Chapter 4 Cyclic Alkanes I.Naming Cycloalkanes A.Making cycloalkanes from alkanes 1)Remove 2 terminal H’s and join the terminal carbons 2)General formula = C n H 2n 3)Names: prefix cyclo- is added to the n-alkane name 4)Homologous series as ring size increases by CH 2 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 cyclopentane methylcyclohexane ethylcyclobutane 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 d)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 II.Physical Properties of Cycloalkanes A.Higher mp, bp, density than linear alkanes B.Stronger London forces due to more symmetric, rigid, cyclic structures cis-dialkylcylohexanestrans-dialkylcyclohexanes

III.Ring Strain A.Forming Rings causes differences from n-alkane regular structure 1)sp 3 hybridization still strives for tetrahedral Carbons = o bond angles 2)Cyclopropane = 60 o, Cyclobutane = 90 o, Cyclopentane = 108 o B.Heats of combustion 1)C n H 2n+2 + O  CO 2 + H 2 O +  E 2)Table 4-2 3)About –157 kcal/mol energy per CH 2 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 180 o for best  bonding 3)C-C DH o = 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 DH o = 62 kcal/mol; also a reactive molecule E.Cyclopentane

1)If planar, bond angles would be 108 o, very close to tetrahedral angles 2)If planar, all H’s would be eclipsing 3)Puckered forms put bond angles at o but relieves eclipsing strain 4)Fast interconversion between half-chair and envelope conformations 5)Ring strain is small, so not particularly reactive F.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)120 o bond angles 2)12 eclipsed H’s EnvelopeHalf-Chair

B.Chair Conformation 1)Bond angles = o 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