What is the difference between a stereogenic center and a chiral center?
A stereogenic center is like an umbrella term, under which a chiral center is defined.
A stereogenic center is just any location in a molecule where the interchange of any two groups gives a new stereoisomer.
A chiral center is specifically a stereogenic center that---if we are talking about a carbon atom:
- is directly connected to four different surrounding atoms or groups of atoms
So, a carbon-based molecule with one chiral center can also be said to have a stereogenic center (which is that same chiral center).
However, one example of a stereoisomer that has no chiral centers is a geometric isomer, which is a kind of diastereomer that exists in more than one distinct form with some form of restricted bond rotation.
trans / cis (or E / Z, respectively) carbon isomers are common geometric isomers. We can see that here:
As a result, a compound like trans-2-butene has no chiral center, but does have some sort of stereogenic center: either carbon 2 or 3 can be specified as such.
If you interchange the methyl and the hydrogen on carbon 2 or 3, you get a new geometric isomer, and you have thus altered the stereogenic center and given a new stereoisomer. (If you alter both carbons 2 and 3 in the same way, you just get the same compound vertically flipped.)