What is branched and bound

Hydrocarbons

In organic chemistry, molecules mainly consist of the elements carbon and hydrogen. Compounds that contain only these two elements are called hydrocarbons. We explain the most important basics around the topic of hydrocarbons in this article.

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Important foundation
Hydrocarbons can be divided into saturated and unsaturated hydrocarbons.

  • Saturated means that all valences (free positions for docking) of the hydrocarbon are occupied. This means that the maximum possible number of hydrogen atoms in a molecule has been reached and no more atoms can be added.
  • Unsaturated hydrocarbons can form multiple bonds (alkenes and alkynes). Their valences are not completely occupied. This means that there is the possibility that further atoms can combine with the hydrocarbon

Alkanes

As already described above, alkanes are saturated hydrocarbons with the general empirical formula $ C_n H_ {2n + 2} $. The name of the alkanes depends on the number of carbon atoms.

The following table provides an overview of the most important alkanes. It is essential to memorize these names as they form the basis for all molecule names.

The alkanes in the table are unbranched. This means that a maximum of two carbon atoms are bound to another carbon atom. The possibility that alkanes are branched still exists.

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Alkenes

Alkenes are unsaturated hydrocarbons that contain double bonds. As a result of these double bonds, fewer hydrogen atoms are bound in the molecule than in alkanes. The general formula is: $ C_n H_2n $.

The name of the alkenes and thus their word endings “-en” are derived from the number of carbon atoms and thus from the corresponding alkane. The alkene with two carbon atoms is therefore ethene.

Alkynes

Alkynes are also unsaturated hydrocarbons which, in contrast to alkenes, do not contain double bonds, but triple bonds and accordingly even fewer hydrogen atoms. The general formula is $ C_n H_ (2n-2) $. Here, too, the name is derived from the number of carbon atoms and thus from the corresponding alkane by using the ending -in instead of the ending -an. An alkyne with two carbon atoms is therefore ethyne.

Melting and boiling points of hydrocarbons

Often one should compare two molecules with regard to their melting and boiling points, i.e. indicate which molecule they are higher for. When a substance melts or boils, the intermolecular interactions are overcome.

The lower the intermolecular interactions, the less energy is required to melt or boil a substance. The melting or boiling point is therefore lower.

In order to be able to estimate the melting or boiling point of a substance, you have to know what kind of intermolecular interactions exist and how strong they are.

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    • With hydrocarbons, the only intermolecular interactions that exist are the van der Waals interactions. These can be assessed according to the following criteria:
    • The longer the chain of hydrocarbons, the stronger the van der Waals forces.
    • The more branched the molecule, the weaker the van der Waals forces.
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With butane molecules, the area on which the van der Waals forces can act is smaller than with octane molecules. Therefore, these interactions are higher with octane than with butane. You can imagine it like an adhesive strip. The longer the tape, the harder it is to pull it off the surface it is stuck to.

The same principle applies to branched molecules: the better the molecules can be stacked one inside the other and the larger the surface on which the van der Waals forces can act, the stronger these interactions. The more branched a molecule is, the more difficult it is for me to attach them to one another.

Solubility of hydrocarbons

If you want to decide whether one substance dissolves in another, you have to know whether the substances are polar or non-polar (you can read more about this in AC). If you know that, you still have to know the following:

Similar dissolves in similar, which means that polar substances only dissolve in polar and non-polar only in non-polar.

Since hydrocarbons only have non-polar bonds, the molecules are also non-polar. So they can only be dissolved in non-polar substances. Since water is a polar substance, hydrocarbons cannot be dissolved in water.

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Solubility of hydrocarbons

This chapter lists the typical reactions of hydrocarbons.

Alkanes typically react under a so-called radical substitution with halogens.

Alkenes and alkynes typically react under a so-called electrophilic addition, breaking a multiple bond.