SCHOOL OF NAURAL SCIENCES
DEPARTMENT OF CHEMISTRY
CHE 2511: BASIC ORGANIC CHEMISTRY
TUTORIAL SHEET 5
28 th
March, 2019
FOR THE WEEK BEGINNING 1 st APRIL, 2019
Question 1
(a) Draw the Newman projections for all conformers of :
(i) 2-methylpropane, viewed along C1-C2 bond.
(ii) 1,2-dichlorobutane, viewed along the C2-C3 bond.
(iii) (S)- 2-hydroxybutanoic acid viewed along C2-C3 bond
(iv) Axial methylcyclohexane (staggered)
(v) Equatorial bromocyclohexane
(vi) Stable boat conformer of methylcyclopentane
(b) Calculate the energy costs for the 2,3-dimethylpentane conformers and plot the labelled potential energy diagram (graph of potential energy vs dihedral angle).
Question 2
Draw the following:
(a) The stable butterfly conformer of methylcyclobutane.
(b) The stable envelope and the stable boat conformer for ethylcyclopentane.
( c) Boat, twist boat and chair conformers of Tert.butylcyclohexane.
(d) Trans -1,2-dimethylcyclopropane conformers.
Question 3
Briefly explain why?
(a) A Cis -1,2-disubstituted cyclohexane must have one group axial and one group equatorial.
(b) A Trans -1,2-disubstituted cyclohexane must have either both groups axial or both groups equatorial.
(c) A 1,2Trans -disubstituted cyclohexane is always more stable than its Cis -isomer.
(d) A 1,3Cis -disubstituted cyclohexane is always more stable than its Trans -isomer.
Question 4
Draw the chair conformations of the following and comment on their relative stability.
(i) Tert.butylcyclohexane (ii) Cis -1,4-di-isopropylcyclohexane
(iii) Trans1,4-di-isopropylcyclohexane (iv) Cis -1,3-di-methylcyclohexane
(v) Trans1,3-dimethylcyclohexane
(vi) Cis -1-chloro-2-methylcyclohexane (vii) Trans1-chloro-2-methylcyclohexane
1

Question 5
Determine the percentages of the Cis -1,3-ditert.butylcyclohexane chair conformers present at
25
0
C. Show all steps clearly. [ Advice: (i) Calculate the SSE difference between the 2 chair conformers; (ii) Calculate the equilibrium constant from SSE difference and (iii) Calculate the ratios and the percentages of the two conformers. Refer to lecture notes and the textbook, Organic
Chemistry by John McMurray]
Useful Information :
Table1: Energy Costs for Interactions in Alkane Conformers
Interaction
H-H eclipsed
H-CH
3
eclipsed
CH
3
-CH
3
eclipsed
Energy Cost(kJ/mol) Interaction
4.0 H-H gauche
6.0
11.0
H-CH
CH
3
3
gauche
-CH
3
gauche
Energy Cost (kJ/mol)
0.0
1.8
4.0
Table2: Energy Costs for 1,3-Diaxial Interactions in Cycloalkane Conformers
1,3-Diaxial
Interaction
H-F
Energy Cost
(kJ/mol)
0.5
1,3-Diaxial
Interaction
H-CH
3
H-CH
2
CH
3
Energy Cost
(kJ/mol)
3.7
H-Cl
H-Br
H-OH
H-COOH
1.4
1.4
2.1
2.9
H-CH(CH
3
)
2
H-C(CH
3
)
3
H-C
6
H
5
H-CH
2
C(CH
3
)
2
3.9
4.6
11.3
6.3
H-CN 0.4 4.2
Energy Costs for Gauche Interactions in Cycloalkane conformers
CH
3
-CH
3
gauche 3.8 kJ/mol
2