In this video, we will focus on drawing structures of alkanes.
Alkanes are a homologous series of saturated hydrocarbons with general formula, CnH2n+2.
Previously, we talked about how to name members in different homologous series. For alkane homologous series, the first member, which has 1 carbon, is methane. The second member has 2 carbons, and we call it ethane. The third member has 3 carbons, it is called propane. The fourth member has 4 carbons. It is butane.
If we are required to draw the structures of these alkanes, how do we go about doing it? When we show all the bonds between the atoms in a molecule, we are writing down the full structural formula of the organic compound.
Let’s start with methane, which has 1 carbon and 4 hydrogens. This is carbon in the periodic table. It has 6 electrons, and it will use its 4 outermost electrons for bonding to achieve stable noble gas octet electronic configuration. This is hydrogen in the periodic table. It has only 1 electron in its first shell, it needs 1 more electron to achieve stable noble gas duplet electronic configuration.
Hence, 1 carbon atom will bond with 4 hydrogens and all the atoms have achieve stable noble gas electronic configuration. Carbon now has 8 outermost electrons, while hydrogen has 2 outermost electrons. When we write the full structural formula, we do not need to draw the electron shell or draw dot and cross to represent the electrons. When drawing the structure, we use a dash, which represents a bond between 2 sharing electrons in a covalent molecule.
Hence, 1 bond means the sharing of 2 electrons.
In methane, carbon atom shares each of its electron with 4 different hydrogen atoms. We replace each of the electron pair with a bond. Ta-da. We have completed the structure of methane.
Let’s try to draw structure of ethane. Ethane has 2 carbons and 6 hydrogens. Similarly, carbon needs to share 4 of its outermost electrons with the other non-metal atoms. This is the dot and cross diagram of ethane. Both carbon share one electron with each other, then each carbon shares the remaining 3 electrons with hydrogen.
Let’s replace each electron pair with a bond. There you go, we have achieved the structure of ethane.
What about propane? Propane has 3 carbons and 8 hydrogens. This is the dot and cross diagram of propane. All 3 carbons share electrons with one another, then each carbon shares their remaining electrons with hydrogen. We then replace each electron pair with a bond. And this is the structure of propane.
Lastly, we have butane, which has 4 carbons and 10 hydrogens. All 4 carbons share electrons with one another, then each carbon shares their remaining electrons with hydrogen. Let’s replace the electron pair with a bond. Ta-da. We have achieved the structure of butane.
If we look at each of these alkanes, you should notice carbon always form 4 bonds while hydrogen always form 1 bond. When you are familiar with structure drawing of organic compounds, you should not need to depend on dot and cross diagram to achieve the full structure formula of organic compounds to save time. Practise more, you will be more confident in writing full structural formula of organic compound.