4.2.1. Saturated and Unsaturated Carbon Compounds

The two characteristic features seen in carbon, that is, tetravalency and catenation, put together give rise to a large number of compounds. Many have the same non-carbon atom or group of atoms attached to different carbon chains. These compounds were initially extracted from natural substances and it was thought that these carbon compounds or organic compounds could only be formed within a living system. That is, it was postulated that a ‘vital force’ was necessary for their synthesis. Friedrich Wöhler disproved this in 1828 by preparing urea from ammonium cyanate. But carbon compounds, except for carbides, oxides of carbon, carbonate and hydrogencarbonate salts continue to be studied under organic chemistry.

Organic compounds are primarily carbon-based compounds that form due to carbon's unique properties. The two key features of carbon that allow for the vast array of organic compounds are:

1. Tetravalency: This means that a carbon atom can form four bonds with other atoms, which allows it to connect with various elements, creating diverse structures with different chemical properties.
2. Catenation: Carbon can form long chains by bonding to other carbon atoms. This enables the formation of straight, branched, or even ring-structured compounds.

Initially, it was believed that organic compounds could only be created in living organisms, a belief that was overturned by chemist Friedrich Wöhler's synthesis of urea from inorganic materials, proving that organic compounds could be formed in a laboratory setting too.

Consider carbon like a versatile Lego block that can connect with different shapes and sizes (other atoms) to create complex structures. Just as you can build various models with a limited number of Lego pieces, the myriad combinations of carbon and other elements yield a vast array of organic compounds. This adaptability is what makes carbon the backbone of life on Earth.

We have already seen the structure of methane. Another compound formed between carbon and hydrogen is ethane with a formula of C2H6. In order to arrive at the structure of simple carbon compounds, the first step is to link the carbon atoms together with a single bond and then use the hydrogen atoms to satisfy the remaining valencies of carbon. For example, the structure of ethane is arrived in the following steps:

1. Step 1: C — C (Connecting carbon atoms with a single bond)
2. Step 2: Each carbon atom remains unsatisfied in their valency, so each is bonded to three hydrogen atoms.

The electron dot structure of ethane is shown in a figure.

Every organic molecule consists of carbon and hydrogen atoms among other elements. Let's break down the example of ethane (C2H6) into steps:

1. Connecting Carbon Atoms: In the first step, we link two carbon atoms with a single bond, known as a covalent bond, which allows them to share their electrons.
2. Fulfilling Valency with Hydrogen Atoms: After linking the carbon atoms, we need to attach hydrogen atoms to fill the remaining valencies. Since each carbon has four valencies, when two carbon atoms bond, we can attach three hydrogen atoms to each carbon to complete the structure, which gives us C2H6.

This method is fundamental in organic chemistry to understand how various carbon compounds are formed and their structures.

Think of constructing a bridge with two supports (the carbon atoms) and connecting strings (hydrogen atoms) that hold them together to ensure stability. Just like a bridge needs to have its supports firmly connected and stable at both ends, carbon compounds need to have their valencies satisfied to maintain stability.

However, another compound of carbon and hydrogen has the formula C2H4 and is called ethene. We follow the same step-wise approach as above. Carbon-carbon atoms linked together with a single bond (Step 1). We see that one valency per carbon atom remains unsatisfied (Step 2). This can be satisfied only if there is a double bond between the two carbons (Step 3). Thus, compounds of carbon having double or triple bonds between the carbon atoms are known as unsaturated carbon compounds and they are more reactive than the saturated carbon compounds.

In organic chemistry, compounds are categorized as saturated or unsaturated:

1. Saturated Compounds: These compounds only contain single bonds between carbon atoms. For example, ethane (C2H6) is a saturated compound because it has single bonds only, which makes it less reactive.

2. Unsaturated Compounds: These contain one or more double or triple bonds. For instance, ethene (C2H4) has a double bond between the two carbon atoms. This double bond makes unsaturated compounds more reactive than their saturated counterparts because of the additional energy and reactivity at the double bond site.

Imagine saturated compounds like fully packed jars of jellybeans, where no more candy can fit in without removing some. Unsaturated compounds are like jars that have room for more jellybeans, as some spaces are available to add more candies (bonds). Therefore, if you want to mix in new flavors (reactants), it's easier to do so in the unsaturated jars.

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