Unit 4: Aliphatic and Aromatic Hydrocarbons

1. Alkanes

Alkanes are saturated aliphatic hydrocarbons containing only carbon-carbon and carbon-hydrogen single covalent bonds. They have the general molecular formula CnH2n+2.

Preparation of Alkanes

Alkanes can be synthesized through various chemical routes depending on the starting material:

1. Catalytic Hydrogenation

Unsaturated hydrocarbons (alkenes or alkynes) react with dihydrogen gas in the presence of finely divided metal catalysts such as Nickel (Ni), Platinum (Pt), or Palladium (Pd) to form alkanes.

Reaction: R-CH=CH2 + H2 (in presence of Ni/Pt/Pd) -> R-CH2-CH3

Important Note: Platinum and palladium catalyze the reaction at room temperature, whereas nickel requires elevated temperatures and pressure (Sabatier-Senderens reaction).

2. Wurtz Reaction

Alkyl halides react with metallic sodium in dry ether medium to form higher symmetrical alkanes containing double the number of carbon atoms present in the reactant.

Reaction: 2 R-X + 2 Na (in dry ether) -> R-R + 2 NaX

Exam Notes & Common Mistakes:

• Wurtz reaction is suitable only for the synthesis of symmetrical alkanes (alkanes with an even number of carbon atoms).
• If a mixture of two different alkyl halides (R-X and R'-X) is used, a mixture of three different alkanes (R-R, R'-R', and R-R') is produced, which are difficult to separate due to close boiling points.

3. Kolbe's Synthesis (Electrolytic Method)

An aqueous solution of sodium or potassium salt of a carboxylic acid undergoes electrolysis to yield an alkane with an even number of carbon atoms at the anode.

Anode Reaction: 2 RCOO- -> R-R + 2 CO2 + 2 e-

Cathode Reaction: 2 H2O + 2 e- -> H2 + 2 OH-

Methane cannot be prepared by Kolbe's electrolytic method because the reaction requires coupling of two alkyl radicals at the anode.

4. From Grignard Reagent

Grignard reagents (Alkyl magnesium halides, RMgX) react with compounds containing active hydrogen (such as water, alcohols, or amines) to undergo protonation, forming alkanes.

Reaction: R-MgX + H-OH -> R-H + Mg(OH)X

Because Grignard reagents are highly reactive toward moisture, they must be prepared and handled in strictly anhydrous conditions.

Reactions of Alkanes

Free Radical Substitution: Halogenation

Alkanes react with halogens (Cl2 or Br2) in the presence of ultraviolet light (hν) or high temperatures (573-773 K) to form haloalkanes. The reaction proceeds via a free radical substitution mechanism involving three distinct steps:

1. Chain Initiation: Homolytic cleavage of the halogen molecule by thermal or light energy generates halogen free radicals.

   Cl-Cl + hν -> 2 Cl•

2. Chain Propagation: The halogen radical abstracts a hydrogen atom from the alkane to form an alkyl radical, which subsequently reacts with another halogen molecule to form the alkyl halide product and regenerate a halogen radical.

   CH4 + Cl• -> •CH3 + HCl

   •CH3 + Cl2 -> CH3Cl + Cl•

3. Chain Termination: The reaction terminates when any two free radicals collide and combine, ending the chain reaction.

   Cl• + Cl• -> Cl2

   •CH3 + •CH3 -> CH3-CH3

   •CH3 + Cl• -> CH3Cl

Reactivity Order of Halogens: F2 > Cl2 > Br2 > I2. Fluorination is explosively violent, while iodination is reversible and requires an oxidizing agent (like HNO3 or HIO3) to consume HI.

2. Alkenes and Alkynes

Alkenes and alkynes are unsaturated aliphatic hydrocarbons containing carbon-carbon double (C=C) and triple (C≡C) bonds, respectively.

Preparation of Alkenes and Alkynes

1. Elimination Reactions

a) Dehydrogenation of Alkanes / Alkenes

Removal of dihydrogen (H2) from alkanes at high temperature over metallic catalysts (like Cr2O3/Al2O3) yields alkenes. Dehydrogenation of alkenes similarly yields alkynes.

Reaction: CH3-CH3 (with heat/catalyst) -> CH2=CH2 + H2

b) Dehydrohalogenation of Alkyl Halides

Heating alkyl halides with alcoholic potassium hydroxide (alk. KOH) leads to the elimination of a halogen atom and a beta-hydrogen atom, yielding alkenes.

Reaction: R-CH2-CH2-X + alc. KOH -> R-CH=CH2 + KX + H2O

Saytzeff's Rule (Zaitsev's Rule): In dehydrohalogenation reactions, when more than one alkene can be formed, the preferred major product is the highly substituted alkene—that is, the alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms.

Example of Saytzeff's Rule: Dehydrohalogenation of 2-bromobutane yields 2-butene (81%, major) and 1-butene (19%, minor).

2. Preparation of Acetylene from Calcium Carbide

Acetylene (ethyne) is industrial prepared by reacting calcium carbide (CaC2) with water.

Reaction: CaC2 + 2 H2O -> H-C≡C-H + Ca(OH)2

Calcium carbide is obtained by heating quicklime (CaO) with coke (C) in an electric furnace.

3. Conversion of Acetylene into Higher Alkynes

Acetylene contains acidic hydrogen atoms attached to sp-hybridized carbon atoms. Reaction of acetylene with strong bases like sodium amide (NaNH2) forms sodium acetylide, which reacts with alkyl halides via nucleophilic substitution to yield higher alkynes.

Step 1: H-C≡C-H + NaNH2 (in liquid NH3) -> H-C≡C- Na+ + NH3

Step 2: H-C≡C- Na+ + R-X -> H-C≡C-R + NaX

Reactions of Alkenes and Alkynes

1. Stereospecific Additions: Cis and Trans Addition

a) Cis-Addition (Alkaline KMnO4)

Reaction of alkenes with dilute, cold alkaline potassium permanganate (Baeyer's Reagent) causes syn-hydroxylation, resulting in cis-addition of two hydroxyl (-OH) groups across the double bond to form vicinal glycols. The purple color of KMnO4 decolorizes during this reaction (test for unsaturation).

Reaction: R-CH=CH-R + H2O + [O] (cold alk. KMnO4) -> R-CH(OH)-CH(OH)-R (cis-diol)

b) Trans-Addition (Bromine Addition)

Addition of bromine (Br2 in CCl4) across an alkene or alkyne occurs via anti-addition (trans-addition). The reaction proceeds through a cyclic bromonium ion intermediate, resulting in bromine atoms attaching from opposite faces of the double bond.

Reaction: R-CH=CH-R + Br2 (in CCl4) -> R-CH(Br)-CH(Br)-R (trans-dibromide)

2. Addition of HX (Hydrohalogenation)

a) Markownikoff's Addition

When an unsymmetrical reagent (HX) adds to an unsymmetrical alkene, the negative part (halide ion, X-) attaches to the double-bonded carbon atom that carries the lesser number of hydrogen atoms.

Markownikoff's Rule Statement: The ionic addition of polar molecules to unsymmetrical alkenes involves the preferential formation of the more stable carbocation intermediate (Tertiary > Secondary > Primary).

Example: CH3-CH=CH2 + HCl -> CH3-CH(Cl)-CH3 (2-chloropropane is the major product).

b) Anti-Markownikoff's Addition (Peroxide Effect / Kharasch Effect)

When hydrogen bromide (HBr) is added to an unsymmetrical alkene in the presence of organic peroxides (R-O-O-R), the addition proceeds opposite to Markownikoff's rule: the bromine atom attaches to the carbon carrying more hydrogen atoms.

Reaction: CH3-CH=CH2 + HBr (in presence of peroxides) -> CH3-CH2-CH2Br (1-bromopropane)

Crucial Note: Anti-Markownikoff addition via the peroxide effect operates strictly through a free radical mechanism and is observed only with HBr (not with HCl or HI, due to unfavorable bond dissociation energies and endothermic propagation steps).

3. Hydration

Addition of water across unsaturated bonds:

• Alkenes: Acid-catalyzed hydration of alkenes yields alcohols according to Markownikoff's rule.

  R-CH=CH2 + H2O (in H+) -> R-CH(OH)-CH3

• Alkynes: Hydration of alkynes in the presence of dilute H2SO4 and mercuric sulphate catalyst (HgSO4) at 333 K yields carbonyl compounds via tautomerization of the initial enol intermediate. Acetylene yields acetaldehyde, while higher alkynes yield ketones.

  HC≡CH + H2O (HgSO4 / H2SO4) -> [CH2=CH-OH] -> CH3-CHO

4. Ozonolysis

Unsaturated hydrocarbons react with ozone (O3) to form cyclic ozonides, which upon oxidative or reductive cleavage with zinc dust and water (Zn/H2O) undergo double/triple bond cleavage to produce carbonyl compounds (aldehydes and ketones).

Reaction: R-CH=CH-R' + O3 -> [Ozonide] + Zn/H2O -> R-CHO + R'-CHO + ZnO

Ozonolysis is an important diagnostic reaction used to determine the exact location of carbon-carbon double or triple bonds in unknown unsaturated hydrocarbons.

5. Formation of Metal Acetylides

Terminal alkynes (alkynes with a H-C≡C- group) possess acidic hydrogens and react with heavy metal complex ions to form insoluble metal acetylides.

• Ammoniacal Silver Nitrate (Tollens' Reagent): Terminal alkynes produce a white precipitate of silver acetylide.

  HC≡CH + 2 [Ag(NH3)2]+ -> Ag-C≡C-Ag (white ppt) + 2 NH4+ + 2 NH3

• Ammoniacal Cuprous Chloride: Terminal alkynes produce a red precipitate of cuprous acetylide.

  HC≡CH + 2 [Cu(NH3)2]+ -> Cu-C≡C-Cu (red ppt) + 2 NH4+ + 2 NH3

Non-terminal alkynes (like 2-butyne, CH3-C≡C-CH3) lack acidic hydrogen atoms and do not form metal acetylides. This reaction serves as a qualitative distinguishing test between terminal and non-terminal alkynes.

3. Aromatic Hydrocarbons

Aromatic hydrocarbons (arenes) are cyclic unsaturated compounds characterized by delocalized pi-electron clouds that exhibit exceptional chemical stability due to aromaticity.

Preparation of Benzene

1. From Phenol

Benzene is prepared by distilling phenol with zinc dust. The zinc acts as a reducing agent, abstracting oxygen to yield benzene and zinc oxide.

Reaction: C6H5OH + Zn (distillation) -> C6H6 + ZnO

2. By Decarboxylation

Heating the sodium salt of benzoic acid (sodium benzoate) with soda lime (a mixture of NaOH and CaO in 3:1 ratio) results in the elimination of a molecule of carbon dioxide (as sodium carbonate) to produce benzene.

Reaction: C6H5COONa + NaOH (in presence of CaO, heat) -> C6H6 + Na2CO3

3. From Acetylene (Cyclic Polymerization)

When acetylene gas is passed through a red-hot iron tube at 873 K, three molecules of acetylene undergo cyclic polymerization to form benzene.

Reaction: 3 HC≡CH (red hot Fe tube, 873 K) -> C6H6

4. From Benzene Sulphonic Acid

Superheated steam hydrolysis of benzene sulphonic acid in the presence of dilute mineral acid (like HCl or H2SO4) removes the sulphonic group to regenerate benzene.

Reaction: C6H5SO3H + H2O (superheated steam / H+) -> C6H6 + H2SO4

Reactions of Benzene (Electrophilic Substitution)

Due to high pi-electron density above and below the aromatic ring, benzene predominantly undergoes electrophilic aromatic substitution (EAS) reactions rather than addition reactions, maintaining its aromatic ring stability.

1. Nitration

Benzene reacts with a nitrating mixture (concentrated HNO3 and concentrated H2SO4) at 323-333 K to yield nitrobenzene.

Reaction: C6H6 + HNO3 (conc. H2SO4) -> C6H5NO2 + H2O

Electrophile Generation: Nitronium ion (NO2+) generated via protonation of HNO3 by H2SO4.

2. Halogenation

Benzene reacts with halogens (Cl2 or Br2) in the presence of a Lewis acid catalyst (such as anhydrous FeCl3, FeBr3, or AlCl3) to yield halo-benzenes.

Reaction: C6H6 + Cl2 (anh. FeCl3) -> C6H5Cl + HCl

Electrophile Generation: Chloronium ion (Cl+) formed by reaction of halogen with Lewis acid: Cl2 + FeCl3 -> Cl+ + FeCl4-

3. Sulphonation

Heating benzene with fuming sulphuric acid (oleum) or concentrated H2SO4 introduces a sulphonic acid group to form benzene sulphonic acid.

Reaction: C6H6 + H2SO4 (SO3 / oleum) -> C6H5SO3H + H2O

Electrophile: Neutral Sulphur Trioxide (SO3).

4. Friedel-Crafts Reaction

a) Alkylation

Treatment of benzene with an alkyl halide (R-X) in the presence of anhydrous aluminium chloride (AlCl3) catalyst leads to the substitution of a ring hydrogen by an alkyl group, yielding alkylbenzene.

Reaction: C6H6 + R-X (anh. AlCl3) -> C6H5-R + HX

Electrophile: Carbocation (R+). Rearrangement of carbocations may occur to form more stable products.

b) Acylation

Treatment of benzene with an acyl halide (RCOCl) or acid anhydride in the presence of anhydrous AlCl3 introduces an acyl group (-COR) to yield an aromatic ketone (e.g., acetophenone).

Reaction: C6H6 + RCOCl (anh. AlCl3) -> C6H5-CO-R + HCl

Electrophile: Acylium ion (R-C≡O+), which is resonance stabilized and does not undergo rearrangement.