Unit 14: Hydrocarbons

14.1 Saturated Hydrocarbons (Alkanes)

Alkanes are saturated hydrocarbons, meaning they contain only single carbon-carbon bonds and carbon-hydrogen bonds. They have the general formula CnH2n+2, where 'n' is the number of carbon atoms. Due to their saturated nature, alkanes are generally unreactive and are often referred to as paraffins (from Latin parum affinis, meaning "little affinity").

Preparation Methods

Alkanes can be prepared through various methods, primarily involving reduction or formation of C-C bonds.

• From Haloalkanes (Alkyl Halides):

  Haloalkanes can be reduced to alkanes using different reducing agents.

  • Reduction with HI and Red Phosphorus:

    Alkyl halides react with hydroiodic acid (HI) in the presence of red phosphorus as a catalyst, typically at elevated temperatures (around 150°C), to yield alkanes.

    R-X + 2HI --(Red P, 150°C)--> R-H + I2 + HX
    

    Example: CH3CH2Br + 2HI --(Red P, 150°C)--> CH3CH3 + I2 + HBr (Bromoethane to Ethane)

  • Wurtz Reaction:

    This method is excellent for preparing symmetrical alkanes (those with an even number of carbon atoms). Two molecules of an alkyl halide react with sodium metal in the presence of dry ether to form a higher alkane.

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

    Example: 2CH3Cl + 2Na --(dry ether)--> CH3-CH3 + 2NaCl (Methyl chloride to Ethane)

    Note: If two different alkyl halides are used, a mixture of three alkanes is formed, making the separation difficult and thus limiting its synthetic utility for unsymmetrical alkanes.

• Decarboxylation of Carboxylic Acids:

  Sodium salts of carboxylic acids, when heated with soda lime (a mixture of NaOH and CaO), undergo decarboxylation to form an alkane with one less carbon atom than the original carboxylic acid.

  RCOONa + NaOH --(CaO, heat)--> RH + Na2CO3
  

  The role of CaO is to keep the mixture dry and prevent the fusion of the glass apparatus at high temperatures.

  Example: CH3COONa + NaOH --(CaO, heat)--> CH4 + Na2CO3 (Sodium acetate to Methane)

• Catalytic Hydrogenation of Alkenes and Alkynes:

  Unsaturated hydrocarbons (alkenes and alkynes) can be converted to alkanes by adding hydrogen in the presence of a catalyst like nickel (Ni), platinum (Pt), or palladium (Pd). This process is known as hydrogenation.

  R-CH=CH-R' + H2 --(Ni/Pt/Pd)--> R-CH2-CH2-R' (Alkene to Alkane)
  R-C≡C-R' + 2H2 --(Ni/Pt/Pd)--> R-CH2-CH2-R' (Alkyne to Alkane)
  

  Example: CH2=CH2 + H2 --(Ni, heat)--> CH3-CH3 (Ethene to Ethane)

Chemical Properties

Alkanes are relatively unreactive due to the strength of their C-C and C-H sigma bonds and their nonpolar nature. Their characteristic reactions are substitution and oxidation.

• Substitution Reactions (Free Radical Mechanism):

  In the presence of UV light or high temperatures, alkanes undergo substitution reactions where hydrogen atoms are replaced by other atoms or groups.

  • Halogenation:

    Alkanes react with halogens (Cl2, Br2) in the presence of UV light or heat. This is a free radical substitution reaction.

    CH4 + Cl2 --(UV light)--> CH3Cl + HCl (Chloromethane)
    

    Further substitution can occur, leading to polychlorinated products like dichloromethane (CH2Cl2), chloroform (CHCl3), and carbon tetrachloride (CCl4).

    The mechanism involves three steps:

    1. Chain Initiation: Halogen molecule breaks into free radicals (e.g., Cl-Cl --(UV)--> 2Cl•).
    2. Chain Propagation: A halogen radical abstracts a hydrogen from the alkane (e.g., CH4 + Cl• --> •CH3 + HCl), and the alkyl radical reacts with a halogen molecule (e.g., •CH3 + Cl2 --> CH3Cl + Cl•).
    3. Chain Termination: Two radicals combine to form a stable molecule (e.g., Cl• + Cl• --> Cl2, •CH3 + •CH3 --> CH3-CH3, •CH3 + Cl• --> CH3Cl).
  • Nitration:

    Alkanes react with concentrated nitric acid at high temperatures (around 400-500°C) to form nitroalkanes. This is also a free radical process.

    R-H + HNO3(conc.) --(heat)--> R-NO2 + H2O
    

    Example: CH4 + HNO3 --(400-500°C)--> CH3NO2 + H2O (Nitromethane)

  • Sulphonation:

    Alkanes react with fuming sulfuric acid (H2SO4 containing SO3) at high temperatures to form alkane sulfonic acids.

    R-H + H2SO4(fuming) --(heat)--> R-SO3H + H2O
    

    Example: CH4 + H2SO4(fuming) --(heat)--> CH3SO3H + H2O (Methanesulfonic acid)

• Oxidation of Ethane:

  Alkanes undergo combustion (complete oxidation) in the presence of excess oxygen, producing carbon dioxide and water, along with a large amount of heat. This makes them excellent fuels.

  2C2H6 + 7O2 --(complete combustion)--> 4CO2 + 6H2O + Heat
  

  In a limited supply of oxygen, incomplete combustion occurs, producing carbon monoxide (CO) or even carbon black (C) instead of carbon dioxide.

  Alkanes can also undergo controlled oxidation under specific conditions to yield alcohols, aldehydes, or carboxylic acids, but these reactions often require specific catalysts and conditions.

14.2 Alkenes

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C). Their general formula is CnH2n. The presence of the pi (π) bond in the double bond makes alkenes more reactive than alkanes, primarily undergoing addition reactions.

Preparation Methods

Alkenes are commonly prepared by elimination reactions.

• Dehydration of Alcohols:

  Alcohols lose a molecule of water when heated with strong dehydrating agents like concentrated sulfuric acid (conc. H2SO4) or phosphoric acid (H3PO4) to form alkenes.

  CH3CH2OH --(conc. H2SO4, 170°C)--> CH2=CH2 + H2O (Ethanol to Ethene)
  

  This is an elimination reaction (E1 or E2 mechanism) where the hydroxyl group and a hydrogen atom from an adjacent carbon are removed.

• Dehydrohalogenation of Alkyl Halides:

  Alkyl halides undergo dehydrohalogenation (removal of HX) when heated with a strong base like alcoholic potassium hydroxide (alc. KOH). A hydrogen atom from a carbon adjacent to the halogen-bearing carbon, along with the halogen, is eliminated to form an alkene.

  CH3CH2Br + alc. KOH --(heat)--> CH2=CH2 + KBr + H2O (Bromoethane to Ethene)
  

  Note: Alcoholic KOH is crucial; aqueous KOH would favor substitution (formation of alcohol).

• Catalytic Hydrogenation of Alkynes:

  Alkynes can be selectively hydrogenated to alkenes using a poisoned catalyst, such as Lindlar's catalyst (palladium on calcium carbonate poisoned with lead acetate and quinoline) or Na/liquid NH3.

  R-C≡C-R' + H2 --(Lindlar's catalyst)--> R-CH=CH-R' (Alkene, predominantly cis)
  

  Example: CH3-C≡C-CH3 + H2 --(Lindlar's catalyst)--> CH3-CH=CH-CH3 (2-Butyne to cis-2-Butene)

Chemical Properties (Electrophilic Addition Reactions)

The electron-rich double bond in alkenes readily undergoes electrophilic addition reactions, where the pi bond breaks and two new sigma bonds are formed.

• Addition of HX (Hydrogen Halides - HCl, HBr, HI):

  Hydrogen halides add across the double bond. The regioselectivity of this addition is governed by Markovnikov's Rule.

  Markovnikov's Rule: When an unsymmetrical reagent (like HX) adds to an unsymmetrical alkene, the positive part of the reagent (H+) adds to the carbon atom of the double bond that has more hydrogen atoms, and the negative part (X-) adds to the carbon atom with fewer hydrogen atoms.

  Example (Markovnikov's Addition): CH3-CH=CH2 + HBr --> CH3-CH(Br)-CH3 (Propene to 2-Bromopropane)

  • Peroxide Effect (Anti-Markovnikov Addition):

    In the presence of organic peroxides, the addition of HBr to unsymmetrical alkenes proceeds via a free radical mechanism, leading to an anti-Markovnikov product. This effect is observed only with HBr, not with HCl or HI.

    CH3-CH=CH2 + HBr --(peroxide)--> CH3-CH2-CH2Br (Propene to 1-Bromopropane)
• Addition of H2O (Hydration):

  Alkenes react with water in the presence of an acid catalyst (like H2SO4) to form alcohols. This reaction also follows Markovnikov's Rule.

  CH2=CH2 + H2O --(H2SO4)--> CH3CH2OH (Ethene to Ethanol)
  

  Example: CH3-CH=CH2 + H2O --(H2SO4)--> CH3-CH(OH)-CH3 (Propene to Propan-2-ol)

• Addition of O3 (Ozonolysis):

  Ozonolysis is a powerful method for determining the position of the double bond in an alkene. Alkenes react with ozone (O3) to form an unstable ozonide, which is then cleaved by reductive workup (e.g., with Zn/H2O or (CH3)2S) to yield aldehydes and/or ketones.

  R-CH=CH-R' + O3 --> Ozonide --(Zn/H2O)--> RCHO + R'CHO
  

  Example: CH3-CH=CH-CH3 + O3 --> Ozonide --(Zn/H2O)--> 2CH3CHO (2-Butene to Acetaldehyde)

  If one of the carbons of the double bond is substituted with alkyl groups, a ketone will be formed.

• Addition of H2SO4 (Sulphuric Acid):

  Alkenes react with cold, concentrated sulfuric acid to form alkyl hydrogen sulfates, following Markovnikov's Rule.

  CH2=CH2 + H2SO4(conc.) --> CH3CH2OSO3H (Ethene to Ethyl hydrogen sulfate)
  

  These alkyl hydrogen sulfates can be hydrolyzed by boiling with water to produce alcohols.

14.3 Alkynes

Alkynes are unsaturated hydrocarbons containing at least one carbon-carbon triple bond (C≡C). Their general formula is CnH2n-2. The triple bond consists of one sigma bond and two pi bonds, making alkynes even more unsaturated and reactive than alkenes.

Preparation Methods

• From Carbon and Hydrogen:

  Acetylene (ethyne), the simplest alkyne, can be prepared by directly combining carbon and hydrogen at very high temperatures in an electric arc.

  2C + H2 --(electric arc)--> C2H2 (Acetylene)
• From Vicinal Dihalides (1,2-Dibromoethane):

  Vicinal dihalides (dihalides where halogens are on adjacent carbon atoms) undergo double dehydrohalogenation when treated with a strong base like alcoholic potassium hydroxide (alc. KOH) to form alkynes.

  CH2Br-CH2Br + 2alc. KOH --(heat)--> CH≡CH + 2KBr + 2H2O (1,2-Dibromoethane to Ethyne)
  

  This reaction typically requires stronger conditions (higher temperature or stronger base) than for alkene formation.

• From Chloroform/Iodoform:

  Chloroform or iodoform can react with silver powder to produce ethyne.

  2CHCl3 + 6Ag --(heat)--> CH≡CH + 6AgCl (Chloroform to Ethyne)
  2CHI3 + 6Ag --(heat)--> CH≡CH + 6AgI (Iodoform to Ethyne)

Chemical Properties

Alkynes undergo addition reactions similar to alkenes, but they can add two molecules of a reagent due to the presence of two pi bonds. Terminal alkynes also exhibit acidic character.

• Addition Reactions:
  • Addition of H2 (Hydrogenation):

    Alkynes can be fully hydrogenated to alkanes by adding two molecules of hydrogen in the presence of a catalyst.

    R-C≡C-R' + 2H2 --(Ni/Pt/Pd)--> R-CH2-CH2-R' (Alkyne to Alkane)
    

    As seen in alkene preparation, selective hydrogenation to alkenes is possible with poisoned catalysts.

  • Addition of HX (Hydrogen Halides):

    Hydrogen halides add across the triple bond, typically in two steps, following Markovnikov's Rule at each step.

    R-C≡CH + HX --> R-CH=CHX (Vinyl halide)
    R-CH=CHX + HX --> R-CX2-CH3 (Geminal dihalide)
    

    Example: CH≡CH + HCl --> CH2=CHCl (Vinyl chloride)
    CH2=CHCl + HCl --> CH3-CHCl2 (1,1-Dichloroethane)

  • Addition of H2O (Hydration):

    Alkynes react with water in the presence of mercuric sulfate (HgSO4) and dilute sulfuric acid (H2SO4) to form enols, which immediately tautomerize to more stable aldehydes or ketones.

    CH≡CH + H2O --(HgSO4/H2SO4)--> [CH2=CHOH] (enol) --> CH3CHO (Ethyne to Acetaldehyde)
    

    For terminal alkynes other than ethyne, this reaction yields ketones following Markovnikov's rule.

    Example: CH3-C≡CH + H2O --(HgSO4/H2SO4)--> [CH3-C(OH)=CH2] (enol) --> CH3-CO-CH3 (Propyne to Propanone)

• Acidic Nature of Terminal Alkynes:

  Terminal alkynes (those with a hydrogen atom directly attached to a triply bonded carbon, e.g., R-C≡C-H) are weakly acidic. This is because the sp-hybridized carbon atom in the triple bond is more electronegative than sp2 or sp3 carbons, allowing it to pull electron density from the C-H bond, making the hydrogen atom slightly acidic and removable as a proton. Internal alkynes (R-C≡C-R') do not exhibit this acidity.

  • Action with Sodium:

    Terminal alkynes react with active metals like sodium to form sodium acetylides, liberating hydrogen gas.

    2R-C≡CH + 2Na --> 2R-C≡CNa + H2 (Sodium acetylide)
    

    Example: 2CH≡CH + 2Na --> 2CH≡CNa + H2 (Sodium acetylide)

  • Action with Ammoniacal Silver Nitrate (Tollens' Reagent):

    Terminal alkynes react with ammoniacal silver nitrate solution to form a white precipitate of silver acetylides.

    R-C≡CH + AgNO3 + NH4OH --> R-C≡CAg(white ppt) + NH4NO3 + H2O
    

    Example: CH≡CH + 2AgNO3 + 2NH4OH --> AgC≡CAg(white ppt) + 2NH4NO3 + 2H2O (Silver acetylide)

  • Action with Ammoniacal Cuprous Chloride:

    Terminal alkynes react with ammoniacal cuprous chloride solution to form a red precipitate of cuprous acetylides.

    R-C≡CH + Cu2Cl2 + NH4OH --> R-C≡CCu(red ppt) + 2NH4Cl + H2O
    

    Example: CH≡CH + Cu2Cl2 + 2NH4OH --> CuC≡CCu(red ppt) + 2NH4Cl + 2H2O (Cuprous acetylide)

14.4 Test of Unsaturation

Alkanes are saturated, while alkenes and alkynes are unsaturated. This difference in saturation can be detected using specific chemical tests.

• Bromine Water Test:

  This test is used to detect the presence of carbon-carbon double or triple bonds. When orange-brown bromine water (Br2 dissolved in water or CCl4) is added to an alkene or alkyne, the bromine adds across the multiple bond, and the orange-brown color of bromine disappears.

  R-CH=CH-R' + Br2(aq) --> R-CHBr-CHBr-R' (Decolorization)
  

  Alkanes, being saturated, do not react with bromine water in the absence of UV light, so the color persists.

• Baeyer's Test (Cold, Dilute Alkaline KMnO4):

  This test uses cold, dilute, alkaline potassium permanganate (KMnO4) solution, which is purple. Alkenes and alkynes react with KMnO4, causing it to decolorize and form a brown precipitate of manganese dioxide (MnO2). This is an oxidation reaction where the multiple bond is cleaved or hydroxylated.

  3R-CH=CH-R' + 2KMnO4 + 4H2O --> 3R-CH(OH)-CH(OH)-R' + 2MnO2(brown ppt) + 2KOH (Decolorization)
  

  Alkanes do not react with cold, dilute KMnO4, so the purple color remains unchanged.

14.5 Comparative Studies

Physical Properties of Alkanes, Alkenes, and Alkynes

While their chemical reactivities differ significantly, hydrocarbons also show variations in their physical properties based on their structure and saturation.

• Boiling Point:
  • For a given number of carbon atoms, the boiling points generally increase with molecular mass.
  • Branching in alkanes decreases the boiling point due to reduced surface area for van der Waals forces.
  • Generally, for hydrocarbons with the same number of carbons, the boiling points are comparable, but there can be subtle differences. Alkynes tend to have slightly higher boiling points than their corresponding alkanes or alkenes due to their more compact linear structure allowing for better packing and stronger van der Waals forces.
  • Order of boiling points (general trend for similar carbon count): Alkynes > Alkanes ≈ Alkenes.
• Solubility:
  • All hydrocarbons (alkanes, alkenes, alkynes) are nonpolar compounds.
  • They are virtually insoluble in water (a polar solvent) but are readily soluble in nonpolar organic solvents such as benzene, ether, and carbon tetrachloride.
  • This follows the "like dissolves like" principle.
• Density:
  • All hydrocarbons are less dense than water (density < 1 g/mL).
  • Their density generally increases with increasing molecular mass (i.e., with an increasing number of carbon atoms).
  • For a given number of carbons, their densities are quite similar.

Kolbe's Electrolysis for Preparation of Hydrocarbons

Kolbe's electrolysis is a method for preparing symmetrical alkanes (and sometimes alkenes or alkynes) by the electrolysis of an aqueous solution of the sodium or potassium salt of a carboxylic acid.

Mechanism:

• At Anode: The carboxylate ion (RCOO-) loses an electron to form an acyloxy radical, which then decarboxylates to form an alkyl radical. Two alkyl radicals then combine to form an alkane. 2RCOO- --> 2RCOO• + 2e-
2RCOO• --> 2R• + 2CO2
2R• --> R-R
• At Cathode: Water is reduced to produce hydrogen gas and hydroxide ions. 2H2O + 2e- --> H2 + 2OH-

Example: Electrolysis of sodium acetate solution yields ethane.

This method is particularly useful for synthesizing symmetrical alkanes with an even number of carbon atoms.