# Stereochemistry A chemical reaction is **stereoselective** if there is an unequal mixture of stereoisomers within the products. **Stereoselective** is different from **stereospecific** ## Stereoisomers **Stereoisomers** are a type of [[Isomers|isomer]] that, unlike [[Isomers|constitutional isomers]] will maintain the same connectivity between [[Atoms and Elements]], but differ in the molecule's **spatial arrangement**. ### Chirality A molecule is **chiral** if it is **not superimposable** on its mirror image - *When a molecule is **superimposable**, it can be aligned with its mirror image.* A **stereogenic center** (or a **chiral center**) is a carbon bonded to four unique substituents - *Since there must be four $\sigma$ (single) bonds present, these centers **must always** have an $sp_3$ hybridization* Chiral molecules must: 1. Have a chiral/stereogenic center 2. have **no symmetry** $2^n$ = max # of stereoisomers $n$ = # of stereogenic center - When naming diastereoisomers, include the R or S configuration - R - Clockwise - S - Counterclockwise ## Cis- & Trans- Isomers The molecular formula and names for stereoisomers will often be the same except for the *trans-* or *cis-* isomers - ***Cis-* Isomers** have **the same** directions of substituents within a ring - ***Trans-* Isomers** have **different** directions of substituents within a ring ![[Cis vs Trans Isomers.png]] ## Enantiomers A **racemic mixture** will contain equal amounts of both enantiomers ## Meso Compounds A **meso compound** It can often be tricky to determine whether or not a molecule is meso. You can use the R/S configuration of the chiral centers present as a sure fire method: ![[Determining Meso Compounds.png]]