Carbon and its Compounds — Study Notes

1. Introduction

Carbon is one of the most abundant elements in nature and is the basis of all known life forms. It forms an enormous number of compounds because it can bond with itself and with other elements in many ways. The compounds of carbon are studied under a separate branch of chemistry called organic chemistry. Carbon is present in carbohydrates, proteins, fats, fuels, plastics, medicines, fibres and millions of other substances. Despite being a non-metal, carbon shows some exceptional properties, including the ability to conduct electricity in the form of graphite and its extremely hard allotrope, diamond.

The unique property of carbon is catenation, which is the ability of carbon atoms to link with other carbon atoms through covalent bonds to form long chains, branched chains and rings. Carbon also forms stable covalent bonds with oxygen, hydrogen, nitrogen, sulphur and halogens. Because the carbon atom has four valence electrons, it forms four covalent bonds to achieve a stable configuration of eight electrons in its outermost shell. This is why carbon compounds are called tetravalent.

In this chapter, we will study the properties of carbon, covalent bonding in carbon compounds, allotropes of carbon, saturated and unsaturated hydrocarbons, functional groups and homologous series, isomerism, important carbon compounds such as ethanol and ethanoic acid, and the chemical properties of carbon compounds including combustion, oxidation, addition and substitution reactions. We will also understand soaps and detergents, which are everyday products made from carbon compounds.

2. Covalent Bonding in Carbon

Carbon has four valence electrons. It can neither gain four electrons (because it would need a lot of energy to hold eight extra electrons) nor lose four electrons (because it would require too much energy to remove four electrons). Therefore, carbon achieves stability by sharing electrons with other atoms. The bond formed by sharing of a pair of electrons between two atoms is called a covalent bond. Compounds formed by covalent bonds are called covalent compounds.

$$H_2: \text{H} - \text{H}$$

$$O_2: \text{O} = \text{O}$$

$$N_2: \text{N} \equiv \text{N}$$

In a single covalent bond one pair of electrons is shared, in a double bond two pairs are shared, and in a triple bond three pairs are shared. Covalent compounds are generally poor conductors of electricity, have low melting and boiling points, and are usually insoluble in water but soluble in organic solvents.

3. Allotropes of Carbon

Allotropes are different forms of the same element with different physical properties but similar chemical properties. Carbon has three important allotropes:

• Diamond: each carbon is bonded to four other carbons in a tetrahedral arrangement forming a rigid three-dimensional network. It is the hardest natural substance and is used in cutting tools and jewellery.
• Graphite: each carbon is bonded to three other carbons forming hexagonal layers, and the layers slide over each other. Graphite is soft, slippery and conducts electricity, and is used as a lubricant and in pencils.
• Fullerenes: carbon clusters such as C60 (buckminsterfullerene), which have a football-like structure and were discovered in 1985.

Versatile Nature of Carbon

Carbon has two main reasons for forming so many compounds: catenation (bonding with carbon atoms) and tetravalency. The bonds formed by carbon are very strong and stable. Moreover, carbon compounds can exist in different structural arrangements, leading to isomerism.

4. Saturated and Unsaturated Carbon Compounds

Carbon compounds are classified into saturated and unsaturated compounds:

• Saturated compounds: contain only single bonds between carbon atoms, for example ethane (C2H6). They are less reactive.
• Unsaturated compounds: contain double or triple bonds between carbon atoms, for example ethene (C2H4) and ethyne (C2H2). They are more reactive and can undergo addition reactions.

Types of Hydrocarbons

Hydrocarbons are compounds made only of carbon and hydrogen. They are classified as:

• Alkanes: saturated hydrocarbons with general formula CnH2n+2. Example: C2H6 (ethane).
• Alkenes: unsaturated hydrocarbons with one double bond, general formula CnH2n. Example: C2H4 (ethene).
• Alkynes: unsaturated hydrocarbons with one triple bond, general formula CnH2n-2. Example: C2H2 (ethyne).

5. Functional Groups and Homologous Series

A functional group is an atom or a group of atoms which determines the chemical properties of an organic compound. The same functional group gives similar chemical properties to different compounds. Some important functional groups are:

A homologous series is a series of compounds in which the same functional group is present and successive members differ by a CH2 unit. For example, the alkanes: CH4, C2H6, C3H8, C4H10. The general formula of the series remains the same, and the physical properties change gradually with increasing molecular mass.

Isomerism

Isomers are compounds which have the same molecular formula but different structures and therefore different properties. For example, butane (C4H10) has two isomers: n-butane and isobutane (2-methylpropane). Isomers are possible only beyond C3 in alkanes.

6. Important Carbon Compounds

Ethanol (C2H5OH)

Ethanol is a member of the alcohol family. It is a colourless liquid with a pleasant smell, miscible with water in all proportions, and is a good solvent. Ethanol is used in the manufacture of medicines, perfumes, dyes and synthetic rubber. It is a constituent of alcoholic beverages. When ethanol is oxidised by a strong oxidising agent, it forms ethanoic acid. Ethanol reacts with sodium to release hydrogen gas:

$$2\text{C}_2\text{H}_5\text{OH} + 2\text{Na} \rightarrow 2\text{C}_2\text{H}_5\text{ONa} + \text{H}_2$$

Ethanoic Acid (CH3COOH)

Ethanoic acid, commonly called acetic acid, belongs to the carboxylic acid family. A 5-8% solution of ethanoic acid in water is called vinegar. Ethanoic acid is a weak acid, turns blue litmus red, and reacts with carbonates to release carbon dioxide. The functional group -COOH is responsible for its acidic nature. Ethanoic acid reacts with alcohols in the presence of concentrated sulphuric acid to form esters (esterification reaction), which have a fruity smell:

$$\text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH} \xrightarrow{\text{H}_2\text{SO}_4} \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}$$

Chemical Properties of Carbon Compounds

• Combustion: carbon compounds burn in oxygen to produce carbon dioxide, water and heat. Saturated compounds burn with a clean blue flame, while unsaturated compounds burn with a sooty yellow flame.
• Oxidation: alcohols are oxidised to carboxylic acids with alkaline potassium permanganate or acidified potassium dichromate.
• Addition reaction: unsaturated compounds add hydrogen, halogen or halogen acids across the double or triple bond. Hydrogenation of vegetable oils is used to make margarine.
• Substitution reaction: in saturated compounds such as methane, one hydrogen atom is replaced by a halogen atom in the presence of sunlight.

$$\text{CH}_4 + \text{Cl}_2 \xrightarrow{\text{sunlight}} \text{CH}_3\text{Cl} + \text{HCl}$$

7. Soaps and Detergents

Soaps are sodium or potassium salts of long-chain carboxylic acids. A soap molecule has a hydrophilic (water-loving) polar head and a hydrophobic (water-fearing) long carbon chain tail. When a soap is added to water, the molecules arrange themselves in a spherical structure called a micelle, with the tails pointing inwards and the heads pointing outwards. The hydrophobic tail dissolves in the dirt/oil, and the head attaches to water, so the dirt can be washed away.

Soaps do not work well in hard water because the calcium and magnesium ions in hard water react with soap to form insoluble scum. Synthetic detergents are ammonium or sulphonate salts of long-chain carboxylic acids. They work well even in hard water because their calcium and magnesium salts are soluble in water. This is why detergents are preferred over soaps for washing clothes in hard water.

Quick Revision Tables

Table 1: Homologous Series of Alkanes

Table 2: Comparison of Soaps and Detergents

Mind Map

Important Diagrams (SVG)

Diagram 1: Structure of a Soap Molecule and Micelle

Diagram 2: Electron Dot Structure of Methane and Ethene

Common Mistakes

1. Writing the molecular formula of an alkane incorrectly; remember the general formula is CnH2n+2 for alkanes, CnH2n for alkenes, and CnH2n-2 for alkynes.
2. Confusing ethanol with ethanoic acid; ethanol has the -OH functional group and is neutral to litmus, while ethanoic acid has -COOH and turns blue litmus red.
3. Stating that carbon can form ionic bonds; carbon always forms covalent bonds because it has four valence electrons.
4. Believing diamond conducts electricity; diamond does not conduct electricity because all four electrons are used in bonding, while graphite conducts due to free electrons.
5. Mixing up addition and substitution reactions; addition occurs in unsaturated compounds (double/triple bonds), while substitution occurs in saturated compounds.
6. Forgetting that hydrogenation of oils converts unsaturated vegetable oils into saturated fats (like margarine) using nickel as a catalyst.
7. Saying detergents are more biodegradable than soaps; soaps are biodegradable while some detergents are not easily biodegradable.

Exam Tips

1. Learn the general formulae of alkanes, alkenes and alkynes, and practise deducing the formula of any member of the homologous series.
2. Memorise the structural formula and uses of ethanol and ethanoic acid, and the equation for esterification.
3. Know the three allotropes of carbon and at least two distinguishing properties of diamond and graphite each.
4. For soap questions, draw the soap molecule (head and tail) and explain micelle formation clearly with a labelled diagram.
5. Practise writing electron dot structures of simple molecules such as CH4, O2, N2, CO2 and H2O.
6. Remember the test for a carboxylic acid: it turns blue litmus red and produces brisk effervescence with baking soda.
7. For addition reactions, remember hydrogenation needs a nickel catalyst and is used to convert oils into fats.

Conclusion

Carbon compounds form the chemistry of life, and understanding them opens the door to organic chemistry, which will be studied in greater depth in higher classes. The concept of covalent bonding, catenation and tetravalency explains why carbon can form an almost limitless number of compounds. Allotropes demonstrate how the same element can show strikingly different physical properties. The functional groups and homologous series bring order and predictability to millions of carbon compounds. Ethanol and ethanoic acid are not just textbook molecules; they are used in industry, medicine and daily life. Finally, soaps and detergents show how an understanding of molecular structure can be applied to solve the practical problem of removing dirt and oil. Mastering this chapter gives you a solid foundation in organic chemistry and its everyday applications.