Hydrocarbons: Saturated, Unsaturated, Aromatic

• A hydrocarbon consists of carbon and hydrogen.

Saturated Hydrocarbons

• Saturated hydrocarbons contain only carbon-carbon single bonds.
• Ethane (CH3-CH3) exemplifies a saturated hydrocarbon.

Unsaturated Hydrocarbons

• Unsaturated hydrocarbons possess carbon-carbon double (C=C) or triple bonds.
• Examples include ethene (CH2=CH2) and ethyne (HC≡HC).

Aromatic Hydrocarbons

• Benzene and its derivatives are classified as aromatic compounds.

Alicyclic Compounds

• Alicyclic or carbocyclic compounds are cyclic compounds composed solely of carbon atoms.
• examples include cyclopropane, cyclobutane and cyclohexane

Heterocyclic Compounds

• Heterocyclic compounds are cyclic structures that incorporate carbon and at least one other element (e.g., N, S, or O) within the ring.
• Examples include Oxirane and Pyridine

Alkanes

• Alkanes are simple organic compounds made of carbon and hydrogen.
• They share the general formula CₙH₂ₙ₊₂ (where n = 1, 2, 3, etc.).
• Carbon atoms in alkane molecules are connected by single covalent bonds, making them saturated hydrocarbons.
• Alkanes have strong carbon-carbon and carbon-hydrogen bonds.
• As a result, alkanes are chemically relatively inert and sometimes referred to as paraffins, meaning "little affinity" in Latin.
• The first three members of the alkane family are methane, ethane, and propane.

Nomenclature of Alkanes

• In the IUPAC system the longest continuous carbon chain with single bonds is selected as the parent chain.
• Numbering starts from the end that includes the maximum number of carbons.
• Examples include methane (CH₄), ethane (C₂H₆), and propane (C₃H₈).

Physical properties of Alkanes

• Alkanes with 1-4 carbons (methane, ethane, propane, and butane) exist as gases due to weak Van der Waals forces.
• Alkanes with C₅ to C₁₇ are liquids, while those with 18+ carbon atoms are solids at 298 K.
• Alkanes are generally colorless and odorless
• Alkanes are insoluble in water and polar solvents, but dissolve in non-polar solvents like ether, benzene, and carbon tetrachloride.
• Boiling points of straight-chain alkanes increase with the number of carbon atoms due to stronger intermolecular Van der Waals forces.

Chemical Properties of Alkanes

• Halogenation occurs at high temperatures (300-500°C) or with diffused sunlight or ultraviolet light.
• Example: CH₄ + CH₂ → CH₃Cl + HCl
• Combustion involves complete oxidation of alkanes to carbon dioxide and water when heated in air, releasing energy exothermically.
• Example: CH₄ + 2O₂ → CH₃Cl + 2H₂O
• Controlled Oxidation of methane and dioxygen at 100 atm over a heated copper tube at 523 K yields methanol.
• Example: 2CH₄ + O₂ → C₂H₃OH
• Aromatization involves converting aliphatic compounds into aromatic ones, also known as reforming.
• Reaction with Steam at 1273 K with nickel catalyst forms carbon monoxide and dihydrogen.
• Example: CH₄ + H₂O → CO + 3H₂; used for industrial dihydrogen production.

Conformations

• Sawhorse projections represent eclipsed and staggered conformations of ethane.
• Newman's projections depict eclipsed and staggered conformations of ethane from a head-on view.

Alkenes

• Alkenes are hydrocarbons with at least one carbon-carbon double bond (C=C).
• They have the general formula CₙH₂ₙ and are known as olefins (oil forming).
• Ethene (C₂H₄) reacts with chlorine to form an oily liquid.

Nomenclature of Alkenes

• Base the hydrocarbon name on the longest carbon chain containing the double bond.
• Number the chain from the end closest to the double bond, indicating its position by the carbon atom from which it originates.
• Precede the parent alkene's name with the double bond's position number, followed by any substituents.
• When a molecule contains two or three double bonds, replace the alkane ending "-ane" with "-diene."

Structure of Alkenes

• In ethylene, the carbon atoms are sp² hybridized.
• These carbons are connected by a σ bond and a π bond.
• The sigma (σ) bond comes from the head-on overlap of sp² hybridized orbitals of the two sp² hybrid orbitals.
• The pi (π) bond is formed by the lateral overlap of the two 2p orbitals of the two carbon atoms.
• Ethylene is a planar molecule.

Physical Properties of Alkenes

• The first three alkene members are gases, the next fourteen are liquids, and higher members are solids.
• Ethene is a colorless gas with a slightly sweet odor
• Other alkenes are colorless and odorless, insoluble in water, and fairly soluble in non-polar solvents like benzene and petroleum ether.
• Alkenes exhibit an increase in boiling point from 20-30K for every additional CH₂ group.

Chemical Properties of Alkenes

• Alkenes react with one mole of dihydrogen gas in the presence of Ni (at 200-250°C), Pt, or Pd (at room temp) catalysts to give an alkane.
• Example: CH₂=CH₂ + H-H → CH₃-CH₃
• Halogens (bromine or chlorine) add to alkenes to form vicinal dihalides in CCl₄ solvent.
• Reactivity order: F > Cl > Br > I.
• Example: CH₂=CH₂ + Br-Br → Br-CH₂-CH₂-Br
• Hydrogen halides (HCl, HBr, HI) add to alkenes to form alkyl halides.
• Reactivity order: HI > HBr > HCl
• Example: CH₂=CH₂ + H-Br → CH₃-CH₂-Br

Markovnikov Rule

• The negative part of an adding molecule attaches to the carbon atom of an unsymmetrical alkene with the fewest hydrogen atoms or largest number of substituents.

Anti-Markovnikov Rule

• In the presence of peroxide, HBr is added to unsymmetrical alkene propene in the opposite way as predicted by the Markovnikov rule.
• This reaction is also called Peroxide or Kharash effect.
• Example: CH₃CH=CH₂ + HBr → CH₃-CH₂-CH₂-Br

Polymerization

• Polythene is obtained by combining many ethene molecules at high temperature/pressure with catalyst.
• The large molecules obtained are called polymers.
• Example: n(CH₂=CH₂) → (-CH₂-CH₂-)ₙ

Ozonolysis

• Ozonolysis involves adding ozone to an alkene, then cleaving the ozonide with Zn-H₂O to form smaller molecules.
• Example: CH₃CH=CH₂ + O₃ → CH₃-CH(O-O)CH₂ → CH₃CHO + HCHO

Oxidation

• Reaction of alkenes with cold, dilute, aqueous potassium permanganate (Baeyer's reagent) makes vicinal glycols.
• Example: CH₂=CH₂ + H₂O + O → CH₂(OH)-CH₂(OH).

Addition of Sulphuric Acid

• Cold concentrated sulphuric acid adds to alkenes to form alkyl hydrogen sulphate.
• Example: CH₂=CH₂ + H-O-S(O)₂-OH → CH₃-CH₂-O-SO₂-OH

Alkynes

• Alkynes are characterized by a triple bond in their molecules with the general formula CₙH₂ₙ₋₂.
• They’re also called Acetylenes, with acetylene (HC≡CH) as the most important member.

Alkyne structure

• Ethyne (HC≡CH) is a linear molecule with sp-hybridized carbon atoms.
• Each carbon atom is attached to each other with a σ-bond and two π-bonds.
• The sigma (σ) bond comes from the overlap of two sp-hybrid orbitals.
• The pi (π) bonds are formed from the separate overlap of the two p-orbitals from the two adjacent carbon atoms

Alkyne Nomenclature

• In the common system, they are named as derivatives of acetylene.
• In IUPAC, they are named as derivatives of corresponding alkanes, with 'ane' replaced by 'yne'.
• The position of the triple bond is indicated by the first triply bonded carbon.

Alkyne Physical Properties

• The first three members (acetylene, propyne, and butynes) are gases.
• The next eight are liquids, while higher members are solids.
• All alkynes are colorless and odorless, except for ethyne, which has an offensive odor.
• Alkynes are weakly polar and nearly insoluble in water, but soluble in organic solvents like ethers, carbon tetrachloride, and benzene.
• Their melting point, boiling point, and density increase with molar mass.

Alkyne Chemical Properties

• Addition of Dihydrogen: Alkynes contain a triple bond, so they add up, two molecules of dihydrogen.
• CH ≡ CH + H₂ → CH₂=CH₂ → CH₃-CH₃
• Addition of Halogens: Alkynes contain a triple bond, so they add up, two molecules of halogen.
• CH ≡ CH + Cl₂ → CH(Cl)=CH(Cl) → CH(Cl)₂-CH(Cl)₂.
• Addition of Hydrogen Halides: Two molecules of hydrogen halides (HCl, HBr, HI) add to alkynes to form gemdihalides (in which two halogens are attached to the same carbon atom).
• CH ≡ CH + HCl → CH₂-CH(Cl)
• Addition of Water: Like alkanes and alkenes, alkynes are also immiscible and do not react with water. However, one molecule of water adds to alkynes on warming with mercuric sulphate and dilute sulphric acid at 333 K to form carbonyl Compounds.
• CH ≡ CH + H₂O → CH₃ – CHO
• Polymerisation: Ethyne on passing through red hot iron tube at 873 K undergoes cyclic polymerisation. Three molecules polymerise to form benzene.
• Oxidation:
• 2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O

Aromatic Hydrocarbons

• Aromatic hydrocarbons are also known as 'arenes'.
• Aromatic compounds are either benzenoids (containing a benzene ring) or non-benzenoids.
• Examples of arenes include benzene, toluene, naphthalene, and biphenyl.
• Benzene and its homologues typically go by their common names are alkyl benzenes.
• Dimethylbenzenes commonly named Xylenes have three isomers: o-xylene, m-xylene, and p-xylene.

Structure of Benzene

• Benzene (C₆H₆) is unsaturated.
• In 1865, Kekulé proposed benzene's cyclic planar structure with six carbons and alternating single-double bonds.
• The Kekule structure indicates the possibility of two isomeric 1,2–dibromobenzenes.
• Kekule's structure fails to explain the stability of benzene.

Resonance and Stability of Benzene

• Kekulé's two structures (A and B) are the main contributing structures.
• A hybrid structure, shown as a circle or dotted circle inside a hexagon, represents benzene.
• The circle stands for six delocalized electrons across the six carbon atoms in the benzene ring.

Aromaticity

• A key feature of sp² hybridized planar rings.
• This feature is where p orbitals permit cyclic delocalization of π electrons.

Conditions of Aromaticity

• Must be cyclic and planar
• Each atom in the aromatic ring must contain a p-orbital. These p-orbitals must be parallel for continuous overlap around the ring.
• The cyclic π molecular orbital must contain (4n + 2) π electrons, following Hückel's rule (n = integer 0, 1, 2, 3, etc.)

Examples of Aromatic Compounds

• Naphthalene (n=2, 10 π electrons), Anthracene (n=3, 14 π electrons), and Phenanthrene (n=3, 14 π electrons).

Preparation of Benzene

• Benzene is commonly sourced commercially from coal tar but can be synthesized in lab settings.
• By passing acetylene through red-hot tube at 500°C (3HC≡CH → Benzene).
• Decarboxylation: By heating sodium salt of benzoic acid with sodalime (NaOH + CaO) (C₆H₅ + NaOH → Benzene + Na₂CO₃).
• By heating phenol with Zinc dust (C₆H₅ + Zn → Benzene + ZnO).

Physical Properities of Benzene

• Benzene is a colourless liquid.
• Benzene is insoluble in water but soluble in alcohol, ether and chloroform
• Itself, benzene is an effective dissolving agent for many substances.
• Benzene burns with a luminous, sooty flame in contrast to alkanes and alkenes which usually burn with a bluish flame.

Addition Reactions

• Under vigorous conditions (high temperature/pressure with nickel catalyst), benzene hydrogenation yields cyclohexane.
• Benzene treated with excess chlorine in the presence of anhydrous AlCl₃ becomes chlorinated into hexachlorobenzene (C₆Cl₆), also known as gammaxane.

Combustion

• When heated in air, benzene burns with sooty flame producing CO₂ and H₂O.
• C₆H₆ + (15/2)O₂ → 6CO₂ + 3H₂O

Directive Influence

• Ortho and para directing groups guide incoming groups to ortho and para positions (e.g., -OH, -NH₂, -NHR, -NHCOCH₃, -OCH₃-CH₃-C₂H₅).
• Meta directing groups guide incoming groups to meta positions (e.g., -NO₂, -CN, -CHO, –COR, –COOH, -COOR, -SO₃H).

Carcinogenicity and Toxicity

• Certain polynuclear hydrocarbons with multiple fused benzene rings are toxic, carcinogenic, and formed via incomplete combustion from tobacco, coal, and petroleum.