Kaplan MCAT Organic Chem Chapter 2: Isomers

What are isomers?

• same molecular formula, but different structures, chem and physical properties

What are structural(constitutional) isomers?

• least similar of all isomers

• only share similar molecular formula, so molecular weights are the same

What are physical properties? chemical properties?

Physical: don’t change the composition of matter like melting point, boiling point, solubility, odor, color and density

Chemical: have to do with the reactivity of the molecule and result in changes in chemical composition

What are stereoisomers?

• same chemical formula and also share the same atomic connectivity(structural backbone)

What are conformational isomers?

• the same molecule, but interconvert by simple bond rotation

• arise from the fact that varying degrees around single bonds can create diff levels of strain

Describe the Newman projection conformations.

• Staggered: lowest energy state; groups 60 deg apart

• Anti staggered: two largest groups opp sides of each other/180 deg apart; most energetically favorable

• Gauche: the two largest groups 60deg apart

• eclipsed: 2 methyl groups 120 deg apart and overlap with the H atoms on the adjacent C

• totally eclipsed: 2 methyl groups directly overlap with 0deg separation; highest energy state; least energetically favorable

• at very low temperatures, conformational interconversions are very slow

Stability of cycloalkanes depends on ring strain based on 3 factors:

• angle strain: bond angles deviate from their ideal values by being stretched or compressed

• torsional strain: cyclic molecules must assume conformations that have eclipsed or gauche interactions

• nonbonded strain:nonadjacent atoms or groups compete for the same space(dominant source of steric strain)

To alleviate strain, cycloalkanes attempt to adopt various nonplanar conformations. Cyclobutane puckers into a

• V shape

• cyclopentane adopts an envelope conformation

• cyclohexane has chair, boat, and twist-boat forms

• most stable for cyclohexane is chair conformation: H atoms perpendicular to the plane of ring are axial(sticking up or down) and those parallel to the plane of the ring are equatorial(sticking out)

• if pointing down→dash, if pointing up→wedge

• with a chair flip, all axial groups become equatorial and vice versa; all dashes remain dashes and all wedges remain wedges

• for substituted rings, the bulkiest group will favor the equatorial position to reduce nonbonded strain

• for nomenclature, if both groups are located on the same side of the ring, molecule is cis; if opposite sides→trans

What are configurational isomers?

• can only change from one form to another by breaking and reforming covalent bonds

• the 2 types are enantiomers and diastereomers; both are considered optical isomers bc the diff spatial arrangement of groups in these molecules affects the rotation of plane-polarized light

• enantiomers: nonsuperimposable mirror images of each other that have the same connectivity but opposite configurations at every chiral center in the molecule

• diastereomers: chiral and share the same connectivity but are not mirror images of each other; this is bc they differ at some of their multiple chiral centers

What is chirality?

• an object is chiral if its mirror image can’t be the same when placed on the original object

• chiral center: carbon with 4 diff substituents

Enantiomers have identical physical and chemical properties with 2 notable exceptions:

• they rotate plane-polarized light in opposite directions and react differently in chiral environments

• at the molecular level, one enantiomer will rotate plane-polarized light to the same magnitude but in the opposite direction of its mirror image→ a compound that rotates the plane of polarized light to the right, or clockwise is dextrorotatory(d-) and labeled (+)

• a compound that rotates light toward the left, or counterclockwise is levorotatory and is labeled (-)

• direction of rotation determined experimentally

What does it mean if a compound is optically active?

• it has the ability to rotate plane-polarized light

• ordinary light is unpolarized, which means that it consists of waves vibrating in all possible planes perpendicular to its direction of propagation; a polarizer allows light waves oscillating only in a particular direction to pass through, producing plane-polarized light

• the amount of rotation depends on the number of molecules that a light wave encounters; this depends on 2 factors: the conc of the optically active compound and the length of the tube through which the light passes

• set standard conditions are 1 g/mL for conc and 1 dm(10cm) for length to compare the optical activities of diff compounds

• specific rotation equation is a=aobs/c(l) where a is specific rotation in degrees, aobs is observed rotation in degrees, c is the conc in g/mL and l is the path length in dm

What is a racemic mixture?

• when both (+) and (-) enantiomers are present in equal concs, they form this→rotations cancel each other out→no optical activity; ex: a solution containing 2M (R)-2-butanol and 2 M (S)-2-butanol

• these solutions will not rotate plane-polarized light

What are diastereomers?

• non-mirror image configurational isomers with diff chemical/physical properties; also rotate plane-polarized light

• occur when a molecule has 2 or more stereogenic centers and differs at some, but not all of these centers

• for any molecule with n chiral centers, there are 2^n possible stereoisomers

• an example of diastereomers include E and Z of the same molecule since those are cis-trans isomers

What are cis-trans isomers(geometric isomers)?

• 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 restricted

• in simple compounds with only 1 substituent on either side of the immovable bond, we use cis and trans; if 2 on same side→cis; if opp→trans

What are meso compounds?

• for a molecule to have optical activity, it must not only have chiral centers within it, but also lack a plane of symmetry

• meso compound: compound with chiral centers and internal plane of symmetry

• a meso compound, with an internal plane of symmetry, will also be optically inactive bc the two sides of the molecule cancel each other out

• meso compounds are made up of 2 halves that are mirror images; as a whole, they are not optically active

• D- and L- tartaric acid are both optically active, but meso version has a plane of symmetry and is not optically active→this means that even though meso-tartaric acid has 2 chiral C atoms, the molecule does not display optical activity

What is relative and absolute configurations of a chiral molecule?

• configuration of a stereoisomer refers to the spatial arrangement of the atoms or groups in the molecule

• relative: its configuration in relation to another chiral molecule(often through chemical interconversion); this is used to determine whether molecules are enantiomers, diastereomers, or the same molecule

• absolute: describes the exact spatial arrangement of these atoms or groups, independent of other molecules

• relative configuration is retained if the bonds of the stereocenter are not broken→position of groups around the chiral carbon are maintained

• absolute configuration is retained if both reactant and products are same E/Z nomenclature

Describe E and Z forms.

• used for compounds with polysubstituted double bonds

• to do this, look for the highest priority substituent attached to each double bonded carbon

• priority is assigned based on the atom bonded to the double bonded carbons: the higher the atomic number, the higher the priority

• if the atomic numbers are equal, priority is determined by the next atoms outward

• the alkene is named Z if the two highest priority subs on each carbon are on the same side of the double bond and E if they are on opp sides

How do you determine R or S for a molecule?

• used for chiral centers in molecules

• 1. assign priority to the 4 substituents, looking only at the atoms directly attached to the chiral center; if atomic numbers are equal, priority determined by the combo of the atoms attached to these atoms; if there is a double bond, it is counted as 2 individual bonds to that atom; if there is a tie→ work outward until broken

• 2. any time two groups are switched on a chiral carbon, the stereochemistry is inverted; if we do this step, we must switch our final answer

• 3. draw a circle from 1 to 2 to 3; if circle is drawn counterclockwise, asymmetric atom →S; if clockwise→R

• 4. name written out: (R) and (S) put in parentheses and sep from the rest of the name by a hyphen; if a compound has more than 1 chiral center, location specified by a number preceding the R or S within the parentheses and without a hyphen

Describe Fischer projections

• horizontal lines indicate bonds that project out from the plane of the page(wedges) and vertical lines are bonds going into the plane of the page(dashes)

• point of intersection of the lines represents a carbon atom

• make sure lowest priority group projects into the page; lowest priority group can be on the top or bottom of the molecule and still project into the page

• switching one pair of subs in a fischer inverts the stereochem of the chiral center; switching two pairs retains the stereochem

• rotating a fischer 90 deg inverts the stereochem of the chiral center; rotating 180 deg retains the stereochem