O- Chem Ch. 2- Isomers

achiral molecule

A carbon atom with only 3 different substituents. - It has a plane of symmetry. - It is superimposable.

chiral center

A carbon with 4 different substituents.

conformational isomers

A class of stereoisomers in which the isomers differ in rotation around single (sigma) bonds. *These are the most similar isomers.

Configurational isomers

A class of stereoisomers that can be interconverted only by breaking bonds. The two categories are enantiomers and diastereomers, which can both be considered optical isomers.

Meso compounds

A molecule with chiral centers that has an internal plane of symmetry. (since they have a plane of symmetry, they are not optically active!) *they are the molecular equivalent of a racemic mixture.

Ring strain

Determines whether cycloalkalnes are fairly stable or unstable and arises from 3 factors: 1. angle strain 2. torsional strain 3. non bonded strain (aka steric strain)

determining R/S from fischer projections

Do it the same way as usual with the lowest priority in the back (vertical line). If it pointing to the side/ horizontal (out of the page) you can use these tricks: 1. Make 0 switches: Determine the order of substituents as normal and draw a circle from 1 to 2 to 3 to determine R/S. The true designation will be opposite of what you determine. 2. Make 1 switch: swap the lowest priority group with one of the groups on the vertical axis. Obtain the R/S designation and the true designation will be opposite. 3. Make 2 switches: swap the lowest priority group wth one of the groups on the vertical axis. Then, switch the other two groups as well. Because we made two switches, this molecule will have the same designation as the initial molecule. This is the same as holding one substituent in place and rotating the other 3 in order.

Straight chain conformations (the different types)

For compounds larger than butane, the name of the confirmation is decided by the relative positions of the two largest substituents about a given carbon- carbon bond. *There is also anti- staggered, in which the two largest groups are exactly opposite each other (the most energetically favorable) The other type of staggered conformation is gauche.

stereoisomers

Have the same chemical formula and connectivity but differ in how the atoms are arranged in space (their wedge and dash pattern). All isomers that are not structural isomers fall under this category.

optical isomers

isomers that affect the rotation of plane- polarized light. ex: enantiomers and diastereomers.

optical activity of enantiomers

one enantiomer will rotate plan polarized light to the same magnitude but opposite direction of its mirror image (assuming concentration and path lengths are equal). - A compound that rotates the plane of polarized light to the right (clockwise), is dextrorotatory (d-) and is labeled (+). - A compound that rotates the plan of polarized light to the left (counter clockwise), is levorotatory (l-) and is labeled (-). *the direction of rotation cannot be determined from the structure of a molecule- it must be determined experimentally. *The amount of rotation depends on the number of modules that a light wave encounters, which depends on the concentration of the optically active compound and the length of the tube through which the light passes. Standard conditions: 1 g/mL concentration and 1 dm (10 cm) length.

chemical properties

properties that have to do with the reactivity of the molecule with other molecules and result in changes in chemical composition. These are generally dictated by the functional groups in the molecule. *This has to do with the reactivity of molecules, resulting in a change in composition.

Cahn- Ingold- Prelog priority rules

*used to determine the highest priority group around a double bond or chiral center. The higher the atomic number of the atom bonded to the double- bonded carbon, the higher the priority. If atomic numbers of two atoms are equal, priority is determined by the next atoms outward. *If there is a double bond when you are going outward to determine priority (in the case of a tie), it is counted as two individual bonds to that atom (so it is counted twice).

fischer projection

- Horizontal lines= wedges (out of page) - Vertical= dashes (into page) - point of intersection= chiral carbon *rotating a Fischer projection by 90 degrees will just convert the stereochemistry of the molecule (R goes to S and S goes to R). By extension, interchanging any TWO pairs of substituents will revert the compound back to its original stereochemistry

Cis- trans isomers (aka geometric isomers)

A specific subtype of diastereomers in which substituents differ in their position around an immovable bond, such as a double bond, or around a ring structure, such as a cycloalkane in which the rotation of bonds is greatly restricted. In compounds with only one substituent on either side of the immovable bond: - Cis= if two substituents are on the same side of the immovable bond - Trans= If the two substituents are on opposite sides of the immovable bond. *For more complicated compounds with polysubstituated double bonds, we use (E)/(Z) nomenclature.

specific rotation/ equation

A standardized measure of light rotation by optically active molecules (rotations measured at different concentrations and tube lengths can be converted to this for comparison by the following equation).

enantiomers

A type of configurational isomer in which the two molecules are NONsuperimposable images of each other. - They have the same connectivity but opposite configurations at every chiral center in the molecule. - They have identical physical and chemical properties with two notable exceptions: 1. optical activity 2. Reactions to chiral environments

Diastereomers

A type of configurational isomer in which the two molecules are chiral and share the same connectivity, but are not mirror images of each other because they differ at some (but not all) of their multiple chiral centers --> this means that they must have multiple chiral centers. *For any molecule with "n" number of chiral centers, there are 2^n possible stereoisomers. *they have different chemical properties and physical properties *They rotate plan polarized light; however, knowing the specific rotation of one diastereogmer gives no indication of the specific rotation of the other.

absolute conformation of a chiral molecule

Describes the exact spatial arrangement of the atoms or groups around the chiral center, independent of other molecules.

Chiral

An object is considered chiral if its mirror image cannot be superimposed (put on top of) on the original object. - The molecule lacks an internal plane of symmetry. *Think of your hands.

physical properties

Characteristics of processes that don't change the composition of matter. *There is no change in the composition of matter. ex: melting point, boiling point, solubility, odor, color, density.

cis/ trans cyclohexane conformations

In rings with more than one substituent: - CIS= both groups are located on the same side of the ring. - TRANS= the groups are located on opposite sides of the ring.

relative configuration of a chiral molecule

It's configuration in relation to another chiral molecule (often through chemical interconversion). We can use the relative configuration to determine whether molecules are enantiomers, disatereomers, or the same molecule.

Achiral

Objects that have mirror images that can be superimposed (put on top of).

structural isomers (aka constitutional isomers)

The only thing that they share is their structural formula (their molecular weights are the same). These are the least similar of all isomers.

optical activity

The rotation of plane polarized light by a chiral molecule. *for a molecule to have optical activity, it must have chiral centers within it AND lack an internal plane of symmetry.

Angle strain

Strain in a cycloalkane which results when bond angles deviate from their ideal values by being stretched or compressed.

Torsional strain

Strain in a cycloalkane which results when cyclic molecules must assume conformations that have eclipsed or gauche interactions (not favorable).

Non bonded strain (aka steric strain)

Strain in a cycloalkane which results when nonadjacent atoms or groups compete for the same space. This is the dominant source of steric strain in the flagpole interactions of the cyclohexane boat confrontation. To alleviate the strain, cycloalkanes attempt to adopt various nonplanar conformations, which are shown in the picture. *the most stable conformation of cyclohexane is the chair conformation!

Newman projection

The molecule is visualized along a line extending through the carbon- carbon bond axis.

Chair conformation of cyclohexane

This is the most stable conformation in which there are equatorial and axial positions, which alternate around the ring (if the wedge on C1 is an axial group, the dash on C2 will also be axial, the wedge on C3 will be axial, and so on). - axial= perpendicular to the plane of the ring (sticking up/down) - equatorial= groups parallel to the plane of the ring (sticking out) *for substituted rings, the bulkiest group will favor the equatorial position to reduce non bonded strain (flagpole interactions) with axial groups. *In rings with more than one substituent, the preferred chair form is determined by the larger group, which will prefer the equatorial position.

(R) and (S) forms

This nomenclature is used for chiral (stereogenic) centers in molecules. The steps are as follows: 1. Assign priority to each group around the chiral carbon using the Cahn- Ingold- Prelog priority rules, looking only at the atoms directly attached to the chiral center. 2. Orient the molecule in space so that the atom with the lowest priority is in the back of the molecule. Since any time that two groups are switched on a chiral carbon, the stereochemistry is reversed, if it is easier to put the highest priority in the back, you can do this but keep in mind that at the end you will switch your answer. 3. Draw a circle from group 1 to 2 to 3 (where 1 is the highest priority and 3 is the lowest priority besides the molecule in the back). - If the circle is counterclockwise, it is labeled (S) - If the circle is clockwise, it is labeled (R)

(E) and (Z) forms

This nomenclature is used for compounds with polysubstituated double bonds. (for molecules with only two substituents around a double bond, we use cis/ trans). Start by identifying the highest priority substituent attached to each double- bonded carbon by using the Cahn- Ingold- Prelog priority rules. - Z= the two highest priority substituents on each carbon are on the same side of the double bond. - E= he two highest priority substituents on each carbon are on opposite sides of the double bond. *"Z= zame side and E= epposite side"

racemic mixture

When both (+) and (-) enantiomers are present in equal concentrations. In these solutions, the rotations cancel each other out., and no optical activity is observed. Since enantiomers have identical physical and chemical properties, you must separate them by reacting both enantiomers with a single enantiomer of another compound, which will lead to 2 diastereomers. ex: Imagine, for example, two enantiomers that contain only one chiral carbon: these compounds could be labeled (+) and (-). If each is reacted with only the (+) enantiomer of another compound, the two products would be (+,+) and (-,+). Because these two products differ at some- but not all- chiral centers, they are necessarily diastereomers, and since diastereomers have different physical properties, you can separate them and the diastereomers can be reacted to regenerate the original enantiomers.

chair flip

When one chair form is converted to the other. In this process, the cyclohexane molecule briefly passes through a fourth conformation called the half chair conformation. After the chair flip, all axial groups become equatorial and all equatorial groups become axial. All dashes remain dashes and all wedges remain wedges. This interconversion can be slowed if a bulky group is attached to the ring.