Steric Hindrance is Like a Fat Goalie

by James

in Alkyl Halides, Organic Chem Study Tips, Organic Chemistry 1, Organic Reactions

Sportswriter Todd Gallagher asked a question that every hockey fan has probably asked at one point. “Could a morbidly obese goaltender succeed in the NHL?”

As he relates in “Andy Roddick Beat Me With a Frying Pan“, he talked Washington Capitals owner Ted Leonisis into letting him run an experiment. He hired Trevor, a local college goaltender, to wear a 1000 pound fat suit, and then let NHL players take target practice. Quote:

Trevor’s goalie crouch was itself unnerving: butt on the ice and legs splayed out in front of him—really the only way someone that large could be situated. There were certainly places to score around his head and shoulders, but he filled most of the net and made it difficult to see the goal line.

1goaliebackPhoto credit: Todd Gallagher

——–

There is simply no way this reaction is going to happen:

2-fatgoalie

Why not?

Pretty much every reaction in organic chemistry boils down to “nucleophile attacks electrophile”.  In order for it to occur, the nucleophile (the lone pair on the CN) has to be able to reach the electrophile (the antibonding orbital of the C-Br bond).

If they can’t meet, it’s not going to happen. And it’s not going to happen because the three alkyl groups attached to the carbon block the approach of the nucleophile to the electrophile: instead of hitting the sigma star orbital (aka the antibonding orbital), the nucleophile electrons bump into the electron clouds around the alkyl groups.  In organic chemistry there’s a phrase for this: steric hindrance. We use this phrase to denote when a reaction is prevented from occurring because the nucleophile and electrophile bump into one another rather than reacting. Think of it as a puck deflecting harmlessly off a goalie’s pads.

 I watched in horror as the Caps began to shoot, but Trevor blocked every single one of their first eleven shots, including a glove save he may not have even been aware of that drew cheers from the crowd. After one particularly brutal slapshot that ripped off the fat suit’s overalls, I checked on Trevor to see how he was doing. “My knee hurts and I can’t breathe.” Great, Trevor! Keep up the good work!

Steric hindrance is the big barrier for the SN2 reaction.* Remove it, and speed up the reaction. How do we do this? Make the carbon smaller. Remove alkyl groups and replace them with hydrogens. The rate of reaction drastically increases.

3-fatgoalie

Just remember: the smaller the goalie, the greater will be the chances to score.

4-fatgoalie

*one way to rationalize the poor reactivity of nucleophiles in hydrogen-bonding (i.e. polar protic) solvents is that they make the nucleophile more sterically hindered. The more electronegative the nucleophile (e.g. fluorine, F), the larger the “fat suit” of solvent molecules hydrogen-bonded to it.

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{ 7 comments… read them below or add one }

brsm

A great analogy, but the product of that doomed SN2 reaction (that you’re not making) should be the nitrile, not the bromide.

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James

Thanks for the correction!

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Todd

This is all way over my head but I am honored my work is being in any way useful to illustrate something academic.

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James

I am honored by your presence, good sir! Thanks for doing the hard work to answer this pressing question for hockey fans everywhere! PS Everyone should buy your book.

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Jon

I found this when I was studying for my final, got distracted by the book’s Amazon preview, and bought it just now so that it’ll be waiting for me in my P.O. box after I’m finished with my exam!

Thank you, Todd!

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Subhadeep Sarkar

superb analogy. Thanks for this article, it made a little portion of the Organic Chemistry clearer to me.

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Shifty

Love this. Though I must admit, as an aspiring biochemist my lack of formal learning in anything chemistry related should be worrying. Nucleophillic attack is my stock answer for everything. I’m also just as rubbish at hockey. Well someone has to populate the lower end of the bell curve.

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