Hydrocarbons

• organic compounds composed of carbon and hydrogen atoms only

• most hydrocarbons are used as fuel (petroleum products) because of its flammable property

• special property to catenate forming seemingly endless chain of carbons (potential combinations estimated up to 10⁶)

• non-polar in nature, thereby most of them are insoluble in water

ex. An oil spill in ocean.

• Petroleum does not mix with water due to opposite polarities.

Classified based on the presence of multiple bonds:

1. Saturated - carbon atoms are bonded with SINGLE bond (1σ)

possesses one sigma bond which is relatively strong type of bond

2. Unsaturated - carbon atoms are bonded with DOUBLE (1σ and 1π) or TRIPLE (1σ and 2π) bonds

   • possesses a sigma and pi bonds; pi bonds are relatively weak type of bond

   • this makes unsaturated hydrocarbons more reactive than saturated ones

   Gasoline is composed of mixtures of paraffins, olefins, naphthenes and benzene.

Classification based on their structures:

1. Open-chain

   • linear structure

   • could be straight or branched chains

2. Closed-chain

   • ring structure

   • could be aliphatic or aromatic

ALIPHATIC HCs (open-chain)

AROMATIC HCs

• are cyclic hydrocarbons with delocalized pi electrons between carbon atoms of ring

• their natural characteristics are described as aromaticity

• contain conjugated pi bonds, that is alternating double bonds

• this special arrangement of pi bonds exhibits resonance (same structure kahit bali-baliktarin), which causes the conjugative stability of the aromatic compounds

According to 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= n, where n = 0 or any positive whole integer

• final answer should be whole number

example:

4n + 2 = 6

4n = 6 - 2

4n = 4

/ 4 = / 4

n = 1

criteria for aromaticity:

• must be cyclic (clove) in structure

• must be flat or planar in configuration

• must have conjugated double bonds

• must follow Hückel’s rule of aromaticity

*tumatalon-talon yung electrons kaya may space in bet.

Benzene (most common aromatic compound)

• generally undergoes electrophilic aromatic substitution reactions

  *hindi nasisira yung structure

• it undergoes substitution reaction wherein the ring system is not destroyed and therefore resist addition of substituent groups within the pi bonds

• this explains the reason why aromatic compounds resembles reactions of saturated rather than unsaturated hydrocarbons

Experiment results

Ignition Test

combustion reaction

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

incomplete combustion reaction:

• hydrocarbons react with insufficient amount of oxygen, instead of producing CO2, produces CO, H₂O, and C as products

When a fuel (e.g. gasoline, kerosene) burns in plenty of air, it receives enough oxygen for complete combustion, thereby producing non-luminous flame

When a fuel burns in a limited space (e.g. car engine), there is no enough oxygen to completely oxidize the fuel, thereby producing black smoke/soot

Baeyer’s Test for Unsaturation

• named after Adolf von Baeyer

• used as a qualitative test for unsaturation

• 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

• used as a qualitative test for unsaturation

• bromine is dissolved either in dichloromethane (CH₂Cl₂), chloroform (CHCl₃), or carbon tetrachloride (CCl₄) 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

addition reaction

• an alkene/alkyne reacts with Br2 producing products of a dibromo for alkene and ultimately a tetrabromo for alkyne

• the weak pi bond in alkenes and alkynes are susceptible to addition reactions

Test for Aromaticity (nitration)

• benzene is nitrated using a nitrating acid (HNO3 + H2SO4) to produce nitrobenzene

• this nitration test predicts the presence of aromatic ring through the formation of yellow colored layer in solution after warming the test sample

• only 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 (transfer proton) 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 → nitro benzene

SUMMARY: