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# Carbon Compounds

Carbon is an element of immense significance in both its elemental and combined form. We are surrounded by compounds made up of carbon and its substituents. The list given below illustrates the importance of carbon compounds in our daily life:

1. Foods [starch, sugar, fats, vitamins, proteins]
2. Fuels [wood, coal, alcohol, petrol]
3. Household and commercial articles [paper, soap, cosmetics, oils, paints]
4. Textile fabrics [cotton, wool, silk, linen, rayon, nylon]
5. Drugs and disinfectants [penicillin, quinine, aspirin, sulfa drugs]
6. Poisons [opium, strychnine]
7. Perfumes [vanillin, camphor]
8. Explosives [nitroglycerine, dynamite, picric acid, TNT]
9. Dyes [indigo, congo red, malachite green]
10. War gases [mustard gas, chloropicrin, lewisite]

The list above consists of compounds having plant or animal origin like sugar, starch, proteins, acetic acid, urea, etc. These are classified as organic compounds and their chemistry is known as "Organic Chemistry". Modern organic chemistry comprises the chemistry of carbon compounds which are natural as well as man made. Much of organic chemistry is devoted to studying compounds of carbon and hydrogen, i.e., hydrocarbons and their derivatives.

Carbon compounds are of a second type which can be prepared from minerals such as oxygen, halogens and metals. These are "inorganic" and result in compounds like carbon dioxide, sodium chloride, copper sulphate, potassium nitrate, sodium carbonate, etc. Their chemistry is referred to as "Inorganic Chemistry".

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## Carbon

Carbon undergoes oxidation by combining with oxygen at higher temperature to form oxides, viz., carbon monoxide (CO) and carbon dioxide (CO2). Carbon monoxide is formed when incomplete combustion of carbon or carbon containing fuels take place.

C + $\frac{1}{2}$ O2 $\rightarrow$ CO(g)

CO is present in automobile exhausts (when there is incomplete combustion), volcanic gases, chimney gases, etc.
C + O2(g) $\rightarrow$ CO2(g) + Heat

2CO + O2(g) $\rightarrow$ 2CO2(g) + Heat

CH4 + 2O2(g) $\rightarrow$ CO2(g) + 2H2O

## Combustion

Combustion means the burning of a substance. It is a process that is highly exothermic, i.e., produces a lot of heat. The products of combustion of carbon and its compounds are heat energy, carbon dioxide and water (vapor).

When a fuel undergoes combustion, the basic requirements should be present. These requirements are as follows:
• A combustible substance: All carbon compounds are combustible except carbon as diamond.
• A supporter of combustion: Atmospheric air or oxygen gas is a supporter of combustion. Combustion does not take place in their absence. Carbon dioxide or nitrogen gases do not support combustion.
• Heating to ignition temperature: A minimum amount of temperature or heat is required to enable a fuel to catch fire. Coal has a high ignition temperature; a matchstick cannot produce enough heat to ignite it. However, a matchstick can ignite paper or LPG gas as it has low ignition temperature.
• When the above conditions are present in any combustion process, proper combustion (energy production) takes place with minimum wastage and pollution.
• For example, if an ideal fuel like LPG (high calorific value and relatively high amounts of branched hydrocarbons) is available, a sufficient and continuous supply of oxygen should be maintained to burn it. If the ignition spark or flame is sufficient then the combustion is smooth and completes as follows.

Most of the carbon compounds like the hydrocarbons when burnt in air or oxygen produce large amounts of heat, carbon dioxide and water vapor. Hence they are used as fuels. For example, methane burns with a blue flame in air.

In a very limited supply of air methane gives carbon black.

Some carbon compounds are very combustible and have an explosive reaction with air, e.g., alkenes. They burn with a luminous flame to produce carbon dioxide and water vapor.

Some hydrocarbon compounds undergo cracking or thermal decomposition. In this process, substances are heated to high temperatures of (500 - 8000C) in the absence of air, and they decompose into a mixture of saturated and unsaturated hydrocarbons and hydrogen.

## Allotrope of Carbon

An element, in different forms, having different physical properties but similar chemical properties is known as allotropy. Carbon shows allotropy. Such different forms are called 'allotrope' of an element or allotropic forms. There are three well known allotropic forms of carbon and they are amorphous carbon, diamond and graphite. The fourth allotropic form of carbon is buckminsterfullerenes which is basically an artificial form of carbon and is made up of 60 C atoms.
A few examples of pure carbons are as follows:
Coal, Coke, Charcoal (or wood charcoal), Animal Charcoal (or bone black), Lamp black, Carbon black, Gas carbon and Petroleum coke

Diamonds and graphite are two crystalline allotropes of carbon. Diamond and graphite both are covalent crystals. But, they differ considerably in their properties.

Comparison of the Properties of Diamond and Graphite

 Diamond Graphite It occurs naturally in free state. It occurs naturally and is manufactured artificially. It is the hardest natural substance known. It is soft and greasy to touch. It has high relative density (about 3.5). Its relative density is 2.3. It is transparent and has high refractive index (2.45). It is black in color and opaque. It is non-conductor of heat and electricity. Graphite is a good conductor of heat and electricity. It burns in air at 900°C to give CO2. It burns in air at 700-800°C to give CO2. It occurs as octahedral crystals. It occurs as hexagonal crystals. It is insoluble in all solvents. It is insoluble in all ordinary solvents

These differences in the properties of diamond and graphite are due to the difference in their structures. In diamond, each C atom is linked to its neighbors by four single covalent bonds. This leads to a three-dimensional network of covalent bonds. In graphite, the carbon atoms are arranged in flat parallel layers as regular hexagons.

Each carbon in these layers is bonded to three others by covalent bonds. Graphite thus acquires some double bond character. Each layer is bonded to adjacent layers by weak van der Waals forces. This allows each layer to slide over the other easily. Due to this type of structure graphite is soft and slippery, and can act as a lubricant.

Graphite is also a good conductor of electricity due to mobile electrons in it.
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