ch264/3 organic chemistry ii cyclohexane rings
DESCRIPTION
CH264/3 Organic Chemistry II Cyclohexane Rings. Dr Andrew Marsh C515 [email protected] Dr David J Fox B510 [email protected]. Today’s Lecture. 1. Cyclohexane conformation 2. Drawing cyclohexane chairs Monosubstituted cyclohexanes - PowerPoint PPT PresentationTRANSCRIPT
CH2641
CH264/3 Organic Chemistry IICyclohexane Rings
Dr Andrew Marsh [email protected]
Dr David J Fox [email protected]
CH2642
Today’s Lecture
1. Cyclohexane conformation
2. Drawing cyclohexane chairs
3. Monosubstituted cyclohexanes
4. Disubstituted cyclohexanes, decalins and steroids
CH2643
Angle strain in rings
24.5° 9.5° 0.5° -5° "angle strain" (divided between two ring bonds)
9.17 6.58 1.24 0.02
38.4 27.5 5.19 0.09
strain per CH2 group (kcal mol-1)
strain per CH2 group (kJ mol-1)
"angle strain" is (109° - ring angle)/2
CH2644
Cyclohexane is ‘strain free’
A planar arrangement of the six methylene groups in cyclohexane does not give a tetrahedral shape for every carbon atom - this is achieved by puckering the ring. Cyclohexane does this by adopting mainly two conformations the CHAIR and the BOAT.
chair boat chair
CGW p.371
Conformational analysis
CH2645
Nobel Prize 1969
CH2646
Cyclohexane is ‘strain free’
HeqHeq
Heq
Hax
Ha
He
HaxHering flip
Heq Heq
HaxHax
view as Newman projection
1,3-diaxial interaction
HeqHeq
Heq
HaxHax
Heq
view as Newman projection
gauche interaction
109° angle allows near strain free cyclic molecule
CH2647
Substituents on cyclohexane
a a
aa
a
eee
e
e
e
axial substituentsequatorial substituents
a
CGW p. 371
CH2648
Ring Flip
Ha
HeHe
Ha
Ha
HeHe
Ha
Ha
He
Ha
He
Ha
He
Ha
He
twist
boat
half chair
Ha
He
Ha
He
Ha
He
Ha
He
Ha
He
Ha
He
boat
4
kJ mol-1
43
25
21
0
CH2649
Chair Conformer
Ha
He
He
HaHa
He
Ha
Heview Ha
He
He
Ha
staggered
Ha
He
He
Ha Ha
He
Ha
He
view Ha
He
He
Ha
CH26410
Boat Conformer
Ha
He
Ha
He
1,4-transannular interaction
H
H
H
H
He
Ha Ha
He
• eclipsing C-C bonds & C-H• no C-H HOMO C-C LUMO donation
view
CH26411
Substituted Cyclohexanes
HeqHeq
He
Ha
Ha
He
Ha
H3C CH3CH3
CH3
CH3
CH3
Hering flip
favoured conformer
Heq Heq
Hax
H3C CH3
CH3
view as Newman projectionunfavourable gauche interaction
1,3-diaxial interaction
HeqHeq
Heq
HaxHax
CH3
CH3
CH3
conc equatorial conformer
conc axial conformerK = >3000, >99.9% equatorial, >20 kJ mol-1 difference
Substituted cyclohexanes: energy difference
Substituent K Axial – equatorial energy difference kJ mol-1
% equatorial
H 1 0 50
OMe 2.7 2.5 73
Me 19 7.3 95
Et 20 7.5 95
iPr 42 9.3 98
tBu >3000 >20 >99.9
110 110 11.7 99
CH26412
CGW p. 375
CH26413
Disubstituted cyclohexanesCH3
CH3
CH3
CH3
anti-1,3-dimethylcyclohexane
syn-1,3-dimethylcyclohexane
H3C
CH3
CH3
CH3
H3C CH3
CH3
CH3
favoured
cis-
trans-
H
H
HH
CH26414
The tert-butyl group is a conformational ‘lock’
OH
OH
OH
H
H
cis-4-t-butylcyclohexanol
trans-4-t-butylcyclohexanol
H
OH
H
OH
H
H
H
OH
H
Decalins
H
H
H
trans-decalin
conformationally locked
Ha
cis-decalin
H
H
HHe
CGW p. 378
Steroids: cholestanol
CH26416
Conformationally locked A–B–C–D rings
HHO
HMe Me
Me
HO
H
H
HH
H
A B
C D
CGW p. 379
CH26417
You should be able to:
(i) Draw cyclohexane as chair conformers
(ii) Ring-flip monosubstituted cyclohexane
(iii) Show which conformer is favoured in mono- and di-substituted cyclohexanes
Outputs