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Overview of Cycloalkanes: Properties, Preparation, and Reactions

Cycloalkanes are cyclic compounds in which carbon atoms form rings. They have the general formula CnH2n and exhibit various physical properties. This compound type can be prepared from dihalides, alkenes, or through addition reactions. Cycloalkanes undergo substitution and addition reactions, leading to the formation of different derivatives. The relative stability theories of cycloalkanes, including Baeyer's strain theory and Sachse Mohr's theory, provide insights into their molecular structure and reactivity.

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Overview of Cycloalkanes: Properties, Preparation, and Reactions

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  1. Cycloalkanes The carbon atoms which are attached to one another to form rings are called cyclic compounds. e.g. cyclopropane, cyclobutane etc. Cycloalkanes have general formula CnH2n or (CH2)n Where n = 3,4,5,6…...etc.

  2. Physical Properties of cycloalkanes :- 1) Cyclopropane & cyclobutane are gases at room temperature ; the remaining cycloalkanes are liquids. 2) Melting & boiling points of cycloalkane show a gradual increase with the increase in molecular weight. e.g. compound bpoC mpO c Cyclopropane Cyclobutane Cyclopentane Cyclohexane - 32.8 12.5 49.3 80.7 - 127.4 - 90.7 - 13.9 6.6 3) Cycloalkanes are insoluble in water but dissolve in ethanol & ether.

  3. • Method of Cycloalkane preparation :- 1) From Dihalides (dehalidation) 2) From Alkenes 3) addition of two unsaturated molecule • Reaction of Cycloalkanes :- 1) Substitution Reactions a) substitution with Cl2& Br2 2) Addition Reaction or Ring opening reaction a) addition of Cl2& Br2 b) addition of acid halide (HBr & HI) c) addition of Hydrogen

  4. • Method of preparation :- 1) From Dihalides :- (dehalidation) Terminal dihalides of alkane when treated with sodium or zinc to form cycloalkanes. H2 C H2 C Cl a) + 2NaCl H2C + 2Na H2C CH2 H2C Cl Cyclopropane 1,3- Dichloropropane H2 C H2C Cl H2C CH2 + 2NaCl + 2Na b) H2 C H2C Cl H2C CH2 1,4- Dichlorobutane Cyclobutane

  5. 2) From alkenes :- When alkenes are treated with methylene iodide ( CH2I2 ) in the presence of a zinc-copper couple, cyclopropane derivatives are formed. H C Zn-Cu H3C CH2 + CH2I2 methylene iodide H3C C H CH2 a) ether Propene C H2 Methylcyclopropane H C Zn-Cu H3C CH CH3 b) + CH2I2 H3C C H C H CH3 ether methylene iodide 2-Butene C H2 1,2-Dimethylcyclopropane

  6. 3) Addition of two unsaturated molecule :- H2C CH2 hv + H2C CH2 H2C CH2 H2C CH2 Ethylene Ethylene Cyclobutane Reacting two molecule of ethylne in presence of light to produce cyclobutane

  7. • Reaction of Cycloalkanes :- 1) Substitution Reactions :- a) Substitution with Cl2 & Br2 HC Cl H2 C e.g. i) + HCl UV light + Cl2 H2C CH2 H2C Cyclopropane CH2 Chlorocyclopropane Cyclopropane react with chlorine in the presence of UV light to give Chlorocyclopropane & Hydrochloric acid.

  8. H C H2C Cl ii) H2C CH2 HCl UV light + + Cl2 H2C CH2 H2C CH2 Chlorocyclobutane Cyclobutane Cyclobutane react with chlorine in the presence of UV light to give Chlorocyclobutane & by product is Hydrochloric acid.

  9. 2) Addition reaction or Ring-opning reaction :- a) Addition of Cl2& Br2 e.g. H2 C + Br2 CCl4 dark H2 C H2 C H2 C Br 1,3- Dibromopropane Br H2C Cyclopropane CH2 Cyclopropane react with Bromine using carbon tetrachloride as a solvent (CCl4) under dark condition to obtained 1,3- Dibromopropane.

  10. b) Addition of acid halide (HBr & HI) :- e.g. H2 C H2 C H2 C H3C 1-Bromopropane Br + HBr Hydrogen bromide H2C Cyclopropane CH2 Cyclopropane reacts with concentrated Hydrogene bromide (HBr) to gives 1-Bromopropane.

  11. c) Addition of Hydrogen :- e.g. i) H2 C Ni 80oC H2 C H3C CH3 + H2 Hydrogen propane H2C Cyclopropane CH2 Cycloppane react with Hydrogen in presence of catalyst Nickel (Ni) to give propane ii) + H2 200oC Ni H2C CH2 H2 C H2 C H3C n-Butane CH3 H2C Cyclobutane CH2 Cyclobutane react with Hydrogen in presence of catalyst Nickel (Ni) to give n-Butane.

  12. Relative stability Theories of cycloalkanes :- 1) Baeyer's strain theory (1885) :- Reactivity & stability of Cycloalkane Baeyer proposed any deviation of bond angle from ideal bond angle value (109.280) will produce a strain in molecule. 2) Sachse Mohr's theory ( Theory of strainless rings) - 1918 :- Sachse Mohr's theory was proposed to explain the reason behind the higher stability of cyclohexane & other higher cycloalkanes. 3) Coulson & Moffitt's Modification (1947) :- Coulson & Moffitt suggested that rehybridization occurs in cyclopropane.

  13. 1) Baeyer's strain theory :- In 1885 Adolf van Baeyer proposed a strain theory which explain reactivity & stability of cycloalkanes. Baeyer proposed any deviation of bond angle from ideal bond angle value (109.280) will produce a strain in molecule. If the bond angles deviate from the ideal bond angle then ring produce strain. Higher the strain, higher the instability. Higher strain produce increase reactivity & increase heat of combustion.

  14. i) Deviation = normal tetrahedral bond angle - actual bond angle. e.g. Deviation of cyclopropane = 109.50- 600= 49.50 ii) Angle strain = 1/2 [109.50- actule bond angle] e.g. cyclopropane = 1/2 [109.5 - 60] = +24.440 cyclobutane = 1/2 [109.5 - 90] = +9.440 cyclopentane = 1/2 [109.5 - 108] = +0.440 iii) Internal bond angle = 180(n - 2) ÷ n n = no. of carbon atoms present in ring. e.g. bond angle (cyclopropane) = 180 (3 - 2) ÷ 3 = 600

  15. • The postulates of Baeyer's strain theory are :- i) In cycloalkanes each carbon atom is SP3hybrid & bonded to other four carbon atoms. The angle between any pair of adjacent bonds should be tetrahedral (109.50). ii) Due to cyclic nature, cycloalkans are planar in nature. As a result the bond angles between adjacent carbon atoms in the ring are not longer equal to the normal tetrahedral angle i.g. 109.50 The angle deviation varies with the size of the ring. iii) Any distortion or deviation from the normal tetrahedral angle causes a strain in the ring & produce instability to resultant molecule. This strain is known as angle strain.

  16. iv) The greater the deviation from the normal angle greater will be angle strain & hence greater the reactivity of cycloalkane. v) Stability of the ring

  17. When carbon is bonded to four other atoms. The angle between any two pair of bonds is the tetrahedral angle 109.50. but the ring of cyclopropane is a triangle with three angles of 600, & the ring of cyclobutane is a square with four angles of 900 . In cyclopropane or cyclobutane. Therefore, one pair of bonds to each carbon can not assume the tetrahedral angle, but must be compressed to 600or900to fit the geometry of the ring.

  18. cyclopropane & cyclobutane undergo ring-opning reactions since these relieve the strain & yield the more stable open-chain compounds. Because the deviation of the bond angles in cyclopropane (109.5 - 60 ) = 49.50. cyclopropane is more highly strained , more unstable, & more prone to undergo ring-opening reactions than cyclobutane.

  19. • Limitation of Baeyer's strain theory :- i) Baeyer was not able to explain the effect of angle strain in larger ring systems. ii) According to Baeyer cyclopentane should be much stable than cyclohexane but practically it is reversed. iii) Larger ring systems are not possible according to Baeyer as they have negative strain but they exist & much stable. iv) Larger ring systems are not planar but puckered to eliinate angle strain. v) Heat of combustion should increase steadily with ring size but the almost exactly the opposite is true.

  20. 2) 2) Sachse Sachse Mohr's theory (Theory of Mohr's theory (Theory of strainless strainless rings) : rings) :- - Sache Mohr's theory was proposed to explain the reason behind the higher stability of cyclohexane & other higher cycloalkanes. According to this theory, carbon atoms in 3-5 membered rings lie in the same plane where as the carbon atoms in six membered & higher rings do not lie in the same plane although they lie in different planes. Sachse & Mohr (1918) proposed that such rings can become free from strain if all the ring carbons are not forced into one plane.

  21. If the ring assumed a folded or puckered condition, the normal tetrahedral angles of 109.50are retained & as a result the strain within the ring is relieved. i.e. Cyclohexane can exist in two non-planar puckered conformations (Chair form & Boat form) both of which are completely free from strain. The chair form of cyclohexane is more stable than the boat form. As the carbon atoms present in six membered & higher rings lie in different planes, there is no alteration in normal tetrahedral angle & therefore, there is no angle strain in bigger/higher rings & are stable.

  22. i.e. chair & boat forms are two stainless rings of cyclohexane which are interconvertible. H Equatorial Hydrogen H Axial Hydrogen Flipping H H H Reflipping H H This side move down H Boat form This side move up H H H Chair form H

  23. • Equatorial hydrogens lie in the plane of the ring carbons. • Axial hydrogens lie (up or down) parallel to the perpendicular axis. • Each carbon atom of cyclohexane is bonded to two hydrogens. • Each of the six carbon atoms of cyclohexane has one equatorial & one axial hydrogen atom. • There are six equatorial hydrogens, & six axial hydrogens. Equatorial Hydrogen H H Axial Hydrogen H H H H H H H H H H

  24. • In the flipping & reflipping between conformations, axial becomes equatorial while equatorial becomes axial. • The bond that are axial in one chair conformer are equatorial in the other chair conformer. • The bonds that are equatorial in one chair conformer are axial in the other chair conformer. pull this carbon down H 4 H H Equatorial Hydrogen H H H 2 1 3 Axial Hydrogen H H Ring flip H 2 H H 3 H H H 6 H H 5 1 5 6 H H H H H H 4 H H push this carbon up

  25. • Six membered & larger membered rings the carbon atoms lie in different plane . • These rings are non planar or puckered rings. • In these puckered rings the normal tetrahedral angle remains unaltered & therefore angle strain is negligible. • Hence these rings are called as strainless rings.

  26. 3) Coulson & Moffitt's modification (1947) : 3) Coulson & Moffitt's modification (1947) :- - Coulson & Moffitt suggested that the rehybridization occurs in cyclopropane where in the carbon-carbon bonds are bent outwords so that the bond angle becomes 1040which consequently reduces the level of ring strain. These bonds are called as 'bent bond' or banana bonds.

  27. • Thus, the C-C bonds have more p-character while the C-H bonds have more S-character. Hence, cyclopropane is much more reactive than alkanes or other higher ring system. • Cyclopropane has C-C-C bond angles of 600 Cyclobutane has C-C-C bond angles have a value 900 The higher cycloalkanes & alkanes have C-C-C bond angles of 109.50 • The small bond angles of cyclopropane indicate that the overlap of SP3 orbitals of carbon is less than the overlap of SP3orbitals of carbon in alkanes (e.g. propane).

  28. • The overlap of SP3orbitals of carbons in cyclopentane, higher cycloalkanes or n-alkanes is maximumbecause in these cases it is possible for the SP3orbitals to overlap along their axes, the bond angles being approximatly equal to 109.50. Fig. Overlap between SP3orbitals in A) Propane : Maximum overlap occurs in propane. B) Cyclopropane : Weak overlap occurs in cyclopropane.

  29. • The C-C bonds in cyclopropane are weaker than the C-C bonds in propane. • They are called Banana bonds or Bent bonds. In organic chemistry, a bent bond also known as a banana bond with geometry somewhat reminiscent of a banana.

  30. THANK YOU

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