Alkanes and Cycloalkanes

HYDROCARBONS
Hydrocarbons are the simplest organic compounds. Containing only carbon and hydrogen, they can be straightchain, branched chain, or cyclic molecules. They also can be saturated (alkanes) or unsaturated with double bond
(alkenes) or triple bond (alkynes).
ALKANES.
Alkanes (also known as paraffins or saturated hydrocarbons) are chemical compounds that consist only of
hydrogen and carbon atoms and are bonded exclusively by single bonds (i.e., they are saturated compounds)
OPEN CHAIN ALKANES
Open-chain alkanes (without rings) have the general formula CnH2n + 2, where n equals the number of carbon
atoms. The following table shows the structures and names for the first 20 unbranched, open-chain alkanes.
IUPAC
name
Molecular
Formula
Condensed Structural
Formula
Methane
CH4
CH4
Ethane
C2H6
CH3 CH3
Propane
C3H8
CH3 CH2 CH3
Butane
C4H10
CH3 (CH2)2 CH3
Pentane
C5H12
CH3 (CH2)3 CH3
Hexane
C6H14
CH3 (CH2)4 CH3
Heptane
C7H16
CH3 (CH2)5 CH3
Octane
C8H18
CH3 (CH2)6 CH3
Nonane
C9H20
CH3 (CH2)7 CH3
Decane
C10H22
CH3 (CH2)8 CH3
Undecane
C11H24
CH3 (CH2)9 CH3
Dodecane
C12H26
CH3 (CH2)10 CH3
Tridecane
C13H28
CH3 (CH2)11 CH3
Tetradecane
C14H30
CH3 (CH2)12 CH3
Pentadecane
C15H32
CH3 (CH2)13 CH3
Hexadecane
C16H34
CH3 (CH2)14 CH3
Heptadecane
C17H36
CH3 (CH2)15 CH3
Octadecane
C18H38
CH3 (CH2)16 CH3
Nonadecane
C19H40
CH3 (CH2)17 CH3
Eicosane
C20H42
CH3 (CH2)18 CH3
Physical and chemical properties.

a) Physical state. The first four alkanes are gases at room temperature. Alkanes from 5 to 17 carbon atoms
are liquids. Solids don't start to appear until about C 18H38.
b) Solubility. Since alkanes have only non- polar covalent bonds, they are virtually insoluble in water, but
dissolve in organic solvents. The liquid alkanes are good solvents for many other covalent compounds.
c) Flammability. Alkanes are used as fuels.
d) Chemical reactivity Alkanes have a very low reactivity. They have a fairly restricted set of reactions:
combustion, halogenation and cracking , (breaking of carbon-carbon bonds).
Examples:
Methane. The simplest hydrocarbon, CH4. It is a colorless and odorless, non- water soluble gas. In nature, it is
the final product of anaerobic putrefaction of plants, known as biogas and it is found in marshes. It is the main
component of natural gas (97%). It is also found in carbon mines. Methane is very dangerous because it is highly
flammable and explosive. Methane is also a greenhouse gas.
Ethane. It is obtained from natural gas and also as a sub product of petroleum distillation. It is used to produce
polymers
Propane and butane. They are the main components of domestic gas used as a fuel in boilers, stoves and room
heaters.
NAMING OPEN CHAIN ALKANES.
UNBRANCHED OPEN CHAIN ALKANES. The first four alkanes have particular names: methane, ethane,
propane and butane. For alkanes with 5 or more carbon atoms, the name is formed by the Greek prefix
corresponding to the number of carbon atoms and the ending ane.
BRANCHED OPEN CHAIN ALKANES
Alkyl Groups
An alkyl is basically an alkane minus one of its hydrogen atoms. For example:
H
H
|
remove one H
|
H-C-H
H-C- or
|
|
H
H
methane
methyl
H H
| |
H-C-C-H
| |
H H
ethane
remove one H
H H
| |
H-C-C| |
H H
ethyl
CH3-
or
CH3CH2-
IUPAC System of Naming Alkanes

The IUPAC (International Union of Pure and Applied Chemistry) is composed of chemists representing the
national chemical societies of several countries. One committee of the IUPAC, the Commission on Nomenclature
of Organic Chemistry, has set a system for naming organic compounds. The last syllable in the name of a
compound designates the family to which it belongs. The alkanes all end in -ane.
IUPAC Rules for Naming the Branched Alkanes

1. The name ending for all alkanes (and cycloalkanes) is -ane.
2. The parent chain is the longest continuous chain of carbons in the structure.
For example, in the branched-chain alkane:

CH3
|
CH3 CH2 CH CH2 CH2 CH3
The parent chain is in bold. It has 6 carbon atoms, therefore, this compound is a derivative of hexane.
3. The carbon atoms of the parent chain are numbered starting from whichever end of the chain gives the
location of the first branch the lower of two possible numbers.
For the example above, the correct direction is from left to right.
CH3
|
CH3CH2CHCH2CH2CH3
1 2 3 4 5 6
The branch is therefore located on the 3rd carbon.

If it was numbered from right to left, the branch would be located on the 4th carbon, which is a higher
number, which is not allowed by IUPAC.
4. a. Name each branch attached to the parent chain according to alkyl groups. In this case, the branch would
be methyl.
b. Attach the name of the alkyl group to the name of the parent as a prefix. Place the location number of the
alkyl group in front of the resulting name. In this case, it would by 3-methylhexane.
c. When two or more groups are attached to the parent chain, name each and locate each with a number. The
numbers must be assigned to give to every attached group the lowest possible number. (The sum of these
numbers must be the lowest possible)
CH3
CH3 CH2CH3 CH3
|
|
|
|
CH3CHCH2CHCH2CHCH2CH2CHCH3
1 2 3 4 5 6 7 8 9 10
6-ethyl-2,4,9,trimethyl decane
Assigning numbers from left to right, the branches are located in carbons: 2,4,6 & 9
If it was numbered from right to left, the branches would be located on the carbons: 2, 5,7 & 9 which is a higher
set of numbers, so the right way to number is from left to right
d. The alkyl substituent names are assembled in alphabetical order. Always use hyphens when separating
numbers from words.
e. Prefixes cyclo, iso and neo are considered as part of the word for alphabetical ordering while prefixes
di, tri, etc or sec, tert, are not.
CH3CH2 CH3
|
|
CH3CH2CH2CHCH2CHCH3
7 6 5 4 3 2 1
This example is 4-ethyl-2-methylheptane
5. When two or more substituents are identical, use prefixes such as di- (2), tri- (3), tetra- (4), and specifiy the
location number of every group. Always separate a number from another number in a name by a comma.
The following is 2,4-dimethylhexane:
CH3 CH3
|
|
CH3CHCH2CHCH2CH3
6. When identical groups are on the same carbon, repeat the number of the carbon in the name. The following
is 2,2-dimethylpentane:
CH3
|
CH3CCH2CH2CH3
|
CH3
7. Sometimes, you may need to go around corners and zigzag to find the longest (parent) chain. The following
is 3,4-dimethylheptane (the parent chain is in bold):
CH3
|
CH3-CH2CH-CH2-CH3
|
CH2-CH-CH3
8. Halogen substituents are treated exactly as alkyl groups
CYCLOALKANES
Cycloalkanes (also called naphthenes) are types of alkanes that have one or more rings of carbon atoms in the
chemical structure of their molecules.
A general chemical formula for cycloalkanes would be CnH2n .
NAMING CYCLOALKANES
Cycloalkanes with a single ring are named analogously to their normal alkane counterpart of the same carbon
count: cyclopropane, cyclobutane, cyclopentane, cyclohexane, etc. The larger cycloalkanes, with greater than 20
carbon atoms are typically called cycloparaffins.
UNBRANCHED CYCLOALKANES
1.
The parent chain is the continuous chain of carbons in the cyclic structure.
Examples
Cyclobutane
cyclopentane
BRANCHED CYCLOALKANES
1. Parent Chain
a. Use the cycloalkane as the parent chain if it has a greater number of carbons than any alkyl substituent.
b. If an alkyl chain off the cycloalkane has a greater number of carbons, then use the alkyl chain as the
parent and the cycloalkane as a cycloalkyl-substituent.
2. Numbering the Cycloalkane

a. If the cycloalkane has only one substituent, assign number 1 to this carbon atom.
b. When numbering the carbons of a cycloalkane, start with a substituted carbon, according to alphabetical
order, so that the substituted carbons have the lowest numbers (the sum of these numbers must be the
lowest possible).
c. Halogen substituents are treated exactly like alkyl groups.
Examples
CH3
CH3
CH2 CH3
methyl cyclobutane
1-ethyl-2-methyl cyclopentane
References
http://www.chemguide.co.uk/organicprops/alkanes/background.html#top
http://hyperphysics.phy-astr.gsu.edu/hbase/organic/hydrocarbon.html
http://library.thinkquest.org/3659/orgchem/alkanes.html
http://www.sciencegeek.net/APchemistry/APtaters/cycloalkanes.htm

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HYDROCARBONS

CH3 CH2 CH3

CH3 (CH2)2 CH3

CH3 (CH2)3 CH3

CH3 (CH2)4 CH3

CH3 (CH2)5 CH3

CH3 (CH2)6 CH3

CH3 (CH2)7 CH3

CH3 (CH2)8 CH3

CH3 (CH2)9 CH3

CH3 (CH2)10 CH3

CH3 (CH2)11 CH3

CH3 (CH2)12 CH3

CH3 (CH2)13 CH3

CH3 (CH2)14 CH3

CH3 (CH2)15 CH3

CH3 (CH2)16 CH3

CH3 (CH2)17 CH3

CH3 (CH2)18 CH3

Physical and chemical properties.

IUPAC System of Naming Alkanes

IUPAC Rules for Naming the Branched Alkanes

For example, in the branched-chain alkane:

The branch is therefore located on the 3rd carbon.

This example is 4-ethyl-2-methylheptane

2. Numbering the Cycloalkane

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