Chapter III

• General properties (reactions) of alicyclic compounds:

• The chemical behavior of cycloalkanes depends on the ring size and the nature of the reagent used ;for example, ring fission (ring opening) occurs with small rings like cyclopropane and cyclobutane to yield an open chain compounds.But those of five and six (cyclopentane and cyclohexane);they are most stable and it is diffuclt to change into open chain compounds.

• 1)Ring fission of alicyclic compounds occurs by :

• a-Catalytic hydrogenation;

• b- Halogenation • c-Reactions with HX ; • d- Oxidation

a-Catalytic hydrogenation: Depends on the ring size,

CH3CH2CH3

H2 / Ni / 80oC

n-butaneH2 / Ni / 200oC

n-pentaneH2 / Ni / 320oC

H2 / Ni / heatno reaction

• b-Halogenation: Both of Cl2 ,Br2 or I2 reacting with cyclopropane and cyclobutane by ring fission;

• in case of cyclopentane and cyclohexane reacting by substitutions to form halogenocycloalkanes.

BrCH2CH2CH2BrBr2

Br2BrCH2CH2CH2CH2Br

Br2

Br

-HBr

Br2

Br

-HBr

• c-Reactions with HX : Cyclopropane and cyclobutane react by ring fission;in case of cyclopentane and cyclohexane ,but it has no effect on five and six membered rings.

c.HI

R.T.

c.HI

R.T.

n-propyl iodide

n-butyl iodide

c.HI

c.HI

R.T.

R.T.

no effect

no effect

• d- Oxidation:• 1-Unsaturated alicyclic compounds: Cyclic

unsaturated hydrocarbons and their derivatives can readily be broken down (ring fission) at the unsaturated linkage (double bond) by oxidation.

• These methods of rupturing the rings are of considerable importance in determining the constitution of cyclic unsaturated compounds, since they are proceed very smoothly by one of these oxidizing agents KMnO4,O3,HNO3.

i-By KMnO4:

KMnO4

OH

OH

CO2H

CO2H

ring f ission

succinic acid

KMnO4

OH

OH

CO2H

CO2H

ring f ission

glutaric acid

ii-By O3:

O

OO

ozonide

CHO

CHO

succinaldehydeO3 H2O

H2O2

O3CHO

CHO

glutaraldehydeO3

CHO

CHO

adipaldehyde

O

adipic acid

HNO3

OHNO3

CH3

CO2H O

CH3

2)Methods of ring contraction of alicyclic compounds :a-On heating with HI:

CH3 + n-hexane

HI , 200oC

CH3

CH3

+HI , 180oC

b-By the effect of HNO2 on cycloalkylamines : Example :

NH2 N+2Cl-

OH

HNO2

OH

H2O

cyclobutylamine

cyclopropylcarbinol

cyclobutanol

NH2

OH

HNO2

cyclopentylamine cyclobutylcarbinol

NH2

OH

HNO2

cyclohexylamine cyclopentylcarbinol

• c-α-Chloroycloketones on treatment with KOH gives cycloalkane carboxylic acid with one carbon less than the starting α-chlorocycloketones.

•

O

Cl

OCO2H

KOH KOH

cyclopentanecarboxylic acid2-chlorocyclohexanone

3 )Methods of ring expansion of alicyclic copmpounds:

OHHI,100oC HI

NH2HNO2 H2 / Ni

OH HI, heat

-H2O-I2

NH2HNO2

then H2 / Ni

HI, heat

-H2O-I2

OH

HNO2

then H2 / Ni

NH2

• b-By deamination of cyclic aminoalcohols with NaNO2 / AcOH to produce cyclic ketones of the next higher ring system.

O

OH

CN

OH

NH2

HCN H2 /NiHNO2

O

It is clear from the above example that ring expansion often takes place if the side chain contains the group –CH2NH2 or –CH2OH ,whilst ring contraction is observed when certain reactive groups (-NH2 or –OH) are present in the nucleus. In any ring changes it is only one carbon atom is included.

• Acetoxylation of the double bonds :• Lead tetraacetate Pb(OAc)4 attacks the double

bonds in alicyclic compounds to form 1,2-diacetates , the products are a mixture of the cis- and trans compounds. For example, cyclohexene forms a mixture of cis- and trans- 1,2- diacetate and these may be hydrolysed to give the corresponding cis- and trans- cyclohexanediol.

H

OAc

H

OAc

H

OH

H

OH

OAc

H

H

OAc

OH

H

H

OH

cis-diacetate cis-diol

trans-diacetate trans-diol

Pb(OAc)

H3O+

H3O+

• Cyclopropane derivatives :• Cyclopropylmethyl ketone was prepared from

1,2-dibromoethane and ethyl acetoacetate disodium salt.

Br

BrCH3

CO2Et

O

Na2+ CO2Et

COCH3 COCH3KOH

HCl

BrCH2CH2CH2COCH3

HBr

bromopropylmethylketone

Br

BrCO2Et

CO2Et

Na2+ CO2Et

CO2Et CO2H

-CO2

H3O+

CO2H

CO2Hmono acid

1,1-diacid

Br

BrCO2Et

CO2Et

Na2+ CO2Et

CO2Et CO2H

H3O+

CO2EtEtO2C

HO2C

-CO2

2,3-dibromopropane

1,1,2-triester 1,2-diacid