Tetrahedral Carbon

A brilliant solution to the problem posed in the preceding section came in 1874 when J. H. van't Hoff proposed that all four valences of carbon are equivalent and directed to the corners of a regular tetrahedron.  If we redraw the structures for C2H5Br as 1, we see that there is only one possible arrangement and, contrary to the impression we got from our earlier structural formulas, the bromine is equivalently located with respect to each of the hydrogens on the same carbon.

A convenient way of representing organic molecules in three dimensions, which shows the tetrahedral relationships of the atoms very clearly, uses the so-called ball-and-stick models. The sticks that represent the bonds or valences form the tetrahedral angles of 109.47^o.