The branched alkanes they are saturated hydrocarbons whose structures do not consist of a straight chain. Straight-chain alkanes are differentiated from their branched isomers by adding a letter n preceding 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, be they main, secondary or tertiary, all have the same dimensions. Why? Because in all the C-C simple bonds are present.
Trees tend to branch out as they grow; so do alkanes. Maintain a constant chain with certain methylene units (-CHtwo-) implies a series of energetic conditions. The more energy the alkanes have, the greater the tendency to branch out.
Both linear and branched isomers share the same chemical properties, but with slight differences in their boiling, melting, and other physical properties. An example of branched alkane is 2-methyl-propane, the simplest of all.
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Branched and linear alkanes have the same general chemical formula: CnH2n + 2. That is, both, for a certain 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, -CHtwo-. Thus the ch3CHtwoCHtwoCHtwoCHtwoCHtwoCH3 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 2º 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 groupings of CH groupstwo. Why? Because the 3rd and 4th carbons are energetically more stable.
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 apolar. The difference, mentioned above, lies in the 3rd and 4th carbons (CHR3 and CR4).
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 others than with the main branch.
All of this results in similar values, but not the same in many of the physical properties.
The liquid and solid phases of alkanes are subject to intermolecular forces under specific conditions of pressure and temperature. As 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 one on top of the 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 contact surface and therefore their melting and boiling points tend to be slightly lower. The more branched the structure, the lower these values will still be.
For example, n-pentane (CH3CHtwoCHtwoCHtwoCH3) has a Peb of 36.1 ºC, while 2-methyl-butane (CH3CHtwo(CH3) CHtwoCH3) and 2,2-dimethylpropane (C (CH3)4) of 27.8 and 9.5ºC.
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.
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 does not matter, since no matter which chain is chosen, they both have 3 C. So their name is 2-methyl-propane. It is an isomer of butane, C4H10.
-Alkane B has at first glance two substituents and a long chain. To -CH groups3 they are numbered in such a way 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 for B. The main chain has 8 C, and the two substituents, a CH3 and a CHtwoCH3 they are more to the left side. Its name therefore is: 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 groups3 on the left. Doing it this way its name is: 5-ethyl-2,2-dimethyl-decane.
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