Isomers

absolute configuration

For a chiral molecule: describes the spatial arrangement of these atoms or groups within the molecule relative to each other. - determined using the R/S naming convention

relative configuration

For a chiral molecule: the relative configuration is its configuration in relation to another chiral molecule - compared between the R and the S enantiomers

asymmetric

carbon atom that lacks a plane or point of symmetry Example) 1-bromo-1-chloroethane -- this molecule is chiral because it is not superimposable on its mirror image

isomers

chemical compounds that have the same molecular formula but differ in structure---that is, in their atomic connectivity or the spatial orientation of their atoms

configuration

describes the spatial arrangement of the atoms or functional groups of a stereoisomer

achiral

molecules that are mirror images of each other that can be superimposed Example) the letter A Example) 1,1-dibromoethane -- has a plane of symmetry -- a simple 180 degree rotation along the y-axis allows the compound to be superimposed upon its mirror image

chiral

- a molecule that is not superimposable upon its mirror image - carbon atoms are chiral only if they have 4 different substituents

cis

- above the double bond - same side

structural isomers

- constitutional isomers - compounds that share only a molecular formula - differ in where and how atoms are connected to each other and thus often have very different chemical and physical properties (melting point, boiling point, solubility, etc.)

cis-trans isomers

- formally known as geometric isomers - compounds that differ in the position of substituents attached to the two carbons that form a double bond - because double bonds cannot rotate, the substituents are fixed relative to each other and to the bond - if the substituents on the carbons are both ABOVE THE DOUBLE BOND, they are on the SAME SIDE, and the double bond is called CIS. - if one is ABOVE and one is BELOW, they are on OPPOSITE SIDES, and it is called a TRANS double bond

(E)

- from German entgegen, meaning opposite - used for alkenes - used if they are on opposite sides "epposite"

(Z)

- from German zusammen, meaning together - used for alkenes (double bonds) - used if two highest priority substituents on each carbon are both above or below the double bond "on ze zame zide"

trans

- one is above - one is below - opposite sides

Fischer projection

- used to represent a 3-dimensional molecule in 2 dimensions - horizontal lines indicate bonds that project out from the plane of the page, while vertical lines indicate bonds behind the plane of the page - can be manipulated by interchanging any two pairs of substituents or by rotating the projection in the plane of the page by 180 degrees, and it will keep its same absolute configuration - if only one pair of substituents is interchanged or if the molecule is rotated by 90 degrees, the mirror image and thus the opposite enantiomer of the original compound is obtained - allow straightforward determination of the configuration at a chiral center

For compounds with more than one substituent on either carbon of the double bond:

1. From the carbon of interest, determine the atomic weight of the first atom encountered along each bond. The group with the highest atomic weight atom has the highest priority. 2. If two atoms are the same, look at the next atom attached to each group; the group that has the second atom with higher molecular weight is higher priority. 3. If two atoms are the same, a double bond takes priority over a single bond. This is a tie-breaker only; higher atomic weight will ALWAYS take priority over a double bond.

C6H14 isomers

Isomers of C6H14 include the following: 1) n-hexane 2) 2-methylpentane 3) 3-methylpentane 4) 2,3-dimethylbutane 5) 2,2-dimethylbutane All have the same formula, but they differ in their carbon framework and in the number and type of atoms bonded to one another.

Determining absolute configuration:

STEP 1: ASSIGN PRIORITY TO THE 4 SUBSTITUENTS 1. From the carbon of interest, determine the atomic weight of the first atom encountered along each bond. The group with the highest atomic weight is higher priority. 2. If two atoms are the same, look at the next atom attached to each group; the group that has the second atom with higher molecular weight is higher priority. 3. If two atoms are the same, a double bond takes priority over a single bond. This is a tie-breaker only; higher atomic weight will always take priority over a double bond. STEP 2: Proceeding from highest priority (1st) to the second lowest (3rd), determine the order of substituents around the wheel as either clockwise or counterclockwise by drawing a loop that connects 1st to 2nd to 3rd priority, ignoring the 4th group. If the order is CLOCKWISE: the chiral center is called R (from Latin rectus, meaning right). If the order is COUNTERCLOCKWISE, it is temporarily called S (from latin sinister, meaning left). STEP 3: Looking at the lowest (4th) priority group: if group 4 is on the vertical line in a Fischer projection or is going into the page, the molecule is oriented correctly, and the designation remains the same. If it is on the horizontal line or coming out of the page, the molecule is in opposite orientation so the temporary designation is swapped to the opposite designation. R and S are placed in parentheses and separated by the rest of the name with a dash

stereoisomerism

Stereoisomers are compounds that have the same connectivity between their atoms and differ from each other only in the way that their atoms are oriented in space. Cis-trans isomers, enantiomers, diastereomers, meso compounds, and conformational isomers are all types of stereoisomers.