CHEM261 Lecture Notes - Dextrorotation And Levorotation, Cyclohexane, Cyclopentane
11 May 2018
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Cycloalkanes Notes:
❏Acyclic → Straight Chains
❏Cyclic → Formation of rings
❏General Formula of Cycloalkanes → CnH2n
❏Name: Cyclo(Number of Carbons).
❏When one substituent present use name of group as prefix →
Chlorocyclobutane.
❏When there are two substituted positions the one that comes first in the alphabet
takes priority 1, therefore labelled as one when counting along the chain. Second
substituted position should be as low as possible. →
1-ethyl-3-methylcyclopentane.
❏Cycloalkane becomes substituent when contains less carbon than alkyl chain.
❏If ring contains three carbons = cyclopropyl. → 3-cyclopropylpentane.
❏Isomers
❏Constitutional Isomers → Isomers whose atoms have different connectivity.
❏Stereoisomers → Isomers whose atoms have the same connectivity but with
difference in atoms spacing.
❏Cis → Same side.
❏Trans → Different sides
❏Cis and Trans are known as Geometric isomers.
❏Heat of combustion → Helps determine stability of compounds.
❏Cyclohexane is most stable. Cyclopropane is least stable (most potential energy).
❏Stability of Cycloalkanes is dependent on
❏1. Torsional strain → Due to eclipsed conformation
❏2. Angle Strain → Deviation from 109.5 Degrees for sp3 hybridization.
❏Torsional strain and Angle strain formulate the ring strain.
❏Conformational Analysis of Cycloalkanes
❏Cyclopropane → Eclipse containing torsional strain. Angles are 60 degrees.
Therefore large angle strain (109.5-60=49.5). C-H bonds contain eclipsed
conformations therefore suffers torsional strain.
❏Due to ring strain cyclopropane → Least stable, therefore most reactive
causing ring to open.
❏Cyclobutane → Appears Elipsed, but is planar. Suffers from
❏Angle strain (109.5-90 = 19.50)
❏Torsional strain for eclipsed conformation.
❏Ring is folded (puckered)
❏Reducing torsional strain, but causing more angle strain (bond
angle = 88)
❏Cyclopentane → puckered to form “envelope” conformation. Torsional strain is
reduced but at the expense of angle strain suffering (104.4)
❏Cyclohexane → If it is planar it would suffer from angle strain (10.5) and torsional
strain. However, it is puckered to form a chair conformation, therefore free from
angle strain (all angles 109.5) Each carbon is tetrahedral.
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Document Summary
When one substituent present use name of group as prefix . When there are two substituted positions the one that comes first in the alphabet takes priority 1, therefore labelled as one when counting along the chain. Second substituted position should be as low as possible. Cycloalkane becomes substituent when contains less carbon than alkyl chain. If ring contains three carbons = cyclopropyl. Constitutional isomers isomers whose atoms have different connectivity. Stereoisomers isomers whose atoms have the same connectivity but with difference in atoms spacing. Cis and trans are known as geometric isomers. Heat of combustion helps determine stability of compounds. Angle strain deviation from 109. 5 degrees for sp3 hybridization. Torsional strain and angle strain formulate the ring strain. C-h bonds contain eclipsed conformations therefore suffers torsional strain. Due to ring strain cyclopropane least stable, therefore most reactive causing ring to open. Cyclobutane appears elipsed, but is planar. Reducing torsional strain, but causing more angle strain (bond angle = 88)
