Ch 7 Cyclic Compounds and Reaction Stereochemistry Flashcards
(35 cards)
Axial bonds
Stick out (up and down)
Equatorial bonds
Stick out (sideways)
Monocyclic Compound
Contains a single ring
-generally more stable with more carbons
Cyclohexane
-Most stable monocyclic compound
-Most common ring occurring in nature
-Has chair conformation (avoids strain)
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Cycloalkanes
-have internal rotations
(in chair conformation: axial bonds can become equatorial, vise versa)
Boat conformation
Unstable form of the chair conformation (everything is eclipsed: high van der Waals repulsions)
-in cyclohexane
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. . . .
Chair conformation
-very stable
-avoids strain
-averaged over time, both axial and equatorial bonds are equivalent
Twist-Boat Conformation
-reduces some repulsion (less eclipsed)
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Monosubstituted Cyclohexanes
Usually in equatorial position
-Conformational diastereomeres
*remember they don’t like being close together when drawing them
Disubstituted Cyclohexane
Planar-ring (may have chair conformation – can’t assume)
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Cis: both groups are facing into/our-page
Trans: facing different directions
*largest group usually takes equatorial
Relative energies of chair conformations
Highest:
half chair
boat
twist-boat
chair
Planar-ring structures
(the normal flat hexagon)
-configuration, not conformation info
-has two chair conformations
-up and down substituent positions are also present in the chair conformations
Meso compounds
-have plane of symmetry
-but also have asymmetric carbons (connections are different)
-“time-averaged planar structure”
Chair Interconversion
Scoot substituents down one carbon and then reflect the mirror image
Cyclopentane
ENVELOPE CONFORMATION
-puckered conformation
-eclipsing hydrogens so higher energy than cyclohexane
Cyclobutane
-lots of angle strain
-puckered conformation (reduces some eclipsing)
Cyclopropane
PLANAR
-can’t pucker
-angle strain and eclipsing strain
-bent bonds (more circular than triangular) have less effective overlap but reduce angle strain
Bicyclic compounds
2 rings share 2+ common atoms (ring fusion)
Oo
Bridgehead carbons - the atoms at which two rings are joined
-adjacent? fused bicyclic compound
-non-adjacent? bridged bicyclic compound
Nomenclature: numbers of carbons in each bridge (bicyclo[3.2.1]octane)
*even when it’s two cyclohexanes put together, make sure you count up the carbons and give it the proper name
Spirocyclic compounds
2 rings share 1 common atom (ring fusion)
O
O
Polycyclic Compounds
organic compounds with many rings joined at common atoms (hard to make synthetically)
Ring fusion with small rings
<= 7
restricted to cis (too much angle strain with trans)
-no trans stereochemistry
Ring fusion with big rings
> =9
cis/trans
-cis can go chair interconversion, trans cannot (usually more stable)
Bredt’s Rule
Bicyclic compounds: small ring’s bridgehead atom cannot have a double bond (too unstable)
-only substituent atoms can have double bonds (think orbitals: too much overlap)
-so resonance structures with bond at bridgehead aren’t important
Steroids
-tetracyclic fused-ring system
-have trans ring fusions (rigid and flat molecule)
-angular methyl groups at carbon 10 and 13 (bridgehead carbons)
