Organic Chemistry MCAT Chapter 2: Isomers

compounds that have the same molecular formula but different structures

isomers

compounds that only have their molecular formula (and therefore molecular weight) in common

structural/constitutional isomers

structural isomers have ___ chemical properties and ___ physical properties

different; different

chemical properties of a molecule are generally dictated by ___

the functional groups in the molecule

compounds that share both molecular formula and atomic connectivity

stereoisomers

two types of stereoisomers

conformational and configurational isomers

compounds that differ only by rotation around single sigma bonds

conformational isomers

stereoisomers that can be interconverted only by breaking bonds

configurational isomers

a model in which a molecule is visualized along a line extending through a carbon-carbon bond axis

Newman projection

varying degrees of rotation around single bonds can create different levels of ___

strain

the most stable straight-chain conformation is the ___ conformation, a type of ___, when the two largest groups are oriented ___ degrees away from each other

anti; staggered; 180

the conformation where groups are the furthest away from each other they can be

staggered

the conformation where groups are overlapped

eclipsed

the conformations in between staggered and eclipsed

skew

the ___ conformation is another type of staggered conformation, when the two largest groups are oriented ___ degrees away from each other

gauche; 60

the ___ conformation is a type of eclipsed conformation, when the two largest groups are oriented ___ degrees away from each other and the molecule is in its ___ state

totally eclipsed; 0; highest-energy

energy required for conformational interconversion is (small/large) and is (easily/hard to) overcome at room temperature

small; easily

all molecules at absolute 0K do not change between ___

conformations

at low temperatures, conformational interconversions proceed ___

slowly

the combination of angle strain, torsional strain, and nonbonded/steric strain in cycloalkanes

ring strain

strain that results when bond angles deviate from their ideal values

angle strain

strain that results when cyclic molecules must assume conformations that have eclipsed or gauche interaction

torsional strain

strain that results when nonadjacent atoms or groups compete for the same space

nonbonded/steric strain

common conformation of cyclobutane

butterfly/puckered

common conformation of cyclopentane

envelope

common conformations of cyclohexane

chair, boat, twist-/skew-boat

nonbonded strain is the dominant source of steric strain in the ___ of the cyclohexane boat conformation

flagpole interactions

most stable conformation of cyclohexane

chair conformation

bonds perpendicular to the plane of the ring in cyclohexane’s chair conformation

axial

bonds parallel to the plane of the ring in cyclohexane’s chair conformation

equatorial

when one chair form is converted to the other

chair flip

fourth conformation of cyclohexane seen during a chair flip

half-chair conformation

for substituted cyclohexanes, the bulkiest group will favor the ___ position to reduce nonbonded strain

equatorial

two categories of configurational isomers

enantiomers and diastereomers

enantiomers and diastereomers are also known as ___

optical isomers

optical isomers are able to ___

rotate plane polarized light

a molecule whose mirror image cannot be superimposed on the original image

chiral

a carbon with four different substituents

chiral center

enantiomers are ___

nonsuperimposable mirror images of each other

diastereomers are ___

nonsuperimposable but not mirror images of each other

both enantiomers and diastereomers are ___

chiral

a carbon with only 3 different substituents is ___

achiral

two enantiomers differ in ___ and ___

optical activity; reactions in chiral environments

light that only has waves oscillating in one direction

plane-polarized light

one enantiomer will rotate plane-polarized light to the ___ magnitude and ___ direction of its mirror image

same; opposite

a compound that rotates plane-polarized light clockwise/to the right

dextrorotatory (+)

a compound that rotates plane-polarized light counterclockwise/to the left

levorotatory (-)

the direction of optical activity is determined ___

only experimentally, not from molecular structure

the amount of rotation depends on ___

the number of molecules encountered by a light wave

the number of molecules encountered by a light wave depends on ___ and ___

the concentration of optically active compound; the length of the tube through which light passes

standard conditions for polarimetry: concentration - ___, path length - ___

1 g/mL; 1 dm

equation to convert rotations of different concentrations and path lengths to standardized specific rotation

specific rotation (degrees) = observed rotation (degrees) divided by concentration (g/mL) times path length (dm)

mixture of both (+) and (-) enantiomers in equal concentrations

racemic mixture

optical activity of racemic mixtures

none

diastereomers have different ___, while enantiomers do not

physical properties

for any molecule with n chiral centers, there are ___ possible stereoisomers

2ⁿ

diastereomers in which substituents differ in their position around an immovable bond or a ring structure

cis-trans/geometric isomers

the terms cis and trans are used in ___ with ___ on either side of the immovable bond

simple compounds; only one substituent

the terms E and Z are used in ___ with ___

complex compounds; polysubstituted double bonds

a molecule with chiral centers with an internal plane of symmetry

meso compound

for a molecule to be optically active and chiral, it must have ___ and lack ___

chiral centers; an internal plane of symmetry

configuration of a molecule in relation to another chiral molecule

relative configuration

exact spatial arrangement of atoms/groups independent of other molecules

absolute configuration

rules for assigning priority to atoms/groups bonded to the double bonded carbon

Cahn-Ingold-Prelog priority rules

the two highest priority substituents are on the same side

(Z) nomenclature

the two highest priority substituents are on opposite sides

(E) nomenclature

first step of determining (R) and (S) absolute configuration

assign priority to atoms attached to chiral center

second step of determining (R) and (S) absolute configuration

invert stereochemistry as needed so the lowest priority group is at the back of the molecule

the stereochemistry is inverted any time ___

two groups are switched on a chiral carbon

third step of determining (R) and (S) absolute configuration

draw a circle from substituent 1 to 2 to 3

if the circle connecting substituents is clockwise the asymmetric atom is called

R

if the circle connecting substituents is counterclockwise the asymmetric atom is called

S

in the Fischer projection, horizontal lines represent ___, vertical lines represent ___, and their points of intersection represent ___

wedges; dashes; carbon atoms

rotating a Fischer projection by ___˚ will invert the stereochemistry of the molecule

90

rotating a Fischer projection by ___˚ will revert the molecule back to its original stereochemistry

180