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What are alkanes?

The alkanes are organic compounds simplest world. They are saturated aliphatic hydrocarbons, made up exclusively of carbon and hydrogen, in which the carbons are linked only by means of simple covalent bonds.

They can be open chain, in which case they have the general formula C n H 2n + 2 . They can also form one or more cycles, in which case the general formula loses two hydrogens for each cycle that is formed (C n H 2n if they have only one cycle, C n H 2n-2 if they have 2, etc.).

Many common organic compounds that we use every day are alkanes. For example, cooking gas and natural gas are made up of one or more gaseous alkanes. Automotive fuel (gasoline, benzine or fuel, depending on the country) is made up of complex mixtures of liquid alkanes, the most important of which are octane isomers.

Properties of alkanes

They are very unreactive

The main characteristic or chemical property of alkanes is that they are quite stable molecules that do not participate in chemical reactions, unless it is under very energetic conditions such as very high temperatures or in the presence of ultraviolet light or free radicals.

They are apolar compounds

Single bonds CC and CH are nonpolar covalent bonds, so alkane molecules cannot have a net dipole moment. Because of this, they are apolar molecules.

They have low boiling and melting points

By the very fact of being apolar, the only intermolecular attractive forces that alkanes present are dispersion forces such as Van der Waals forces. As these forces are very weak, they are easy to break to convert solids to liquids and these to gases at low temperature.

They are less dense than water

Alkanes are the least dense of the organic compounds, and they are always less dense than water. For this reason, they always float on the surface of the water when the two are mixed.

They are generally colorless

Gaseous and liquid alkanes are colorless and translucent. However, when they solidify, they generally form white opaque amorphous solids, as in the case of some paraffins and some plastics.

They are not soluble in water

The rule of thumb for solubility is that like dissolve like. Water is a polar solvent while all alkanes are completely nonpolar, so they are not soluble in water.

They are soluble in nonpolar organic solvents

Using the same argument as above, alkanes are indeed soluble in nonpolar solvents such as benzene or cyclohexane. In fact, liquid alkanes are part of nonpolar organic solvents.

They cannot undergo addition reactions

Being saturated hydrocarbons, alkanes cannot undergo addition reactions. The only exception is cyclopropane, which has a three-membered ring that is highly stressed and can break easily.

They are combustible

One of the few chemical reactions in which alkanes participate is in combustion, and that is precisely the main use that is given to them.

Types of alkanes

Depending on the connectivity between carbon atoms, alkanes can be:

  • Linear
  • Branched
  • Cyclic (cycloalkanes)
  • Bicyclic or polycyclic alkanes
  • Spirans

Linear alkanes

They are the simplest of the alkanes and therefore of all organic compounds. They all have the molecular formula C n H 2n + 2 and are characterized by having a single chain of carbon atoms linked one after another.

Branched alkanes

They also have the general formula C n H 2n + 2 but unlike linear alkanes, the chain of carbon atoms branches at least at one point. Branched alkanes are chain isomers of linear alkanes, since they only differ in the order in which the carbon atoms are linked.

Cycloalkanes

In cycloalkanes, the chain ends of linear alkanes join together to form a cyclic chain. To form this additional CC bond it is necessary to remove one hydrogen from each terminal carbon, so the general formula for these compounds is C n H 2n . The smallest possible cycloalkane is the one with three carbon atoms called cyclopropane (C 3 H 6 ).

Like open chain alkanes that can have branches, cycloalkanes can also have substituent groups in the form of open chains.

Bicyclic and polycyclic alkanes

There are many cyclic alkanes in which two or more rings share two or more carbon atoms. These compounds are called polycycles. The molecular formula of polycycles depends on how many cycles the structure has.

In the simplest case, bicycles, the formula is C n H 2n-2 since a pair of hydrogens must be lost to close each cycle. For polycycles with more than two fused cycles, the formula will be equal to that of the alkanes minus one pair of hydrogens for each cycle formed.

Spirans

Spirans are a special class of bicycles in which the two rings share only one carbon atom. In these cases, the two cycles are in planes perpendicular to each other, so the structure looks as if one of the cycles is rotated with respect to the other.

Nomenclature of alkanes

Linear Alkane Nomenclature

The nomenclature of alkanes, and indeed that of all organic compounds, is based on the nomenclature of linear alkanes. These are named simply according to the number of carbon atoms in the chain.

The first four are given common names that are methane, ethane, propane, and butane, while the rest are systematically named by prefixing the number of carbons (penta, hexa, hepta, etc.) to the alkane _ano ending. .

Number of carbons

Molecular formula

Semi-developed formula

Name 

1

CH4

CH4

Methane

2

C2H6

CH3CH3

Ethane

3

C3H8

CH3CH2CH3

Propane

4

C4H10 

CH3 (CH2) 2CH3

Butane

5

C5H12

CH3 (CH2) 3CH3

Pentane

6

C6H14

CH3 (CH2) 4CH3

Hexane

7

C7H16

CH3 (CH2) 5CH3

Heptane

8

C8H18

CH3 (CH2) 6CH3

Octane

9

C9H20

CH3 (CH2) 7CH3

Nonano 

10

C10H22

CH3 (CH2) 8CH3

Dean

Branched Alkanes Nomenclature

Branched alkanes are named based on the name of linear alkanes. The process involves selecting one of the possible carbon chains as the main chain and the rest of the branches remain as substituent groups.

The process consists of the following steps:

Step 1: Identify the main chain.

The selection is made according to the following criteria in order of priority:

  1. The longest string is selected.
  2. If there is more than one, the one with the most branches is selected from among them.
  3. If there is more than one that has the same length and the same number of branches, the one that, when numbered, gives the smallest possible combination of locators is selected.
  4. If two or more chains have the same length, the same number of branches and all have the same locators, the one that assigns the smallest locators to the branches that appear first in alphabetical order is selected.
  5. In case all of the above is equal, then anyone can be chosen.

The main chain gives the main name to the alkane, as if it were a linear alkane.

Step 2: Number the main chain.

The carbon atoms of the main chain should be listed in sequence from one end to the other, following these rules:

  1. The numbering that assigns the smallest combination of locators to the branches is selected.
  2. If the two numberings give the same combination of locators, the one that assigns the smallest locators to the branches that appear first in alphabetical order is selected.
  3. In case both are the same, then it does not matter which one is chosen.

Step 3: Name the branches and order them alphabetically.

Branches are named as alkyl radicals, substituting the _ano end of the corresponding linear alkane for the _yl end.

Number of carbons

Alkyl radical 

Name

1

–CH3

Methyl radical

2

–CH2CH3 

     “ethyl 

3

–CH2CH2CH3

     “propyl

4

–CH2 (CH2) 2CH3

     “butyl

5

–CH2 (CH2) 3CH3

     “pentyl

6

–CH2 (CH2) 4CH3

     “hexyl

7

–CH2 (CH2) 5CH3 

     “heptyl

8

–CH2 (CH2) 6CH3

     “octyl

9

–CH2 (CH2) 7CH3

     “nonyl

10

–CH2 (CH2) 8CH3

     “say it

Step 4: The compound name is constructed.

The name is constructed by naming all the branches in alphabetical order (removing the final letter “o” from the alkyl), preceded by its locator, followed by the name of the main chain.

If any branch is repeated, the locators of each one are placed, separated by commas, and the name of the branch is prepended with a prefix that indicates how many times it is repeated (di for 2, tri for 3, and so on).

Example:

Name the following branched alkane:

STEP 1: Selecting the main chain.

This chain has 16 carbons, so the main chain will be called hexadecane.

STEP 2: Number main chain.

 

It is numbered from left to right because they give the smallest locators.

STEP 3: Name all branches.

There are three ethyl radicals, three methyl radicals, and two propyl radicals.

STEP 4: Build the name

The name of the compound will be:

8,9,10-triethyl-3,4,5-trimethyl-6,7-dipropylhexadecane

Examples of alkanes

Some additional examples of alkanes are presented in the following table.

Name

Molecular formula or structure

Radical 

Number of carbons

Methane 

CH4

Methyl 

1

Ethane

C2H6

Ethyl 

2

Propane

C3H8

n-propyl 

3

n-Butane

C4H10

n-butyl 

4

Isobutane

C4H10

Isobutyl

4

n-Pentane

C5H12

n-pentyl

5

Isopentane

C5H12

Isopentyl

5

Neopentane

C5H12

Neopentyl

5

n-Hexane

C6H14

n-hexyl

6

Polyethylene

CH3 (CH2) nCH3

—-

> 100

Cyclopentane

C5H10

Cyclopentyl

5

Cyclohexane

C6H12 

cyclohexyl

5

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