Linear alkanes structure, properties, nomenclature, examples

The linear alkanes are saturated hydrocarbons whose general chemical formula is n-C_nH_{2n + 2}. As they are saturated hydrocarbons, all their bonds are simple (C-H) and are composed only of carbon and hydrogen atoms. They are also called paraffins, differentiating them from branched alkanes, which are called isoparaffins..

These hydrocarbons lack branches and rings. More than lines, this family of organic compounds are more like chains (Straight chain alkane); or from a culinary angle, to spaghetti (raw and cooked).

The raw spaghetti comes to represent the ideal and isolated state of linear alkanes, although with a pronounced tendency to break; while those cooked, regardless of whether they are al dente or not, approach their natural and synergistic state: some interacting with others in a whole (the pasta dish, for example).

These types of hydrocarbons are found naturally in natural gas and in oil fields. The lightest ones have lubricating characteristics, while the heavy ones behave like an undesirable asphalt; soluble, however, in paraffins. They serve very well as solvents, lubricants, fuels, and asphalt.

Article index

• 1 Structure of linear alkanes
  • 1.1 Groups
  • 1.2 Lengths of your chains
  • 1.3 Conformations
• 2 Properties
  • 2.1 Physical
  • 2.2 Interactions
  • 2.3 Stability
• 3 Nomenclature
• 4 Examples
• 5 References

Structure of linear alkanes

Groups

It was mentioned that the general formula of these alkanes is C_nH_{2n + 2}. This formula is the same in fact for all alkanes, whether linear or branched. The difference then in the n- which precedes the formula for alkane, whose denotation means "normal".

It will be seen later that this n- it is unnecessary for alkanes with a carbon number equal to or less than four (n ≤ 4).

A line or chain cannot consist of a single carbon atom, so methane (CH₄, n = 1). If n = 2, we have ethane, CH₃-CH₃. Note that this alkane consists of two methyl groups, CH₃, linked together.

Increasing the number of carbons, n = 3, gives the alkane propane, CH₃-CH_two-CH₃. Now a new group appears, CH_two, called methylene. No matter how large the linear alkane is, it will always have only two groups: CH₃ and CH_two.

Lengths of their chains

When n or the number of carbons in the linear alkane increases, there is a constant in all the resulting structures: the number of methylene groups increases. For example, suppose linear alkanes with n = 4, 5 and 6:

CH₃-CH_two-CH_two-CH₃ (n-butane)

CH₃-CH_two-CH_two-CH_two-CH₃ (n-pentane)

CH₃-CH_two-CH_two-CH_two-CH_two-CH₃ (n-hexane)

The chains become longer because they add CH groups to their structures_two. Thus, a long or short linear alkane differs by how many CH_two separates the two terminal groups CH₃. All these alkanes have just two of these CH₃: at the beginning of the string and at the end of it. If I had more, it would imply the presence of branches.

Likewise, the total absence of CH groups can be seen, present only in the branches or when there are substituent groups linked to one of the carbons of the chain.

The structural formula can be summarized as follows: CH₃(CH_two)_n-2CH₃. Try to apply and illustrate it.

Conformations

Some linear alkanes can be longer or shorter than others. This being the case, n can have a value of 2 a ∞; that is, a chain composed of infinite CH groups_two and two CH groups₃ terminals (theoretically possible). However, not all strings are "accommodated" in the same way in space..

It is here that the structural conformations of alkanes arise. What they owe? To the rotability of the C-H bonds and their flexibility. When these links rotate or rotate around an internuclear axis, the chains begin to flex, fold or wind, moving away from their original linear characteristic..

Linear

In the image above, for example, a thirteen-carbon chain is shown at the top that remains linear or extended. This conformation is ideal, since it is assumed that the molecular environment minimally affects the spatial arrangement of the atoms in the chain. Nothing disturbs her and she has no need to bend.

Rolled or folded

In the middle of the image, the 27-carbon chain experiences an external disturbance. The structure, to be more "comfortable", rotates its links in such a way that it is folding on itself; just like a long spaghetti.

Computational studies showed that the maximum number of carbons that a linear chain can have is n = 17. From n = 18, it is impossible that it does not begin to bend or coil.

Mixed

If the chain is very long, there may be regions of the chain that remain linear while others have been bent or wound. Of all, this is perhaps the closest to reality conformation.

Properties

Physical

Being hydrocarbons, they are essentially apolar, and therefore hydrophobic. This means that they cannot mix with water. They are not very dense because their chains leave too many empty spaces between them.

Likewise, their physical states range from gaseous (for n < 5), líquidos (para n < 13) o sólidos (para n ≥ 14), y dependen de la longitud de la cadena.

Interactions

Linear alkane molecules are apolar, and therefore their intermolecular forces are of the London scattering type. The chains (probably adopting a mixed conformation), are then attracted by the action of their molecular masses and the instantaneous induced dipoles of their hydrogen and carbon atoms..

It is for this reason that when the chain becomes longer, and therefore heavier, its boiling and melting points increase in the same way..

Stability

The longer the chain, the more unstable it is. They generally break their links to form smaller chains from a large one. In fact, this process is known as cracking, highly used in oil refining.

Nomenclature

To name linear alkanes, just add the n- indicator before the name. If n = 3, as with propane, it is impossible for this alkane to present any branching:

CH₃-CH_two-CH₃

Which does not happen after n = 4, that is, with n-butane and the other alkanes:

CH₃-CH_two-CH_two-CH₃

OR

(CH₃)_twoCH-CH₃

The second structure corresponds to isobutane, which consists of a structural isomer of butane. To differentiate one from the other, the n- indicator comes into play. Thus, the n-butane refers to the linear isomer only, without branches.

The higher the n, the greater the number of structural isomers and the more important it will be to use n- to refer to the linear isomer.

For example, octane, C₈H₁₈ (C₈H_{8 × 2 + 2}), has up to thirteen structural isomers, since many branches are possible. The linear isomer, however, is named: n-octane, and its structure is:

CH₃-CH_two-CH_two-CH_two-CH_two-CH_two-CH_two-CH₃

Examples

They are mentioned below and to finish some linear alkanes:

-Ethane (C_twoH₆): CH₃CH₃

-Propane (C₃H₈): CH₃CH_twoCH₃

-n-Heptane (C₇H₁₆): CH₃(CH_two)₅CH₃.

- n-Dean (C₁₀H₂₂): CH₃(CH_two)₈CH₃.

-n-Hexadecane (C₁₆H_{3. 4}): CH₃(CH_two)₁₄CH₃.

-n-Nonadecane (C₁₉H₄₀): CH₃(CH_two)₁₇CH₃.

-n-Eicosane (C_twentyH₄₂): CH₃(CH_two)₁₈CH₃.

- n-Hectane (C₁₀₀H₂₀₂): CH₃(CH_two)₉₈CH₃.

References

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5. Garcia Nissa. (2019). What Are Alkanes? Study. Recovered from: study.com
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