write four different formulae by using different type of weight
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WRITING THE STRUCTURAL FORMULAE OF ORGANIC MOLECULES
Already we have seen the bonding nature of carbon in the previous section. Now let us familiarize ourselves with the structural formulae of some organic molecules. On this page, we will build them up with variety of combinations of different atoms using valence rules. While doing so we should keep in our mind that it is customary that every organic molecule must contain at least one carbon atom.
The organic compounds are categorized into various types based on the functional group or groups present. The simplest of all are saturated acyclic hydrocarbons otherwise known as alkanes. It is possible to get most of the organic compounds by modifying these alkanes.
Now let us write the structural formulae of alkanes.
HOMOLOGOUS SERIES
Before writing the formulae of alkanes, it is necessary to understand the meaning of homologous series.
A Homologous series refers to organic compounds with a similar general formula and possessing similar chemical properties due to presence of same functional group and exhibiting a regular gradation of physical properties with increase in molecular weight.
The difference between the formulae of two successive compounds in a homologous series is CH2.
WHAT ARE ALKANES?
Alkanes constitute homologous series of saturated acyclic hydrocarbons represented by the general formula CnH2n+2.
Where n = number of carbon atoms.
The hydrocarbons are organic compounds containing only carbon and hydrogen as constituent atoms.
The word saturated indicates the presence of only single bonds between two carbon atoms, C-C.
That means, in alkanes, all the carbon atoms must be sp3 hybridized and form four single bonds with other atoms. In this case other atoms refer to carbon and hydrogen only.
STEPWISE CONSTRUCTION OF ALKANE MOLECULES
Construction of CH4 (methane) molecule:
Let us start with the first alkane containing one carbon (n=1). Substitution of this value in the general formula, CnH2n+2 gives CH4. It is named as methane.
It can be constructed by connecting a carbon with four hydrogen atoms. Remember that the carbon is tetravalent. The stepwise construction of methane molecule is shown below.
construction of methane molecule
Construction of C2H6 (ethane) molecule:
The second alkane contains two carbons and its formula is C2H6. It is named as ethane. There is a C-C bond in this molecule. The stepwise construction of ethane molecule is shown below.
Remember that each carbon in any given alkane forms only four single bonds.
construction of ethane molecule
Also note that the shaded area, which shows how the ethane molecule differs by a 'CH2' group from methane molecule. We can also think that the ethane molecule is obtained by inserting a CH2 group in between a C and H of methane.
Construction of C3H8 (propane) molecule:
Now let us move on to the third alkane named as propane containing three carbons. Its formula is C3H8. The three carbon atoms are linked to each other linearly and the 8 hydrogens are attached to them as shown below.
construction of propane molecule
Construction of butane & isobutane molecules:
The next formula containing four carbon atoms is C4H10. However the 4 carbon atoms can be arranged in a linear chain or in branched manner. Therefore two structurally different alkanes containing 4 carbons are obtained as shown below. These molecules with linear and branched chains are named as butane and isobutane respectively. They possess same molecular formula but differ in structures and hence are known as structural isomers.
construction of butane & isobutane molecules
It is very important to note that all of the following structures are one and same and indicate the different ways of writing the structure of isobutane.
different ways of writing isobutane
Construction of isomers of C5H12: