Organic Chemistry (Alkanes and Alkenes)


Organic chemistry is the study of carbon compounds except for the oxides of carbon, carbonates, hydrogencarbonates, and carbides of metals. Organic compounds are grouped into groups with the same general formula, same functional group and same chemical properties. Such a group is called a homologous series.

Homologous series

A homologous series is a group of compounds with the same general formula and same chemical properties. Each member of a homologous differs from the next member by the presence of an additional -CH2 group. Examples of homologous series are:

  • Alkanes
  • Alkenes
  • Alcohols
  • Carboxylic acids
  • Esters
  • e.t.c

Hydrocarbons

Hydrocarbons are organic compounds which contain carbon and hydrogen only. Alkanes and alkenes are hydrocarbons. Alcohols, Carboxylic acids and Esters are not hydrocarbons because they also contain oxygen.

Alkanes

Alkanes are hydrocarbons with the general formula CnH2n+2 whereby n is the number of carbon atoms in the compound.

Examples of alkanes are:

Name Formula m.p.(°C) b.p.(°C)
Methane CH4 -182 -162
Ethane C2H6 -183 -88
Propane C3H8 -188 -42
Butane C4H10 -138 0
Pentane C5H12 -130 36
Hexane C6H14 -95 69
Heptane C7H16 -90 98
Octane C8H18 -57 126
Nonane C9H20 -54 151
Decane C10H22 -30 174

Structural formulae shows the exact arrangement of the atoms in a molecule. For example:

Name Molecula Formula Structural formula
Methane CH4 CH4
Ethane C2H6 CH3CH3
Propane C3H8 CH3CH2CH3

Displayed structural formula or graphic formula shows the arrangement of atoms and bonds in a plane. For example

Name Molecula Formula Displayed formula
Methane CH4 Methane
Ethane C2H6 Ethane
Propane C3H8 Propane

General properties of alkanes

  • Alkanes are generally inert, which means they do not react most chemical reagents under nomal conditions.
  • They burn in excess air to form carbon dioxide and water.
  • They undergo substitution reactions with halogens.
  • They are saturated.

Saturation

A saturated compound is a compound in which all its atoms exert their usual combining with other atoms. This means that for a hydrocarbon to be saturated, all its carbon atoms must make 4 single bonds, since carbon has a combining power or valency of 4.

Alkanes are saturated. Saturated compounds are generally react by substitution, by which a new atom replaces an existing atom in the compound.

Uses of alkanes

  • methane is part of biogas, natural gas and coal gas which are used as fuels.
  • methane is also used to make printers’ ink, paints, and carbon paper.
  • Propane and butane are liquified and used as cooking gas.
  • Petrol, paraffin (kerosene), diesel are used as fuels for engines.
  • wax is used as a lubricant.
  • asphalt is used in making roads.

Methane

Methane is the first compound in the alkanes homologous series. Methane is formed in marshes and coal mines by the action of bacteria on organic matter. It is also the main component of biogas. Coal gas contains approximately 30% methane, and natural gas contains about 90% methane.

Laboratory preparation of methane

Preparation of methane

Methane is prepared in the laboratory by heating anhydrous sodium ethanoate with an alkaline such sodalime (sodium and calcium hydroxide):

CH3COONa(s) + NaOH(s) → CH4(g) + Na2CO3(s)

Properties of methane

  • It is a colourless and odourless gas.
  • It is only slightly soluble in water.
  • It is neutral to litmus.
  • It is less dense than air.
  • It does not react with acids or alkalis.
  • It burns in air to form carbon dioxide and water.
  • It release large quantities of heat when it burns, which makes it suitable for use as a fuel.

Combustion of methane

In excess oxygen, methane burns with a faintly luminous flame, forming carbon dioxide and water. A mixture of methane and air is explosive as follows:

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

In limited oxygen, carbon monoxide and carbon (soot) are formed as follows:

CH4(g) + O2(g) → C(s) + 2H2O(g)

2CH4(g) + 3O2(g) → 2CO(g) + 4H2O(g)

Reactions of methane and chlorine

When place in bright sunlight or spark, a mixture of chlorine and methane explodes to produce carbon and hydrogen chloride as follows:

CH4(g) + 2Cl2(g) → C(s) + 4HCl(g)

In diffused sunlight, substitution of hydrogen occurs. One chlorine atom substitutes one hydrogen atom of methane to produce chloromethane:

CH4(g) + Cl2(g) → HCl(g) + CH3Cl(g) (chloromethane)

Chlorine again substitutes one carbon atom on chloromethane to form dichloromethane:

CH3Cl(g) + Cl2(g) → HCl(g) + CH2Cl2(l) (dichloromethane)

And so on:

CH2Cl2(l) + Cl2(g) → HCl(g) + CHCl3(l) (trichloromethane or chloroform)

CHCl3(l) + Cl2(g) → HCl(g) + CCl4(l) (trichloromethane or carbon tetrachloride)

Alkenes

Alkenes are hydrocarbons with the general formula CnH2n. An alkene has a double bond between two of its carbon atoms. An alkene with one carbon atom doesn’t exist.

Examples of alkenes are:

Name Formula
Ethene C2H4
Propene C3H6
Butene C4H8
Pentene C5H10
Hexene C6H12
Heptene C7H14
Octene C8H16
Nonene C9H18
Decene C10H20

Structural formulae of the first two alkenes:

Name Molecular Formula Structural Formula
Ethene C2H4 CH2CH2
Propene C3H6 CH3CH2CH

Displayed formulae of the first two alkenes:

Name Molecular Formula Displayed Formula
Ethene C2H4 Ethene
Propene C3H6 Propene

General properties of alkenes

  • Alkenes are generally very reactive.
  • They burn in excess air to form carbon dioxide and water.
  • They undergo addition reactions with halogens.
  • They are unsaturated.

Unsaturation

An unsaturated compound is a compound in which some of the atoms are not exerting their full combining power on other atoms. In hydrocarbons, this is due to the presence of a double bond forcing carbon to form less than four single bonds.

A double bond is weaker than two single bonds. This make unsaturated compounds very reactive. Alkenes are unsaturated.

Ethene

Laboratory preparation of ethene

Ethene is prepared in the laboratory by dehydration of ethanol. Ethanol vapour is passed over hot aluminium oxide at about 400°C and collected over water.

CH3CH2OH(g) → C2H4(g) + H2O(g)

Properties of ethene

  • Ethene is a colourless.
  • It has a faint sweet smell.
  • It is slightly soluble in water.
  • It is neutral to litmus.
  • It is approximately as dense as air.

Combustion of ethene

Ethene burns in air with a bright smoky flame:

C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(g)

Addition reactions of ethene

Hydrogen reacts with ethene when passed over a catalyst of heated nickel or platinum to form ethane. This a simple example of hydrogenation of alkenes.

C2H4(g) + H2(g) → C2H6(g)

Chlorine and bromine also adds to ethene under the influency of sunlight to form dichloroethane and dibromoethane respectively:

C2H4(g) + Cl2(g) → C2H4Cl2(l)

C2H4(g) + Br2(g) → C2H4Br2(l)

Ethene and steam react at about 300°C, 70 atm pressure and catalyst phosphoric acid to form ethanol.

C2H4(g) + H2O(g) → C2H5OH(g)

Ethene decolourises pink acidified potassium manganate(VII) to produce ethanediol, CH2OHCH2OH.

Hydrogen chloride adds slowly to ethene to form chloroethane:

C2H4(g) + HCl(g) → C2H5Cl(g)

Uses of ethene

  • manufacture of polythene
  • manufacture of ethanol

Tests for hydrocarbons

Alkanes and alkenes can be distinguished by the fact that alkenes decolourises bromine water.

Related Page

Organic Chemistry