Chemistry 2213a Fall 2012 Western University
Topic 2a. Organic Structure #1: Alkanes & Cycloalkanes
• Alkanes and cycloalkanes are generally unreactive due to the strength and
non-polarity of the C-C and C-H bonds. They don’t do much except burn.
• Despite their low reactivity, alkanes
and cycloalkanes are important in
because they act as molecular
scaffolds – frameworks that can be
used to attach other functional H
groups. They also illustrate many H H
important concepts of structure.
A. Constitutional Isomerism
• Alkanes are acyclic hydrocarbons and have the general formula CnH2n+2.
• Recall, however, that the molecular formula of a substance does not always
indicate its structure because of constitutional isomerism. Constitutional
isomers are compounds that have the same molecular formula but different
bonding sequence (atom connectivity). Organic Structure #1: Alkanes & Cycloalkanes 2
• Constitutional isomers are different chemical compounds, so they have
different chemical and physical properties, e.g.the boiling points of various
CH 3 CH 3
CH 3CH 2 H2CH 2CH 3 CH 3CHCH 2H 3 CH 3CCH 3
BP (deg C) = 36 28 9 CH 3
• The number of possible isomers rapidly increases with molecular size, as there
would be more possible combinations of bonding sequences. This structural
diversity is key to the centrality of organic chemistry in biological systems.
C 3 8 1 C8H18 18
C H 2 C H 35
4 10 9 20
C 5 12 3 C10 22 75
C 6 14 5
C H 9 C H 366,319
7 16 20 42
• Review constitutional isomers from first-year chemistry or from the textbook
and practice drawing them. Organic Structure #1: Alkanes & Cycloalkanes 3
• Different constitutional isomers will have a different name. To name
compounds, follow the IUPAC rules in sections 3.3 and 3.5 of the textbook.
We’ll see many more examples later on.
Br 8-ethyl-3,3,4-trimethylundecane Organic Structure #1: Alkanes & Cycloalkanes 4
B. Representing 3D Structures
• Recall the common conventions used for drawing a 3-D molecule on a 2-D
surface. For example, ethane can be represented as:
1.Dot-Line-Wedge is most versatile and widelyHusCd.C H
2.Newman projections: molecule viewed along one C-C H
axis. Rear bonds along edge of circle. Very good for showing
conformations. Note that the rear circle is the rear atom, and
bonds meet at the front atom.
3.Fischer projections: useful for molecules with several chiral
carbons (more later). The C chain is written vertically, with east-west bonds
above the plane, and north-south below plane. Fischer projections are
frequently used in biochemistry.
H C H H C H
H C H H C H
H H Organic Structure #1: Alkanes & Cycloalkanes 5
• Recall that conformations are different, short-lived spatial arrangements of
atoms or groups caused by the rotation of single bonds (σ). That is,
conformations are different shapes of the same constitutional isomer.
• Newman projections can be used to show conformations, such as for 1,2-
Cl Cl Cl HCl Cl
Cl H H
H H H H H H H H
H H Cl Cl
• All representations are the same molecule or chemical compound, but in
different conformations. However, they are different in energy.
• Because these rapidly interconvert (10) at room temperature, they cannot
be isolated and are NOT isomers. Organic Structure #1: Alkanes & Cycloalkanes 6
• Cycloalkanes have the general formula C H . n 2n
• Cycloalkanes from C to3C hav5 nearly (but not quite) all C's in a plane, with
the substituents above and below the plane. C and4C rings5bend a bit out of
the plane to reduce the eclipsed interactions, even though this increases angle
strain (make a model of cyclohexane).
C 3 6 C4H 8 C5H 10 C 6 12
cyclopropane cyclobutane cyclopentane cyclohexane
• Nomenclature: the ring is numbered sequentially, with 6
numbers chosen so that the sum of substituent numbers
gives the lowest possible value, i.e. 4-chloro-1-isopropyl- 4 2
3 Organic Structure #1: Alkanes & Cycloalkanes 7
E. Cyclohexane Conformations
• Recall that there are two distinct conformations, the boatand the chair, in
which all carbons have normal tetrahedral geometry (all bonds 109.5°).
• The boat conformation is not very favourable.
• The chair conformation is the most favourable. However, cyclohexane rings