Organic 3: Alkanes/Cycloalkanes - Conformations & cis-trans Stereoisomers Flashcards
Conformations
different spatial arrangements of a molecule that are generated by rotation about single bonds
Conformational analysis
study of how conformational factors affect the structure of a molecule and its properties
Staggered conformation
each C-H bond of one carbon bisects an H-C-H angle of the other carbon
Eclipsed conformation
each C-H bond of one carbon is aligned with a C-H bond of the other carbon
Gauche
spatial relationship term used when the torsion angle is 60 degrees
Anti
spatial relationship term used when the torsion angle is 180 degrees
Torsional strain
conformations in which the torsion angles between adjacent bonds are other than 60 degrees
Steric strain
torsional and other sources of strain in molecules
Activation energy
the minimum energy that a reacting system must possess above its most stable state in order to undergo a chemical or structural change
Transition state
the point of maximum potential energy encountered in reactants as they proceed to products
van der Waals strain (steric hindrance)
destabilization of a molecule that results when 2 of its atoms are too close to each other; contributes to the total steric strain
Strain energy
deviations from the ideal values for bond angles/distances, etc will destabilize a particular structure and increase its potential energy
Angle strain
strain a molecule has because one or more of its bond angles deviate from the ideal value
Envelope
Nonplanar conformation of cyclopentane in which 4 of the carbon atoms are coplanar; the 5th carbon is out of the plane of the other 4
Half-chair
Nonplanar conformation of cyclopentane in which there are 3 coplanar carbons, with 1 carbon atom displaced above that plane and another below it
Chair conformation
most stable conformation of cyclohexane; with C-C-C bond angles of 111 degrees, it is nearly free of angle strain; all of its bonds are staggered, making it free of torsional strain as well
Boat
much less stable nonplanar conformation of cyclohexane; bond angles are approximately tetrahedral; relatively free of angle strain; destabilized by torsional strain associated with eclipsed bonds on 4 of its carbons; two “flagpole” hydrogens contribute a small amount of van der Waals strain as well
Twist boat (skew boat)
more stable nonplanar conformation of cyclohexane; reduces both torsional strain and van der Waals strain by a rotation about the carbon-carbon bond
Axial hydrogens
6 of cyclohexane’s hydrogens have their bonds parallel to a vertical axis that passes through the ring’s center; alternately directed up or down on adjacent carbons
Equatorial hydrogens
6 of cyclohexane’s hydrogens are located approximately along the equator of the molecule; 4 bonds to each carbon are arranged tetrahedrally
Ring inversion (chair-chair interconversion)
one chair conformation is converted to another chair
Intermediate
not a potential energy maximum, but a local minimum on the potential energy profile
1,3-diaxial repulsions
repulsive forces between axial substituents on the same side of a cyclohexane ring
Standard free energy
one of the fundamental equations of thermodynamics concerns systems at equilibrium and relates the equilibrium constant K to the difference in this between the products and the reactants
cis
when substituents are on the same side they are ___ to each other
trans
when substituents are on opposite sides they are ___ to each other
Stereoisomers
isomers that have their atoms bonded in the same order (same constitution) but they differ in the arrangement of atoms in space; those of the cis-trans type are sometimes referred to as geometric isomers
Polycyclic
compounds that contain more than 1 ring
Spiro
a compound with one atom common to 2 rings
Bridged
compound with 2 atoms common to 2 or more rings
Heterocyclic compound
a substance with a ring that contains at least one heteroatom
