered, which could terminate a chain process to form dead polymer that could be revived into living polymer at will. This has made it possible to produce

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polymers of desired properties with high reproducibility.

Since the present article is not about polymers alone, the examples considered so far are sufficient to show the importance of free radicals in polymerisation. They clearly demonstrate the contribution of free radical chemistry to the outcome of World War II and to the development of polymer chemistry.

Manufacture of Chemicals

Free radical reaction based industry is not confined to the manufacture of polymers alone. Several very useful chemicals are being produced by employing free radical reactions. We shall illustrate with two examples - (i) the process for simultaneous production of phenol and acetone, and (ii) the manufacture of nylon-6.

The Manufacture of Phenol (Hercules Process)

This process involves oxidation of cumene with dioxygen to its hydroperoxide, which is eventually converted to phenol and acetone. The oxidation takes place in the presence of a catalytic

amount of hydrogen bromide. Cumene hydroperoxide produced is treated with 10-2S% H2S04 at about SS-60°C to yield phenol

and acetone, which have many applications. The main reactions involved are given in Schemes 2 and 3. There are several notable

features in these reactions.

(i) The initiating reaction uses HBr in catalytic quantity.

Oxygen, itself a diradical and a major reactant here, abstracts hydrogen from HBr to generate bromine (Br-) and hydroperoxy (H-O-O·) radicals: Both are reactive radicals and are capable of abstracting hydrogen from a C-H bond; this is what happens in their next reaction with cumene (7).

(ii) Cumene has three types of hydrogen - five aromatic hydrogens on the benzene ring, six methyl hydrogens, and one methine hydrogen on the isopropyl side chain. The methine hydrogen is the most vulnerable, because it is attached to benzylic tertiary carbon and has therefore the lowest bond dissociation

Box 1. Uses of some Polymers

Polyethylene

--tCH2-CH2~

Bottles, tubing, sheets

Polyvinyl chloride

CI

--fCH2-

6H* Rain coats, shower

curtains, garden hose, rigid

clear bottle, swimming

pool liner

Polystyrene

Ph

--fCH2-6H* Moulded objects, electrical

insulation

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Manufacture of phenol and acetone (Hercules Co.)

----1._ H-O-O 0 + Br 0 (6)

Me

Me--6 + H-O-O Ph/ •

Me I Me_C + H+

/ 'O .... OH Ph

(InO) (InO)

Me

Me--6 + InH Ph/ •

2

Me I Me--C 0

/ '0_0 Ph

3

(7)

(8)

Me I Me_C +

Ph/ 'O .... OH

Me I Me_C

/ 'O_OH Ph

5

(10)

Baeyer - Villiger rearrangement

+OH2

Me I Me_C

Ph/ •

Me--t Ph / ....... 0 .....

Me

Me--d Ph -----l~. / ....... 0 .....

Me, -----l... Ph -OH + C=O Me' Me 06 7

Scheme 2.

heat

(9)

---~~~ 2 H-O H-O-O-H Hydrogen peroxide

(13)

Hydroxy radical

Scheme 3.

(11 )

(12)

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energy. The resultant cumene radical 2 is stabilised by resonance

and steric obstruction by bulky substituents. (This is one reason

why it is formed most easily).

(iii) In the next reaction (8), oxygen adds to 2 to form the alkyl

peroxy radical 3. This is an example of addition of one radical to another, which would have been a termination process if both were radicals each with one electron. However, since O2 is a diradical, the adduct 3 turns out to be a radical with one electron.

(iv) Peroxy radicals in general are quite reactive and 3, like H-0-0', abstracts a hydrogen from cumene (9) to form 2 again, as

in (7), completing the cycle, and 3 getting converted to

hydroperoxide 5.

(v) The reactions depicted in equations (7-9) are repeated till

all the cumene has reacted. Note that each reaction is highly

selective, which is essential for obtaining the desired product in

the purest possible state with none or very little by-products.

(vi) In the reaction of equation (7), HBr is produced again

which is reused for the reaction in (6). Thus, small amount of HBr is sufficient to initiate the reaction.

(vii) The hydrogen peroxide produced in reaction 2 can

decompose in to two hydroxy radicals, which also can bring

about the oxidation of cumene by abstracting the hydrogen (equations 13 and 14)

Manufacture of Nylon-6 (Toray Process)

In this process caprolactam, the monomer required for making

the polymer nylon-6, is produced by free radical nitrosation of cyclohexane. The nitrosating compound is nitrosyl chloride,

NOCI (Cl-N =0). Its chemical properties make it suitable for this purpose. The important reaction steps involved in the

process are depicted in Scheme 4.

A salient aspect of this reaction, which is different from the oxidation of cumene in the previous example, is that nitrosation

-RE-S-O-N-A-N-C-E--' -M-a-rc-h--2-0-0-5------------~------------------------------n-

Scheme 4.

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Manufacture ofNylon-6 (Toray Co.)

~c hv • • CI-NO ~ CI + NO (15)

O~:'CI-.8

0" + "NO-0" +HCI

9 ONO 10

{

·~I = highly reactive

NO = slow reactive

Selective (16)

(17)

... ONOH --- (18)

11

O

NOH (N~O +H+~~ (19)

12 Beckmann rearrangement

(N~ H r~ 1 ~ ~o ---.. -NiC-(CH2)S-NHtC- (20)

Nylon - 6

is a non-chain radical reaction. The other noteworthy characteristics of this process are

(i) Only chlorine radical abstracts the hydrogen atom of cyclohe­xane. The nitrosyl (NO') radical does not perform this task (16).

(ii) Cyclohexyl radical (9) does not abstract NO from NOCI. If this were to happen, the reaction could have taken the charac­teristics of a chain process. Abstraction of a group, even if it is only diatomic, such as NO, is difficult. Therefore, only single atom abstraction reactions are commonly observed, and they usually involve H (and halogens - CI, Br, and I - in some cases).

(iii) Chlorocyclohexane is not formed by abstracting CI from NOCI. This is because cyclohexyl radical is present in very small concentration at any given point of time during the reaction, and the chlorine radical, which is also in low concentration but

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far more reactive than NO·, is rapidly used up in H-abstraction reaction of (16) and is not available to couple with cyclohexyl

radical. (This is important in atmospheric chemistry also, which

will be considered in the next part of this series).

On the other hand, NO· combines with 9, as it cannot undergo any other reaction in this case, and produces nitrosocyclehexane (10). (In Barton reaction also, a similar coupling of NO and car­bon radical takes place. We shall consider this in another part).

(iv) Nitrosoalkanes containing a hydrogen atom at the alpha­position undergo tautomerisation to oximes. This occurs very

rapidly in the presence of an acid or a base catalyst.

R >=NOH

R1

Though this is an equilibrium reaction, oximes are more stable

and once formed, they do not normally revert to the nitroso

compound.

The compound 11 is cyclohexanone oxime, identical to the one

obtained by the reaction of cyclohexanone with hydroxylamine.

(v) Cyc1ohexanone oxime (11) undergoes Beckmann rearrange­

ment under acid catalysis to caprolactam, which is then polymerised to obtain nylon-6. These are ionic reactions.

All the radical reactions that are depicted here exhibit very high selectivity, making it possible to obtain the desired products in excellent yields, with required purity, and at competitive costs,

compared to the alternative processes.

In the next part, we shall consider the role of free radicals in

causing diseases as well as fighting them to keep us healthy, and related free radical reactions.

Suggested Reading

[1] B Giese,Radicais in Organic

Synthesis, Pergamon, New

York, 1986.

[2] A F Parsons,Anlntroduction

to Free Radical Chemistry,

Blackwell Science, Oxford,

2000.

[3] P Renaud and M P Sibi

(Editors),Radicalsin Organic

Synthesis, Vols.l and 2,

Wiley-VCn, Wein-heim,

2001.

[4] V R Gowarikar, N V Viswa­

nathanandJ Sreedhar,Poly­

mer Science, New Age Inter­

national, New Delhi, 1999.

[5] C Walling, J. Chem. Educ.,

Vo1.63,pp.99-102,1986.

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