ORGANIC CHEMISTRY - Chapter 3: Alkanes and Cycloalkanes: Conformations and Cis-Trans Stereoisomers

conformations

different spatial arrangements of a molecule that are generated by rotation about single bonds

conformational analysis

the study of how conformational factors affect the structure of a molecule and it's properties

staggered conformation of ethane

eclipsed conformation of ethane

What is the most stable conformation of ethane?

staggered

What is the least stable conformation of ethane?

eclipsed

types of staggered conformations

gauche (60 degree torsion angle)
anti (180 degree torsion angle)

torsional strain

ethane conformations in which torsion angles between adjacent bonds are not 60 degrees

Which type of bonds in ethane produce the most torsional strain?

eclipsed bonds

steric strain

comprised of additional sources of strain and torsional strain

potential energy maxima

conformations appearing at the highest points of the curve
eclipsed for ethane

potential energy minima

conformations appearing at the lowest points of the curve
staggered for ethane

conformers

conformations that correspond to potential energy minima

transition state

the point of maximum potential energy encountered by the reactants as they proceed to products

What is the transition state for the conversion of one staggered conformation of ethane to another?

eclipsed conformation

two ways to measure speed of a reaction

half-life
Arrhenius equation

van der waals strain / steric hindrance

the destabilization of a molecule that results when two of its atoms are too close to eachother

Why is the gauche conformation less stable than anti conformation?

van der waals strain / steric hindrance

Higher alkanes with unbranched carbon chains are most stable in WHAT conformation?

all anti-conformations

angle strain

the strain a molecule has because one or more of its bond angles deviate from the ideal value

What is the ideal bond angle for alkanes?

109.5 degrees

Baeyer strain theory

predicts cycloalkanes beyond cyclopentane should become increasingly strained and less stable

Which cycloalkane is planar?

cyclopropane

Which cycloalkane is nonplanar?

ALL except cyclopropane

Does cyclopentane have angle strain?

no

Does cyclopentane have torsional strain?

yes, significant because five bonds eclipsed on both the top and bottom faces of the ring

Where is torsional strain relieved in cyclopentane?

nonplanar conformations

What is the most stable conformation of cyclohexane?

chair conformation
all staggered bonds (no torsional strain)
barely any angle strain (111 degree angles)

boat conformation

bond angles are approx. tetrahedral (free of angle strain)
destabilized by torsional strain from eclipsed bonds on four carbons

twist/skew boat conformation

more stable than the boat conformation

axial

bonds parallel to a vertical axis that passes through the ring's center

equatorial

located approximately along the equator of the molecule

ring inversion / chair-chair interconversion

process in which one chair conformation is converted to another

What are the intermediates of cyclohexane?

twist conformations

intermediate

not a potential energy maximum but a local minimum on the potential energy profile

Which conformation of cyclohexane is highest in energy?

half-chair
because they have the most eclipsing interactions

What is the most important result of ring inversion?

any substituent that is axial in the original chair conformation becomes equatorial in the ring-inverted form and vice versa

energy diagram for ring inversion in cyclohexane

When two conformations of a molecule are in equilibrium with each other, which one dominates?

the one with the lower free energy

What are 1,3 diaxial repulsions in substituted cyclohexane comparable to?

van der waals strain in the gauche conformations of alkanes

The relative amounts of the two conformations depend on ....

effective size of the substituent

What is the size of the substituent, in the context of cyclohexane conformations, related to?

the degree of branching at the atom connected to the ring

cis

substituents on the same side

trans

substituents on the opposite sides

stereoisomers

isomers that have their atoms bonded in the same order but differ in arrangement of atoms in space

T/F: Any substituent is more stable in an axial orientation.

FALSE
Any substituent is more stable in an EQUATORIAL orientation rather than an axial one.

If a disubstituted cyclohexane has two different substituents, which is the most stable conformation?

the chair that has the larger substituent in an equatorial orientation

polycyclic

compounds that contain more than one ring

How are polycyclic compounds classified?

according to minimum number of bond cleavages required to generate a noncyclic structure

spiro compound

one atom is common to two rings

names of spiro alkanes

spiro[number.number]alkane

numbers (in ascending order) are number of carbons unique to each ring

When substituents are present in spiro compounds, where does numbering begin?

in the smaller ring, adjacent to the spiro carbon and proceeds consecutively around the smaller ring away from the spiro carbon, through it, and then around the larger ring

bridged compound

two atoms are common to two or more rings

bridged bicyclic alkanes names

bicyclo[number.number.number]alkane

parent alkane = same number of carbons as the total in the bicyclic skeleton
bracketed numbers = number of carbons in the three bridges in descending order
- numbering beings at the bridgehead position and proceeds consecutively in direction of the largest bridge and continues thru next largest, atoms in smallest bridge are numbered last

fused-ring compounds

contain rings that share a common side
(for classification and naming, they are placed in the bridge category)

Are all cyclic compounds hydrocarbons?

NO

heteroatom

substances that include an atom other than carbon

heterocycle

a ring that contains at least one heteroatom

heterocyclic compound

a substance based on a heterocyclic ring

ethylene oxide

tetrahydrofuran

pyrrolidine

piperidine