Ch 7 Cyclic Compounds and Reaction Stereochemistry Flashcards
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)
Enantiomer reaction
Enantiomers react at identical rates with an achiral reagent or catalyst (a reaction and its reverse have to have identical transition states)
React at different rates with chiral reagents or catalysts (principle of enantiomeric differentiation) – go through diasteriomeric transition states (because of different orientations) that have different free energies (so different rates)
Making enantiomers
Chiral products from achiral reagents: enantiomers are formed at identical rates (racemate!)
Achiral products from chiral reagents: enantiomers are formed at different rates
-preference is hard to predict
-catalysts usually focus on making one enantiomer
Diastereomer reactions
-have different reactivities with any reagent (chiral or achiral)
-transition states are diasteromeric -> so different standard free energies -> so different reaction rates (cis and trans may have different reactivities with different reagents)
-products are formed at different rates (but their enantiomers form a racemate)
Transition state
an in-between molecule in a reaction that is the most unstable and highest energy point in the entire process
Faces - in context of double and triple bonds
top face vs bottom face –
-syn addition: on same face (both wedge, both dash)
-anti addition: on opposite faces (wedge/dash)
Substitution reaction
one group replaces another
-configuration is retained or inverted (or mix): to actually see it, one of the products must have a stereocenter
Stereoselective reaction
reaction where a stereoisomer is formed in excess to another
– Bromine Addition –
cis isomer-> racemate
trans isomer- -> meso compound
Stereospecificity
different reactant stereoisomers give different product stereoisomers
-all stereospecific are stereoselective
-not all stereoselective are stereospecific
Addition reaction
can occur with syn or anti stereochemistry (or a mix)
Meso compound
have chiral center but overall achiral (so have plane of symmetry)
-superimposable on mirror image (so identical compounds)
Diastereomer
-not enantiomer but stereoisomer (same connectivity, different orientation)
-non-superimposable
