Carbon and Its Compounds

Carbon and Its Compounds

Activity 4.1

• Listing and categorizing ten daily-used items into those made of metal, glass/clay, and others.
• Most items in the 'others' category are made of carbon compounds.
• Burning a carbon compound produces $CO_2$, which can be tested using lime water.

Introduction to Carbon

• Carbon is a versatile element crucial in elemental and combined forms.
• Carbon is the basis of food, clothes, medicines, books, and living structures.
• Carbon constitutes only a small fraction of the Earth's crust (0.02% as minerals) and atmosphere (0.03% as $CO_2$).

Bonding in Carbon – The Covalent Bond

• Ionic compounds have high melting/boiling points and conduct electricity in solution or molten state.
• Most carbon compounds are poor conductors of electricity.
• Carbon compounds generally have low melting and boiling points, indicating weak intermolecular forces.
• Bonding in carbon compounds doesn't give rise to ions.
• Carbon has 4 valence electrons and needs to gain or lose 4 electrons to achieve a noble gas configuration.
• Gaining 4 electrons to form $C^{4-}$, is difficult because the nucleus struggles to hold 10 electrons with only 6 protons.
• Losing 4 electrons to form $C^{4+}$ requires a large amount of energy to remove 4 electrons.
• Carbon overcomes this by sharing valence electrons, forming covalent bonds.

Formation of Molecules by Sharing Electrons

• Hydrogen (atomic number 1) forms $H_2$ molecules by sharing one electron each to attain helium's configuration.
• A shared pair of electrons forms a single covalent bond, represented by a line (—).
• Chlorine (atomic number 17) forms $Cl_2$ molecules (diatomic).
• Oxygen (atomic number 8) forms double bonds by sharing two electrons to complete its octet.
• Nitrogen (atomic number 7) forms triple bonds by sharing three electrons to complete its octet, forming $N_2$.
• Ammonia has the formula $NH_3$.

Methane

• Methane (formula $CH_4$) is a simple carbon compound widely used as fuel (bio-gas, CNG).
• Carbon, being tetravalent, shares its 4 valence electrons with 4 hydrogen atoms.
• Covalent bonds: Bonds formed by sharing an electron pair between atoms.
• Covalently bonded molecules have strong intramolecular bonds but weak intermolecular forces.

Allotropes of Carbon

• Carbon exists in different allotropic forms with varying physical properties (e.g., diamond and graphite).
• In diamond, each carbon atom is bonded to four others in a rigid 3D structure.
• In graphite, each carbon atom is bonded to three others in the same plane, forming hexagonal arrays.
• Graphite has a double bond satisfying carbon's valency and is arranged in layers.
• Diamond is the hardest substance, while graphite is smooth, slippery, and a good conductor of electricity.
• Synthetic diamonds can be made under high pressure and temperature.
• Fullerenes (e.g., $C_{60}$) have carbon atoms arranged in a football shape.

Versatile Nature of Carbon

• The number of known carbon compounds is in the millions.

Catenation

• Carbon's ability to form bonds with other carbon atoms results in large molecules with long or branched chains, or rings.
• Carbon atoms can be linked by single, double, or triple bonds.
• Saturated compounds have only single bonds; unsaturated compounds have double or triple bonds.
• Carbon-carbon bonds are strong and stable.

Tetravalency

• Carbon can bond with four other atoms (carbon or other monovalent elements).
• Carbon forms strong bonds with oxygen, hydrogen, nitrogen, sulfur, chlorine, etc., leading to stable compounds.
• The small size of carbon facilitates strong bonds by enabling the nucleus to hold shared electron pairs tightly.

Organic Compounds

• Organic compounds have a non-carbon atom or group attached to different carbon chains.
• Initially, it was thought a 'vital force' was needed to synthesize organic compounds.
• Friedrich Wöhler disproved this in 1828 by preparing urea from ammonium cyanate.
• Carbon compounds excluding carbides, carbon oxides, and carbonate/hydrogencarbonate salts are studied under organic chemistry.

Saturated and Unsaturated Carbon Compounds

• Ethane ($C<em>2H</em>6$) is formed by linking two carbon atoms with a single bond, then satisfying the remaining valencies with hydrogen.
• Propane ($C<em>3H</em>8$) has all valencies satisfied by single bonds.
• Ethene ($C<em>2H</em>4$) requires a double bond between the two carbon atoms.
• Ethyne ($C<em>2H</em>2$) requires a triple bond between the two carbon atoms.
• Unsaturated compounds are more reactive than saturated compounds.

Chains, Branches, and Rings

• Carbon chains can contain many carbon atoms (e.g., methane, ethane, propane, butane, pentane, hexane).
• Butane can have two different structures with the same formula ($C<em>4H</em>{10}$): straight chain and branched chain.
• Structural isomers have identical molecular formulas but different structures.
• Cyclic compounds have carbon atoms arranged in a ring (e.g., cyclohexane, $C<em>6H</em>{12}$).
• Benzene ($C<em>6H</em>6$) has a specific ring structure.
• Hydrocarbons contain only carbon and hydrogen.
  • Alkanes: Saturated hydrocarbons.
  • Alkenes: Unsaturated hydrocarbons with one or more double bonds.
  • Alkynes: Unsaturated hydrocarbons with one or more triple bonds.

Heteroatoms and Functional Groups

• In a hydrocarbon chain, hydrogen atoms can be replaced by halogens, oxygen, nitrogen, or sulfur (heteroatoms).
• Heteroatoms or groups containing them confer specific properties to the compound (functional groups).

Common Functional Groups

• Halo- (Chloro/bromo): -Cl, -Br
• Alcohol: -OH
• Aldehyde: -CHO
• Ketone: -CO-
• Carboxylic acid: -COOH

Homologous Series

• A series of compounds with the same functional group substituting for hydrogen in a carbon chain.
• The chemical properties are similar across the series.
• Successive compounds differ by a -$CH_2$ unit.
• The molecular mass increases successively by 14u.
• Alkenes have the general formula $C<em>nH</em>{2n}$.
• Physical properties show a gradation with increasing molecular mass (e.g., melting and boiling points).

Nomenclature of Carbon Compounds

• Names are based on the basic carbon chain, modified by prefixes or suffixes indicating the functional group.
  • Identify the number of carbon atoms.
  • Indicate the functional group with a prefix or suffix.
  • If the suffix starts with a vowel, remove the final 'e' from the carbon chain name.
  • Replace 'ane' with 'ene' (double bond) or 'yne' (triple bond) for unsaturated compounds.

Chemical Properties of Carbon Compounds

Combustion

• Carbon burns in oxygen to give carbon dioxide, heat, and light.

• Most carbon compounds release a large amount of heat and light on burning.
  $C + O<em>2 \rightarrow CO</em>2 + \text{heat and light}$
  $CH<em>4 + 2O</em>2 \rightarrow CO<em>2 + 2H</em>2O + \text{heat and light}$
  $CH<em>3CH</em>2OH + 3O<em>2 \rightarrow 2CO</em>2 + 3H_2O + \text{heat and light}$

• Saturated hydrocarbons give a clean flame, while unsaturated compounds give a yellow flame with black smoke.

• Incomplete combustion results in a sooty flame.

• Coal and petroleum combustion produces sulfur and nitrogen oxides, which are major pollutants.

Oxidation

• Carbon compounds can be easily oxidized on combustion.

• Alcohols can be converted to carboxylic acids.
  $CH<em>3CH</em>2OH \xrightarrow{\text{alkaline } KMnO<em>4 + \text{heat}} CH</em>3COOH$

• Oxidizing agents add oxygen to other substances (e.g., alkaline potassium permanganate).

Addition Reaction

• Unsaturated hydrocarbons add hydrogen in the presence of catalysts (Pd, Ni) to give saturated hydrocarbons.
• This is used in the hydrogenation of vegetable oils.
• Vegetable oils have long unsaturated carbon chains, while animal fats have saturated carbon chains.

Substitution Reaction

• Saturated hydrocarbons are fairly unreactive but undergo substitution in the presence of sunlight.
• Chlorine can replace hydrogen atoms one by one.
  $CH<em>4 + Cl</em>2 \xrightarrow{\text{sunlight}} CH_3Cl + HCl$

Some Important Carbon Compounds: Ethanol and Ethanoic Acid

Properties of Ethanol

• Ethanol: Commonly called alcohol, used in alcoholic drinks and medicines.
• It is a good solvent and soluble in water in all proportions.
• Ethanol can cause drunkenness, and pure ethanol can be lethal.

Reactions of Ethanol

• Reaction with sodium: $2Na + 2CH<em>3CH</em>2OH \rightarrow 2CH<em>3CH</em>2O^-Na^+ + H_2$

• Leads to the evolution of hydrogen and the formation of sodium ethoxide.

• Dehydration of ethanol: At 443 K with concentrated sulfuric acid, ethanol produces ethene.
  $CH<em>3CH</em>2OH \xrightarrow{\text{Hot Conc. } H<em>2SO</em>4} CH<em>2=CH</em>2 + H_2O$

• Concentrated sulfuric acid acts as a dehydrating agent.

Properties of Ethanoic Acid

• Ethanoic acid: Commonly called acetic acid, part of the carboxylic acids group.
• 5-8% solution in water is called vinegar (used as a preservative).
• Pure ethanoic acid freezes in cold climates (glacial acetic acid).

Reactions of Ethanoic Acid

• Acidic nature: Carboxylic acids are weak acids, unlike mineral acids like HCl.

• Esterification: Reacts with absolute ethanol in the presence of an acid catalyst to give an ester.
  $CH<em>3COOH + CH</em>3CH<em>2OH \rightleftharpoons CH</em>3COOC<em>2H</em>5 + H_2O$

• Esters are sweet-smelling and used in perfumes and flavoring agents.

• Saponification: Esters react with sodium hydroxide to form alcohol and sodium salt of carboxylic acid.
  $CH<em>3COOC</em>2H<em>5 + NaOH \rightarrow C</em>2H<em>5OH + CH</em>3COONa$

• Reaction with a base: Reacts with sodium hydroxide to give a salt (sodium ethanoate) and water.
  $NaOH + CH<em>3COOH \rightarrow CH</em>3COONa + H_2O$

• Reaction with carbonates and hydrogencarbonates: Produces a salt, carbon dioxide, and water.
  $2CH<em>3COOH + Na</em>2CO<em>3 \rightarrow 2CH</em>3COONa + H<em>2O + CO</em>2$
  $CH<em>3COOH + NaHCO</em>3 \rightarrow CH<em>3COONa + H</em>2O + CO_2$

Soaps and Detergents

Soap Action

• Soap molecules have hydrophilic (water-interacting) and hydrophobic (oil-interacting) ends.
• They form micelles, with the hydrophobic ends towards the oil droplet and the hydrophilic ends facing outward.
• This helps pull out dirt in water, cleaning clothes.

Micelles

• Soap micelles keep the hydrocarbon portion out of the water, forming clusters.
• Oily dirt is collected in the center of the micelle.
• Micelles stay in solution due to ion-ion repulsion.
• Soap solutions appear cloudy due to the scattering of light by the micelles.

Hard Water

• Soaps react with calcium and magnesium salts causing hardness in water, forming scum.
• Detergents are sodium salts of sulfonic acids or ammonium salts that do not form insoluble precipitates.
• Detergents are effective in hard water and used in shampoos and cleaning products.