Nylon-6
Nylon 6 is an awful lot like our friend nylon 6,6. You can look at the
picture if you don't believe me.
But making nylon 6 is lot different from nylon 6,6. First of all, nylon 6
is only made from one kind of monomer, a monomer called caprolactam. Nylon 6,6
is made from two monomers, adipoyl chloride and hexamethylene diamine.
If you want to see caprolactam in 3-D,
But enough small talk. Let's get on with the business of turning
caprolactam into nylon 6. Nylon 6 is made by heating caprolactam to about 250
oC with about 5-10% water thrown in. So what happens to caprolactam
when there's water around? The carbonyl oxygen looks around, and sees a water
molecule, and sees how easy it would be to steal one of the water's hydrogen
atoms. Now as is often the case, a little thing like this that seem harmless
enough can grow into something much bigger. If you watch, you'll see that
caprolactam's greed is going to get the better of it.
The carbonyl oxygen donates a pair of electrons to the hydrogen atom of
water, thus stealing the hydrogen from the water. This gives us a protonated
carbonyl, and a free hydroxyl group. Keep this hydroxyl group in mind, because
it is going to come back to haunt greedy ol' caprolactam. But first, let's
remember that the carbonyl oxygen now has a positive charge. It doesn't like
this, so it swipes a pair of electrons from the carbonyl double bond, leaving
the positive charge on the carbonyl carbon atom.
But carbocations are not happy critters. Putting a carbocation in a
molecule is just begging for some nucleophile to come along and attack it.
Nucleophile? Did someone say nucleophile? I think there's one nearby. It's that
old hydroxide ion that was left when caprolactam stole the proton from the water
molecule. This little hydroxide ion never really worked through the negative
emotions of having lost its proton to caprolactam. Still harboring a lot of
hostility, it attacks the carbocation.
The molecule formed is now an unstable gem diol. Unstable? Of course.
Didn't I tell you that caprolactam's greed would be its undoing? A mad
reshuffling of electrons happens next. The nitrogen atom donates a pair of
electrons to a hydrogen atom on one of the hydroxyl groups, stealing it away.
The electrons that the hydrogen shared with its oxygen shift to form a double
bond between the oxygen and the carbon atom. And lastly, the electrons shared by
the carbon and the nitrogen shift completely to the nitrogen, severing the
carbon-nitrogen bond.
Alas, the circle is broken, and caprolactam is no more! Like many junk
bond dealers in the eighties, it has paid dearly for its greed. What we're left
with is a linear amino acid.
But our story is far from over. You see, that linear amino acid can react
with a caprolactam molecule, a lot like the water molecule did. Caprolactam
molecules aren't very bright. Witnessing one of their own destroyed by greed
doesn't make them any less greedy. They just try to steal what they can from
their fallen sibling, like greedy little buzzards. Ever avaricious, a
caprolactam molecule will steal the acid hydrogen form the linear amino acid.
The carbonyl oxygen donates a pair of electrons to that hydrogen, stealing it
away from the amino acid.
And as expected, the electrons rearrange to form the carbocation, just as
before:
This carbocation is still an open invitation to any nucleophile around,
but this time, there's a new nucleophile on the block. That's the amino acid
that just lost its acid hydrogen. It too has a lot of hostility towards the
thieving caprolactam, and attacks just like we saw the hydroxide ion attack
earlier.
This gives us an ammonium species, and this particular one is very
unstable. The electrons play musical chairs. Showing no elemental loyalty, the
ring nitrogen steals a hydrogen from the ammonium nitrogen. In addition, the
bond joining the carbon and the nitrogen is severed, opening the ring. Another
greedy caprolactam molecule bites the dust.
But we're not through yet. That carboxylate group at the end of the
molecule is going to sweep around and steal the alcohol hydrogen.
This makes a new carbonyl group in the middle of the molecule, and
regenerates the carboxylic acid. (Here's a little secret: No one really knows
the order of the last two steps. They might happen in the reverse order. We just
know that both of them happen before everything is over.) If you want to see a
movie of the whole propagation process, . (Remember, this is
a step growth
polymerization.)
Now that we have an acid again, it is sure to react with another
avaricious caprolactam molecule, and then another, and another, until we get
long chains of nylon 6.
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