Carbon Compounds

Organic and inorganic compounds are two important types of compounds. Carbon is the essential element in organic compounds. Compounds having carbon as a constituent element are generally called carbon compounds.

Important inorganic carbon compounds include carbon dioxide, carbon monoxide, carbide salts, carbonate salts and bicarbonate salts.

Bonds in Carbon Compounds

Most carbon compounds have comparatively low melting and boiling points. Their melting and boiling points are generally below 300°C.

• Intermolecular attractive forces are weak.
• Most carbon compounds are poor conductors of electricity.
• Most carbon compounds do not contain ionic bonds.
• Their chemical bonds generally do not produce ions.
• Carbon compounds mainly contain covalent bonds.

Carbon and Covalent Bond

Atomic number of carbon = 6. Its electronic configuration is 2,4 and its valency is 4.

Carbon cannot easily lose four electrons to form C⁴⁺ and cannot easily gain four electrons to form C⁴⁻. It therefore attains stability mainly by sharing electrons with other atoms.

Figure 9.5 – Electron-dot and line structure of methane molecule

Single, Double and Triple Bonds

Two carbon atoms can be bonded together by one, two or three covalent bonds. These are called single bond, double bond and triple bond respectively.

• One shared pair → single bond
• Two shared pairs → double bond
• Three shared pairs → triple bond

Examples:

Figure 9.6 – Ball and stick model and space filling model of methane molecule

Carbon: A Versatile Element

Carbon forms an extremely large number of compounds because of the special nature of its covalent bonds.

Catenation

• Carbon forms long chains.
• Chains may be straight or branched.
• Carbon atoms may form closed ring structures.
• Carbon–carbon bonds are strong and stable.

Tetravalency

Carbon being tetravalent can form bonds with four other atoms, carbon or any other element. This results in a very large number of carbon compounds.

Figure 9.13 – Two isomeric compounds with molecular formula C₄H₁₀

Hydrocarbons

Hydrocarbons are compounds containing carbon and hydrogen as the only two elements.

Saturated hydrocarbons

Compounds in which carbon atoms are joined only by single bonds are called saturated compounds. Saturated hydrocarbons are also called alkanes.

Unsaturated hydrocarbons

Carbon compounds having a double bond or triple bond between two carbon atoms are called unsaturated compounds.

• Double bond → alkene
• Triple bond → alkyne
• Unsaturated hydrocarbons are generally more reactive than saturated hydrocarbons.

Figure 9.15 – Various types of hydrocarbons

Straight, Branched and Ring Structures

The carbon skeleton determines the shape of a carbon compound.

• A straight chain is formed when carbon atoms are joined next to one another.
• A branched chain contains a branch in the carbon skeleton.
• A closed chain forms a ring structure.

Structural isomerism

For C₄H₁₀, two different structural formulae are possible: one straight-chain and one branched-chain compound.

Figure 9.14 – Ring structure of cyclohexane

Functional Groups

Atoms such as halogens, oxygen, nitrogen and sulphur can substitute one or more hydrogen atoms in a hydrocarbon chain. The atom that substitutes hydrogen is called a hetero atom.

Figure 9.16 – Some functional groups in carbon compounds

Homologous Series

The series of compounds formed by joining the same functional group in place of a particular hydrogen atom on carbon chains having sequentially increasing length is called a homologous series.

• Successive members differ by one –CH₂– unit.
• Molecular mass of successive members differs by 14 u.
• Members have the same functional group.
• Chemical properties are similar.
• The carbon chain length increases successively.

Figure 9.17 – Initial members of homologous series

General Formulae

The important homologous series have the following general formulae.

Remember: successive members of a homologous series differ by –CH₂– and their molecular masses differ by 14 u.

IUPAC Nomenclature – Step 1

IUPAC stands for International Union for Pure and Applied Chemistry. It provides a systematic method for giving unique names to carbon compounds.

Step 1

• Draw the structural formula and count the carbon atoms in the longest straight chain.
• Select the corresponding parent alkane.
• For a double bond, change the ending “ane” to “ene”.
• For a triple bond, change the ending “ane” to “yne”.

Figure 9.18 – IUPAC nomenclature of straight-chain compounds: Step 1

IUPAC Nomenclature – Steps 2 and 3

Step 2

If a functional group is present, replace the last letter “e” of the parent name by the condensed name of the functional group as the suffix. Halogens are written as prefixes.

Step 3

• Number the carbon chain from one end to the other.
• Give number 1 to carbon in –CHO or –COOH when present.
• Otherwise, number the chain so that the carbon carrying the functional group gets the smaller number.
• Separate a number and a letter by a small horizontal line.

Figures 9.19 and 9.20 – IUPAC nomenclature: Steps 2 and 3

Chemical Properties – Combustion

Carbon compounds burn in the presence of oxygen and produce heat and light. Carbon dioxide and water are common products.

Saturated and unsaturated compounds

• Saturated compounds generally burn with a clean blue flame.
• Unsaturated compounds generally burn with a yellow flame and release black smoke or soot.
• Hot carbon particles emit yellow light.
• If oxygen supply is limited, even saturated compounds may give a yellow flame.

Oxidation

During combustion, carbon compounds combine with oxygen and are completely oxidised. Substances that provide oxygen to other substances are called oxidants or oxidizing agents.

• Potassium permanganate is a common oxidizing agent.
• Potassium dichromate is another common oxidizing agent.

Oxidation of ethanol

Ethanol is oxidised by alkaline potassium permanganate to form ethanoic acid. The pink colour of potassium permanganate disappears while the oxidant is being consumed.

Addition Reaction

Unsaturated compounds can react with hydrogen in the presence of platinum or nickel catalyst to form saturated compounds.

Hydrogenation of an unsaturated compound in presence of Pt/Ni

This reaction is used for hydrogenation of vegetable oils. Hydrogenation converts unsaturated chains into saturated chains and vanaspati ghee is formed.

Substitution Reaction

Saturated hydrocarbons react with chlorine in the presence of sunlight. Chlorine atoms replace hydrogen atoms one by one.

Ethanol

Ethanol has molecular formula C₂H₅OH or CH₃CH₂OH.

• It is a colourless liquid at room temperature.
• Boiling point is 78°C.
• It is also called alcohol or spirit.
• It is soluble in water in all proportions.
• Its aqueous solution is neutral to litmus.
• It is a good solvent and is used in tincture iodine, cough mixtures, tonics and medicines.

Methanol

Methanol is poisonous. Small quantities can affect vision and may be lethal. Ethanol mixed with methanol is called denatured spirit; blue dye is added for recognition.

Ethanol – Reaction with Sodium and Dehydration

Reaction with sodium

Products: sodium ethoxide and hydrogen gas.

Dehydration of ethanol

When ethanol is heated at 170°C with excess concentrated sulphuric acid, one molecule of water is removed and ethene is formed.

Concentrated sulphuric acid acts as a dehydrating agent.

Ethanoic Acid

Ethanoic acid has formula CH₃COOH and common name acetic acid.

• It is a colourless liquid.
• Boiling point = 118°C.
• Its aqueous solution is acidic.
• It turns blue litmus red.
• Vinegar contains 5–8% aqueous acetic acid.
• Pure ethanoic acid freezes around 17°C and is called glacial acetic acid.

Reaction with sodium hydroxide

Ethanoic Acid – Carbonate and Bicarbonate

With sodium carbonate

Effervescence is produced because carbon dioxide gas is released. Carbon dioxide turns lime water milky.

With sodium bicarbonate

Esterification

Ethanoic acid reacts with ethanol in presence of an acid catalyst and ethyl ethanoate is formed.

Figure 9.25 – Esterification reaction

Esters generally have a sweet odour and are used in fragrances and flavouring agents.

Saponification

General reaction:

When fat is heated with sodium hydroxide, soap and glycerin are formed.

Macromolecules

Natural macromolecules

• Polysaccharides – starch and cellulose
• Proteins – form a large part of animal bodies and help in physiological processes
• Nucleic acids – control heredity at the molecular level
• Natural rubber

Polysaccharides provide food and materials such as fibres and paper. Proteins and nucleic acids perform important functions in living organisms.

Polymers and Polymerization

Figures 9.26 and 9.27 – Synthesis of polyethylene and various polymers with uses

Examples of Synthetic Polymers

Homopolymer and copolymer

• Homopolymer: formed by repetition of a single type of monomer.
• Copolymer: formed from two or more different monomers.
• Polymer structures may be linear, branched or cross-linked.

Board Exam Quick Revision – Definitions

• Covalent bond – chemical bond formed by sharing of two valence electrons between two atoms.
• Catenation – ability of carbon atoms to form strong covalent bonds with other carbon atoms.
• Hydrocarbon – compound containing carbon and hydrogen as the only two elements.
• Saturated compound – carbon compound in which carbon atoms are joined by single bonds.
• Unsaturated compound – carbon compound containing double or triple bonds.
• Functional group – atom or group of atoms containing hetero atoms that gives specific chemical properties.
• Homologous series – series having the same functional group and sequentially increasing carbon-chain length.
• Addition reaction – reaction in which atoms or groups are added to an unsaturated compound.
• Substitution reaction – reaction in which one atom or group is replaced by another.

Board Exam Quick Revision – Alcohol, Acid and Polymer

• Esterification – formation of an ester by reaction between a carboxylic acid and an alcohol.
• Saponification – reaction of an ester with sodium hydroxide to form sodium carboxylate and alcohol.
• Polymer – macromolecule formed by regular repetition of a small unit.
• Monomer – small unit that repeats regularly to form a polymer.
• Polymerization – reaction by which monomers are converted into a polymer.

Most Important Reactions