Chapter 4 Cyclic Alkanes
I.
Naming Cycloalkanes
A.
B.
CH
3
Making cycloalkanes from alkanes
1) Remove 2 terminal H’s and join the terminal carbons
2) General formula = CnH2n
3) Names: prefix cyclo- is added to the n-alkane name
4) Homologous series as ring size increases by CH2
CH3
CH2
CH2
cyclopentane
Rules for naming cycloalkanes
1) Monosubstituted
a) The substituted C is designated as C1
b) Name as n-alkane chain, but with cyclo- prefix
methylcyclohexane
c) Don’t need to number substituent
ethylcyclobutane
2) Polysubstituted
Cl a)
Assign numbers to have lowest possible total numbers
b) If 2 substituents could possible have the same number, alphabetize to
determine which is the lowest numbered substituent
1-ethyl-1-methylcyclopentane
and
1-chloro-2-methyl-5-ethylcycloheptane
3)
Disubstituted cycloalkanes have isomers
a) Two possible arrangements for substituents
b) Both on the same face of the ring = cis
c) One on each face of the ring = trans
R
H
R
R
R
H
cis-dialkylcylohexanes
d)
II.
H
R
R
R
R
H
trans-dialkylcyclohexanes
Stereoisomers = molecules with the same formula and same
connectivity, but different spatial arrangements of atoms
i. Structural or Constitutional isomers had different connectivity
ii. Conformations of the same molecule have different spatial
arrangements, but they can interconvert by bond rotation
iii. Stereoisomers interconvert only by bond breakage
iv. Have different physical and chemical properties
Physical Properties of Cycloalkanes
A.
B.
Higher mp, bp, density than linear alkanes
Stronger London forces due to more symmetric, rigid, cyclic structures
III. Ring Strain
A.
Forming Rings causes differences from n-alkane regular structure
1) sp3 hybridization still strives for tetrahedral Carbons = 109.5o bond angles
2) Cyclopropane = 60o, Cyclobutane = 90o, Cyclopentane = 108o
B.
Heats of combustion
1) CnH2n+2 + O2 ------ CO2 + H2O + DE
2) Table 4-2
3) About –157 kcal/mol energy per CH2 group in n-alkanes
4) Cycloalkanes give off more heat than expected: they are more unstable
5) Potential Energy diagram
C.
Cyclopropane
1) All H’s eclipsed =
eclipsing strain
1) Bond angle strain = not
180o for best s bonding
3) C-C DHo = 65 kcal/mol
(vs 90 normally)
4) Reactive molecule
D.
Cyclobutane
1)
2)
3)
4)
E.
Eclipsing strain somewhat relieved by puckered structure
Rapid flipping occurs between puckered forms
Bond angle strain present, but less than in cyclopropane
C-C DHo = 62 kcal/mol; also a reactive molecule
Cyclopentane
1)
2)
3)
4)
5)
F.
If planar, bond angles would be 108o, very close to tetrahedral angles
If planar, all H’s would be eclipsing
Puckered forms put bond angles at 104.4o but relieves eclipsing strain
Fast interconversion between half-chair and envelope conformations
Ring strain is small, so not particularly reactive
Envelope
Half-Chair
Classification of rings based on size
1) Small rings: (C3, C4) high ring strain
2) Common rings: (C5, C6,C7) small or no ring strain
3) Medium rings: (C8-C12) some ring strain
4) Large rings: (> C13) no ring strain, virtually like n-alkane structures
IV. Cyclohexane
A.
Planar structure not stable
1) 120o bond angles
2) 12 eclipsed H’s
B.
Chair Conformation
1)
2)
3)
4)
C.
Bond angles = 107.5o
All H’s are staggered
No ring strain, heat of combustion identical to n-hexane
Very common and stable structural unit in Organic Chemistry
Other conformations
1) Boat
a) 6.9 kcal/mole unstable
b) 8 eclipsed H’s
c) Transannular Strain
2)
D.
Twist Boat
a) Removes some transannular strain
b) 1.4 kcal/mol more stable than boat
Potential Energy Diagram