Alkynes, Alkene, Alkanes, Aromatic

Topics: Hydrogen, Benzene, Methane Pages: 13 (1698 words) Published: October 8, 2012
Alkane, Alkene, Alkyne & Aromatic

Learning Objectives
2. 3.

Determine the IUPAC name, common name and structure of an alkanes, alkenes and cycloalkanes. Identify the physical properties of alkanes. Describe briefly natural sources and importance of alkanes Describe reactions of alkanes Propose a mechanism on free radicals substitution

4. 5.

6. Describe the preparation and reactions of alkenes.

7. Propose mechanism on preparation; dehydration of


8. Define carbocation, electrophile and Markovnikov’s Rule. 9. Propose mechanism on reaction addition of cold sulphuric

acid and Hydrogen halide


Nomenclature of Alkanes

Sources & Importance of Alkanes
A. Natural Gas  Consists of 90-95% methane with lesser amounts of ethane and other low-molecular-weight hydrocarbons. B. Petroleum  Liquid mixture of literally thousands of different hydrocarbons.  The most important processes in the petroleum refining are fractional distillation, catalytic cracking and catalytic reforming. C. Coal  Raw material for the production of organic compounds  Synthesis gas, a mixture of carbon monoxide and hydrogen, can be derived from natural gas, coal or petroleum.

Fractional distillation of petroleum. The lighter, more volatile fractions are removed from higher up the column, and the heavier, less volatile fractions are removed from lower down

The fractions obtained from crude oil

Physical Properties
 Low molecular-weight alkanes are gases at room temperature

and atm pressure. Higher molecular weight alkanes are liquids  Alkanes are non-polar compounds and the only forces of

attraction between their molecules are dispersion forces, weak electrostatic interactions between temporary induced dipoles of adjacent atoms or molecules.

 Among a set of alkanes constitutional isomers, the least

branched isomer generally has the highest boiling point; the most branched isomer generally has the lowest boiling point.  Branched-

chain alkanes unbranched-chain isomers.






 Alkanes cannot form hydrogen bonds with water and are

therefore insoluble.

1. Combustion
Oxidation of alkanes to carbon dioxide and water is the basis for their use as energy sources of heat and power. e.g.

CH4 (g) + 2O2(g)  CO2(g) + 2H2O (l)

; -890 kJmol-1

Incomplete combustion gives the poisonous gas, carbon monoxide

To obtain “more” of lower molecular mass alkanes, which are more volatile. e.g. Al2O3/SiO2

2CH3CH2CH3 (g)
Vapour passed at


CH4 (g) + CH3CH=CH2 (g) + CH2=CH2 (g) + H2

When treated with halogens, under elevated temperature or exposure to sunlight, alkanes undergo a substitution reaction to yield haloalkanes. CH4 + Cl2 light or heat

CH3Cl + CH2Cl2 + CHCl3 + CCl4 + HCl

Mechanism of the chlorination of methane
Chain initiation Cl2 + CH4 CH3 + Cl2 2Cl HCl + CH3 CH3Cl + Cl Cl2 C2H6 CH3Cl Chain propagation Cl

Chain termination 2Cl 2 CH3 Cl + CH3


! mechanism

! mechanism

! mechanism


Nomenclature of Alkenes

Alkenes are more reactive than alkanes

The double bond of alkenes consists of a σ-bond and a π-bond. The π-bond is above and below the line joining the nuclei of the bonded carbon atoms and therefore accessible to electrophiles. This is why C=C bond is reactive.

Sources of Alkenes
Dehydration of alcohol, RCH2CH2OH
(a) Al2O3 at 400oC or (b) Excess conc. H2SO4, 170oC or better, (c) H3PO4 (l)

Dehydrohalogenation of haloalkanes
NaOH/KOH in ethanol

! mechanism

Alkanes from petroleum oil
‘CRACKING’ is used industrially. Alkane vapour is passed over a heated catalyst, e.g. SiO2 +Al2O3

(This reaction gives a fair Yield with a 2o halogenoalkane, A good yield with a 3o halogenaoalkane and a very poor yield with a 1o halogenoalkane

RCH=CH2 An alkene

Physical Properties of Alkenes...
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