When does a SN1 reaction only yield one enantiomer, and not a racemic mixture?

[ihaz3gs]

I'm curious about how a SN1 reaction can yield only one product (if it can, at all), rather than two (ultimately leading to a retention of stereochemistry).

Would sterical reasons and/or its orbitals also have any play into this?

Thanks!

 

[chemisttree]

If the back side is blocked this happens. Substituted adamantyl compounds would be a perfect example of this type of reaction. Also seen if the reaction center is located at the bridgehead carbon of norbornane, for example. If a reaction is kineticically controlled, approach from one side can dominate as well. This is usually the case where a solid surface is involved in the rate-determining step.