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Alkanes
Alkanes
Background:
With one carbon atom and four
hydrogen atoms. methane is
the simplest alkane.
Hydrocarbons having no double or triple bonds and no
functional groups
, are classified as alkanes or cycloalkanes, depending on whether the carbon atoms of the molecule are arranged only in chains or also in rings.
Alkanes provide a framework on which functional groups can be attached in order to create other classes of organic compounds. The alkanes and cycloalkanes members of a larger class of compounds referred to as
aliphatic compounds
which by their definition contain no aromatic rings.
Alkanes and cycloalkanes can also be referred to as
saturated hydrocarbons
, meaning that they have no capacity to carry additional hydrogen atoms without changing their carbon chain structure.
Physical Properties:
Melting / Boiling Points
Melting points and boiling points of alkanes generally increase with molecular weight and with the length of the main carbon chain. The melting/boiling points of alkanes are the lowest of any organic compounds of comparable molecular size. Branched alkanes have even lower melting/boiling points than equivalently sized normal alkanes.
Under standard conditions alkanes with a carbon chain length of four or less are gasses. Alkanes with carbon chains of lenght 5 to 16 are liquids and those with carbon chains of length 17 or more are solids.
Solubility
Alkanes are insoluble in water because the polar water molecules are not attracted to the non-polar alkane molecules. However alkanes are soluble in non-polar solvents such as Carbon Tetrachloride,CCl
4
.
Density
Alkanes are less dense than water.
Chemical Properties:
The general formula for alkanes is: C
n
H
2n+2
In the oil industry alkanes are referred to as "paraffins" which implies "lack of affinity." This reflects the fact that alkanes are largely inert and immune to reactions with most acids and bases and with both polar and non-polar agents.
Alkanes however, are suseptable to reaction with oxygen (combustion) and with halogens (substitution). They can also be modified through processes referred to as cracking, isomerization and reformation.
Uses and Applications:
Short chain alkanes are clean burning and make useful, for instance methane, ethane, propane, and butane are all common names used for fuel gases used in barbeques, heating systems, combustion engines and lighters.
Kerosene and gasoline are a well known alkane based liquid fuels.
Paraffin wax is mixture of solid phase alkanes where the primary chain consists of 17 or more carbons.
Alkanes are also make excellent feedstock for use in the custom manufacture of more complex organic compounds including pharmeceuticals, fertilizers, polymers and gasoline.
Important Compounds and Derivatives:
Alkanes are used as the feed stock for the production of organic compunds of all kinds.
The most common types of alkane reactions are combustion reactions and substitution reactions.
Combustion Reactions
Alkanes all combine completely with oxygen to form C0
2
+ H
2
0, while releasing significant quantities of heat and light.
For instance the combustion of propane in a gas barbeque can be described using the following balanced equation.
C
3
H
8
+ 5O
2
--> 3CO
2
+ 4H
2
0 + HEAT
Substitution Reactions
Alkanes react with
halogens
to form
alkyl halides
plus the corresponding
hydrogen halide
.
Reactions involving F
2
usually proceed spontaneously releasing considerable energy whereas reactions involving Cl
2
and Br
2
requite the input of heat or UV light to cause them to proceed. The bromination of ethane is described by the following balanced equation.
C
2
H
6
+ Br
2
--> C
2
H
5
Br + HBr
where the application heat or UV light is used as a catalyst.
The end result of this reaction is the "substitution" of a bromide ion for a single hydrogen ion on the original alkane.
Laboratory Preparation:
Laboratory preparation of alkanes is usually unnecessary since they can be inexpensively purchased as petroleum distillates. Processes such the
Fischer-Tropsch Process
can be used to generate alkanes from elemental carbon or from CO
2
, however it is very difficult to produce alkanes of any molecular purity without distillation.
Industrial Preparation:
Alkanes are commonly derived from the
hydrocracking
and
fractional distillation
of crude oil or petroleum.
Naming Conventions:
The following table lists the IUPAC names and the chemical formula for the first 10
normal alkane
compounds.
Molecular Formula
Structural Formula
IUPAC Name
CH
4
CH
4
Methane
C
2
H
6
CH
3
-CH
3
Ethane
C
3
H
8
CH
3
-CH
2
-CH
3
Propane
C
4
H
10
CH
3
-(CH
2
)
2
-CH
3
Butane
C
5
H
12
CH
3
-(CH
2
)
3
-CH
3
Pentane
C
6
H
14
CH
3
-(CH
2
)
4
-CH
3
Hexane
C
7
H
16
CH
3
-(CH
2
)
5
-CH
3
Heptane
C
8
H
18
CH
3
-(CH
2
)
6
-CH
3
Octane
C
9
H
20
CH
3
-(CH
2
)
7
-CH
3
Nonane
C
10
H
22
CH
3
-(CH
2
)
8
-CH
3
Decane
Branching
Alkane structures can become quite complex, especially when the number of carbon atoms involved becomes large. To begin naming a branched alkane you start by searching out the longest possible continuous carbon chain within the structure. Than gives you the base name taken from the above table.
Lets say that you find the longest chain contains five carbons. Then your base name will become "pentane".
Now look for all the branches that exist relative to the main chain, and at the same time record the following:
the length of each branch;
the number assigned to the main chain carbon to which each branch is attached.
When numbering the carbon atoms in your main chain, start from the end that is closest to a branch. The first carbon is numbered "1", the second "2" and so on.
If you find a single branch consisting of CH
3
attached to the #2 carbon in the main chain your alkane name will become:
2-methylpentane
The prefix "methyl" is derived from "methane" the single carbon alkane, but the "ane" suffix is changes to "yl" to denote that this is a branch chain and not the main chain. The number "2" before the prefix "methyl" tells us that the branch is attached to the #2 carbon on the main pentane chain.
Now suppose we were to add an additional CH
3
group to the #3 carbon atom in the pentane chain. The compound name now becomes:
2,3-dimethylpentane
The "3" is added to indicate that the branch attaches to the third carbon in the pentane chain and the "di" before the methyl indicates that there are two methyl branches involved, tri would indicate 3 and tetra 4, etc ...
Finally lets suppose that the second CH
3
branch is now a C
2
H
5
branch. Our new name will become:
3-ethyl-2-methylpentane
The numbers again indicate the location of the attachment carbons in the main chain and the new branch is named "ethyl" based on the
change of suffix rule
applied to the name ethane. Note that ethyl comes before methyl because prefixes are assigned in alphabetical order. Even if the methyl were to become dimethyl, the order would be based on the alphabetical realtionship between ethyl and methyl, on not on that between ethyl and dimethyl.
History:
Alkanes are found naturally mixed together in petroleum, which was formed by the action of high temperatures and pressures, in the absence of oxygen, on the remains of sea creatures.
Bibliography:
IUPAC Naming examples for Alkanes
Detailed IUPAC Naming Rules for Alkanes and Cycloalkanes
Wikipedia article on alkanes
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