ch 4: rotational conformtations

quiz 3 stuffs

eclipsed

__________ conformations are so unstable because electrons are pushing against each other. They have torsional strain. they do not have empty antibonding orbitals like staggered do

stable

lower energy = more ______

staggered conformations

are low in energy

steric

_______ strain is because of the crowding of atoms because the groups are more than just a hydrogen (higher energy)

steric and torsional

both of these together make the most strain. torsional just by itself is way more than if there was only steric

longest

molecules occupy the most stable conformer the __________.

maxima

conformations are always on the _______

minima

staggered conformations are always on _______. they are lower in energy

more

bigger substituent with eclipsed interaction= ______ energy

steric

______ strain can exist in both staggered conformations and eclipsed conformations

gauche

_______ interactions are in staggered conformations with steric strain

cycloalkanes

have a 3rd type of strain. most have angle strain

sp3

carbon atoms in alkanes are __ hybridized. ideal bond angle for this is 109.5

angle strain

molecules that are not close to the optimal bond angle have a special kind of strain called…

NOT

to optimize bond angles, most cycloalkanes are ___ flat in their most stable conformation. none of these molecules exist flat

steric strain

can occur in both staggered and eclipsed. when there is not enough space and substituents crowd each other out

torsional strain

found only in eclipsed conformations, when there is repulsion between electrons in bonds that do not share an atom

angle strain

when the bond angles of an atom are less than ideal- leads to poor orbital overlap (makes bonds weaker)

3 & 4

membered rings are more reactive

cyclohexane

lowest energy (most stable is 6 membered ring). going from 3 membered ring down the energy decreases, and going from biggest ring down to cyclohexane energy goes up

cyclopropane

is highly reactive and has severe angle strain (bond angles of 60) and torsional strain. all bonds are eclipsed

cyclobutane

has significant angle strain (88 degree bond angles) and some torsional strain, not quite eclipsed

cyclopentane

has very little angle strain, but does have one important area where torsional strain occurs. envelope shape. bottom of envelope has torsional strain

high angle strain

means it is ready to react and open up ring (3 and 4)

cyclohexane

considered to have 0 ring strain in its optimal conformation; the chair.

• no angle strain, angles must be 109.5

• no torsional strain, all adjacent C-H bonds must be staggered

boat

the _____ is the least stable conformation of a cyclohexane. there is no angle strain, but there is torsional strain and steric strain

flagpole interactions

steric strain of two hydrogens at ends of the boat that come close to each other

axial groups

point either straight up or down and alternate around the ring. up at peaks, down at valleys

axial

in the cyclohexane, the chair conformation has six ____ atoms

equatorial

_________ substituents are positioned at angles parallel to the sets of parallel lines making up the chair itself. they also alternate up and down. always opposite of axial it is near

equatorial

six of the atoms in the chair conformation are __________.

monosubstituted cyclohexane

one substituent on cyclohexane. anytime you add substituents besides H, you introduce conformation with steric strain

flip

all axial positions become equitorial and all equitorial become axial (up and down does not change)

rotating

flipping a chair is the result of C-C bonds _______.

axial

substituent causes additional steric strain. equitorial does NOT have steric strain!

1, 3 diaxial strain

methyl group added. type of gauche interaction ( steric with staggered)

stable

equitorial

larger

size is important, _________ substituent = more pronounced steric strain

disubstituted cyclohexanes

when multiple substituents are present, the positioning of the groups on the chair must be shown by using solid or dashed wedges or by showing groups in either axial or equitorial positions

wedge

up

dash

down

equitorial

preference is for largest group to be equitorial

steroisomers

molecules that differ in the three dimensional spaitial arrangement of their atoms, but not in the connectivity of their atoms

cis

in same direction/ same side of the ring

trans

opposite; one is up and one is down/ opposite sides of ring

cis/ trans isomerism

not constitutional isomers. only applies to molecules with restricted rotation like rings and pi bonds