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
Br2 BrCH2CH2CH2CH2Br
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 CO2HH3O+
CO2EtEtO2C
HO2C
-CO2
2,3-dibromopropane
1,1,2-triester 1,2-diacid
