alicyclic chemistry

• Alicyclic chemistry


  The chemistry of ring shaped compounds

  
• Cycloalkanes


  Cyclic alkanes; their carbon atoms arranged in a ring

  
• Cycloalkanes


  • Have fewer hydrogen atoms than acyclic compounds
  • Have a general formula CnH2n
• Naming simple cycloalkanes


  The prefix cyclo is added to the alicyclic alkane name

  
• Structure of cycloalkanes


  Shown as a regular polygon with the number of vertices equal to the number of C's

  
• The rules for naming cycloalkanes are the same as for alkanes but with two additional rules

  
• Rule 1 for naming cycloalkanes

  Decide whether the cyclic or acyclic portion contains more carbons. This determines the base name

  
• Alkyl substituted cycloalkane or cycloalkane substituted alkane


  If the number of carbon atoms in the ring is equal to or greater than the number in the substituent, the compound is named as an alkyl-substituted cycloalkane

  
• Rule 2 for naming cycloalkanes

  Carbons are numbered to give the lowest numbers for substituted carbons

  
• Numbering cycloalkanes


  Starts at the most substituted carbon, and goes around in order to give the lowest numbers

  
• When there are more acyclic than cyclic carbons


  The cyclic part becomes a cycloalkyl substituent

  
• Physical properties of cycloalkanes

  • Boiling and melting points increase with increasing molecular mass and are higher than comparable alkanes
  • The more rigid structure of cycloalkanes permit greater attractive interactions
  • Higher density is also due to rigid structure which allows the molecule to 'pack' more effectively than corresponding alkanes thereby increasing mass per unit volume
• Geometric isomerism in cycloalkanes

  • Substituted cycloalkanes can give rise to geometric isomers (cis and trans isomers)
  • If two non-hydrogen substituents are on the same side of the ring, it is the cis isomer. If they are on opposite sides it is the trans isomer

  
• Baeyer's theory of angle strain


  • The difference between a tetrahedral angle (109.5) and the internal angle of a polygon is used as a measure of stability
  • When carbon is bonded to four other atoms, the angle between any pair of bonds is the tetrahedral angle 109.5°
  • If a cycloalkane requires bond angles different to 109.5° the sp3 orbitals cannot overlap as efficiently as possible. This gives rise to angle strain (Bayer strain)
  • The greater the deviation from this angle the more unstable a molecule is and thus the more prone it is to ring opening reactions

  
• Rings of all sizes 3C to 30C can now be easily prepared
• Where the Baeyer theory fails


  • The angles used for each ring are based on the assumption that the rings of all cycloalkanes are planar (flat)
  • Cycloalkanes in reality adopt a puckered three dimensional conformations that allow all the bond angles to be nearly tetrahedral
  • Cyclopentane should be more stable than cyclohexane. However, experiments reveal it is in fact the reverse
  • Larger ring systems are not possible as they have negative strain but they do exist and are stable

  
• Types of strain


  • Angle strain - expansion or compression of bond angles away from most stable
  • Torsional/bond strain – due to the eclipsing of bonds on neighboring atoms
  • Steric strain - repulsive interactions between non-bonded atoms in close proximity
• Calculating ring strain


  • Ring strain is calculated with heats of combustion
  • The energy released is the heat of combustion. This value can be converted in useful information
• Ring strain per CH2

  • Cyclopropane: 9.2 kcal
  • Cyclobutane: 6.6 kcal
  • Cyclopentane: 1.3 kcal
  • Cyclohexane: 0.0 kcal
  • Cycloheptane: 0.9 kcal
  • Open chain: 0.0 kcal

  
• Cyclopropane


  • Most strained cycloalkane due to angle strain and torsional strain
  • Bonding overlap is reduced because the enforced 60° bond angle leads to poor overlap of the sp3 orbitals (bonds are bent and therefore weakened)
  • The three membered ring has to be planar, and all the C-H bond are eclipsed

  
• Cyclobutane


  • Neither planar, nor a perfect square so it does not have 90° angles
  • Adopts a slightly puckered conformation (one carbon atom is about 25° above) with bond angles of 88°
  • This increases angle strain but reduces torsional strain

  
• Cyclopentane


  • Planar cyclopentane would have very little angle strain but a large amount of torsional strain
  • Adopts a puckered 'envelope' conformation, which reduces the torsional strain
• Cyclohexane


  • By far the most common cycloalkane in nature and also in organic chemistry
  • Adopts a puckered structure with zero ring strain
  • Has two conformations: chair and boat

  
• Cyclohexane chair conformation


  • All the bond angles are 109.5° and all the C-H bonds are eclipsed (Zero ring strain)
  • Has alternating atoms in a common plane and tetrahedral angles between all carbons

  
• Cyclohexane boat conformation


  • Avoids any angle strain, but there is torsional strain
  • The two hydrogens at the ends of the boat are in close contact, causing torsional strain and the flagpole hydrogens are eclipsed

  
• Cyclohexane conformations


  • The chair is the lowest energy conformation
  • The half chair is the point of highest energy, and is not a stable conformation
• Axial and equatorial positions in cyclohexane

  • Six of the C-H bonds point straight up and down (axial bonds)
  • Six of the C-H bonds point out from the ring (equatorial bonds)
  • Each carbon atom has one axial and one equatorial hydrogen
  • Each face of the ring has three axial and three equatorial hydrogens in an alternating arrangement

  
• Conformational mobility of cyclohexane

  Chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions by a ring-flip
• The chair conformation with the methyl in the axial position can interconvert into a chair conformation with the methyl in equatorial position (ring flip)
• Conformations of monosubstituted cyclohexanes


  • The axial substituent has 2 gauche interactions, adding 1.8 kcal/mol of energy
  • The equatorial substituent has no gauche interactions and is more stable

  
• Axial-equatorial energy differences for substituents

  • -F: 0.2 kcal/mol
  • -CN: 0.2 kcal/mol
  • -Cl: 0.5 kcal/mol
  • -Br: 0.6 kcal/mol
  • -OH: 1.0 kcal/mol
  • -COOH: 1.4 kcal/mol
  • -CH3: 1.7 kcal/mol
  • -CH2CH3: 1.8 kcal/mol
  • -CH(CH3)2: 2.1 kcal/mol
  • -C(CH3)3: 5.4 kcal/mol
• Conformations of disubstituted cyclohexanes

  • There is great steric interference when there are two large substituents in axial positions oriented on C1 and C3 (or C1 and C5)
  • The less stable conformation has both substituents in axial positions (diaxial)
  • The more stable conformation has both substituents in equatorial positions (diequatorial)

  
• Trans-1,3-dimethylcyclohexane


  • Does not have a conformation with a 1,3-diaxial interaction between two methyl groups
  • Both conformations are the same since they both contain a methyl group in an axial and equatorial position

  
• Cis-1,3-dimethylcyclohexane


  • The stable conformer has the diequatorial conformation
  • The trans isomer must have one methyl group in an axial position and is higher in energy than the cis isomer

  
• Substituents of different sizes

  • Energy difference between axial and equatorial positions is generally higher for larger (bulkier) groups than for a smaller group
  • If both substituents cannot be equatorial, the larger group goes equatorial, and the smaller goes axial
• Extremely bulky groups


  • A group such as tert-butyl will always go to the equatorial position because it is so bulky
  • If two t-butyl groups are in the same cyclohexane both will take equatorial positions
  • If they cannot, the molecules is forced into the twist boat conformation which will be less crowded and lower in energy