Special Features of Carbon
The number of carbon compounds known at present is more than 5 million. Many newer carbon compounds are being isolated or prepared every day. Even though the abundance of carbon is less, the number of carbon compounds alone is more than the number of compounds of all the elements taken together. Why is it that this property is seen in carbon and in no other elements? Because carbon has some unique features such as:
· Tetra valency
· Multiple bonds
Catenation is binding of an element to itself or with other elements through covalent bonds to form open chain or closed chain compounds. Carbon is the most common element which undergoes catenation and forms long chain compounds. Carbon atom links repeatedly to itself through covalent bond to form linear chain, branched chain or ring structure.
This property of carbon itself is the reason for the presence of large number of organic carbon compounds. So organic chemistry essentially deals with catenated carbon compounds.
For example, Starch and Cellulose contain chains of hundreds of carbon atoms. Even plastics what we use in our daily life are macromolecules of catenated carbon compounds.
Another versatile nature of carbon is its tetravalency. The shell electronic configuration of carbon is 2,4 (Atomic no: 6). It has four electrons in its outermost orbit. According to Octet Rule, carbon requires four electrons to attain nearest noble gas (Neon) electronic configuration. So carbon has the tendency to share its four electrons with other atoms to complete its octet. This is called its tetravalency. Thus carbon can form four covalent bond with other elements.
For example, in methane, carbon atom shares its four valence electrons with four hydrogen atoms to form four covalent bonds and hence tetravalent.
As seen above, the tetravalent carbon can form four covalent bonds. With this tetravalency, carbon is able to combine with other elements or with itself through single bond, double bond and triple bond. As we know, the nature of bonding in a compound is the primary factor which determines the physical and chemical characteristics of a compound. So the ability of carbon to form multiple bonds is the main reason for the formation of various classes of carbon compounds. Table 4.2 shows one of such classes of compounds called ‘hydrocarbons’ and the type of bonding in them.
When one or more hydrogen in hydrocarbons is replaced by other elements like O, N, S, halogens, etc., a variety of compounds having different functional groups are produced. You will study about them in your higher class.
Isomerism is another special feature of carbon compounds especially found in catenated organic compounds. Let us consider the molecular formula of an organic compound C2H6O. Can you name the compound? You can’t. Because the molecular formula of an organic compound represents only the number of different atoms present in that compound. It does not tell about the way in which the atoms are arranged and hence its structure. Without knowing the structure, we can’t name it.
A given molecular formula may lead to more than one arrangement of atoms. Such compounds are having different physical and chemical properties. This phenomenon in which the same molecular formula may exhibit different structural arrangement is called isomerism. Compounds that have the same molecular formula but different structural formula are called isomers (Greek, isos = equal, meros = parts).
The given formula C2H6O is having two kinds of arrangement of atoms as shown below.
Both the compounds have same molecular formula but different kind of arrangements. In compound ‘a’, the oxygen atom is attached to a hydrogen and a carbon. It is an alcohol. Whereas in compound ‘b’, the oxygen atom is attached to two carbon atoms and it is an ether. These compounds have different physical and chemical properties. You will study about isomerism in detail in higher classes.
Allotropy is a property by which an element can exist in more than one form that are physically different and chemically similar. The different forms of that element are called its allotropes. Look at the materials given below. They are charcoal, graphite and diamond.
Are they equally hard? Are they cost same? Definitely not. Diamond is shiny, costliest and hardest of all. Charcoal and graphite are soft and dark. But chemically they are all similar. Yes. They are made of only carbon. They are called allotropes of carbon.
The main reason for the existence of allotropes of an element is its method of formation or preparation.
Carbon exists in different allotropic forms and based on their physical nature they are classified as below.
· In diamond, each carbon atom shares its four valence electrons with four other carbon atoms forming four covalent bonds.
· Here the atoms are arranged in repeated tetrahedral fashion which leads to a three dimensional structure accounting for its hardness and rigidity.
· In graphite, each carbon atom is bonded to three other carbon atoms through covalent bonds in the same plane.
· This arrangement forms hexagonal layers which are held together one over other by weak Vander Waals forces.
· Since the layers are held by weak forces, graphite is softer than diamond.
· Each carbon has four covalent bonds.
· Hard, heavy and transparent.
· It has tetrahedral units linked in three dimension.
· It is non-conductor of heat and electricity.
· Each carbon has three covalent bonds.
· Soft , slippery to touch and opaque.
· It has planar layers of hexagon units.
· It is conductor of heat and electricity.
The third crystalline allotrope of carbon is fullerene. The best known fullerene is Buckminster fullerene, which consists of 60 carbon atoms joined together in a series of 5- and 6- membered to form spherical molecule resembling a soccer ball. So its formula is C60.
This allotrope was named as Buckminster fullerene after the American architect Buckminster fuller. Because its structure remembered the framework of dome shaped halls designed by Fuller for large international exhibitions, it is called by the pet name Bucky Ball. A large family of fullerenes exists, starting at C20 and reaching up to C540.
In amorphous form of carbon, carbon atoms are arranged in random manner. These form of carbon are obtained when wood is heated in the absence of air. Table 4.4 enlists some amorphous forms of carbon and their features.