The branched alkanes are saturated hydrocarbons whose structures do not consist of a linear chain. At alkanes of linear chains are branched isomers thereof difference by adding one letter n predating the name. Thus, n-hexane means that the structure consists of six carbon atoms aligned in a chain.
The branches of a bare tree canopy (lower image) could be compared to those of branched alkanes; however, the thickness of its chains, whether they are major, minor or tertiary, have all the same dimensions. Why? Because in all the simple bonds C – C are present.
Trees tend to branch out as they grow; so do alkanes. Maintaining a constant chain with certain methylene units (–CH 2 -) implies a series of energetic conditions. The more energy the alkanes have, the greater the tendency to branch out.
Both the linear and branched isomers share the same chemical properties, but with slight differences in their boiling points , melting points, and other physical properties. An example of a branched alkane is 2-methylpropane, the simplest of all.
Branched and linear alkanes have the same general chemical formula: C n H 2n + 2 . That is to say, both, for a given number of carbon atoms, have the same number of hydrogens. Therefore, the two types of compounds are isomers : they have the same formula but different chemical structures.
What is observed first in a linear chain? A finite number of methylene groups, –CH 2 – . Thus, CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 is a straight chain alkane called n-heptane.
Note the five consecutive methylene groups. Also, it should be noted that these groups make up all the chains, and therefore are of the same thickness but with variable lengths. What else can be said about them? Which are 2nd carbons, that is, carbons linked to two others.
For said n-heptane to branch, it is necessary to rearrange its carbons and hydrogens. How? The mechanisms can be very complex and involve the migration of atoms and the formation of positive species known as carbocations (–C + ).
However, on paper it is enough to arrange the structure in such a way that there are 3rd and 4th carbons; in other words, carbons bonded to three or four others. This new ordering is more stable than the long clusters of CH 2 groups . Why? Because the 3rd and 4th carbons are more energetically stable.
Chemical and physical properties
The branched and linear alkanes, having the same atoms, retain the same chemical properties. Their bonds remain simple, C – H and C – C, and with little difference in electronegativities , so their molecules are nonpolar. The difference, mentioned above, lies in the 3rd and 4th carbons (CHR 3 and CR 4 ).
However, as the chain branches into the isomers, the way the molecules interact with each other changes.
For example, the way to join two linear branches of a tree is not the same as putting two highly branched one on top of the other. In the first situation there is a lot of surface contact, while in the second the “gaps” between the branches predominate. Some branches interact more with each other than with the main branch.
All of this results in similar values, but not the same in many of the physical properties.
Boiling and melting points
The liquid and solid phases of alkanes are subject to intermolecular forces under specific conditions of pressure and temperature. Since the molecules of the branched and linear alkanes do not interact in the same way, neither will their liquids or solids be the same.
Melting and boiling points increase with the number of carbons. For linear alkanes, these are proportional to n . But for branched alkanes, the situation depends on how branched the main chain is, and what the substituent or alkyl groups are (R).
If the linear chains are considered as rows of zigzags, then they will fit perfectly on top of each other; but with the branched ones, the main chains hardly interact because the substituents keep them apart from each other.
As a result, branched alkanes have a smaller molecular interface, and therefore their melting and boiling points tend to be slightly lower. The more branched the structure, the smaller these values will still be.
For example, n-pentane (CH 3 CH 2 CH 2 CH 2 CH 3 ) has a Peb of 36.1 ° C, while 2-methyl-butane (CH 3 CH 2 (CH 3 ) CH 2 CH 3 ) and 2,2-dimethyl-propane (C (CH 3 ) 4 ) of 27.8 and 9.5 ° C.
Using the same reasoning, branched alkanes are slightly less dense, due to the fact that they occupy a greater volume, due to the decrease in surface contact between the main chains. Like linear alkanes, they are immiscible with water and float above it; that is, they are less dense.
Nomenclature and examples
Five examples of branched alkanes are shown in the image above. Note that the branches are characterized by having 3rd or 4th carbons. But what is the main chain? The one with the highest number of carbon atoms.
-In A it is indifferent, since no matter which chain is chosen, both have 3 C. So, its name is 2-methyl-propane. It is an isomer of butane, C 4 H 10 .
-Alkane B has at first glance two substituents and a long chain. The -CH 3 groups are numbered such that they have the least number; therefore, the carbons start counting from the left side. Thus, B is called 2,3-dimethyl-hexane.
-For C the same applies as in B. The main chain has 8 C, and the two substituents, a CH 3 and a CH 2 CH 3 are located further to the left side. Its name is therefore: 4-ethyl-3-methyloctane. Note that the -ethyl substituent is mentioned before the -methyl by its alphabetical order.
-In the case of D, it does not matter where the carbons of the main chain are counted. Its name is: 3-ethyl-propane.
-And finally for E, a slightly more complex branched alkane, the main chain has 10 C and it starts to count from any of the CH 3 groups on the left. In doing so its name is: 5-ethyl-2,2-dimethyl-decane.
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