Alicyclics
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Alicyclics Aliphatic compounds containing rings, cycloalkanes, cycloalkyl halides, cycloalkyl alcohols, cyclic ethers, cycloalkenes, cycloalkadienes, etc. Cycloalkanes. cyclopropane cyclobutane cyclopentane cyclohexane. methylcyclopentane 1,1-dimethylcyclobutane.

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Alicyclics

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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-methylcyclohexene1,3-cyclobutadiene


cyclohexanolethyl cyclopentyl ether

cyclohexyl alcohol


  • Cycloalkanes, syntheses:

  • Modification of a ring compound:

  • 1. reduction of cycloalkene

  • 2. reduction of cyclic halide

  • a) hydrolysis of Grignard reagent

  • b) active metal & acid

  • 3. Corey House

  • B. Ring closures


A. Modification of a cyclic compound:

H2, Ni

Sn, HCl

Mg; then H2O


Li

CuI

+ CH3CH2-Br

CH2CH3

must be 1o

Corey-House


  • ring closures

  • CH2=CH2 + CH2CO, hv 

  • Br-CH2CH2CH2CH2CH2-Br + Zn 

  • etc.


  • cycloalkanes, reactions:

  • halogenation

  • 2. combustion

  • 3. cracking

  • 4. exceptions

Cl2, heat

+ HCl


exceptions:

H2, Ni, 80o

CH3CH2CH3

Cl2, FeCl3

Cl-CH2CH2CH2-Cl

H2O, H+

CH3CH2CH2-OH

conc. H2SO4

CH3CH2CH2-OSO3H

HI

CH3CH2CH2-I


exceptions (cont.)

+H2, Ni, 200o CH3CH2CH2CH3

??????????


internal bond deviation heat of

angles from 109.5 combustion

60o-49.5o 166.6

90o-19.5o164.0

108o-1.5o158.7


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 109.5o 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 H2/Ni at a higher temperature, but does not react like cylcopropane in the other exceptional reactions.


internal bond deviation heat of

angles from 109.5 combustion

60o-49.5o 166.6

90o-19.5o164.0

108o-1.5o158.7

120o+11.5o157.4

128.5o+19o158.3

135o+25.5o158.6


Cyclohexane does not have any angle strain! It isn’t a flat molecule. By rotating about the carbon-carbon bonds, it can achieve 109.5o bond angles.


conformations of cyclohexane

chair twist boat

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


CH3 in axial positionCH3 in equatorial position

is more stable


  • Cycloalkenes, syntheses:

  • Modification of a ring compound:

  • 1) dehydrohalogenation of an alkyl halide

  • 2) dehydration of an alcohol

  • 3) dehalogenation of vicinal dihalides

  • (B. Ring closures)


KOH(alc)

H+, Δ

cyclohexene

Zn


  • Cycloalkenes, reactions:

  • addition of H28. hydroboration-oxid.

  • addition of X29. addition of free radicals

  • addition of HX10. addition of carbenes

  • addition of H2SO411. epoxidation

  • addition of H2O,H+12. hydroxylation

  • addition of X2 + H2O13. allylic halogenation

  • oxymerc-demerc. 14. ozonolysis

  • 15. vigorous oxidation


H2, Pt

Br2, CCl4

trans-1,2-dibromocyclohexane


HBr

H2SO4

H2O, H+

Markovnikov orientation


Br2 (aq.)

H+, dimer.

HF, 0o


HBr, peroxides

polymerization

CH2CO, hν

Peroxybenzoic acid


KMnO4

cis-1,2-cyclohexanediol

HCO3H

trans-1,2-cyclohexanediol

Br2, heat


stereoselective


cyclic alcohols, halides, ethers as expected:

PBr3

Na

H+

CH3COOH +

NaOCl


NaOH

2o alkyl halide => E2

Mg

H2O

conc. HI, heat

conc. HBr, heat

2 Br-CH2CH2-Br

1,4-dioxane


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 cyclopentene oxide

(oxirane) (methyloxirane)

Synthesis:

C6H5CO3H

cis-2-butene

β-butylene oxide


  • epoxides, reactions:

  • acid catalyzed addition

OH

CH2CH2

OH

H2O, H+

OH

CH3CH2-O-CH2CH2

CH3CH2OH, H+

OH

CH2CH2

Br

HBr


2. Base catalyzed addition

OH

CH2CH2

OH

CH3CH2-O-CH2CH2-OH

H2N-CH2CH2-OH

CH3CH2CH2CH2-OH


mechanism for acid catalyzed addition to an epoxide


mechanism for base-catalyzed addition to an epoxide:


acid catalyzed addition to unsymmetric epoxides?

OH

+ H2O, H+ CH3CHCH2

OH

which oxygen in the product came from the water?

18OH

CH3CHCH2

OH

+ H218O, H+


CH3

O

+ CH3OH,H+ CH3CHCH2

OH

Br

+ HBr  CH3CHCH2

OH


Base?

18OH

+ Na18OH, H218O CH3CHCH2

OH

OCH3

+ CH3OH, CH3ONa CH3CHCH2

OH

NH2

+ NH3 CH3CHCH2

OH


Acid:

Z

+ HZ CH3CHCH2

OH

Base:

Z

+ Z-, HZ CH3CHCH2

OH


“variable transition state”

Z

acid:— C — C —

OH

Bond breaking is occurring faster than bond making, making the carbon slightly positive. C δ+ : 3o > 2o > 1o

δ+

δ+

base:

Z

— C — C —

O

Bond breaking is occurring at the same time as bond breaking, there is no charge on the carbon. Steric factors are most important: 1o > 2o > 3o

δ-


Acid:

Z

+ HZ CH3CHCH2

OH

Cδ+: Z to 2o carbon

Base:

Z

+ Z-, HZ CH3CHCH2

OH

steric factors: Z to 1o carbon


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