Chapter 4 Cyclic Alkanes

I. Naming CycloalkanesA. 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

B. Rules for naming cycloalkanes

1) Monosubstituted

a) The substituted C is designated as C1

b) Name as n-alkane chain, but with cyclo- prefix

c) Don’t need to number substituent

2) Polysubstituted

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

CH3 CH3CH2 CH2

cyclopentane

methylcyclohexaneethylcyclobutane

Cl

1-ethyl-1-methylcyclopentane and 1-chloro-2-methyl-5-ethylcycloheptane

3) Disubstituted cycloalkanes have isomersa) Two possible arrangements for substituentsb) Both on the same face of the ring = cisc) One on each face of the ring = trans

d) Stereoisomers = molecules with the same formula and same connectivity, but different spatial arrangements of atomsi. Structural or Constitutional isomers had different connectivityii. Conformations of the same molecule have different spatial

arrangements, but they can interconvert by bond rotationiii. Stereoisomers interconvert only by bond breakageiv. Have different physical and chemical properties

II. Physical Properties of CycloalkanesA. Higher mp, bp, density than linear alkanesB. Stronger London forces due to more symmetric, rigid, cyclic structures

cis-dialkylcylohexanes trans-dialkylcyclohexanes

H

H

R

RR

R

R

RR

H

H

R

III. Ring StrainA. Forming Rings causes differences from n-alkane regular structure

1) sp3 hybridization still strives for tetrahedral Carbons = 109.5o bond angles2) Cyclopropane = 60o, Cyclobutane = 90o, Cyclopentane = 108o

B. Heats of combustion

1) CnH2n+2 + O2 ------ CO2 + H2O + E2) Table 4-2

3) About –157 kcal/mol energy per CH2 group in n-alkanes4) Cycloalkanes give off more heat than expected: they are more unstable5) Potential Energy diagram

C. Cyclopropane1) All H’s eclipsed =

eclipsing strain1) Bond angle strain = not

180o for best bonding3) C-C DHo = 65 kcal/mol

(vs 90 normally)4) Reactive molecule

D. Cyclobutane

1) Eclipsing strain somewhat relieved by puckered structure

2) Rapid flipping occurs between puckered forms

3) Bond angle strain present, but less than in cyclopropane

4) C-C DHo = 62 kcal/mol; also a reactive molecule

E. Cyclopentane

1) If planar, bond angles would be 108o, very close to tetrahedral angles2) If planar, all H’s would be eclipsing3) Puckered forms put bond angles at 104.4o but relieves eclipsing strain4) Fast interconversion between half-chair and envelope conformations5) Ring strain is small, so not particularly reactive

F. Classification of rings based on size1) Small rings: (C3, C4) high ring strain2) Common rings: (C5, C6,C7) small or no ring strain3) Medium rings: (C8-C12) some ring strain4) Large rings: (> C13) no ring strain, virtually like n-alkane structures

IV. CyclohexaneA. Planar structure not stable

1) 120o bond angles2) 12 eclipsed H’s

Envelope Half-Chair

B. Chair Conformation

1) Bond angles = 107.5o

2) All H’s are staggered

3) No ring strain, heat of combustion identical to n-hexane

4) Very common and stable structural unit in Organic Chemistry

C. Other conformations

1) Boat

a) 6.9 kcal/mole unstable

b) 8 eclipsed H’s

c) Transannular Strain

2) Twist Boat

a) Removes some transannular strain

b) 1.4 kcal/mol more stable than boat

D. Potential Energy Diagram