Alicyclics Aliphatic compounds containing rings cycloalkanes cycloalkyl halides

Alicyclics Aliphatic compounds containing rings, cycloalkanes, cycloalkyl halides, cycloalkyl alcohols, cyclic ethers, cycloalkenes, cycloalkadienes, etc.

Cycloalkanes cyclopropane cyclobutane cyclopentane cyclohexane

methylcyclopentane 1, 1 -dimethylcyclobutane trans-1, 2 -dibromocyclohexane

cycloalkenes 3 4 2 5 1 6 cyclopentene 3 -methylcyclohexene 1, 3 -cyclobutadiene

cyclohexanol cyclohexyl alcohol ethyl cyclopentyl ether

Cycloalkanes, syntheses: A. Modification of a ring compound: B. 1. reduction of cycloalkene C. 2. reduction of cyclic halide D. a) hydrolysis of Grignard reagent E. b) active metal & acid F. 3. Corey House G. B. Ring closures

A. Modification of a cyclic compound: H 2, Ni Sn, HCl Mg; then H 2 O

Li Cu. I + CH 3 CH 2 -Br must be 1 o Corey-House CH 2 CH 3

B. ring closures C. CH 2=CH 2 + CH 2 CO, hv D. Br-CH 2 CH 2 CH 2 -Br + Zn E. etc.

cycloalkanes, reactions: 1. halogenation Cl 2, heat 2. 2. combustion 3. 3. cracking 4. 4. exceptions + HCl

exceptions: H 2, Ni, 80 o Cl 2, Fe. Cl 3 H 2 O, H+ conc. H 2 SO 4 CH 3 CH 2 CH 3 Cl-CH 2 CH 2 -Cl CH 3 CH 2 -OH CH 3 CH 2 -OSO 3 H HI CH 3 CH 2 -I

exceptions (cont. ) + ? ? ? ? ? H 2, Ni, 200 o CH 3 CH 2 CH 3

internal bond angles deviation from 109. 5 heat of combustion 60 o -49. 5 o 166. 6 90 o -19. 5 o 164. 0 108 o -1. 5 o 158. 7

Cyclopropane undergoes addition reactions that other cycloalkanes and alkanes do not. This is because of angle strain in the small ring. Because the bond angles are less than the optimal 109. 5 o for maximum overlap, the bonds are weaker than normal carbon-carbon single bonds and can be added to. Cyclobutane has angle strain that is less than that for cyclopropane, reacts with H 2/Ni at a higher temperature, but does not react like cylcopropane in the other exceptional reactions.

internal bond angles deviation from 109. 5 heat of combustion 60 o -49. 5 o 166. 6 90 o -19. 5 o 164. 0 108 o -1. 5 o 158. 7 120 o +11. 5 o 157. 4 128. 5 o +19 o 158. 3 135 o +25. 5 o 158. 6

Cyclohexane does not have any angle strain! It isn’t a flat molecule. By rotating about the carbon-carbon bonds, it can achieve 109. 5 o bond angles.

conformations of cyclohexane chair boat twist boat

The chair conformation of cyclohexane is free of both angle strain and torsional strain (deviation from staggered). This is the most stable conformation.

The boat conformation is free of angle strain, but has a great deal of torsional strain (eclipsed). To relieve the strain, it twists slightly to form the twist boat:

a = axial positions in the chair conformation e = equatorial positions

CH 3 in axial position CH 3 in equatorial position is more stable

Cycloalkenes, syntheses: A. Modification of a ring compound: B. 1) dehydrohalogenation of an alkyl halide C. 2) dehydration of an alcohol D. 3) dehalogenation of vicinal dihalides E. (B. Ring closures)

KOH(alc) H+ , Δ cyclohexene Zn

Cycloalkenes, reactions: 1. addition of H 2 10. hydroboration-oxid. 2. addition of X 2 11. addition of free radicals 3. addition of HX 12. polymerization 4. addition of H 2 SO 4 13. addition of carbenes 5. addition of H 2 O, H+ 14. epoxidation 6. addition of X 2 + H 2 O 15. hydroxylation 7. dimerization 16. allylic halogenation 8. alkylation 17. ozonolysis 9. oxymerc-demerc. 18. vigorous oxidation

H 2, Pt Br 2, CCl 4 trans-1, 2 -dibromocyclohexane

HBr H 2 SO 4 H 2 O, H+ Markovnikov orientation

Br 2 (aq. ) H+, dimer. HF, 0 o

HBr, peroxides polymerization CH 2 CO, hν Peroxybenzoic acid

KMn. O 4 cis-1, 2 -cyclohexanediol HCO 3 H trans-1, 2 -cyclohexanediol Br 2, heat

stereoselective

cyclic alcohols, halides, ethers as expected: PBr 3 Na CH 3 COOH + Na. OCl H+

Na. OH Mg 2 o alkyl halide => E 2 H 2 O conc. HI, heat conc. HBr, heat 1, 4 -dioxane 2 Br-CH 2 -Br

Alicyclic compounds are chemically like their open chain analogs. The exceptions are for small ring compounds where angle strain may give rise to reactions that are not typical of other molecules.

Epoxides: ethylene oxide propylene oxide (oxirane) cyclopentene oxide (methyloxirane) Synthesis: C 6 H 5 CO 3 H cis-2 -butene β-butylene oxide

epoxides, reactions: 1) acid catalyzed addition H 2 O, H+ CH 3 CH 2 OH, H+ HBr OH CH 2 OH OH CH 3 CH 2 -O-CH 2 OH CH 2 Br

2. Base catalyzed addition OH CH 2 OH CH 3 CH 2 -O-CH 2 -OH H 2 N-CH 2 -OH CH 3 CH 2 CH 2 -OH

mechanism for acid catalyzed addition to an epoxide

mechanism for base-catalyzed addition to an epoxide:

acid catalyzed addition to unsymmetric epoxides? + H 2 O, H+ OH CH 3 CHCH 2 OH which oxygen in the product came from the water? 18 OH + H 218 O, H+ CH 3 CHCH 2 OH

+ CH 3 OH, H+ + HBr CH 3 O CH 3 CHCH 2 OH Br CH 3 CHCH 2 OH

Base? 18 OH + Na 18 OH, H 218 O CH 3 CHCH 2 OH OCH 3 + CH 3 OH, CH 3 ONa CH 3 CHCH 2 OH + NH 3 NH 2 CH 3 CHCH 2 OH

Acid: Z + HZ CH 3 CHCH 2 OH Base: Z + Z-, HZ CH 3 CHCH 2 OH

“variable transition state” acid: Z δ+ —C—C— ‡ OH δ+ base: Z ‡ —C—C— O δ- Bond breaking is occurring faster than bond making, making the carbon slightly positive. C δ+ : 3 o > 2 o > 1 o Bond breaking is occurring at the same time as bond breaking, there is no charge on the carbon. Steric factors are most important: 1 o > 2 o > 3 o

Acid: Z + HZ CH 3 CHCH 2 OH Cδ+: Z to 2 o carbon Base: Z + Z-, HZ CH 3 CHCH 2 OH steric factors: Z to 1 o carbon