Head-Tail Placement

Head-tail placement has to do with which carbon the R group is placed on (this contrasts with tacticity where which side the R group is on is the issue.) 
 H   H
 |   |
 C - C 
 |   |
 H   R
The carbon with the R on it can be called the "head", and then to maintain consistency, the carbon, and the carbon without the R should be called the tail. Sperling calls a polystyrene connection with two benzene groups on adjacent carbons a "head-to-head", so from this it follows that the presence of an R group on a carbon makes that carbon a "head" carbon. 
 H   H   H   H 
 |   |   |   | 
 C - C - C - C     Head to Tail placement
 |   |   |   |
 H   R   H   R
    |-----|


 H   H   H   H
 |   |   |   |
 C - C - C - C    Head to Head placement
 |   |   |   |
 H   R   R   H
    |-----|

 H   H   H   H
 |   |   |   |
 C - C - C - C    Tail to Tail placement
 |   |   |   |
 R   H   H   R
    |-----|
I've never seen an author talk about tail-to-tail placement. They always say "head-to-tail" or "head-to-head." Strictly speaking, tail-to-tail is just as legimate as head-to-head.

Assume a reaction where head-to-head is highly unlikely (steric hindrance maybe.) If tail-to-tail occurs, as shown above, then that leaves a head at the end, and since another head can't react with it, only a tail will come in and react. But this head to tail leaves another head in the end, so you can see we're in a loop that forces head-tail reactions. A table of scenarios is provided:

head-to-head head-to-tail tail-to-tail what to call the outcome
easy easy easy random orienticity
not likely easy easy head-tail orienticity
easy easy not likely head-tail orienticity


Now, there may be a situation where head-head, head-tail, and tail-tail are possible, but the probabilities are different for each one. I don't know if research has been carried out in this area.



Last Update- September 2, 1995- wld