1. Bonding in Carbon-The Covalent bond
A covalent bond, also known as a molecular bond, is a chemical bond that shares electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between the atoms that share electrons is known as covalent bonding. For many molecules, the sharing of electrons enables each atom to attain the equivalent of a full outer shell, corresponding to a stable electronic configuration.
A carbon-carbon bond is a covalent bond between two carbon atoms. The most common form is the single bond that is composed of two electrons, one from each of the two atoms. Carbon is one of the few elements that can form long chains of its own atoms, a property called catenation.
Bonding to attain noble-gas configuration
Allotropes of carbon
Carbon is capable of forming many allotropes due to its valency. Well-known forms of carbon include diamond and graphite.
Diamond is a well known allotrope of carbon. The hardness and high dispersion of light of diamond makes it useful for both industrial applications and jewelry. Diamond is the hardest known natural mineral. This makes it an excellent abrasive and makes it hold polish and luster extremely well. No known naturally occurring substance can cut (or even scratch) a diamond, except another diamond.
Graphite conducts electricity, due to delocalization of the pi bond electrons above and below the planes of the carbon atoms. These electrons are free to move, so are able to conduct electricity. However, the electricity is only conducted along the plane of the layers.
Buckminsterfullerenes:
They are named for the resemblance of their allotropic structure to the geodesic structures devised by the scientist and architect Richard Buckminster "Bucky" Fuller. Fullerenes are molecules of varying sizes composed entirely of carbon, which take the form of a hollow sphere, ellipsoid, or tube.
Versatile Nature of Carbon
Organic compounds are made up of carbon, oxygen, hydrogen, and few other elements. However, the number of organic compounds is far bigger than inorganic compounds that do not form bonds.
The distinct nature of carbon atom and its capacity to form bonds with other atoms leads to such huge number of organic compounds.
The versatile nature of carbon can be best understood with its features such as, tetravalency and catenation. In this section let us learn more about versatility of carbon.
Carbon is a versatile element and is found in many different chemical compounds, including those found in space. Carbon is versatile because it can form single, double, and triple bonds. It can also form chains, branched chains, and rings when connected to other carbon atoms.
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.
- Catenation: The property of forming long chains by self-linking with other carbon atoms to form long chains, rings, double or triple bonds is called catenation.
- Isomerism: Compounds with same molecular formula but different structural formula are called isomers. An isomerism commonly seen is due to difference in the arrangement of atoms or groups of atoms & is called structural isomerism. The 4 types of structural isomerism are:
- Chain isomerism
- Position
- Functional
- Metamerism
- Tetravalency: Carbon has 4 electrons in its valence shell. Energy considerations do not allow it to gain or lose 4 electrons; therefore it forms covalent bonds with other elements to complete its octet. This accounts for its tetravalency and explains its ability to form a variety of compounds.
Hydrocarbons
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These are organic compounds containing only carbon and hydrogen and are represented by the general formula C x H y where x and y are whole numbers. Other organic compounds are derived from these parent compounds by replacement of one or more hydrogen atoms, e.g. the addition of one -CH₂ group can result in the formation of a new compound.
Homologous series of compounds is a family of carbon compounds which:
- Have the same general formula.
- Show a difference of 14 u in their molecular mass.
- Show gradation in the physical properties.
- Show similarity in the chemical properties.
- Are characterised by the same functional group.
- Differ from the previous member by a –CH₂ group.
Example, CH3OH, C2H5OH etc.
Rules to name carbon compounds:
- Choose the longest unbranched carbon chain.
- Determine the functional group present.
- Number the carbon atoms such that the C atom to which the functional group is present gets the smallest number.
- If the same substituent occurs more than once, the location of each point on which the substituent occurs is given & the number of times the substituent group is indicated by a prefix (di, tri, tetra etc.).
Chemical Properties of Carbon Compounds
Carbon is an element which is used in some form or the other in our everyday life. It is derived from the Latin word carbo meaning coal. Symbol C represents carbon and its atomic number is 6. It is a non-metal and tetravalent (four electrons are available to form covalent chemical bonds).Carbon can form bonds with other small atoms, which includes other carbon atoms. However, under normal conditions, carbon is weakly reactive. Carbon can react with oxygen and metals only at very high temperatures forming carbon oxides and carbides respectively.
Carbon has many allotropes meaning various forms of an element. Graphite and diamond are some of the examples.
Hydrocarbons are those compounds consisting of only hydrogen and carbon atoms. In this topic, we will see some chemical properties of hydrocarbons.
Chemical properties of hydrocarbons:
Though there are millions of carbon compounds, the reactions they go through are limited. Some important reactions of carbon compounds are substitution, addition, polymerisation, combustion, and thermal cracking. We will now see some of the chemical reactions of hydrocarbons.
- Combustion
- Saturated compounds (Alkanes) give clean blue flame with sufficient air; unsaturated compounds (Alkenes/Alkynes) give yellow sooty flame.
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- Oxidation
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- Addition: Unsaturated compounds undergo addition reaction e.g. ethene forms ethane.
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- Substitution: Saturated compounds undergo substitution reaction where an atom or group of atoms replaces another atom.
E.g. in the presence of sunlight, chlorine replaces hydrogen in hydrocarbons.
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4. Some important Carbon compounds
Carbon is the most significant element present in earth’s crust, both in its elemental form and in the combined form.
It is a key component of all known life on Earth. Complex molecules are made up of carbon bonded with other elements, especially oxygen, hydrogen and nitrogen, and carbon can bond with all of these because of its four valence electrons.
Ethanol and Ethanoic acid are two important compounds used in our daily life. In this chapter we will learn about these two important organic compounds of Carbon.
Ethanol:
Ethanol is considered as one of the most important organic compound. The molecular formula of ethanol is C2H5OH. It is also called as ethyl alcohol.
Ethyl alcohol (commonly referred to as alcohol), may serve as a source of energy in small quantities. But in large amounts it affects the nervous system.
Physical properties of Ethanol:
- Ethanol is liquid at room temperature.
- It is a colourless inflammable and sweet smelling liquid.
- Good solvent, used in medicines like tincture of iodine, used in all alcoholic preparations.
- It Is miscible with water.
- Pure ethanol is called absolute alcohol.
- Ethanol can cause drowsiness upon consumption, even with small quantities of dilute ethanol.
- Extremely poisonous when consumed in pure form (absolute alcohol).
Chemical properties of Ethanol:
- Ethanol can be manufactured through fermentation of molasses.
- The process involves slow decomposition of a complex organic compound like molasses into simpler compounds including ethanol, by means of microorganisms like yeast.
- Reaction with sodium: Ethanol readily reacts with sodium to form sodium ethoxide and hydrogen gas.
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- Reaction with concentrated sulphuric acid: Ethanol on heating to a temperature of 443 K with excess concentrated sulphuric acid, gives ethene.
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Ethanoic Acid:
Ethanoic acid, also known as acetic acid or methanecarboxylic acid is one of the most common organic acids and known in the form of vinegar. It is a colourless, pungent smelling liquid which is sour to taste. The molecular formula of Ethanoic Acid is CH₃COOH. It is also present in a number of fruit juices. In the combined state it occurs in many oils and essential oils.
Physical properties of Ethanoic acid:
- Ethanoic acid is a colourless corrosive liquid with a pungent odour.
- The melting point of pure ethanoic acid is 17 0C.
- Ethanoic acid freezes at 290 K and is known as glacial acetic acid.
- Miscible with water, ether and ethyl alcohol.
- 5-8% solution of acetic acid in water is called vinegar.
- All carboxylic acids are weak acids- undergo partial ionisation.
Chemical properties of Ethanoic acid:
- Esterification: Esters are sweet smelling substances used in perfumes and artificial flavours. Formed when carboxylic acid reacts with alcohol in presence of acid catalyst
CH₃COOH + CH₃CH₂OH → CH₃COOC₂H₅ (ester)
- Saponification: Esters react in the presence of an acid or a base to give back the alcohol and carboxylic acid. This is called saponification reaction. This is reverse reaction of esterification reaction.
C₂H₅OH + CH₃COONaàCH₃COOC₂H₅ NaOH
- Reaction with base: Carboxylic acids react with bases to give salt and water
CH₃COOH + NaOH → CH₃COONa + H₂O (sodium acetate and water)
- Reaction with carbonates and hydrogen carbonates
2 CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂
Soap is a sodium salt or potassium salt of long chain fatty acids having cleansing action in water. They are using as cleansing agents to remove dirt, oil from the skin and clothes.
Detergents have almost the same properties as soaps but they are more effective in hard water. Detergents are generally ammonium or sulphonate salts of long chain carboxylic acids.
Usage of cleansing agents goes back to around 3000 years in history, when soap nut powder was used. In many parts of India, even now, soap nut powder is used as a natural soap to remove oil. Due to a better understanding of the role of hygiene and the promotion of popular awareness of the relationship between cleanliness and health, industrially manufactured bar soaps became available in the late eighteenth century.
Soaps:
Soaps are water-soluble sodium or potassium salts of fatty acids. Animal fat or vegetable oil act as glyceride or glyceryl ester and sodium hydroxide and potassium hydroxide act as bases.
Detergents:
Detergents are generally ammonium or sulphonate salts of long chain carboxylic acids. They are water-soluble cleansing agents which combines with impurities and dirt to make them more soluble and differs from soap in not forming a scum with the salts in hard water.
Cleansing action of Soaps and detergents:
The cleaning action of soap is due to micelle formation and emulsion formation. Inside water a unique orientation forms clusters of molecules in which the hydrophobic tails are in the interior of the cluster and the ionic ends on the surface of cluster. This results in the formation of micelle.
Soap in the form of micelle cleans the dirt as the dirt will be collected at the centre of micelle. This property of soap makes it an emulsifier. The dirt suspended in micelles is easily rinsed away. This is known as cleaning action of soap.
- The soap molecule is generally represented as RCOONa.
- In solution, it ionizes to form RCOO- and Na+. Each soap molecule has a polar head group (carboxylate ion, COO- group) and a long non-polar hydrocarbon tail (R group from long chain fatty acid). The polar head attracts the polar water molecule and is called hydrophilic end and the non-polar tail attracts the water insoluble oily or greasy dirt particles.
- When a dirty cloth is placed in soap solution, the long non-polar hydrocarbon tail of soap molecules points towards the oily dirt particles and the polar heads point towards the water.
- This forms a spherical structure with polar parts of the molecule on the surface and non-polar parts in the centre. This spherical structure is called micelle.
- This micelle is attracted towards water and carries the oily dirt particles along with it. This causes the dirt particles to detach from the fibres of the cloth. In this manner, clothes become free from dirt or dust.