# Organic Chemistry/Cycloalkanes

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## Overview

A *cycloalkane* is a regular alkane with a ring or loop. An example is
cyclohexane, which is a ring of 6 carbon atoms, each bonded to 2
hydrogen atoms (C~6~H~12~).

We briefly discussed cycloalkanes in the
alkanes unit of this book, but
in this unit, we\'ll be going into much greater detail about
cycloalkanes as well as cycloalkenes. There are a number of properties
that are unique to these structures and thus they deserve special
attention.

Because of their cyclical nature, cycloalkanes do not have the freedom
of rotation that regular alkanes possess. Stereochemistry plays a very
important role in both the limitations of movements, but also, in some
instances the limitations of reactions that can take place. Often these
limitations are due less to the cyclical nature itself so much as the
lack of freedom of rotation found in such alkanes.

A good example is the restrictions placed on
**E2**
elimination reactions, which we\'ll cover later in the book.

## Drawing Cycloalkanes

!First four simple
cycloalkanes{width="400"}
The above image of the first four simple cycloalkanes shows that each
carbon has 2 hydrogens attached. As you can see, the other 2 bonds are
to adjacent carbons. While the shapes show the basic 2-dimensionaly,
top-down shape, only cyclopropane and cyclobutane have all of their
carbons on a plane. Cylcopentane and cyclohexane, as well as higher
cycloalkanes, all have a 3-dimensional geometry. Cyclohexane is
particularly important and it will be discussed separately.

!Cyclopentane and cyclohexane without atoms
labeled
More often, cycloalkanes, like many other organic structures, are drawn
such that their carbons and hydrogens aren\'t labeled. Cyclopentane and
cyclohexane (see above) are generally drawn as a pentagon and a hexagon,
respectively.

## Naming of Cycloalkanes

Naming of cycloalkanes is similar to alkanes. The smallest cycloalkane
is \"Cyclopropane\", so as you might imagine, this is a ring of 3
carbons. If the number of carbons on the cycloalkane is higher than the
number of carbons on any of its substituents (that is, carbon chains
attached to carbons on the cycloalkane), then the base name is prefixed
with \"cyclo\" and the rest of the name the same as an alkane with the
same number of carbons.

!3-cyclopentylhexane

If, as in the above image, the cycloalkane has fewer carbons than a
carbon chain it\'s connected to, then the longest carbon chain is the
primary and the cyclic portion receives the substituent \"-yl\" ending
and prefixes the primary name. Thus the name 1-ethylbutylcyclopentane,
while technically an accurate description, is not the correct name.

!3-chloro-1,1-dimethylcyclohexane and
2-bromo-3-chloro-1,1-dimethylcyclohexane

Numbering of carbons in substituted cycloalkanes is fairly
straight-forward. If there is only one substituent, it is the 1-carbon.
If there are more than one, then numbering proceeds, clockwise or
counter-clockwise, such that the lowest number after 1 goes to the least
(or a least) substituted carbon. I know that sounds tricky, so we\'ll
look at the example on the left, 3-chloro-1,1-dimethylcyclohexane. the
carbon with the chlorine bond is the least substituted carbon, so it
gets the lowest number after 1 and 1 goes to the nearest carbon with
more than 1 substituent. So, the 1 is applied to the methyls.
Substituents are still placed in alphabetic order, as you can see from
the second example,
2-**b**romo-3-**c**hloro-1,1-di**m**ethylcyclohexane.

## Ring Strain in Cycloalkanes

Carbons with 4 single bonds want the bonds to be at a tetrahedral angle
of 109.5°. In 1885, Adolf von Baeyer proposed that since carbons prefer
this angle, that only rings of 5 and 6 members should be possible.
Baeyer made the mistake of thinking in 2-dimensions and assuming that
all rings are planar. In fact, only cyclopropane and cyclobutane are
flat, resulting in their bond angles of 60° and 90°, respectively. But
for some time, it was believed that these smaller cycloalkanes could not
be created and that ones of more than 6 carbons would have strain
because their angles would be too far in excess of the 109.5° preferred
angle.

!First four simple cycloalkanes in 3-dimensional stick
representation

In the above figure, the first four simple cycloalkanes are again
represented, but this time the 3 dimensional representations are shown.
Notice how cyclopentane has 4 carbons on a plane and the 5th is slightly
off the plane, giving it the shape of an open envelope.

In fact, rings larger than 3 carbons have 3 dimensional shapes that
relieve this bond strain, but it\'s important to note that bond strain
does affect stability. For cyclopropane and cyclobutane, the strain
energy is about 110 kJ/mol. Cyclobutane can enter a \"puckered\"
formation that slightly relieves some torsional strain. Cyclopentane,
which is non-planar can remove some of the strain and has only about 25
kJ/mol of strain. Cyclohexane, because of its chair conformation, can
maintain perfect tetrahedral angles, resulting in no strain. As the ring
size goes up further, angle strain can be avoided at the cost of
introducing some eclipsing strain, and *vice versa*, so that some strain
exists, but the most strained of these, cyclononane, has only about 50
kJ/mol of strain. Ring strain pervades for ring sizes up to 13 members.
After that, there are enough carbons that ring strain is removed
completely.

!Cyclopentane in a 3-dimensional stick
representation

But angle strain isn\'t the only issue involved in ring strain.
Torsional strain is also a factor. When the hydrogen bonds in a ring
eclipse each other, this creates additional strain. In the case of
cyclopropane the hydrogens eclipse each other, greatly adding to the
strain. In fact, this is the cause of most of the strain found in
cyclopentane. Cyclopentane has bond angles very close to 109.5°, but
because 4 of its carbons are planar, there it has torsional strain that
makes it less stable. Looking at the cyclopentane along 2 of the bonds
on the plane, with the non-planar carbon closest to us, in the figure
above, one can see that the bend in the bonds allows the hydrogens to
move slightly out eclipse, reducing the torsional strain somewhat.

Cyclohexane is the lowest membered ring that\'s able to have both 109.5°
bond angles and maintain a staggered conformation, allowing it to be
strain free.

## Conformations of Cyclohexane

There are 3 main conformations of Cyclohexane, the chair, the reverse
chair and the boat. The chair conformations have no angular, torsional
or steric strain. However the boat conformation does have steric strain,
as the hydrogens at the high points on the front and back of the boat
are slightly overlapping.

## Conformations of Mono- and Di-substituted Cyclohexane

**TO DO**

## The Effect of Conformation on Reactions with Cyclohexane

Different conformations of cyclohexane can react differently for certain
reactions. For example, the **E2** elimination
reaction,
which we\'ll cover later in this book, requires that a halogen atom and
a hydrogen atom be on adjacent carbons and coplanar from each other, or
180° (anti). In cyclohexanes with multiple substituents attached to its
carbons, it\'s possible for that the hydrogen and halogen can be on
adjacent carbons, but never be coplanar, thus preventing the reaction
from occurring.

In other reactions, steric interference from adjacent substituents, for
example, a large alkyl group on an adjacent carbon, can prevent a
nucleophile from attacking a desired location. For these, and other
reasons, it\'s important to understand the 3-dimensional geometry and
structure of the reactants as well as the mechanisms of the reactions
that are taking place.

## Large Ring Systems

**TO DO**

## Polycyclic Ring Systems

In some compounds, two or more rings are fused together in the same
molecule. This can occur in both aliphatic and aromatic compounds.

If the two rings meet at a single carbon atom, where that carbon atom is
part of both rings, the compound is called a spiro
compound. The two rings are roughly (but
not exactly) perpendicular to each other because of the usual
tetrahedral bonding of four ligands around carbon.

If the two rings are bridged at two or more carbon atoms, the carbon
atoms form part of a bridge, and the compound is a bicycloalkane (for
two rings) or a polycycloalkane (for three or more rings).

An aromatic bicyclic compound is naphthalene
(C~10~H~8~), which is a pair of benzene rings fused across a
carbon-carbon bond. An aliphatic (non-aromatic) bicyclic compound is
decalin (C~10~H~18~), formally known as
decahydronaphthalene, which is a pair of cyclohexane rings fused across
a C-C bond.

### Naming conventions for bicyclic rings

A bicyclic alkane ring system is composed of three parts:

1.  The longer carbon chain (**a**)
2.  The shorter carbon chain (**b**)
3.  The carbon-carbon bridge (**c**)

Note that $a >= b >= c$ always.

A bicyclic ring is named according to the **total number of carbons** in
the following form:

:   **bicyclo\[a.b.c\]alkane**

In this formula, substitute \[a.b.c\] with the lengths of the carbon
chains, and substitute \"alkane\" with the total number of carbons
(heptane, octane, nonane, etc.).

A famous example of a bicyclic compound is
norbornane (see image below). The formal name
for norbornane is bicyclo\[2.2.1\]heptane. There are two carbons to the
left of the bridge, two carbons to the right of the bridge, and one
carbon on the bridge itself. Therefore, the numbers in the brackets are
\[2.2.1\]. Note that the two carbons at the base of the bridge are not
counted in the brackets.

!Norbornane.{width="300"}

## Introduction to Heterocyclic Compounds

!A sampling of single-ring heterocylic
compounds

Heterocyclic compounds are cyclic compounds that contain atoms other
than carbon in their ring. The figure above shows just a few single-ring
heterocyclic compounds. The ones shown are simply 5 and 6 membered
rings. There are smaller and larger rings and there are also multiple
ring heterocycles.

Heterocyclic compounds play a big role in organic chemistry and because
they have different electron configurations from carbon, they react
differently from carbon rings and differently from each other. Later in
this book we will discuss several specific heterocyclic compounds that
are very commonly used in organic chemistry and we\'ll discuss their
individual characteristics.

For now, it\'s simply important to know what a heterocyclic compound is.

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\|\| \<\< Haloalkanes \|
Alkanes \| Alcohols \>\>

