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Level 181

Conformations of Alkanes


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conformations
different arrangement of atoms that result from bond rotations
rotating single bonds => same...different...
same connectivity, different 3D orientation
conformations of alkanes
staggered and eclipsed
staggered because the hydrogens are farthest apart
which of the conformations of alkanes is more stable
Steric strain
An increase in energy resulting when atoms are forced too close to one another
three different ways to draw alkanes
wedge and dash, sawhorse, newman projection
wedge and dash views
molecules from the side
sawhorse views
molecules from the side but all with straight, solid lines
newman projection views
straight on from the front carbon
dihedral angle
60 degrees when staggered, 0 degrees when eclipsed
potential energy diagrams for conformations
when bond is rotated 60 degrees clockwise, energy changes; highest when molecules are eclipsed, but amounts can vary
conformations of cycloalkanes
talk about bond angles; what they want; stability; shape/confirmation
cyclopropane
bond angle is 60 but wnats 109.5 because of sp3 hybridization; unstable and flat
cis cycloalkane isomers
same side = cis
trans cycloalkane isomer
opposite sides (wedge and dash) = trans
cyclobutane
bond and is ninety degrees; wants 109.5 because of sp3 still; unstable; more stable than cyclopropane; puckered confirmation
cyclopentane
bond angle is 108 but wants 109.5; really stable; envelope confirmation
cyclohexane
most stable and most important ring; bond and is 109.5; chair and boat confirmations (6 total confirmations)
parallel lines
know how to draw a good chair! Key?
axial bonds
bonds that are straight up and down
equatorial bonds
outward bonds with a little up or down direction
down
if axial is up, equatorial is...
every cyclohexane has _ axial hydrogens
six; they alternate up, down, up, down, up, down
very rapidly
chair flip/chair-chair-interconversion happens
includes...
ramification of the chair-chair interconversion
up
after flipping, if a given H atom started up, it will end up
equitorial
after flipping, if a given H atom started axial, it will end up
axial
after flipping, if a given H atom started equatorial, it will end up
substituted cyclohexanes
axial vs. equatorial preference
what is more stable?
larger groups want to be equatorial so they have more space