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Chapter III

Chapter III. General properties (reactions) of alicyclic compounds:

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Chapter III

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  1. 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 cyclobutaneto 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.

  2. 1)Ring fission of alicyclic compounds occurs by : • a-Catalytic hydrogenation; • b- Halogenation • c-Reactions with HX ; • d- Oxidation

  3. a-Catalytic hydrogenation: Depends on the ring size,

  4. 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.

  5. 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.

  6. 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.

  7. i-By KMnO4:

  8. ii-By O3:

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

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

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

  12. 3) Methods of ring expansion of alicyclic copmpounds :

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

  14. Acetoxylation of the double bonds : • Lead tetraacetatePb(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.

  15. Cyclopropane derivatives : • Cyclopropylmethyl ketone was prepared from 1,2-dibromoethane and ethyl acetoacetate disodium salt.

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