Mystery Molecule and Jack Dunitz on Fluorine

Jack Dunitz (one of the greatest chemical crystallographers) visited our lab today. I had told people beforehand that I would ask him what the mystery molecule was and prophesied that he would get it immediately. He did.
This gives me a chance to record the enormous personal debt I owe to Jack with whom I spent a year in Zurich. He is deeply loved by the many people who have passed through his lab. He now works almost exclusively with theoretical tools rather than equipment and today told us about Fluorine – or more precisely organofluorine compounds. Substituting hydrogen by fluorine in hydrocarbons (aliphatic or aromatic) makes almost no difference to physical properties (except density), but despite their similar properties the fluorocarbons and hydrocarbons don’t mix. In fact perfluoro butane in butane has one of the highest activity coefficients (10). But even the molecular volume is almost unaltered. In some directions the fluorine is actually smaller than the hydrogen.
So there are still many simple observations in chemistry that we don’t understand. With Gautam Desiraju one of Jack’s most engaging was that over the whole of reported chemical space there are more compounds with an even number of carbon atoms than odd (Nature closed access reference) – see report in New Scientist where he is quoted as:
“It’s much more intriguing if you don’t offer an explanation,”
I’ll leave you with the puzzle he greeted me with when I started in Zurich. “If a golf ball is hit with an infinitely massive golf club moving with velocity V, what will be the velocity of the ball after the collision”. The answer is simple and logical.

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13 Responses to Mystery Molecule and Jack Dunitz on Fluorine

  1. Bill says:

    If the club is infinitely massive, inelasticity of collision doesn’t matter, so — Vball = Vclub? I feel as though I am missing something.

  2. pm286 says:

    (1) I should have said that the collision is perfectly elastic. If this doesn’t make sense, then yes you are missing something. It’s not a trick. I didn’t get it first time either.

  3. Russ says:

    Caveat: I am an organic chemist, so I dropped physics like a bad habit years ago.
    But anyway, my guess is that if the club head is infinitely massive, then it is unable to move, and V must equal zero and therefore it can’t really hit the ball.
    Maybe I’m oversimplifying.

  4. pm286 says:

    (3) Russ, that’s brilliant. But I shall invoke an infinitely powerful superbeing to wield the club. Assume it’s (say) 1000 times more massive than the ball and there is a precise answer (in the ideal physics world at least – I play golf once a year and the velocity of the ball is either zero or has a large crossproduct with the club (i.e. it goes in an unintended direction).

  5. Propter Doc says:

    You like tormenting people! If the club is 1000 times more massive than the ball, then can we state that the velocity of the ball would be 1000 times the velocity of the club?
    As for the molecule. Is it a monohydrate?

  6. pm286 says:

    (5) I don’t like tormenting people! I hope the problems generate discussion – I assumed that the crystal would be something that you might ask your crystallographer. If nothing else it is a good way of getting to know her.
    You can state that the velocity of the ball would be 1000 times the velocity of the club. You would be wrong!

  7. Russ says:

    I was hoping someone would pipe up with the answer, because now it’s kind of bugging me. It would seem the standard high school physics doesn’t apply to infinitely massive golf clubs wielded by superbeings. Or if it does, the golf ball flies off at infinite speed and/or blows up in a blinding flash of energy (amounting to it’s mass times the speed of light squared).
    Put a nerd out of his misery here – what’s the answer?

  8. Matt says:

    The ball would have infinite velocity?
    Momentum must be conserved, and as momentum = mass x velocity,
    mass of club x velocity of club = mass of ball x velocity of ball
    if the club has infinite mass and the ball doesn’t, then the ball must have infinite velocity.
    if something really heavy hits something really light, the light object moves really fast.

  9. Chris says:

    If the club has an infinite mass then it also has an infinitely massive gravitational field, the ball would become irretrievably trapped on the club. So its velocity would be the same as the club V.

  10. pm286 says:

    (9) Chris – that’s lovely – and quite defensible. But these questions date back before the theory of relativity. Questions like “an infinitely smooth elephant which can be approximated to a sphere”. I will pursue the maths on the other post

  11. The ball doesn’t go infinitely faster than the club because kinetic energy must be conserved (in addition to momentum). In fact, the final velocity of the the ball is 2*V. It can be seen intuitively without any algebra by thinking about it from the club’s frame of reference: the club sees a tiny ball approaching it with velocity -V. Since the club doesn’t even move due to it’s infinite mass, the ball must bounce with velocity +V, which is +2V from the original frame of reference. In fact, this is the same that happens when a ball bounces off the Earth. 😉

  12. Oops, I hadn’t noticed that you posted the solution on another post…

  13. pm286 says:

    (13) Don’t worry – It’s difficult to keep up with multiple postings

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