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Cycloalkanes• Alkanes with closed ring(s) of C atoms
• General formula: CnH2n (C3H6, C4H8, C5H10, etc.)
• Naming: use cyclo- prefix before alkane name
cyclopropanen = 3
cyclopentanen = 5
cyclobutanen = 4
cyclohexanen = 6
Naming substituted cycloalkanes:• 1 substituent: no numbering necessary• 2 or more substituents: highest alpha priority on C #1
CH3
methylcyclopentane
CH3
Cl1-chloro-3-methylcyclohexane
C3H6 C4H8 C5H10 C6H12
1-chloro-5-methylcyclohexane
Dashes and Wedges Bonds
H3C
CH3
H
HHH
Wedge
Indicates bond coming out of the page toward you
Dashes
Indicates bond going into the page away from you
Coming out at you (like 3-D movie)
Going into the page away from you
Stereoisomers• Cycloalkane rings have two faces
• Stereoisomers: same connectivity, but different arrangement of atoms in space
• Geometric Isomers: type of stereoisomer involving rings or multiple bonds with substituents on two different carbons
– Designated as either cis or trans and must be included in the name
Cl
Cl
Cl
Cl
cis isomer – substituents on
same face of the ring
cis-1,2-dichlorocyclobutane
trans isomer – substituents on
opposite faces of the ring
trans-1,2-dichlorocyclobutane
CC
C CH
ClH
Cl
H
H
H
H
CC
C CH
ClH
H
Cl
H
H
H
Cl
ClCl
Cl
Cl
Cl
Cl
Cl
Conformational Analysis
Newman projections: looking down a C-C bond
Steric strain: atoms/substituents “bumping into” each other• More steric strain higher in energy, less stable
staggered eclipsed
Conformational isomers: differ only by rotation about single bonds• Also called conformers or rotamers• Interconvert easily at room temperature
H3C
H
CH3
HHH
H3C
CH3
H
HHHDash-wedge diagram
180° rotation
H3C
CH3
H
HHH
CH3
HH
CH3
HH
CH3
H H
H
CH3
H
H3C
H
CH3
HHH
is more stable than
Strain in Cycloalkanes
What if cycloalkane rings were flat?
Bond angles: 60° 90° 108° 120°
Tetrahedral bond angle = 109.5°
Flat rings would require all C-H bonds to be eclipsed
Result: When possible, rings will compensate to relieve both kinds of strain
Bond angle strain
Torsional (twisting) strain
Cyclopropane and Cyclobutane
Cyclopropane
Bond angles 60°
All C-H bonds eclipsed
No way to relieve strain
UNSTABLE
Cyclobutane
Puckers to stagger C-H bonds
Puckering decreases bond angles to less than 90°
UNSTABLE
H H
HH
H
H
HH
Cyclopentane
Bond angles close to tetrahedral (108°)
Puckers to stagger C-H bonds
“envelope”conformation
STABLE
H
H
HH
H
H
H
H
HH
CyclohexaneHH
HH
H H
H
H
HH
HH
Puckers to relieve bond angle strain and torsional strain
Result: All bond angles ~109°, all C-H bonds staggered
STABLE
“chair”conformation
HH
HH
H H
H
H
HH
HH
axial bonds: straight up or downequatorial bonds: ~ in plane of ring
axial positions: crowded, only H can fitequatorial positions: plenty of room
Cyclohexane “ring flip”• Axial and equatorial positions can be swapped by ring flipping
(Ea ~ 50 kJ/mol)
• Intermediate is a boat conformation, higher energy than chair
Br
H
H
H H
H
H
Cl
H
H
H
H
H Cl
HH
HH
H
Br
H H
HH
HBr
HH
H H
H
Cl
HH
HH
cis-1-bromo-4-chlorocyclohexane
Br axialCl equatorial
Br equatorialCl axial
“boat”
Drawing chairs from flat pictures
CH3
CH3
CH3
H
CH3
H
H
H
CH3
BrBr
CH3
cis-1,2-dimethylcyclohexane
trans-1-bromo-2-methylcyclohexane
Optical Isomers• Optical isomers are non-superimposable mirror images
– interact differently with polarized light
– type of stereoisomer
I
HCl
Br
I
HCl
Br
bromochloroiodomethane
* *
F
HCH3
Cl
F
HH3C
Cl
1-chloro-1-fluoroethane
* *
C* = stereocenter: C with 4 different substituentshint that molecule may have optical isomers
mirror plane in molecule no optical isomers
Br plane of symmetry:molecule halves are mirror images
No optical isomers
Chirality: Chiral vs. Achiral
Chiral Achiral
right shoe left mit right-handed glass plate scissors
tennisracket
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