CHM136H1 Chapter Notes - Chapter 4: Cycloalkane, Alicyclic Compound, Substituent
CHM136H1(S) – Textbook à Chapter 4 à 4.1 – 4.8
• Cycloalkanes – saturated cyclic hydrocarbons à alicyclic compounds
o Are like any other alkanes in that both are non-polar and pretty
inert
o Less flexible than open-chain alkanes à no freedom of bond
rotation compared to open-chain alkanes because it can only do
so by breaking open the ring
§ However, larger cycloalkanes do have more rotational
freedom à e.g., large rings composed of 25 or more C’s
are very floppy
o General formula: (CH2)n or CnH2n
o Shown as polygons in skeletal drawings
§ A few examples:
o Substituted cycloalkanes named by rules similar to those for
open-chain alkanes:
1) Find the parent ring
a. First, count how many C atoms in ring and in largest
substituent
2
i. If # of C’s in ring ≥ # of C’s in substituent à
compound named as an alkyl-substituted
cycloalkane
ii. If # of C’s in ring < # of C’s in substituent à
compound named as a cycloalkyl-substituted
alkane
2) Number the substituents, write the name
a. For alkyl- or halo-substituted cycloalkane, choose first
point of attachment of substituent as carbon 1 and
number substituents so that the 2nd substituent has as
low a number as possible à if ambiguity still exists,
number so that the 3rd, 4th, etc. substituent has as low a
number as possible
i.e.,
i. When 2 or more different alkyl groups could take
on the same numbers, number according to
alphabetical order of substituents
3
e.g.,
ii. Always treat halogens the same way you would
for alkyl groups
o Have a top and bottom face when viewed edge-on à makes cis-
trans isomerism possible in substituted cycloalkanes
§ cis- = both substituents on same face of ring
§ trans- = substituents on opposite faces of ring
§ ex.:
vs.
- These 2 isomers have same atom connectivity but differ in spatial
arrangement of atoms à why these isomers are called stereochemical
isomers/stereoisomers, where stereochemistry refers to the 3-D aspects
of chemical structures and reactivity
- Both isomers are stable compounds that don’t interconvert without
breaking and reforming bonds*
• 1885: A. von Baeyer suggested that small and large rings are unstable due
to angle strain (strain that arises when bond angles deviate from the
ideal 109° value)
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CHM136H1 Full Course Notes
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Document Summary
However, larger cycloalkanes do have more rotational freedom e. g. , large rings composed of 25 or more c"s are very floppy: general formula: (ch2)n or cnh2n, shown as polygons in skeletal drawings. 2 e. g. : always treat halogens the same way you would for alkyl groups, have a top and bottom face when viewed edge-on makes cis- trans isomerism possible in substituted cycloalkanes. Cis- = both substituents on same face of ring. Trans- = substituents on opposite faces of ring. These 2 isomers have same atom connectivity but differ in spatial arrangement of atoms why these isomers are called stereochemical isomers/stereoisomers, where stereochemistry refers to the 3-d aspects of chemical structures and reactivity. 3: amount of strain in compound = total energy of compound energy of strain-free reference compound. Easiest way to do this for a cycloalkane is to measure its heat of combustion, because the more strain energy compound contains, the more heat is released through combustion.
