The semi-developed formula, Also known as the semi-structural formula, it is one of the many possible representations that can be given to the molecule of a compound. It is very recurrent in organic chemistry, especially in academic texts, since it shows the correct ordering of a molecule and its covalent bonds.
Unlike the developed formula, which is the same as the structural formula, this one does not show the C-H bonds, omitting them to simplify the representation. From this formula, any reader will be able to understand what the backbone of a molecule is; but not its geometry or any of the stereochemical aspects.
To clarify this point we have above the semi-developed formula of 2-methylheptane: a branched alkane whose molecular formula is C8H18, and that obeys the general formula CnH2n + 2. Note that the molecular formula says absolutely nothing about the structure, while the semi-developed formula already allows us to visualize what this structure is like..
Also, note that the C-H bonds are omitted, highlighting only the C-C bonds that make up the carbon chain or skeleton. It will be seen that for simple molecules, the developed formula coincides with the condensed formula; and even with the molecular.
Article index
The molecular formula of methane is CH4, as it has four C-H bonds and is tetrahedral in geometry. These data are provided by the structural formula with wedges outside or below the plane. For methane, the condensed formula also becomes CH4, as well as the empirical and the semi-developed. This is the only compound for which this uniqueness is fulfilled..
The reason the semi-developed formula for methane is CH4, it is because their C-H bonds are not written; doing so would correspond to the structural formula.
The semi-developed formula for propane is CH3-CHtwo-CH3, having only two C-C bonds. Its molecule is linear, and if you notice it, its condensed formula is exactly the same: CH3CHtwoCH3, the only difference being that the C-C bonds are omitted. For propane, it is true that both the semi-developed and the condensed formula coincide.
In fact, this is true for all linear chain alkanes as will continue to be seen in the following sections.
The semi-developed formula of butane is CH3-CHtwo-CHtwo-CH3. Note that it can be written on the same line. This formula strictly speaking corresponds to that of the n-butane, indicating that it is the linear and unbranched isomer. The branched isomer, 2-methylpropane, has the following semi-developed formula:
This time it can no longer be written or represented on the same line. These two isomers share the same molecular formula: C4H10, which does not serve to discriminate one from another.
Again we have another alkane: pentane, whose molecular formula is C5H12. The semi-developed formula of n-pentane is CH3-CHtwo-CHtwo-CHtwo-CH3, easy to represent and interpret, without the need to place the C-H links. CH groups3 are called methyl or methyls, and CHtwo are the methylenes.
Pentane has other branched structural isomers, which can be seen in the lower image represented by their respective semi-developed formulas:
The isomer to is 2-methylbutane, also called isopentane. Meanwhile, the isomer b is 2,2-dimethylpropane, also known as neopentane. Its semi-developed formulas make the difference between the two isomers clear; But it doesn't say much about what such molecules would look like in space. For this, formulas and structural models would be needed.
Semi-developed formulas are not only used for alkanes, alkenes or alkynes, but for any type of organic compound. Thus, ethanol, an alcohol, has a semi-developed formula: CH3-CHtwo-OH. Note that a C-O bond is now represented, but not the O-H bond. All hydrogen bonds are neglected.
Linear alcohols are easy to represent as are alkanes. In short: all semi-developed formulas for linear molecules are easy to write.
Ethers can also be represented with semi-developed formulas. For the case of dimethyl ether, whose molecular formula is CtwoH6Or, the semi-developed becomes: CH3-O-CH3. Note that dimethyl ether and ethanol are structural isomers, since they share the same molecular formula (count the C, H and O atoms).
Semi-developed formulas for branched compounds are more tedious to represent than linear ones; but even more so are cyclic compounds, such as cyclohexane. Its molecular formula corresponds to the same as for hexene and its structural isomers: C6H12, since the hexagonal ring counts as an unsaturation.
To represent cyclohexane, a hexagonal ring is drawn at whose vertices the methylene groups will be located, CHtwo, just as seen below:
The developed formula of cyclohexane would show the C-H bonds, as if the ring had television "antennas".
The molecular formula of phosphorous acid is H3PO3. For many inorganic compounds, the molecular formula is sufficient to give you an idea of the structure. But there are several exceptions, and this is one of them. With the fact that the H3PO3 is a diprotic acid, the semi-developed formula is: HPO (OH)two.
That is, one of the hydrogens is linked directly to the phosphorous atom. However, the formula H3PO3 also admits a molecule with semi-developed formula: PO (OH)3. Both are, in fact, what is known as tautomers.
Semi-developed formulas in inorganic chemistry are very similar to condensed ones in organic chemistry. In inorganic compounds, because they do not have C-H bonds, and because they are in principle simpler, their molecular formulas are usually enough to describe them (when they are covalent compounds).
Semi-developed formulas are very common when the student is learning the rules of nomenclature. But once assimilated, in general, chemistry notes are crammed with skeletal-type structural formulas; that is, not only are the C-H links omitted, but time is also saved by ignoring the C.
For the rest, condensed formulas are more recurrent in organic chemistry than semi-developed ones, since the former do not even need to write the links as in the latter. And when it comes to inorganic chemistry, these semi-developed formulas are less widely used..
Yet No Comments