Organic Chemistry

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  • #1

Me and my friends have been cracking our heads over this question:

Why does PhenylEthene solidify when exposed to air?

Me and my friends have come up with an explanation:

The oxygen in the air oxidises the phenylethene to a diol. The hydrogen bonds existing between the molecules bring them closer, thus it solidifies.

My teacher's explanation was that PhenylEthene undergoes an addition polymerisation reaction and the oxygen in the air acts as an initiator/catalyst for the reaction/.

My question is, why is the first explanation unacceptable and secondly, is oxygen really an initiator or catalyst for addition polymerisation reactions?

Answers and Replies

  • #2
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What mechanism would you propose for your reaction?

Your teacher's explanation involves a free radical process, the unsaturated compound, such as oil, will form a thin flexible plastic which is the polymer formed from the radical reactions. If you were to propose a radical mechanism, how would you explain your diol, it's quite implausible right?

The diols would probably interact intramolecularly, this would mean relatively less intermolecular hydrogen bonding and I'm not quite sure how one would make out a long range hydrogen bonding, even of an alcohol of phenylethene. You may want to explore melting points of alcohols in general, it would make a great school project, although I think that alcohols have generally high mps.
  • #3
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Phenylethene is nothing but styrene. And as far as I'm aware, the polymerization of styrene to polystyrene goes via a free radical mechanism. So, I believe the catalyst is light (possibly in the UV range) rather than oxygen (I can't see what role oxygen would play in this polymerization).

As for your explanation, I see the following problems :

1. Oxidation of an alkene to a diol is not a spontaneous reaction. It often needs a strong oxidizer like KMnO4 and a catalyst (in addition to water and possibly H+)

2. Intermolecular H-bonds in diols may, if at all, lead to dimerization. The geometry doesn't appear conducive to metwork formation. Also, intermolecular H-bonding in 1,2-diols, I believe, is relatively weak due to the formation of intramolecular H-bonds.

EDIT : Looks like GCT's covered essentially the same ground. I defer to his judgement on this.
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  • #4
ok... so my teacher was wrong for proposing that oxygen is an initiator of the addition polymerisation lol.

Thanks for your explanations. I appreaciate your help.