Carbon and its Compounds

Carbon is an important non-metallic element. It is the sixth most abundant element in the universe. It can exist in the free state or in the form of its compounds. It is the major chemical constituent of most organic matter. Carbon is the second most common element in the human body after oxygen. Carbon is present in coal, oil and natural gas.

Carbon atoms can form compounds by combining with other carbon atoms as well as atoms of other elements. Carbon has the unique property of forming long chains of carbon atoms. These long chains serve as a backbone on which various groups can attach to give a large variety of compounds.

Properties of Carbon

In the structure of a carbon atom, there are 4 electrons in the second shell. The electronic configuration of carbon is 2,4. To complete its octet, carbon requires four more electrons. But due to unfavorable energy considerations, it cannot gain four electrons by ion formation and hence attain the electronic configuration of neon. Due to the same reason, it is also not possible for carbon to lose these four electrons and attain the noble gas configuration of helium. However, it can form covalent bonds by sharing these four electrons.

Carbon can form four covalent bonds. It is tetravalent in nature. The sharing of four more electrons from other atoms completes the octet of carbon atom and it attains the stability by forming four covalent bonds.

Carbon can form bonds with atoms of other elements such as hydrogen (H), nitrogen (N), oxygen (O), sulphur (S) and halogens (X), etc. It also has the property of self combination i.e. bond formation with the other carbon atoms. Thus, carbon can form long chains of carbon atoms. This unique property of forming long chains is known as catenation.

The carbon-carbon covalent bond is strong in nature.

In addition to the single covalent bonds, carbon can also form multiple bonds - double or triple bonds with other carbon, oxygen or nitrogen atoms to give a large variety of compounds. The number of compounds formed is so large that a separate branch of chemistry, called organic chemistry, is devoted to the study of these compounds.

Allotropes of Carbon

Carbon occurs in free state (not combined with any other element) in three allotropic forms. Allotropes are different forms of the same element in the same physical state. Earlier only two allotropic forms - graphite and diamond were known. Another allotropic form, fullerene, has been discovered few years back.

Diamond

Diamonds are formed inside the earth under the conditions of high temperature (about 1500°C) and high pressure (about 70,000 atmospheres). In a diamond crystal, each carbon atom is linked to four other carbon atoms by covalent bonds in a tetrahedral fashion. This results in a three dimensional arrangement.

The three-dimensional network of covalently bonded carbon atoms provides a rigid structure to diamonds. This rigidity makes diamond a very hard substance.

The density of diamond is high. It has a value of 3.51 g cm^–3. The melting point of diamond (in vacuum) is also very high - 3500°C because a large amount of heat energy is required to break the three-dimensional network of covalent bonds.

Since all the four electrons are covalently bonded and there are no free electrons in diamond, hence it does not conduct electricity. But diamond is a good conductor of heat.

Graphite

In contrast to diamond, graphite is soft, black and slippery solid. It has a metallic luster. It is also a good conductor of electricity and heat.

In contrasts to diamond, which has a three-dimensional tetrahedral arrangement of carbon atoms, graphite contains layers of carbon atoms. In each layer, a particular carbon atom is linked to three other carbon atoms in a trigonal planar arrangement with a bond angle of 120°. Thus, three electrons of carbon are covalently bonded to the other three carbon atoms. The fourth electron, which does not participate in bonding, is free. These electrons of various carbon atoms are free to move along between the layers and hence are able to conduct electricity.

The bonding between these layers of carbon atoms is weak. Hence, these layers can slide one over the other. This property makes graphite a good solid lubricant. The density of graphite is less than that of diamond. It has a value of 2.2g cm^-3. The melting point of graphite (in vacuum) is about 3700°C. Graphite can be converted to diamond by applying very higher atmospheric pressure and temperature.

Fullerenes

Fullerenes were discovered in 1985 by Robert F. Curl, Harold W. Kroto and Richard E. Smalley. They were awarded the Nobel Prize in Chemistry in 1996 for this discovery. Fullerenes have closed structures like a football. A typical fullerene, named as buckminsterfullerene has 60 carbon atoms.

Fullerenes are formed when vaporized carbon condenses in an atmosphere of an inert gas.

In addition to the above three allotropic forms, carbon also exists in three microcrystalline or amorphous forms of graphite. They are charcoal, coke and carbon black.

Compounds of Carbon

The compounds of carbon can be classified as organic and inorganic compounds.

Most of the inorganic compounds are obtained from various minerals. For example, limestone, marble and dolomite contain carbon as carbonates. The other inorganic compounds are carbides of metal (e.g. CaC₂, calcium carbide), HCN, CS₂ and oxides of carbon such as CO₂ and CO. 

The organic compounds are obtained from natural sources such as plants and animals, coal and petroleum.

The properties of organic and inorganic compounds are different from each other. Organic compounds are generally low melting solids or liquids. They dissolve in organic solvents such as benzene, alcohol, chloroform, etc. but are generally insoluble in water. The inorganic compounds are generally solids which have high melting and boiling points. They generally dissolve in water but are insoluble in organic solvents.

Oxides of Carbon

The two important oxides of carbon are carbon monoxide (CO) and carbon dioxide (CO₂). 

Carbon monoxide is formed when carbon or hydrocarbons are burned in a limited supply of oxygen.

2C(s) + O₂(g) → 2CO(g)

It is a colourless and odourless gas. It has a melting point of –199°C and boiling point of –192°C. It is a major air pollutant and is released in large quantities from automobile engines. Its low level poisoning causes headache and drowsiness whereas its large amounts can cause even death. It is toxic because it reduces the oxygen carrying capacity of blood by binding with hemoglobin, the red pigment of blood.

Carbon dioxide is formed when carbon containing substances are burnt in excess of oxygen.

C(s) + O₂(g) → CO₂(g)

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O 

It is also produced by heating of carbonates.

CaCO₃(s) → CaO(s) + CO₂(g)

It is also released as a by product in the fermentation of sugar to produce alcohol (ethanol).

C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g)

Carbon dioxide is colourless and odourless gas. It is present in very small amount (0.03%) in the atmosphere.

Hydrocarbons

Hydrocarbons are compounds which contain only carbon and hydrogen. The main source of hydrocarbons is petroleum.

Aliphatic Hydrocarbons

The word aliphatic is derived from the Greek word aleiphar meaning fat. Aliphatic hydrocarbons were named so because they were derived from fats and oils.

Hydrocarbons can be acyclic compounds, which are straight chain compounds, or cyclic compounds, which have rings of carbon atoms. 

Aromatic Hydrocarbons

The word aromatic is derived from the word aroma meaning fragrance. The aromatic compounds have a characteristic smell. Structurally, they include benzene and its derivative.

The aliphatic hydrocarbons can be divided into two categories: saturated hydrocarbons and unsaturated hydrocarbons. In saturated hydrocarbons, carbon atoms are linked to each other by single bonds whereas in unsaturated hydrocarbons, multiple bond (double and triple bonds) are present between carbon atoms.

Saturated Hydrocarbons (Alkanes)

Methane (CH₄) is the simplest alkane in which four hydrogen atoms are linked to the carbon atom in a tetrahedral fashion. If instead of a hydrogen atom, the carbon atom is further linked to another carbon atom, you get another alkane, namely ethane (C₂H₆). Similarly, more carbon atoms can link with each other and the carbon chain can further extend to give a variety of hydrocarbons.

The general formula of alkanes is C_nH_2n+2 where n is the number of carbon atoms in the alkane molecule. Alkanes are colourless and odorless compounds. They have very low reactivity. Many of these compounds are gases or liquids.

Isomerism in Alkanes

Isomers are compounds which have the same molecular formula but have different structures. The first three hydrocarbons have only one isomer because there is only one way in which one, two or three carbon atoms can link to each other. But when there are four carbon atoms, they can join in two different ways. Corresponding to the above two carbon skeletons, there are two hydrocarbons - butane and isobutane.

The number of possible structures in which different carbon atoms can link to each other increases with the increase in number of carbon atoms in the alkane molecules.

IUPAC Nomenclature of Alkanes

Earlier organic compounds were known by their popular or common names which mostly originated from the sources of these compounds. But as the number of these compounds increased, it became difficult to correlate the structure and name of the compound. This let to the need of a systematic nomenclature of compounds.

For IUPAC naming, you must have idea about word root of carbon skeleton.

1. meth
2. eth
3. prop
4. but
5. pent
6. hex
7. hept
8. oct

For straight chain alkanes higher than butane, the suffix - ane is added to the Greek root for the number of carbon atoms. For example, pent - for five, hex - for six and so on.

C₃H₈, word root + ane → Prop + ane → Propane

Unsaturated Hydrocarbons

Unsaturated hydrocarbons contain carbon-carbon double or triple bonds. Unsaturated hydrocarbons having carbon-carbon double bonds (C=C) are called alkenes bond whereas those having carbon-carbon triple bonds (C≡C) are known as alkynes.

Alkenes

The simplest alkene, ethene has two carbon atoms joined by a double bond. Its molecular formula is C₂H₄.

Homologous series of alkenes can be represented by the general formula CnH_2n where n represents the number of carbon atoms in the alkene molecule.

Alkynes

The simplest alkyne is ethyne and it has molecular formula C₂H₂. Its common name is acetylene. It is used to ripen the fruits such as banana, mango, etc. It is also used along with oxygen in oxy-acetylene torch which is used for welding purposes.

The general formula for the homologous series of alkynes is C_nH_2n-2 where n is the number of carbon atoms in the alkyne molecule.

Functional Derivatives of Hydrocarbons

Functional derivatives of hydrocarbons are those compounds which are derived from hydrocarbons by replacing one or more hydrogen atoms with the functional groups.

A functional group is an atom or a group of atoms which is responsible for characteristic properties of a compound. The double and triple bonds which respectively give the alkenes and alkynes their characteristic properties are functional groups.

Since each of these functional groups exhibits the characteristic properties and reactions, all the compounds having the same functional group show the same chemical reactions and constitute one class of compounds. For example, haloalkanes such as chloromethane, chloroethane, chloropropane which have the halo (chloro) functional group show the characteristic reactions of the halo (chloro) group and hence, constitute the class of compounds known as haloalkanes.