PRPM112 LAB: EX.7

• HYDROCARBONS


  • organic compounds composed of carbon and hydrogen atoms only
  • most hydrocarbons are used as fuel (petroleum products) because of its flammable property
  • have a special property to catenate forming seemingly endless chain of carbons (potential combinations estimated up to 106
  • are non-polar in nature, thereby most of them are insoluble in water
• • saturated 


  – carbon atoms are bonded with single bond
• • unsaturated


  – carbon atoms are bonded with double or triple bonds
• unsaturated hydrocarbons more reactive than saturated ones
• open-chain


  – acyclic; linear structure; could be straight or branched chains
• • closed-chain 


  – cyclic; ring structure; could aliphatic or aromatic
• pi bonds are relatively weak type of bond
• HYDROCARBONS are generally classified into two: aliphatic and aromatic hydrocarbons.
• ALKANES


  “paraffins”

  single

  sp3

  saturated

  CnH2n + 2

  -ane

  1 σ bond
• ALKENES

  • “olefins”
  • double
  • sp2
  • unsaturated
  • CnH2n
  • -ene
  • 1 σ bond, 1 π bond
• ALKYNES

  • triple
  • sp
  • unsaturated
  • CnH2n - 2
  • -yne
  • 1 σ bond, 2 π bonds
• AROMATIC HCs

  • are cyclic hydrocarbons with delocalized pi electrons between carbon atoms of ring
  • their natural characteristics are described as aromaticity
  • criteria for aromaticity:

  • must be cyclic in structure

  • must be flat or planar in configuration

  • must have conjugated double bonds

  • must follow Hückel’s rule of aromaticity
• Hückel's Molecular Orbital Theory,


  a compound is particularly stable if all of its bonding molecular orbitals are filled with paired electrons
• the number of pi electrons = 4n + 2, where n = 0 or any positive whole integer
• benzene, being the most common example of an aromatic compound, generally undergo electrophilic aromatic substitution reactions
• IGNITION TEST


  benzene = luminous = present soot

  cyclohexane = luminous = present soot

  gasoline = non-luminous = absent soot

  kerosene = non-luminous = absent soot

  n-hexane = luminous = present soot
• combustion reaction


  – hydrocarbon reacts with oxygen to produce carbon dioxide, water, and heat

  𝐶𝑥𝐻𝑦 + 𝑛 𝑂2 ↔ 𝑥 𝐶𝑂2 + 𝑦 2 (𝐻2𝑂)
• LUMINOUS FLAME


  yellow

  incomplete

  present soot

  do not produced much energy; do not produce much heat
• NON-LUMINOUS FLAME


  blue

  complete

  absent soot

  burns efficiently; produces hotter flame
• incomplete combustion reaction


  – hydrocarbons react with insufficient amount of oxygen, instead of producing CO2 , produces CO, H2O and C as products
• BAEYER’S TEST FOR UNSATURATION

  benzene = purple-colored solution

  cyclohexane = purple-colored solution

  gasoline = dark-brown precipitate

  kerosene = reddish-brown precipitate

  n-hexane = purple-colored solution

  
  

  
  

  [*Brown precipitate is a positive result. Most of the time, solution turns to reddish-brown (still indicate a positive result).]
• BAEYER’S TEST FOR UNSATURATION


  • named after Adolf von Baeyer
  • used as a qualitative test for unsaturation (presence of double or triple bonds)
  • Baeyer's reagent is an alkaline solution of potassium permanganate (KMnO4 ) (strong oxidizer)
  • reaction with double or triple bonds (-C=C- or -C≡C-) in an organic material causes the color to fade from purple to brown precipitate
• oxidation reaction 


  – an alkene/alkyne is oxidized by KMnO4 producing products of a diol (for alkene), an alkane with four hydroxyl groups (for alkyne), a manganese dioxide (MnO2 ) and permanganate ion (MnO4 2- )
• BROMINE TEST FOR UNSATURATION

  benzene = yellow-colored solution

  cyclohexane = yellow-colored solution

  gasoline = clear pinkish solution

  kerosene =clear colorless solution

  n-hexane = yellow-colored solution
• BROMINE TEST FOR UNSATURATION 


   used as a qualitative test for unsaturation (presence of double or triple bonds)
• addition reaction 


  – an alkene/alkyne reacts with Br2 producing products of a dibromo for alkene and ultimately a tetrabromo for alkyne
• TEST FOR AROMATICITY (NITRATION)

  benzene = pale yellow-colored layer in solution

  cyclohexane = clear colorless solution

  gasoline = deep yellow-colored layer in solution

  kerosene =yellow-colored layer in solution

  n-hexane = clear colorless solution

  
  

  [*Yellow colored layer in solution is a positive result.]
• benzene is nitrated using a nitrating acid (HNO3 + H2SO4 ) to produce nitrobenzene
• nitration test predicts the presence of aromatic ring through the formation of yellow colored layer in solution after warming the test sample
• aromatic compounds will proceed through electrophilic aromatic substitution
• electrophilic aromatic substitution reaction:

  ➢ nitration of benzene firstly involves the formation of a very powerful electrophile, the nitronium ion, which is linear

  ➢ this occurs following the interaction of sulfuric and nitric acid

  ➢ sulfuric acid is stronger and it protonates the nitric acid on the OH group so that a molecule of water can leave

  ➢ benzene attacks the positively charged nitrogen atom of the electrophile, where one of the N=O bonds is broken at the same time

  ➢ this is followed by rapid loss of a proton to regenerate the aromaticity
•  bromine is dissolved either in dichloromethane, chloroform, or carbon tetrachloride and the alkene/alkyne sample is added to it
•  reaction with double or triple bonds (-C=C- or -C≡C-) in an organic material causes the color to fade from brown to clear colorless solution
• weak pi bond in alkenes and alkynes are susceptible to addition reactions