- Structure of linear alkanes
- Groups
- Lengths of their chains
- Conformations
- Linear
- Rolled up or folded
- Mixed
- Properties
- Physical
- Interactions
- Stability
- Nomenclature
- Examples
- References
The linear alkanes are saturated hydrocarbons whose general chemical formula is n -C n H 2n + 2. As they are saturated hydrocarbons, all their bonds are simple (CH) and are made up 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 similar to chains (Straight chain alkane); or from a culinary angle, to spaghetti (raw and cooked).
If raw spaghetti were less brittle, they would have an even closer resemblance to linear alkanes. Source: Pixabay.
The raw spaghetti comes to represent the ideal and isolated state of linear alkanes, although with a pronounced tendency to break; while cooked ones, 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 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.
Structure of linear alkanes
Groups
It was mentioned that the general formula for these alkanes is C n H 2n + 2. This formula is the same in fact for all alkanes, whether linear or branched. The difference then in the n- that precedes the alkane formula, whose denotation means "normal".
It will be seen later that this n- 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 (CH 4, n = 1) is ruled out for explanation. If n = 2, we have ethane, CH 3 -CH 3. Note that this alkane consists of two methyl groups, CH 3, linked together.
Increasing the number of carbons, n = 3, gives the alkane propane, CH 3 -CH 2 -CH 3. Now a new group appears, CH 2, called methylene. No matter how large the linear alkane is, it will always have only two groups: CH 3 and CH 2.
Lengths of their chains
When 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:
CH 3 -CH 2 -CH 2 -CH 3 (n- butane)
CH 3 -CH 2 -CH 2 -CH 2 -CH 3 (n- pentane)
CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3 (n- hexane)
The chains become longer because they add CH 2 groups to their structures. Thus, a long or short linear alkane differs in how many CH 2 separates the two terminal CH 3 groups. All these alkanes have only two these CH 3: at the beginning of the chain 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: CH 3 (CH 2) n-2 CH 3. Try to apply and illustrate it.
Conformations
Structural conformations of linear alkanes. Source: Gabriel Bolívar.
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 2 groups and two terminal CH 3 groups (in theory it is possible). However, not all strings are "arranged" in the same way in space.
It is here that the structural conformations of alkanes arise. What they owe? To the rotability of the CH bonds and their flexibility. When these links rotate or rotate around an internuclear axis, the chains begin to flex, fold, or coil away from their original linear characteristic.
Linear
In the upper image, 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 over.
Rolled up or folded
In the middle of the image, the twenty-seven 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; such as a long spaghetti.
Computational studies have shown 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 twist.
Mixed
If the chain is very long, there may be regions of it that remain linear while others have been bent or wound. Of all, this is perhaps the closest to reality conformation.
Properties
Physical
As they are 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), liquid (for n <13) or solid (for n ≥ 14), and depend on the length of the chain.
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 make smaller chains from a large one. In fact, this process is known as cracking, which is highly used in oil refining.
Nomenclature
To name linear alkanes it is enough to add the indicator n- before the name. If n = 3, as with propane, it is impossible for this alkane to present any branching:
CH 3 -CH 2 -CH 3
Which does not happen after n = 4, that is, with n-butane and the other alkanes:
CH 3 -CH 2 -CH 2 -CH 3
OR
(CH 3) 2 CH-CH 3
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, n-butane refers only to the linear isomer, without branches.
The higher n, the greater the number of structural isomers and the more important it is to use n- to refer to the linear isomer.
For example, octane, C 8 H 18 (C 8 H 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:
CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3
Examples
They are mentioned below and to finish some linear alkanes:
-Ethane (C 2 H 6): CH 3 CH 3
-Propane (C 3 H 8): CH 3 CH 2 CH 3
- n- Heptane (C 7 H 16): CH 3 (CH 2) 5 CH 3.
- n -Decane (C 10 H 22): CH 3 (CH 2) 8 CH 3.
- n- Hexadecane (C 16 H 34): CH 3 (CH 2) 14 CH 3.
- n -Nonadecane (C 19 H 40): CH 3 (CH 2) 17 CH 3.
- n -Eicosane (C 20 H 42): CH 3 (CH 2) 18 CH 3.
- n -Hectane (C 100 H 202): CH 3 (CH 2) 98 CH 3.
References
- Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
- Morrison, RT and Boyd, R, N. (1987). Organic Chemistry. (5th Edition). Editorial Addison-Wesley Interamericana.
- Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. Amines. (10th edition.). Wiley Plus.
- Jonathan M. Goodman. (1997). What Is the Longest Unbranched Alkane with a Linear Global Minimum Conformation? J. Chem. Inf. Comput. Sci. 1997, 37, 5, 876-878.
- Garcia Nissa. (2019). What Are Alkanes? Study. Recovered from: study.com
- Mr. Kevin A. Boudreaux. (2019). Alkanes. Recovered from: angelo.edu
- List of straight-chain Alkenes.. Recovered from: laney.edu
- Helmenstine, Anne Marie, Ph.D. (September 7, 2018). Name the First 10 Alkanes. Recovered from: thoughtco.com
- Quirky Science. (March 20 2013). Straight chain alkanes: predicting properties. Recovered from: quirkyscience.com
- Wikipedia. (2019). Higher alkanes. Recovered from: en.wikipedia.org