Chapter 4 Flashcards
what are conformations
different spatial arrangements of a molecule that are generated by rotation about single bonds
what is conformational analysis
the study of how conformational factors affect the structure of a molecule and its properties
steric hindurence
when large atoms restrict the amount of possible arrangments
what is anticonformations
when large mols are 180 degrees apart instead of 60
what are the 3 most common ways to show conformations
wedge and dash
sawhorse
newman projection
dashes, wedges and lines in wedge and dash drawings
dash - going “dashing” away from you
wedge - coming towards you
straight line - on the plane
what are the basics of newman projections
bonds on the back carbon are shown sticking out of the circle, those with lines on top of the circle are on the front of the carbon
stagged
when bonds to the axis of rotation are not aligned
eclipse
when bonds to the axis of rotation ARE alligned
how do Newman projections differ
by their rotations of the front and back carbon atoms (angles basically)
what type of newman projection has the highest energy repulsion between bonds
eclipsed
what type of newman projection has the lowest energy repulsion between bonds
anti
which newman projection is least stable and why
eclipsed
because it has the highest energy
which newman projection is the most stable and why
anti
because it has the lowest NG
0 degree NP
eclipsed
60 degree NP
gauche
180 degree NP
anti
ethane conformations
are infinite and correspond to changes in the torsional angle
what do conformations that are not staggered have
torsional strain
in newman projections, what conformation are molecules usually in
staggered conformations, hardly ever in eclipsed conformations
what are the two different staggered conformations for butane
gauche (higher NG, steric strain on CH3, least stable)
anti (most stable, least NG)
types of strains in newman projections
3
torsional strain
steric hindrance
steric strain
torsional strain
results from eclipsed bonds
steric hindrance
when 2 atoms are too close together (AKA van der Waals strain)
steric strain
the combination of torsional strain and steric hindrance
energy of an eclipse
high (less stable)
energy of staggered
low (more stable)
what characteristics do the lowest energy conformation of alkanes have
all bonds staggered
with simple alkanes, has zig-zag arrangement of carbon atoms
what bond angle do tetrahedral carbons prefer
109.5
what type of cycloalkane has the highest angle strain and what is the bond angle
cyclopropane
60 degrees
what do all other cycloalkanes besides tetrahedral carbons try to do to the angle strain
minimize it, and are therefore not planar
why is heat of combustion important
gives us a way to measure the relative stability of cycloalkanes
what does the lowest heat of combustion per CH2 group correspond to
the most stable cycloalkane
bent/banana bonds
when strong sp3-sp3-s bonds cannot be formed because of 60 degree bond angles, the bonds are called bent/banana
how is torsional strain in cyclopropane
it is high because the C-H bonds are eclipsed
what is the general conformation for cyclobutane and why
a nonplanar “puckered” conformation
to minimize the torsional strain
how is the angle strain of planar cyclopentane and why
low angle strain because the natural bond angle is 108
what type of strain do cyclopentane have and why
torsional strain because the C-H bonds are eclipsed
cyclopentane and stability
2nd most stable cycloalkane, they relieve some of the torsional strain but not all
what is the most stable cycloalkane and what conformation is it most stable in
cyclohexane, chair conformation
what are the two groups of bonds in cyclohexanes
- axial
- equitorial
equitorial hydrogens
horizontal
axial hydrogens
vertical up and down
how many chair conformations of cyclohexane are there and what do they do
there are 2, and they rapidly interconvert
what happens when cyclohexane inverts from original chair conformation
the original conformation becomes equitorial in the inverted form, and vice versa
general rule for ring inversions
procedes through highest energy half-chair conformations and the twist and boat conformations
general rule for interconversion of methylcyclohexanes
methyl group is either equatorial or axial
what confirmation of methylcyclohexane is most favored
equatorial methyl
what destabilizes the conformation of methylcyclohexane
van der waals strain between the methyl and axial hydrogen atoms destabilizes an axial methyl
where is methyl more stable in inversions
when it is equitorial and has 0 interactions
chair inversion of fluorocyclohexane (vs. methyl)
less crowding because F is a smaller substituent
difference in energy is lower than methyl
chair inversion of t-butylcyclohexane (vs. methyl)
more crowding because butyl is a larger substituent (tertiary butyl)
difference in energy is much higher than methyl
isomers
different compounds that have the same molecular formula
constitutional isomers
differ in connectivity
stereoisomers
have the same connectivity but different arrangements of the atoms in space
what are the 2 possible orientations for sustituents
trans and cis
cis substituents
on the same side of the ring
trans substituents
on the opposite sides of the ring
which type of substituent has higher energy and why
cis, because van der waals strain, more hindrance, less stable, more NG
what type of stereoisomer is most stable
trans
cis-1,4-dimethylcyclohexane
both cis isomers when inverted, have the same energy
diaxial and stability relationship
least diaxial is most stable
trans-1,4-Dimethylcyclohexane
one chair conformation of trans isomers has both methyls equatorial
more stable
more stable than cis chair conformations
cis conformations of 1,2-dimethylcyclohexane
are equal in NG, one methyl group is equatorial and the other is axial
what does the lowest chair conformation of the trans isomer of dimethylcyclohexane have
both methyls equitorial
with 2 different substituents, what will happen with conformation
the lowest conformation will have a larger substituent in the equatorial postion
spirocyclic systems
have 2 rings with one common atom
how do you name spirocyclic systems
spiro(number.number)alkane
alkane = number of carbons in the rings
how do you number the carbons in spirocyclic systems
increasing order starting from the top of the left ring, going counter clockwise and up to the top of the right ring
what are bridged compounds
2 nonadjacent atoms common to 2 or more rings
how do you name bridged compounds
bycyclo(number.number.number)alkane
parent alkane = number of carbons
numbers: number of carbon atoms between the bridgehead atoms in descending order
fused ring compounds
where two rings share a common side
how to name fused rings
bicyclo(number.number.number)alkane
same as bridges compounds pretty much
heterocyclic compounds
contains an atom other than carbon in the ring (heteroatoms)
