Chemical substances containing carbon are referred to as carbon compounds.
Carbon is a versatile element that forms the basis for all living organisms and many of the things we use.
The amount of carbon present in the earth’s crust and in the atmosphere is quite meagre. The earth’s crust has only 0.02% carbon in the form of minerals (like carbonates, hydrogencarbonate, coal and petroleum) and the atmosphere has 0.03% of carbondioxide.
Bonding in Carbon – The Covalent Bond
A covalent bond is formed by equal sharing of electrons from both the participating atoms. A covalent bond forms when the difference between the electronegativities of two atoms is too small for an electron transfer to occur to form ions.
Characteristics of Covalent compound.
- These compounds are poor conductors of electricity.
- These compounds have low melting and boiling points as compared to ionic compounds.
- The forces of attraction between the molecules are not very strong. Since these compounds are largely non-conductors of electricity, we can conclude that the bonding in these compounds does not give rise to any ions.
Elements forming ionic compounds attain the noble gas configuration by either gaining or losing electrons from the outermost shell. In the case of carbon, it has four electrons in its outermost shell and needs to gain or lose four electrons to attain noble gas configuration.
The atomic number of carbon is 6 and the electronic configuration of carbon can be written as 1s22s22p2.
If it were to gain or lose electrons –
(i) It could gain four electrons forming C4– anion. But it would be difficult for the nucleus with six protons to hold on to ten electrons, that is, four extra electrons.
(ii) It could lose four electrons forming C4+ cation. But it would require a large amount of energy to remove four electrons leaving behind a carbon cation with six protons in its nucleus holding on to just two electrons.
Carbon overcomes this problem by sharing its valence electrons with other atoms of carbon or with atoms of other elements. The shared electrons ‘belong’ to the outermost shells of both the atoms and lead to both atoms attaining the noble gas configuration.
Lewis Dot Structure of certain compounds
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Methane is widely used as a fuel and is a major component of bio-gas and Compressed Natural Gas (CNG).   |
Versatile Nature of Carbon
The nature of the covalent bond enables carbon to form a large number of compounds. Carbon’s versatility is best appreciated through properties like tetravalency and catenation.
(i) Tetravalency: Carbon has a valency of four so it is capable of bonding with four other atoms of carbon or atoms of some other mono-valent element
(ii) Catenation: Carbon has the unique ability to form bonds with other atoms of carbon, giving rise to large molecules. This property is called catenation.
(a) Compounds may have long straight chains, branched chains or even carbon atoms arranged in rings.
In cyclopropane, atoms are connected to form a ring and is saturated compound.
Benzene is cyclic in nature with chemical formula, C6H6, i.e., each carbon atom in benzene is arranged in a six-membered ring and is bonded to only one hydrogen atom. It includes 3-double bonds which are separated by a single bond. It is unsaturated compound.
(b) Carbon atoms may be linked by single, double or triple bonds.
Compounds of carbon, which are linked by only single bonds between the carbon atoms are called saturated compounds. Some examples, are shown below in the table.
Compounds of carbon having double or triple bonds between their carbon atoms are called unsaturated compounds.
Allotropes of carbon
Allotropy or allotropism is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of the elements.
The element carbon occurs in different forms in nature with widely varying physical properties. Both diamond and graphite are formed by carbon atoms, the difference lies in the manner in which the carbon atoms are bonded to one another.
In diamond, each carbon atom is bonded to four other carbon atoms forming a rigid three-dimensional structure.
In graphite, each carbon atom is bonded to three other carbon atoms in the same plane giving a hexagonal array. One of these bonds is a double-bond, and thus the valency of carbon is satisfied. Graphite structure is formed by the hexagonal arrays being placed in layers one above the other.
These two different structures result in diamond and graphite having very different physical properties even though their chemical properties are the same.
Diamond is the hardest substance known while graphite is smooth and slippery.
Graphite is also a very good conductor of electricity unlike other non-metals.
Diamonds can be synthesized by subjecting pure carbon to very high pressure and temperature. These synthetic diamonds are small but are otherwise indistinguishable from natural diamonds.
Fullerenes form another class of carbon allotropes. The first one to be identified was C-60 which has carbon atoms arranged in the shape of a football. Since this looked like the geodesic dome designed by the US architect Buckminster Fuller, the molecule was named fullerene.
Homologous Series
A homologous series is a sequence of compounds with the same functional group and similar chemical properties in which the members of the series differ by molecular formula of CH₂ and molecular mass of 14u (members of the series can be branched or unbranched)
For example, the homologous series of alkanes.
CH4 and C2H6 - these differ by a –CH2- unit
C2H6 and C3H8 - these differ by a –CH2- unit
The general formula of this series is CnH2n+2, where n = 1, 2, 3, 4……
Similarly, the homologous series for alkenes.
C2H4 and C3H6 - these differ by a –CH2- unit
C3H6 and C4H8 - these differ by a –CH2- unit
C4H8 and C5H10 - these differ by a –CH2- unit
The general formula for alkenes can be written as CnH2n, where n = 2, 3, 4.
Nomenclature of Carbon Compounds
The names of compounds in a homologous series are based on the name of the basic carbon chain modified by a “prefix” means “phrase before” or “suffix” means “phrase after” indicating the nature of the functional group.
Naming a carbon compound can be done by the following method –
(i) Identify the number of carbon atoms in the compound.
For example, a compound having three carbon atoms would have the name propane.
(ii) In case a functional group is present, it is indicated in the name of the compound with either a prefix or a suffix.
(iii) If the name of the functional group is to be given as a suffix, and the suffix of the functional group begins with a vowel a, e, i, o, u, then the name of the carbon chain is modified by deleting the final ‘e’ and adding the appropriate suffix.
For example, a three-carbon chain with a ketone group would be named in the following manner –
Propane – ‘e’ = propan + ‘one’ = propanone.
(iv) If the carbon chain is unsaturated, then the final ‘ane’ in the name of the carbon chain is substituted by ‘ene’ or ‘yne’.
For example, a three-carbon chain with a double bond would be called propene and if it has a triple bond, it would be called propyne.
Carbon also forms bonds with other elements such as halogens, oxygen, nitrogen and sulphur, in a hydrocarbon chain, by replacing the hydrogen with these elements, such that the valency of carbon remains satisfied. In such compounds, the element replacing hydrogen is referred to as a heteroatom.
Chemical Properties of Carbon Compounds
1. Combustion reaction:
Carbon compounds when burns in presence of oxygen gives carbon dioxide along with the release of heat and light. Such a reaction is called combustion reaction.
Saturated hydrocarbons will generally give a clean flame while unsaturated carbon compounds will give a yellow flame with lots of black smoke.
2. Oxidation reaction:
The addition of oxygen and removal of hydrogen during chemical reaction is called oxidation reaction. Or
Oxidation is a chemical reaction that occurs in an atom or compound and results in the loss of one or more electrons.
Alkaline potassium permanganate or acidified potassium dichromate are capable of adding oxygen to others, hence known as oxidizing agents.
A substance that tends to bring about oxidation to other substances by being itself reduced by gaining the electrons is known as oxidizing agent.
3. Addition reaction:
The reactions in which two molecules react to form a single product having all the atoms of the combining molecules are called addition reactions.
The hydrogenation reaction is an example of the addition reaction. In this reaction, hydrogen is added to a double bond or a triple bond in the presence of a catalyst like nickel, palladium or platinum.
(Catalysts are substances that cause a reaction to occur or proceed at a different rate without the reaction itself being affected.)
4. Substitution reaction:
The reaction in which an atom or group of atoms in a molecule is replaced or substituted by different atoms or group of atoms is called substitution reaction.
In alkanes, hydrogen atoms are replaced by other elements.
CH4 + Cl2→ CH3Cl + HCl (in the presence of sunlight)
Some important Carbon Compounds – Ethanol and Ethanoic Acid
(A) Properties of Ethanol
Ethanol is a liquid at room temperature.
Ethanol is commonly called alcohol and is the active ingredient of all alcoholic drinks.
In addition, because it is a good solvent, it is also used in medicines such as tincture iodine, cough syrups, and many tonics.
Ethanol is also soluble in water in all proportions.
Consumption of small quantities of dilute ethanol causes drunkenness. However, intake of even a small quantity of pure ethanol (called absolute alcohol) can be lethal.
Reactions of Ethanol
1. Reaction with sodium:
Alcohols react with sodium leading to the formation of alkoxide and the evolution of hydrogen gas.
2Na + 2CH3CH2OH → 2CH3CH2O–Na+ + H2
(Sodium ethoxide)
2. Reaction to give unsaturated hydrocarbon:
Heating ethanol at 443 K with excess concentrated sulphuric acid results in the dehydration of ethanol to give ethene –
The concentrated sulphuric acid can be regarded as a dehydrating agent which removes water from ethanol
(B) Properties of Ethanoic Acid
Ethanoic acid is commonly called acetic acid and belongs to a group of acids called carboxylic acids.
5-8% solution of acetic acid in water is called vinegar and is used widely as a preservative in pickles.
The melting point of pure ethanoic acid is 290 K and hence it often freezes during winter in cold climates. This gave rise to its name glacial acetic acid.
Reactions of ethanoic acid:
1. Esterification reaction:
Esters are most commonly formed by reaction of an acid and an alcohol.
Ethanoic acid reacts with absolute ethanol in the presence of an acid catalyst to give an ester –
Generally, esters are sweet-smelling substances. These are used in making perfumes and as flavouring agents.
On treating with sodium hydroxide, which is an alkali, the ester is converted back to alcohol and sodium salt of carboxylic acid. This reaction is known as saponification because it is used in the preparation of soap.
Soaps are sodium or potassium salts of long chain carboxylic acid.
CH3COOC2H5 + NaOH → C2H5OH + CH3COONa
2. Reaction with a base:
Ethanoic acid reacts with a base such as sodium hydroxide to give a salt (sodium ethanoate or commonly called sodium acetate) and water:
NaOH +CH3COOH → CH3COONa +H2O
3. Reaction with carbonates and hydrogencarbonates:
Ethanoic acid reacts with carbonates and hydrogencarbonates to give rise to a salt, carbon dioxide and water. The salt produced is commonly called sodium acetate.
2CH3COOH +Na2CO3 → 2CH3COONa + H2O + CO2
CH3COOH + NaHCO3 → CH3COONa + H2O + CO2
Soaps and Detergents
Most dirt is oily in nature and as you know, oil does not dissolve in water.
The molecules of soap are sodium or potassium salts of long-chain carboxylic acids. The ionic-end of soap interacts with water while the carbon chain interacts with oil. The soap molecules, thus form structures called micelles, where one end of the molecules is towards the oil droplet while the ionic-end faces outside. This forms an emulsion in water. The soap micelle thus helps in pulling out the dirt in water and we can wash our clothes clean.
How Soap works?
Micelles
Soaps are molecules in which the two ends have differing properties, one is hydrophilic, that is, it interacts with water, while the other end is hydrophobic, that is, it interacts with hydrocarbons.
When soap is at the surface of water, the hydrophobic ‘tail’ of soap will not be soluble in water and the soap will align along the surface of water with the ionic end in water and the hydrocarbon ‘tail’ protruding out of water.
Inside water, these molecules have a unique orientation that keeps the hydrocarbon portion out of the water. Thus, clusters of molecules in which the hydrophobic tails are in the interior of the cluster and the ionic ends are on the surface of the cluster. This formation is called a micelle. Soap in the form of a micelle is able to clean, since the oily dirt will be collected in the centre of the micelle.
The micelles stay in solution as a colloid and will not come together to precipitate because of ion-ion repulsion. Thus, the dirt suspended in the micelles is also easily rinsed away.
The soap micelles are large enough to scatter light. Hence a soap solution appears cloudy.
→ Detergents are generally sodium salts of sulphonic acids or ammonium salts with chlorides or bromides ions, etc. Both have long hydrocarbon chain. The charged ends of these compounds do not form insoluble precipitates with the calcium and magnesium ions in hard water. Thus, they remain effective in hard water. Detergents are usually used to make shampoos and products for cleaning clothes.