Cycloalkanes• Saturated cyclic HC• Alicyclic (aliphatic cyclic)• General formula (CH2)n
Naming cycloalkanes• Rule 1: Find parent– Count the number of C atoms in ring and largestsubstituent chain– If number of C in ring is = or > than number insubstituent alkyl-substituted cycloalkane– If number of C in largest substituent is > than inring cycloalkyl-substituted alkane
• Rule 2: number substituent and write name– Choose point of attachment as C number 1 andnumber substituent on ring so that 2nd substituenthas low a number as possible.– Ambiguity exists, number so that 3rd and 4thsubstituent has a low number as possible untilpoint of difference found– 2 or more different alkyl groups could potentiallyreceive the same number present, numberalphabetically
Cis-trans isomerism in cycloalkanes• Non-polar• Fairly inert• Less flexible than open chain alkanes• Larger cycloalkanes have higher rotationalfreedom• Ring C=25 and > floppy and indistinguishablefrom open chain alkane• 2 faces (top & bottom)• Isomerism stereoisomer: same compounddifferent 3D orientation (cis-trans isomers
Stability of cycloalkanes: ring strain• C3,4 :strain• C5 :less strain• C6 :strain free• Strain that contribute to the overall energy ofcycloalkane:– Angle strain: strain due to expansion orcompression of bond angle– Torsional strain– Steric strain
Cyclopropane• Rigid• Planar• No bond rotation unless bond broken• Most strain• 60o angle- angle strain• Torsional strain• Bond weaker and more reactive
Cyclobutane• Less angle strain than cyclopropane• More torsional strain bcoz more H• Not flat.• 1 C atom lies 25o above plane increaseangle strain but reduce torsional strain untilminimum E balance between 2 opposingeffects achieved
Cyclopentane• Planar:– No angle strain -108o– High torsional strain• Puckered, non-planar conformation:– 4 C in plane– 1 C bent out of plane– increase angle strain– Decrease torsional strain
Cyclohexane• Most common cycloalkane• Strain free: Chair conformation (no angle ortorsional strain) – 2D drawing• Twist boat conformation– No angle strain– Have steric and torsional strain– 23kJ/mol higher in E than chair conformation
Cyclohexane• Axial and equatorial bond– 6 axial perpendicular to ring– 6 equatorial in “plane” of ring around equator– Each C has axial and equatorial– Each face has 3 axial and 3 equatorial– Ring flip chain conformation interconvert at roomT– Exchange axial and equatorial position
Conformation of monosubtitutedcyclohexane• Ring flip two conformation of monosubstitutedcyclohexane not equally stable• Equatorial conformation more stable bcoz nointeraction• Axial conformation– Steric strain caused by 1,3 diaxial interaction– Similar to gauche butane ; 3.8 kJ/mol x2 = 7.6 kJ/molstrain– More bulky alkyl group thus more steric strain
Conformation of disubstitutedcyclohexane• Cis-1,2- dimethylcyclohexane– On same face– First:• 1 gauche interaction (3.8 kJ/mol)• 2 CH3-H diaxial interaction (7.6kJ/mol)• = 11.4 kJ/mol– Second (ring flip):• 1 gauche interaction (3.8 kJ/mol)• 2 CH3-H diaxial interaction (7.6kJ/mol)• = 11.4 kJ/mol
Conformation of disubstitutedcyclohexane• Trans-1,2- dimethylcyclohexane– First (equatorial)• 1 gauche (3.8 kJ/mol)• No diaxial– Second (axial)• Four 1,3-diaxial interaction (2 x 7.6 kJ/mol =15.2kJ/mol)– Almost exclusively exist in diequatorialconformation
Polycyclic rings• Two or more rings fused together along acommon bond• Bicyclo [2.2.1] heptane
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