Versatile Nature Of Carbon

Carbon's ability to form an enormous number of compounds is attributed to its unique properties, making it a highly versatile element. While chemists know millions of carbon compounds, all other elements combined form a significantly smaller number of compounds.

Two key factors contributing to carbon's versatility are:

1. Catenation: The unique ability of carbon atoms to form bonds with other carbon atoms, creating long chains, branched chains, or rings. This property is called catenation. Carbon atoms can be linked by single, double, or triple bonds.
   • Compounds with only single bonds between carbon atoms are called saturated compounds (e.g., alkanes).
   • Compounds with double or triple bonds between carbon atoms are called unsaturated compounds (e.g., alkenes, alkynes). Unsaturated compounds are generally more reactive than saturated compounds.

   No other element exhibits catenation to the same extent as carbon. The carbon-carbon bond is very strong and stable, allowing the formation of large molecules.

2. Tetravalency: Carbon has a valency of four, meaning it can form covalent bonds with four other atoms. Carbon readily forms strong bonds with other carbon atoms and with atoms of other elements like hydrogen, oxygen, nitrogen, sulphur, and halogens (chlorine, bromine, etc.). The small size of the carbon atom allows its nucleus to hold shared electron pairs strongly, contributing to the strength and stability of these bonds.

These two properties, tetravalency and catenation, allow carbon to form a vast range of compounds with diverse structures and properties.

Organic Compounds: Carbon compounds (except carbides, carbon oxides, carbonates, and hydrogencarbonates) are traditionally studied under organic chemistry. Historically, it was thought they could only be produced by living systems, but Friedrich Wöhler's synthesis of urea disproved this.

Saturated And Unsaturated Carbon Compounds

Hydrocarbons (compounds containing only carbon and hydrogen) can be classified based on the types of bonds between carbon atoms.

When drawing the structure of a hydrocarbon:

1. First, link the carbon atoms together with single bonds.
2. Then, use hydrogen atoms to satisfy the remaining valencies of each carbon atom (carbon forms 4 bonds).

If all the valencies of carbon atoms are satisfied by single bonds (either C-C or C-H), the compound is saturated (an alkane).

Example: Ethane ($\text{C}_2\text{H}_6$). Two carbon atoms linked by a single bond. Each carbon needs 3 more bonds, satisfied by 3 hydrogen atoms.

C—C becomes $\text{H}_3\text{C}—\text{CH}_3$.

If carbon atoms are linked by double or triple bonds, the compound is unsaturated (an alkene or alkyne). These compounds have fewer hydrogen atoms for a given number of carbon atoms compared to saturated compounds.

Example: Ethene ($\text{C}_2\text{H}_4$). Two carbon atoms linked by a single bond require 4 more hydrogen atoms. With only 4 available, the carbon atoms must form a double bond (C=C) to satisfy their valencies (each C forms 2 bonds with H and 2 with the other C). $\text{H}_2\text{C}=\text{CH}_2$.

Example: Ethyne ($\text{C}_2\text{H}_2$). Two carbon atoms linked by a single bond require 6 hydrogen atoms. With only 2 available, the carbon atoms must form a triple bond (C$\equiv$C) to satisfy their valencies (each C forms 1 bond with H and 3 with the other C). H—C$\equiv$C—H.

Unsaturated compounds are generally more reactive than saturated compounds due to the presence of double or triple bonds.

Chains, Branches And Rings

Carbon atoms can form diverse structures through catenation:

• Straight chains: Carbon atoms are linked sequentially in a line (e.g., methane, ethane, propane, butane, pentane, hexane).
• Branched chains: The carbon chain has side branches (e.g., isobutane, a structural isomer of butane).
• Rings (Cyclic compounds): Carbon atoms are arranged in a closed ring structure. These can be saturated (e.g., cyclohexane, $\text{C}_6\text{H}_{12}$) or unsaturated (e.g., benzene, $\text{C}_6\text{H}_6$).

Compounds with the same molecular formula but different structural formulas are called structural isomers (e.g., butane, $\text{C}_4\text{H}_{10}$, exists as straight-chain butane and branched isobutane). The number of structural isomers increases as the number of carbon atoms increases.

Hydrocarbons are classified based on saturation and structure:

• Alkanes: Saturated hydrocarbons with only single bonds (general formula $\text{C}_n\text{H}_{2n+2}$).
• Alkenes: Unsaturated hydrocarbons with at least one double bond (general formula $\text{C}_n\text{H}_{2n}$ for acyclic alkenes with one double bond).
• Alkynes: Unsaturated hydrocarbons with at least one triple bond (general formula $\text{C}_n\text{H}_{2n-2}$ for acyclic alkynes with one triple bond).

No. of C atoms	Name	Formula	Structure
1	Methane	$\text{CH}_4$
2	Ethane	$\text{C}_2\text{H}_6$
3	Propane	$\text{C}_3\text{H}_8$
4	Butane	$\text{C}_4\text{H}_{10}$
5	Pentane	$\text{C}_5\text{H}_{12}$
6	Hexane	$\text{C}_6\text{H}_{14}$

Will You Be My Friend?

Carbon readily forms bonds not only with hydrogen and other carbon atoms but also with other elements such as halogens (Cl, Br, I), oxygen, nitrogen, and sulphur. These elements replace one or more hydrogen atoms in a hydrocarbon chain. The atom or group of atoms that replaces hydrogen is called a heteroatom.

Heteroatoms are often part of specific groups of atoms called functional groups. The presence of a functional group largely determines the chemical properties of the carbon compound, regardless of the length of the carbon chain.

Heteroatom	Class of compounds	Formula of functional group
Cl/Br	Haloalkane (Chloro/bromoalkane)	—Cl, —Br
Oxygen	Alcohol	—OH
	Aldehyde
	Ketone
	Carboxylic acid

The bond from the functional group attaches to the carbon chain by replacing a hydrogen atom.

Homologous Series

As carbon chains can have varying lengths, a series of compounds can be formed where the same functional group is attached to carbon chains of different lengths. Such a series is called a homologous series.

Members of a homologous series have similar chemical properties (due to the same functional group) and show a gradual change in physical properties as the molecular mass increases.

Successive members of a homologous series differ by a constant unit, typically a $\text{—CH}_2\text{—}$ group.

Examples of Homologous Series:

• Alkanes: Methane ($\text{CH}_4$), Ethane ($\text{C}_2\text{H}_6$), Propane ($\text{C}_3\text{H}_8$), Butane ($\text{C}_4\text{H}_{10}$), etc.
  • Difference between $\text{CH}_4$ and $\text{C}_2\text{H}_6$: $(\text{C}_2\text{H}_6) - (\text{CH}_4) = \text{CH}_2$. Molecular mass difference: $(2\times 12 + 6\times 1) - (1\times 12 + 4\times 1) = 30 - 16 = 14$ u (Mass of $\text{CH}_2$ is $12 + 2\times 1 = 14$ u).
  • Difference between $\text{C}_2\text{H}_6$ and $\text{C}_3\text{H}_8$: $(\text{C}_3\text{H}_8) - (\text{C}_2\text{H}_6) = \text{CH}_2$. Molecular mass difference: $(3\times 12 + 8\times 1) - (2\times 12 + 6\times 1) = 44 - 30 = 14$ u.

  General formula for alkanes: $\text{C}_n\text{H}_{2n+2}$ (where $n$ is the number of carbon atoms, $n \ge 1$).

• Alkenes: Ethene ($\text{C}_2\text{H}_4$), Propene ($\text{C}_3\text{H}_6$), Butene ($\text{C}_4\text{H}_8$), etc.
  • Difference between $\text{C}_2\text{H}_4$ and $\text{C}_3\text{H}_6$: $(\text{C}_3\text{H}_6) - (\text{C}_2\text{H}_4) = \text{CH}_2$. Molecular mass difference: $(3\times 12 + 6\times 1) - (2\times 12 + 4\times 1) = 42 - 28 = 14$ u.

  General formula for acyclic alkenes with one double bond: $\text{C}_n\text{H}_{2n}$ (where $n \ge 2$).

• Alkynes: Ethyne ($\text{C}_2\text{H}_2$), Propyne ($\text{C}_3\text{H}_4$), Butyne ($\text{C}_4\text{H}_6$), etc.
  • Difference between $\text{C}_2\text{H}_2$ and $\text{C}_3\text{H}_4$: $(\text{C}_3\text{H}_4) - (\text{C}_2\text{H}_2) = \text{CH}_2$. Molecular mass difference: $(3\times 12 + 4\times 1) - (2\times 12 + 2\times 1) = 40 - 26 = 14$ u.

  General formula for acyclic alkynes with one triple bond: $\text{C}_n\text{H}_{2n-2}$ (where $n \ge 2$).

• Alcohols: Methanol ($\text{CH}_3\text{OH}$), Ethanol ($\text{C}_2\text{H}_5\text{OH}$), Propanol ($\text{C}_3\text{H}_7\text{OH}$), Butanol ($\text{C}_4\text{H}_9\text{OH}$). They all contain the —OH functional group.

Within a homologous series, physical properties like melting point, boiling point, and solubility show a gradual change as the molecular mass increases. Chemical properties are similar because the functional group is the same.

Nomenclature Of Carbon Compounds

A systematic method exists for naming carbon compounds based on the number of carbon atoms and the type of functional group present. This naming system ensures clarity and consistency.

The names of compounds are based on the parent hydrocarbon chain's name, modified by prefixes or suffixes indicating functional groups.

Steps for Naming Simple Carbon Compounds:

1. Identify the number of carbon atoms in the longest continuous chain. This determines the parent name (e.g., 3 carbons = propane, 4 carbons = butane).
2. Identify the functional group(s) present.
3. Modify the parent name based on the functional group using prefixes or suffixes (Table 4.4). The suffix replaces the final '-e' of the parent alkane name if the functional group suffix starts with a vowel. Prefixes are added before the parent name.

Example: A compound with 3 carbon atoms and an alcohol group (—OH). Parent hydrocarbon is propane. Alcohol suffix is '-ol'. Remove '-e' from propane and add '-ol'. Name: Propanol.

Example: A compound with 3 carbon atoms and a double bond (C=C). Parent hydrocarbon is propane. Alkene suffix is '-ene'. Replace '-ane' of propane with '-ene'. Name: Propene.

Example: A compound with 3 carbon atoms and a ketone group (—CO—). Parent hydrocarbon is propane. Ketone suffix is '-one'. Remove '-e' from propane and add '-one'. Name: Propanone.

Example: A compound with 2 carbon atoms and a bromine atom (—Br). Parent hydrocarbon is ethane. Haloalkane prefix is 'bromo-'. Name: Bromoethane.

Chemical Properties Of Carbon Compounds

Carbon compounds exhibit a range of chemical reactions, including combustion, oxidation, addition, and substitution.

Combustion

Carbon and most carbon compounds burn (combust) in the presence of oxygen to produce carbon dioxide and water, releasing a large amount of heat and light. This property makes them excellent fuels.

Carbon compound + Oxygen $\longrightarrow$ Carbon dioxide + Water + Heat + Light

Examples:

• Burning of carbon (coal): $\text{C} + \text{O}_2 \longrightarrow \text{CO}_2 + \text{heat and light}$
• Burning of methane (natural gas): $\text{CH}_4 + 2\text{O}_2 \longrightarrow \text{CO}_2 + 2\text{H}_2\text{O} + \text{heat and light}$
• Burning of ethanol: $\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \longrightarrow 2\text{CO}_2 + 3\text{H}_2\text{O} + \text{heat and light}$

The nature of the flame produced depends on the saturation of the hydrocarbon and the availability of oxygen:

• Saturated hydrocarbons generally burn with a clean, blue flame when there is sufficient oxygen (complete combustion).
• Unsaturated hydrocarbons often burn with a yellow, sooty flame due to incomplete combustion, producing soot (unburnt carbon).
• If the supply of air is limited, even saturated hydrocarbons can undergo incomplete combustion and produce a sooty flame.

Fossil fuels (coal and petroleum), formed over millions of years from biomass, contain impurities like nitrogen and sulphur. Their combustion produces oxides of sulphur and nitrogen, which are major air pollutants.

A flame is produced when gaseous substances burn. Luminous (yellow, sooty) flames are seen when unburnt carbon particles in the flame glow due to heat. Clean (blue) flames indicate complete combustion of fuel gases.

Oxidation

Carbon compounds can undergo oxidation, which involves the gain of oxygen. While combustion is a form of oxidation (rapid oxidation), controlled oxidation reactions are also possible.

Certain substances called oxidising agents can add oxygen to other substances or remove hydrogen. Examples of oxidising agents are alkaline potassium permanganate ($\text{KMnO}_4$) and acidified potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$).

Example: Oxidation of alcohols to carboxylic acids. Alkaline $\text{KMnO}_4$ or acidified $\text{K}_2\text{Cr}_2\text{O}_7$ can oxidise ethanol ($\text{CH}_3\text{CH}_2\text{OH}$) to ethanoic acid ($\text{CH}_3\text{COOH}$).

$\text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{Alkaline KMnO}_4 \text{ or Acidified K}_2\text{Cr}_2\text{O}_7}{\text{Heat}} \text{CH}_3\text{COOH}$

In this reaction, oxygen is added to ethanol (replacing hydrogen or inserting oxygen atoms), converting the alcohol functional group (—OH) to a carboxylic acid functional group (—COOH). The colour of the oxidising agent (e.g., purple $\text{KMnO}_4$) disappears as it is consumed in the reaction.

Addition Reaction

Unsaturated hydrocarbons (alkenes and alkynes) containing double or triple bonds are more reactive than saturated hydrocarbons. They can undergo addition reactions, where atoms or groups of atoms add across the double or triple bond.

Example: Hydrogenation of unsaturated hydrocarbons. Unsaturated hydrocarbons react with hydrogen gas ($\text{H}_2$) in the presence of a catalyst (like palladium or nickel) to form saturated hydrocarbons.

$\text{C}_2\text{H}_4\text{(ethene)} + \text{H}_2 \xrightarrow{\text{Ni catalyst}}{\text{Heat}} \text{C}_2\text{H}_6\text{(ethane)}$

This reaction is commonly used in the industry to convert vegetable oils (which contain long unsaturated carbon chains) into saturated animal fats (like vanaspati ghee). Nickel acts as a catalyst, speeding up the reaction without being consumed itself.

Catalysts are substances that increase the rate of a reaction without participating in the reaction itself.

Dietary advice often recommends choosing vegetable oils containing unsaturated fatty acids over animal fats containing saturated fatty acids, as excessive consumption of saturated fats is considered harmful to health.

Substitution Reaction

Saturated hydrocarbons (alkanes) are generally quite unreactive. However, they can undergo substitution reactions under specific conditions, where one or more hydrogen atoms are replaced by other atoms or groups of atoms.

Example: Reaction of methane ($\text{CH}_4$) with chlorine ($\text{Cl}_2$) in the presence of sunlight. Chlorine atoms can substitute for hydrogen atoms in methane, forming chloromethane ($\text{CH}_3\text{Cl}$) and hydrochloric acid ($\text{HCl}$).

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

This reaction is called a substitution reaction because a hydrogen atom in methane is substituted (replaced) by a chlorine atom. This reaction can continue, leading to the substitution of multiple hydrogen atoms by chlorine.

Some Important Carbon Compounds – Ethanol And Ethanoic Acid

Among the millions of carbon compounds, some have significant commercial importance and are frequently encountered in daily life. Ethanol and ethanoic acid are two such examples.

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Properties Of Ethanol

Ethanol (chemical formula $\text{CH}_3\text{CH}_2\text{OH}$ or $\text{C}_2\text{H}_5\text{OH}$) is a common alcohol. It is a liquid at room temperature, soluble in water in all proportions.

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Properties and Uses:

• Active ingredient in alcoholic drinks.
• Good solvent, used in medicines (e.g., tincture iodine, cough syrups) and tonics.
• Consumption of dilute ethanol can cause drunkenness, but pure ethanol ('absolute alcohol') is poisonous and can be lethal. Long-term alcohol consumption leads to serious health problems.
• Methanol ($\text{CH}_3\text{OH}$), a simpler alcohol, is even more poisonous. It gets oxidised in the liver to methanal (formaldehyde), which damages cells and can cause blindness or death.
• Ethanol produced for industrial use is often 'denatured' (made unfit for drinking) by adding poisonous substances (like methanol) and dyes.
• Ethanol is used as a fuel, sometimes as an additive to petrol. Burning ethanol produces carbon dioxide and water, making it a cleaner fuel.
• Sugarcane is used to produce molasses, which is fermented to produce ethanol.

Chemical Reactions of Ethanol:

• Reaction with Sodium: Ethanol reacts with active metals like sodium to produce sodium ethoxide and hydrogen gas.

  $2\text{Na} + 2\text{CH}_3\text{CH}_2\text{OH} \longrightarrow 2\text{CH}_3\text{CH}_2\text{O}^-\text{Na}^+ + \text{H}_2\text{(g)}$

  Hydrogen gas is produced, which can be tested with a burning splint (pop sound).

• Dehydration to Unsaturated Hydrocarbon: Heating ethanol with excess concentrated sulphuric acid (a dehydrating agent) at 443 K ($170^\circ$C) removes a water molecule, forming ethene (an alkene).

  $\text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{Hot Conc. H}_2\text{SO}_4} \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O}$

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Properties Of Ethanoic Acid

Ethanoic acid (chemical formula $\text{CH}_3\text{COOH}$) is a carboxylic acid. It is commonly known as acetic acid.

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Properties and Uses:

• Pure ethanoic acid has a melting point of 290 K (17°C), so it often freezes in cold climates, earning it the name 'glacial acetic acid'.
• A 5-8% solution of acetic acid in water is called vinegar and is widely used as a food preservative (e.g., in pickles).
• Carboxylic acids are organic acids. Compared to mineral acids (like $\text{HCl}$), which ionise completely, carboxylic acids are weak acids (ionise only partially). This can be shown by comparing the pH of dilute ethanoic acid and dilute $\text{HCl}$ of the same concentration using a universal indicator (which shows $\text{HCl}$ is a stronger acid).

Chemical Reactions of Ethanoic Acid:

• Esterification: Ethanoic acid reacts with alcohols (like ethanol) in the presence of an acid catalyst (like concentrated sulphuric acid) to form an ester. Esters are generally sweet-smelling substances used in perfumes and flavouring agents.

  $\text{CH}_3\text{COOH} + \text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{Acid catalyst}} \text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O}$

  (Ethanoic acid) + (Ethanol) $\longrightarrow$ (Ethyl ethanoate, an ester) + (Water)

  The reaction is reversible.

  Esters can be converted back to alcohol and a sodium salt of the carboxylic acid by treating them with an alkali (like $\text{NaOH}$). This reaction is called saponification and is used to make soaps.

• Reaction with Bases: Like mineral acids, ethanoic acid reacts with bases (like $\text{NaOH}$) to form a salt (sodium ethanoate/acetate) and water.

  $\text{CH}_3\text{COOH} + \text{NaOH} \longrightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O}$

• Reaction with Carbonates and Hydrogencarbonates: Ethanoic acid reacts with metal carbonates and hydrogencarbonates to produce a salt (acetate), carbon dioxide gas, and water, similar to mineral acids.

  $2\text{CH}_3\text{COOH} + \text{Na}_2\text{CO}_3 \longrightarrow 2\text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2$

  $\text{CH}_3\text{COOH} + \text{NaHCO}_3 \longrightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2$

  The produced $\text{CO}_2$ gas causes effervescence and turns lime water milky, confirming its presence.

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Soaps And Detergents

Soaps and detergents are cleansing agents that work by interacting with both oily dirt and water, allowing the dirt to be washed away.

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Soaps: Soaps are sodium or potassium salts of long-chain carboxylic acids. A soap molecule has two distinct parts:

• A long hydrocarbon chain (non-polar), which is hydrophobic (water-repelling) and interacts with oil or dirt.
• An ionic end (polar, e.g., $\text{—COO}^-\text{Na}^+$), which is hydrophilic (water-attracting) and interacts with water.

When soap is dissolved in water, it forms structures called micelles. In a micelle, the hydrophobic hydrocarbon tails cluster together in the interior, away from water, while the hydrophilic ionic ends point outwards, towards the water. Oily dirt particles are trapped in the centre of the micelle by the hydrocarbon tails.

The micelles, with their charged ionic surfaces, remain suspended in water as a colloid and do not precipitate due to electrostatic repulsion between micelles. This allows the oily dirt enclosed within the micelles to be easily rinsed away with water, cleaning the surface.

Soaps work well in soft water. However, in hard water (water containing calcium and magnesium salts), soap reacts with these ions to form an insoluble precipitate called scum. This scum is a white, curdy solid that reduces the effectiveness of soap and leaves behind residue. This is why more soap is needed in hard water, and foam formation is reduced.

Detergents: Detergents are another class of cleansing agents that overcome the problem of scum formation in hard water. They are typically sodium salts of sulphonic acids or ammonium salts with chloride or bromide ions. Like soap, detergent molecules also have a long hydrocarbon chain and a charged end.

The key difference is that the charged ends of detergents do not form insoluble precipitates with calcium and magnesium ions in hard water. Therefore, detergents remain effective cleansers even in hard water. Detergents are widely used in shampoos and products for washing clothes.