# Organic Chemistry/Alkanes

![](Go_To_Organic_Chemistry_Contents_.png "Go_To_Organic_Chemistry_Contents_.png")

Alkanes are the simplest organic molecules, consisting solely of
singly-bonded carbon and hydrogen atoms. Alkanes are used as the basis
for naming the majority of organic compounds (their **nomenclature**).
Alkanes have the general formula C~n~H~2n+2~. Although their
reactivities are often rather uninteresting, they provide an excellent
basis for understanding bonding, conformation, and other important
concepts which can be generalized to more \"useful\" molecules.

# Introduction

thumb\|2,2-dimethylpropane or neopentane.\
An example of an alkane Alkanes
are the simplest and the least reactive
hydrocarbon species containing only carbons
and hydrogens. They are commercially very important, for being the
principal constituent of gasoline and lubricating oils and are
extensively employed in organic chemistry; though the role of pure
alkanes (such as hexanes) is delegated mostly to solvents.

The distinguishing feature of an alkane, making it distinct from other
compounds that also exclusively contain carbon and hydrogen, is its lack
of unsaturation. That is to say, it
contains no double or triple bonds, which are highly reactive in organic
chemistry.

Though not totally devoid of reactivity, their lack of reactivity under
most laboratory conditions makes them a relatively uninteresting, though
very important component of organic chemistry. As you will learn about
later, the energy confined within the carbon-carbon bond and the
carbon-hydrogen bond is quite high and their rapid oxidation produces a
large amount of heat, typically in the form of fire.

As said it is important, not considered very important component in the
chemistry.

## Introductory Definitions

**Organic compounds** contain **carbon** and **hydrogen** by definition
and usually other elements (e.g. **nitrogen** and **oxygen**) as well.
(CO~2~ is not an organic compound because it has no hydrogen).

**Hydrocarbons** are organic compounds that contain carbon and hydrogen
only.

**Alkanes** are hydrocarbons or organic compounds made up of only
carbon-carbon single bonds.Hence they are saturated. (as opposed to
double and triple bonds). The simplest alkane is
**methane.**

## Methane

```{=html}
<div class="noprint" style="border:1px solid gold; background:cornsilk; padding: 4px; text-align: center; float: right;">
```
`<small>`{=html} *On WP:*\
Alkanes `</small>`{=html}

```{=html}
</div>
```
Methane, (CH~4~, one carbon bonded to four hydrogens) is the simplest
organic molecule. It is a gas at standard temperature and pressure
(STP).

  -------------------------------------------------------------------------
  ![](Methane-2D-flat-small.png "Methane-2D-flat-small.png"){width="150"}
  Methane
  -------------------------------------------------------------------------

This is a flattened, two-dimensional representation of methane that you
will see commonly. The true three-dimensional form of methane does not
have any 90 degree angles between bonded hydrogens. The bonds point to
the four corners of a tetrahedron, forming
cos^-1^(-1/3) ≈ 109.5 degree bond angles. !3D rendering of methane, a
regular tetrahedral
shape.

## Ethane

Two carbons singly bonded to each other with six hydrogens is called
**ethane**.

![](Ethane-flat.png "Ethane-flat.png"){width="80"}

Ethane is the second simplest hydrocarbon molecule. It can be thought of
as two methane molecules attached to each other, but with two fewer
hydrogen atoms. Note that, if we were simply to create a new bond
between the carbon centers of two methane molecules, this would violate
the octet rule for the involved atoms.

There are several common methods to draw organic molecules. They are
often used interchangeably, although some methods work better for one
situation or another. It is important to be familiar with the common
methods, as these are the \"languages\" organic chemists can use to
discuss structure with one another.

# Drawing alkanes

When writing out the alkane structures, you can use different levels of
the shorthand depending on the needs at hand in hand. For example,
pentane can be written out. Its formula is C~5~H~12~.

![](Pentane.svg "Pentane.svg"){width="150"},

or CH~3~--CH~2~--CH~2~--CH~2~--CH~3~,

or CH~3~(CH~2~)~3~CH~3~,

or minimized to
![](Pentane-2D-Skeletal.svg "Pentane-2D-Skeletal.svg"){width="100"}

## Line drawing shorthand

Although non-cyclic alkanes are called straight-chain alkanes they are
technically made of linked chains. This is reflected in the line-drawing
method. Each ending point and bend in the line represents one carbon
atom and each short line represents one single carbon-carbon bond. Every
carbon is assumed to be surrounded with a maximum number of hydrogen
atoms unless shown otherwise.

  -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  ![](Propane-2D-Skeletal.svg "Propane-2D-Skeletal.svg"){width="60"} ![](Butane_simple.svg "Butane_simple.svg"){width="85"} ![](Pentane-2D-Skeletal.svg "Pentane-2D-Skeletal.svg"){width="100"}
  *Propane, butane, pentane*
  -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Structures drawn without explicitly showing all carbon atoms are often
called \"skeletal\" structures, since they represent the skeleton or the
backbone of the molecule. In organic chemistry, carbon is very
frequently used, so chemists know that there is a carbon atom at the
endpoints of every line that is not specifically labeled.

# Conformations

**Conformers**, also called **conformational isomers**, or **rotational
isomers**,or**rotomers** are arrangements of the same molecule made
transiently different by the rotation in space about one or more single
bonds.

Other types of isomer can only be converted from one form to another by
*breaking* bonds, but conformational isomers can be made simply by
*rotating* bonds.

## Newman projections

Newman projections are drawings used to represent different positions of
parts of molecules relative to each other in space. Remember that single
bonds can rotate in space if not impeded. Newman projections represent
different positions of rotating molecule parts.

+----------------------------------+----------------------------------+
| Conformers interconvert readily, | ![](Spinning                     |
| normally thousands of times a    | newman.png "Spinningnewman.png") |
| second as parts of molecules     |                                  |
| spin.                            |                                  |
+----------------------------------+----------------------------------+
| In the following drawings,       | ```{=html}                       |
| methyl groups are on the front   | <div align=center>               |
| and back ends of the molecule    | ```                              |
| and a circle represents all that | ![](Methylnewman.png "Me         |
| lies between.                    | thylnewman.png")`<small>`{=html} |
|                                  | *Note: This is how methyl groups |
|                                  | are represented in Newman        |
|                                  | projections* `</small>`{=html}   |
|                                  |                                  |
|                                  | ```{=html}                       |
|                                  | </div>                           |
|                                  | ```                              |
+----------------------------------+----------------------------------+

```{=html}
<center>
```
+----------------------------------+----------------------------------+
| ```{=html}                       | ```{=html}                       |
| <center>                         | <center>                         |
| ```                              | ```                              |
| !                               | ! |
| "Staggerednewmanprojection.png") |                                  |
|                                  | ```{=html}                       |
| ```{=html}                       | </center>                        |
| </center>                        | ```                              |
| ```                              |                                  |
+----------------------------------+----------------------------------+
| ```{=html}                       | ```{=html}                       |
| <center>                         | <center>                         |
| ```                              | ```                              |
| **Staggered conformation**       | **Eclipsed conformation**\       |
|                                  | (front end overlaps the back and |
| ```{=html}                       | also unstable)                   |
| </center>                        |                                  |
| ```                              | ```{=html}                       |
|                                  | </center>                        |
|                                  | ```                              |
+----------------------------------+----------------------------------+

```{=html}
</center>
```
## Conformations and energy

Different conformations have different potential energies. The staggered
conformation is at a lower potential energy than the eclipsed
conformation, and is favored. In ethane, the barrier to rotation is
approximately 25 kJ/mol, indicating that each pair of eclipsed hydrogens
raises the energy by about 8 kJ/mol. This number also applies to other
organic compounds which have hydrogen atoms at similar distances from
each other. At very low temperatures all conformations revert to the
stabler( due to minimized vibration of atoms at it\'s mean position) ,
lower energy staggered conformation.

## Steric effects

Steric effects have to do with size. Two bulky objects run into each
other and invade each others space. If we replace one or more hydrogen
atoms on the above Newman projections with a methyl or other group, the
potential energy goes up especially for the eclipsed conformations.

Lets look at a Newman projection of butane as it rotates
counterclockwise around its axes.

```{=html}
<center>
```
+---------+---------+---------+---------+---------+---------+---------+
| ```     | ![{widt | ```     | "){widt | ```     | "){widt | ```     |
| !       | h="50"} | !       | h="50"} | !       | h="50"} | !       |
| [](Newm |         | [](Newm |         | [](Newm |         | [](Newm |
| anproje |         | anproje |         | anproje |         | anproje |
| ction1. |         | ction2. |         | ction3. |         | ction4. |
| png "Ne |         | png "Ne |         | png "Ne |         | png "Ne |
| wmanpro |         | wmanpro |         | wmanpro |         | wmanpro |
| jection |         | jection |         | jection |         | jection |
| 1.png") |         | 2.png") |         | 3.png") |         | 4.png") |
|         |         |         |         |         |         |         |
| ```     |         | ```     |         | ```     |         | ```     |
| {=html} |         | {=html} |         | {=html} |         | {=html} |
| </      |         | </      |         | </      |         | </      |
| center> |         | center> |         | center> |         | center> |
| ```     |         | ```     |         | ```     |         | ```     |
+---------+---------+---------+---------+---------+---------+---------+
| ```     |         | ```     |         | ```     |         | ```     |
| {=html} |         | {=html} |         | {=html} |         | {=html} |
| <       |         | <       |         | <       |         | <       |
| center> |         | center> |         | center> |         | center> |
| ```     |         | ```     |         | ```     |         | ```     |
| Anti    |         | E       |         | Gauche  |         | E       |
|         |         | clipsed |         |         |         | clipsed |
| ```     |         |         |         | ```     |         |         |
| {=html} |         | ```     |         | {=html} |         | ```     |
| </      |         | {=html} |         | </      |         | {=html} |
| center> |         | </      |         | center> |         | </      |
| ```     |         | center> |         | ```     |         | center> |
|         |         | ```     |         |         |         | ```     |
+---------+---------+---------+---------+---------+---------+---------+

```{=html}
</center>
```
When the larger groups overlap they repel each other more strongly than
do hydrogen, and the potential energy goes up.

## Entropy

Entropy, represented as a **ΔS**, is a mathematical construct that
represents disorder or probability. Natural systems want to find the
lowest energy or organization possible, which translates to the highest
entropy.

*A note about potential energy: If you are rusty on this, remember the
analogy of a big rock pushed to the top of a hill. At the top it has a
maximum of potential energy. When you push it and allow it to roll down
the hill the potential energy stored in it is transformed into kinetic
energy that can be used to generate heat or smash something.*

Notice that statistically, the ethane molecule has twice as many
opportunities to be in the gauche conformation as in the anti
conformation. However, because the Gauche configuration brings the
methyl groups closer together in space, this generates high energy
steric interactions and do not occur without the input of energy. Thus,
the butane molecules shown will almost never be found in such
unfavorable conformations.

# Preparation of Alkanes

### Wurtz reaction

Wurtz reaction is coupling of haloalkanes using sodium metal in solvent
like dry ether

`<B>`{=html} 2R-X + 2Na → R-R + 2Na^+^X^−^`</B>`{=html}

##### Mechanism

The reaction consists of a halogen-metal exchange involving the free
radical species R• (in a similar fashion to the formation of a Grignard
reagent and then the carbon-carbon bond formation in a nucleophilic
substitution reaction.)

One electron from the metal is transferred to the halogen to produce a
metal halide and an alkyl radical.

:   R-X + M → R• + M^+^X^−^

The alkyl radical then accepts an electron from another metal atom to
form an alkyl anion and the metal becomes cationic. This intermediate
has been isolated in a several cases.

:   R• + M → R^−^M^+^

The nucleophilic carbon of the alkyl anion then displaces the halide in
an S~N~2 reaction, forming a new carbon-carbon covalent bond.

:   R^−^M^+^ + R-X → R-R + M^+^X^−^
:   **COREY-HOUSE reactioN**
:   \[Also called as \'coupling of alkyl halides with organo metallic
    compounds\'\]
:   It is a better method than wurtz reaction. An alkyl halides and a
    lithium dialkyl copper are reacted to give a higher hydrocarbon
:   R\'-X + R~2~CuLi\-\-\--\>R-R\' + R-Cu + LiX
:   (R and R\' may be same or different)
:   It

### Clemmensen reduction

Clemmensen reduction is a reduction of ketones (or aldehydes) to alkanes
using zinc amalgam and hydrochloric acid !The Clemmensen
reduction{width="300"}
The Clemmensen reduction is particularly effective at reducing
aryl-alkyl ketones. With aliphatic or cyclic ketones, zinc metal
reduction is much more effective

The substrate must be stable in the strongly acidic conditions of the
Clemmensen reduction. Acid sensitive substrates should be reacted in the
Wolff-Kishner reduction, which utilizes strongly basic conditions; a
further, milder method is the Mozingo reduction. As a result of
Clemmensen Reduction, the carbon of the carbonyl group involved is
converted from sp^2^ hybridisation to sp^3^ hybridisation. The oxygen
atom is lost in the form of one molecule of water.

### Wolff-Kishner reduction

!The Wolff-Kishner
reduction{width="400"}
The Wolff--Kishner reduction is a chemical reaction that fully reduces a
ketone (or aldehyde) to an alkane. Condensation of the carbonyl compound
with hydrazine forms the hydrazone, and treatment with base induces the
reduction of the carbon coupled with oxidation of the hydrazine to
gaseous nitrogen, to yield the corresponding alkane.

##### Mechanism

!The mechanism of Wolff-Kishner
reduction{width="600"}

The mechanism first involves the formation of the hydrazone in a
mechanism that is probably analogous to the formation of an imine.
Successive deprotonations eventually result in the evolution of
nitrogen. The mechanism can be justified by the evolution of nitrogen as
the thermodynamic driving force. This reaction is also used to
distinguish between aldehydes and ketones.

### Mozingo Reduction

A thioketal is first produced by reaction of the ketone with an
appropriate thiol. The product is then hydrogenolyzed to the alkane,
using Raney nickel
![](Mozingo_reaction.svg "Mozingo_reaction.svg"){width="600"}

# Properties of Alkanes

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```
`<small>`{=html} *On WP:*\
Alkane properties `</small>`{=html}

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</div>
```
Alkanes are **not very reactive** when compared with other chemical
species. This is because the backbone carbon atoms in alkanes have
attained their octet of electrons through forming four covalent bonds
(the maximum allowed number of bonds under the octet rule; which is why
carbon\'s valence number is 4). These four bonds formed by carbon in
alkanes are sigma bonds, which are more stable than other types of bond
because of the greater overlap of carbon\'s atomic orbitals with
neighboring atoms\' atomic orbitals. To make alkanes react, the input of
additional energy is needed; either through heat or radiation.

Gasoline is a mixture of the alkanes and unlike many chemicals, can be
stored for long periods and transported without problem. It is only when
ignited that it has enough energy to continue reacting. This property
makes it difficult for alkanes to be converted into other types of
organic molecules. (There are only a few ways to do this). Alkanes are
also **less dense than water**, as one can observe, oil, an alkane,
floats on water.

Alkanes are **non-polar solvents**. Since only C and H atoms are
present, alkanes are nonpolar. Alkanes are
immiscible in water but freely miscible in
other non-polar solvents. Alkanes consisting of weak dipole dipole bonds
can not break the strong hydrogen bond between water molecules hence it
is not miscible in water. The same character is also shown by alkenes.
Because alkanes contain only carbon and hydrogen, combustion produces
compounds that contain only carbon, hydrogen, and/or oxygen. Like other
hydrocarbons, combustion under most circumstances produces mainly carbon
dioxide and water. However, alkanes require more heat to combust and do
not release as much heat when they combust as other classes of
hydrocarbons. Therefore, combustion of alkanes produces higher
concentrations of organic compounds containing oxygen, such as aldehydes
and ketones, when combusting at the same temperature as other
hydrocarbons.

The general formula for alkanes is C~N~H~2N+2~; the simplest possible
alkane is therefore methane, CH~4~. The next simplest is ethane,
C~2~H~6~; the series continues indefinitely. Each carbon atom in an
alkane has sp³ hybridization.

Alkanes are also known as paraffins, or collectively as the paraffin
series. These terms are also used for alkanes whose carbon atoms form a
single, unbranched chain. Branched-chain alkanes are called
isoparaffins.

**Methane** through **Butane** are very flammable gases at standard
temperature and pressure (STP). **Pentane** is an extremely flammable
liquid boiling at 36 °C and boiling points and melting points steadily
increase from there; octadecane is the first alkane which is solid at
room temperature. Longer alkanes are waxy solids; candle wax generally
has between C~20~ and C~25~ chains. As chain length increases ultimately
we reach polyethylene, which consists of carbon chains of indefinite
length, which is generally a hard white solid.

## Chemical properties

Alkanes react only very poorly with ionic or other polar substances. The
pKa values of all alkanes are above 50, and so they are practically
inert to acids and bases. This inertness is the source of the term
paraffins (Latin para + affinis, with the meaning here of \"lacking
affinity\"). In crude oil the alkane molecules have remained chemically
unchanged for millions of years.

However redox reactions of alkanes, in particular with oxygen and the
halogens, are possible as the carbon atoms are in a strongly reduced
condition; in the case of methane, the lowest possible oxidation state
for carbon (−4) is reached. Reaction with oxygen leads to combustion
without any smoke; with halogens, substitution. In addition, alkanes
have been shown to interact with, and bind to, certain transition metal
complexes.

Free radicals, molecules with unpaired electrons, play a large role in
most reactions of alkanes, such as cracking and reformation where
long-chain alkanes are converted into shorter-chain alkanes and
straight-chain alkanes into branched-chain isomers.

In highly branched alkanes and cycloalkanes, the bond angles may differ
significantly from the optimal value (109.5°) in order to allow the
different groups sufficient space. This causes a tension in the
molecule, known as steric hinderance, and can substantially increase the
reactivity. The same is preferred for alkenes too.

# Introduction to Nomenclature

Before we can understand reactions in organic chemistry, we must begin
with a basic knowledge of naming the compounds. The
IUPAC nomenclature
is a system on which most organic chemists have agreed to provide
guidelines to allow them to learn from each others\' works.
Nomenclature, in other words, provides a foundation of language for
organic chemistry.

The names of all alkanes end with *-ane*. Whether or not the carbons are
linked together end-to-end in a ring (called *cyclic alkanes* or
*cycloalkanes*) or whether they contain side chains and branches, the
name of every carbon-hydrogen chain that lacks any double bonds or
functional groups will end with the suffix *-ane*.

Alkanes with unbranched carbon chains are simply named by the number of
carbons in the chain. The first four members of the series (in terms of
number of carbon atoms) are named as follows:

1.  CH~4~ = **methane** = one hydrogen-saturated carbon
2.  C~2~H~6~ = **ethane** = two hydrogen-saturated carbons
3.  C~3~H~8~ = **propane** = three hydrogen-saturated carbons
4.  C~4~H~10~ = **butane** = four hydrogen-saturated carbons

Alkanes with five or more carbon atoms are named by adding the suffix
*-ane* to the appropriate numerical multiplier, except the terminal *-a*
is removed from the basic numerical term. Hence, C~5~H~12~ is called
*pentane*, C~6~H~14~ is called *hexane*, C~7~H~16~ is called *heptane*
and so forth.

Straight-chain alkanes are sometimes indicated by the prefix *n-* (for
normal) to distinguish them from branched-chain alkanes having the same
number of carbon atoms. Although this is not strictly necessary, the
usage is still common in cases where there is an important difference in
properties between the straight-chain and branched-chain isomers: e.g.
*n-hexane* is a neurotoxin while its branched-chain isomers are not.

## Number of hydrogens to carbons

This equation describes the relationship between the number of hydrogen
and carbon atoms in alkanes:

:   **H = 2C + 2**

where \"C\" and \"H\" are used to represent the number of carbon and
hydrogen atoms present in one molecule. If C = 2, then H = 6.

Many textbooks put this in the following format:

:   **C~n~H~2n+2~**

where \"C~n~\" and \"H~2n+2~\" represent the number of carbon and
hydrogen atoms present in one molecule. If C~n~ = 3, then H~2n+2~ =
2(3) + 2 = 8. (For this formula look to the \"n\" for the number, the
\"C\" and the \"H\" letters themselves do not change.)

Progressively longer hydrocarbon chains can be made and are named
systematically, depending on the number of carbons in the longest chain.

## Naming carbon chains up to twelve

-   methane (1 carbon)
-   ethane (2 carbons)
-   propane (3 carbons)
-   butane (4 carbons)
-   pentane (5 carbons)
-   hexane (6 carbons)
-   heptane (7 carbons)
-   octane (8 carbons)
-   nonane (9 carbons)
-   decane (10 carbons)
-   undecane (11 carbons)
-   dodecane (12 carbons)

The prefixes of the first three are the contribution of a German
Chemist, August Wilhelm Hoffman, who also suggested the name quartane
for 4 carbons in 1866. However, the but- prefix had already been in use
since the 1820s and the name quartane never caught on. He also
recommended the endings to use the vowels, a, e, i (or y), o, and u, or
-ane, -ene, -ine or -yne, -one, and -une. Again, only the first three
caught on for single, double, and triple bonds and -one was already in
use for ketones. Pent, hex, hept, oct, and dec all come from the ancient
Greek numbers (penta, hex, hepta, octa, deka) and oddly, non, from the
Latin novem. For longer-chained alkanes we use the special IUPAC
multiplying affixes. For example, pentadecane signifies an alkane with
5+10 = 15 carbon atoms. For chains of length 30, 40, 50, and so on the
basic prefix is added to -contane. For example, C~57~H~116~ is named as
heptapentacontane. When the chain contains 20-29 atoms we have an
exception. C~20~H~42~ is known as icosane, and then we have, e.g.
tetracosane (eliding the \"i\" when necessary). For the length 100 we
have \"hecta\" but for 200, 300 \... 900 we have \"dicta\", \"tricta\",
and so on, eliding the \"i\" on \"icta\" when necessary; for 1000 we
have \"kilia\" but for 2000 and so on, \"dilia\", \"trilia\", and so on,
eliding the \"i\" on \"ilia\" when necessary.

We then put all of the prefixes together in reverse order. The alkane
with 9236 carbon atoms is then hexatridinoniliane.

## Isomerism

The atoms in alkanes with more than three carbon atoms can be arranged
in many ways, leading to a large number of potential different
configurations (isomers). So-called \"normal\" alkanes have a linear,
unbranched configuration, but the *n-* isomer of any given alkane is
only one of potentially hundreds or even possibly millions of
configurations for that number of carbon and hydrogen atoms in some sort
of chain arrangement.\
Isomerism is defined as the compound having same moleculer formula the
formula which present the different moleculer formula arrangement are
called as Isomerism.\
e.g.- The molecular formula for butane is C~4~H~10~.

The number of isomers increases rapidly with the number of carbon atoms
in a given alkane molecule; for alkanes with as few as 12 carbon atoms,
there are over three hundred and fifty-five possible forms the molecule
can take!

:   {\| class=\"wikitable\"

\|- ! \# Carbon Atoms !! \# Isomers of Alkane \|- \| 1 \| 1 \|- \| 2 \|
1 \|- \| 3 \| 1 \|- \| 4 \| 2 \|- \| 5 \| 3 \|- \| 6 \| 5 \|- \| 7 \| 9
\|- \| 8 \| 18 \|- \| 9 \| 35 \|- \| 10 \| 75 \|- \| 11 \| 159 \|- \| 12
\| 355 \|}

## Branched chains

Carbon is able to bond in all four directions and easily forms strong
bonds with other carbon atoms. When one carbon is bonded to more than
two other carbons it forms a branch.

  ------------------------------------------------------------------ ------------------------------------------------------------
  !Isobutane{width="200"}   !Neopentane{width="200"}
  ------------------------------------------------------------------ ------------------------------------------------------------

Above you see a carbon bonded to three and four other carbons.

:   Note: a methane group is called a **methyl** group when it is bonded
    to another carbon instead of a fourth hydrogen. --CH~3~

The common system has naming convention for carbon chains as they relate
to branching.

:   **n-alkanes** are linear
:   **iso-alkanes** have one branch R~2~CH---
:   **neo-alkanes** have two branches R~3~C---

*Note: \"R\" in organic chemistry is a placeholder that can represent
any carbon group.*

## Constitutional isomers

One of the most important characteristics of carbon is its ability to
form **several relatively strong bonds** per atom. It is for this reason
that many scientists believe that carbon is the only element that could
be the basis for the many complicated molecules needed to support a
living being.

One carbon atom can have attached to it not just the one or two other
carbons needed to form a single chain but can bond to up to four other
carbons. It is this ability to bond multiply that allows isomerism.

**Isomers** are two molecules with the same molecular formula but
different physical arrangements. **Constitutional isomers** have their
atoms arranged in a different order. A constitutional isomer of butane
has a main chain that is forked at the end and one carbon shorter in its
main chain than butane.

  ------------------------------------- ----------------------------------------------------------------------------------------------
  !Butane   !Isobutane (2-methyl-propane)"){width="100"}
  ------------------------------------- ----------------------------------------------------------------------------------------------

## Naming Alkanes

There are several ways or systems for the **nomenclature**, or naming,
of organic molecules, but just two main ones.

1.  The traditional, non-systematic names. Many of these linger on,
    especially for simpler or more common molecules.
2.  The systematic **IUPAC**
    (*eye-YOU-pack (International Union of Pure And Applied Chemistry)*)
    names.

The IUPAC system is necessary for complicated organic compounds. It
gives a series of unified rules for naming a large compound by
conceptually dividing it up into smaller, more manageable nameable
units.

Many traditional (non-IUPAC) names are still commonly used in industry,
especially for simpler and more common chemicals, as the traditional
names were already entrenched.

## IUPAC naming rules

```{=html}
<div class="noprint" style="border:1px solid gold; background:cornsilk; padding: 4px; text-align: center; float: right;">
```
`<small>`{=html} *On WP:*\
IUPAC naming
`</small>`{=html}

```{=html}
</div>
```
1.  Find the longest carbon chain, identify the end near which the most
    substituents are located, and
    number the carbons sequentially from that end. This will be the
    parent chain.
2.  Consider all other carbon groups as substituents.
3.  Alphabetize the substituents.
4.  Number the substituents according to the carbon to which they are
    attached. If numbering can be done in more than one way, use the
    numbering system that results in the smallest numbers.

Substituents are named like a parent, and replacing the *-ane* ending
with *-yl*.

### Numbering

![](237trimethyloctanelines.png "237trimethyloctanelines.png")

The above molecule is numbered as follows:

![](237trimethyloctane.png "237trimethyloctane.png")

2,3,7-Trimethyloctane

![](267trimethyloctane.png "267trimethyloctane.png")

**Not** 2,6,7-Trimethyloctane. Remember, number so as to give the
smallest numbers to the substituents.

### Alphabetizing

![](3ethyl3methylpentane.png "3ethyl3methylpentane.png")

3-Ethyl-3-methylpentane

*Ethyl* is listed before *methyl* for alphabetizing purposes.

## Branched Substituents

### Naming branched substituents

![](3-(1-methylethyl)-2,4-dimet.png "3-(1-methylethyl)-2,4-dimet.png")

3-(1-methylethyl)-2,4-dimethylpentane

The main chain in the drawing is numbered 1-5. The main part of the
branched substituent, an ethyl group, is numbered 1\' and 2\'. The
methyl substituent off of the ethyl substituent is not numbered in the
drawing.

To name the compound, put the whole branched substituent name in
parentheses and then number and alphabetize as if a simple substituent.

## Common system

Some prefixes from the common system are accepted in the IUPAC system.

For alphabetization purposes, **iso-** and **neo-** are considered part
of the name, and alphabetized. **Sec-** and **tert-** are not considered
an alphabetizable part of the name.

(In the following images, the **R-** represents any carbon structure.)

### Iso-

**Iso-** can be used for substituents that branch at the second-to-last
carbon and end with two methyls. An isobutyl has four carbons total:

![](Isobutyl.png "Isobutyl.png") *Isobutyl*

### Sec-

![](Sec-.png "Sec-.png")

**Sec-** can be used for substituents that branch at the first carbon .

### Neo-

**Neo-** refers to a substituent whose second-to-last carbon of the
chain is trisubstituted (has three methyl groups attached to it). A
neo-pentyl has five carbons total.

![](Neopentyl.png "Neopentyl.png") *Neopentyl*

### Tert-

![](Tert-.png "Tert-.png")

**Tert-** is short for **tertiary** and refers to a substituent whose
first carbon has three other carbon groups attached to it.

# See also

1.  Cycloalkanes
    

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