Cycloalkanes

Report
Cycloalkanes
 All C atoms in the ring are sp3.
 General formula for cycloalkanes
is CnH2n.
 Physical properties are similar to
those of the compact, branched
alkanes.
Nomenclature of Cycloalkanes
 Count the number of C atoms in the
ring and use a prefix of cyclo- .
 If there is only one substituent, no
numbering is needed.
 If there are two or more substituents,
number the C’s in the ring to give the
lowest possible numbers for the
substituents.
 A cycloalkane may be named as the
main chain or as a substituent.
Nomenclature of Cycloalkanes
3-cyclopropyl-1,1-dimethylcyclohexane
3-sec-butyl-1,1-dimethylcyclohexane
1,1-dimethyl-3-(1-methylpropyl)cyclohexane
cis-trans Isomerism of
Cycloalkanes
 Although cycloalkanes do not
have double bonds, they do have
two “faces.”
 Substituents pointing toward the
same face are cis.
 Substituents pointing toward
opposite faces are trans.
cis-trans Isomerism of
Cycloalkanes
trans-1,2-dichlorocyclopentane
cis-1,2-dichlorocyclopentane
Ring Strain in Cycloalkanes
 Five- and six-membered rings are
the most stable.
 Components of ring strain
 angle strain
 when bond angles are <109.5°
 torsional strain (aka steric strain)
 when eclipsed conformations result
 Quantified by comparing heats of
combustion data.
Ring Strain in Cyclopropane
 Angle strain is large
 bond angle is 60° instead of 109.5°
 Torsional (steric) strain is present
 3-membered ring results in an eclipsed
conformation that is “locked in.”
 Heat of combustion is 38.5 kJ per -CH2higher than that of a long-chain
alkane.
 Total ring strain is 115 kJ.
Ring Strain in Cyclopropane
torsional strain
angle strain
Ring Strain in Cyclobutane
 Angle strain is large
 bond angle is 88° instead of 109.5°
 Torsional strain is present
 4-membered ring results in an
almost-eclipsed conformation.
 Heat of combustion is 27.5 kJ per
-CH2- higher than that of a longchain alkane.
 Total ring strain is 110 kJ.
Ring Strain in Cyclobutane
C-C bond angles are 88°, the result of a nonplanar
geometry that relieves a little torsional strain.
Ring Strain in Cyclopentane
 Cyclopentane flips through puckered
“envelope” conformations.
 Allows bond angles of nearly 109.5°.
 Reduces torsional strain by allowing
movement away from a totally eclipsed
(planar) conformation.
 Heat of combustion is only 5.4 kJ per CH2- higher than that of a long-chain
alkane.
 Total ring strain is only 27 kJ.
Not Much Ring Strain in
Cyclopentane
Cyclohexane Has NO Ring Strain
 Cyclohexane can assume various
puckered conformations.
 Allows bond angles of 109.5°.
 Chair conformation has no eclipsing.
 Heat of combustion is the same as that
of a long-chain alkane.
 Total ring strain is 0 kJ.
 The six-membered ring is ubiquitous in
nature.
 carbohydrates
 steroids
 plant products
Chair Conformation of
Cyclohexane
Boat Conformation of
Cyclohexane
The
hydrogens in
the boat
conformation
are all
eclipsed,
leading to
steric strain.
Twist-Boat Conformation of
Cyclohexane
The hydrogens are not totally eclipsed.
This is the second most stable conformation. It is
this conformation that is generally meant when the
term “boat” is used.
Comparison of Chair and Boat
Conformations of Cyclohexane
Half-Chair Conformation of
Cyclohexane
Conformational Energies of
Cyclohexane
At room
temperature,
cyclohexane
flips through all
of these
conformations,
but spends the
most time in
the chair
conformations.
Ring Flipping in Cyclohexane
 At room temperature, cyclohexane “flips” from
one chair conformation to the other.
 When this happens, atoms in axial positions
become atoms in equatorial positions, and vice
versa.
axial
equatorial
1,3-diaxial Interaction in
Cyclohexane



A substituent in an axial position is gauche to the ring
C’s that are two away. This form of steric hindrance
is called a 1,3-diaxial interaction.
When the ring flips, the substituent is in an equatorial
position and anti to the ring C’s that are two away.
This means the equatorial positions are lower energy
and more favored for substituents.
1,3-diaxial Interaction in
Cyclohexane
 When there are two substituents
on the ring in a 1,3 arrangement,
the steric hindrance is even more
severe.
 The cis isomer will be lower in
energy than the trans isomer.
 When the substituents are not the
same, the larger substituent goes to
the equatorial position.
1,3-diaxial Interaction in
Cyclohexane
 The cis isomer will be lower in energy
than the trans isomer.
cis
trans
1,3-diaxial Interaction in
Cyclohexane
 When the substituents are not the
same, the larger substituent goes to
the equatorial position.
1,3-diaxial Interaction in
Cyclohexane
 The t-butyl group is so bulky that
it will ALWAYS go to the
equatorial position.
 t-butyl groups on C1 and C4 will
cause the twist-boat conformation to
be lower energy than either of the
chair conformations.
Other Interactions in Cyclohexane
 Which is more stable, the cis or trans isomer of
1-ethyl-2-methylcyclohexane?
Nomenclature of Bicyclo
Compounds
 Count the number of C atoms in the
rings (ignore substituents).
 This compound will be a bicyclo
decane.
Nomenclature of Bicyclo
Compounds
 Find the bridge carbons. Here they are
numbered 1 and 6.
 Count the carbons in the two chains
that connect the bridge carbons. Here,
there are four carbons in each.
Nomenclature of Bicyclo
Compounds
 Finally, count the carbons in the bridge
and between the bridge carbons. Here
there are none.
 The name of this compound is
bicyclo[4.4.0]decane
Nomenclature of Bicyclo
Compounds
bicyclo[2.2.1]heptane
bicyclo[3.2.2]nonane

similar documents