Chemistry

UNIT 2: ALKANES

Key unit competency

Relate the physical and chemical properties of the alkanes to the preparation

methods, uses and isomerism.

Learning objectives

• Name straight chain alkanes up to carbon-20

• Define homologous series

• Use IUPAC system to name straight and branched alkanes

• Describe the preparation methods of the alkanes

• Prepare and collect methane gas

• Respect of procedure in experiment to carry out preparation of methane or propane

• Describe and explain the trend in physical properties of homologous series ofalkanes

• Be aware of the dangers associated with combustion reactions of the alkanes

• Write reaction for free radical mechanism for a photochemical reaction

• State the chemical properties of the alkanes

• Develop practical skills,interpret results make appropriate deductions.

• Appreciate the importance of the alkanes in daily life

• Appreciate the dangers caused by the alkanes to the environment as major

sources of air contaminants

• State the uses of the alkanes

Alkanes are the simplest class of organic compounds. They are made of carbon and

hydrogen atoms only and contain two types of bonds, carbon-hydrogen (C-H) and

carbon-carbon (C-C) single covalent bonds. They do not have functional groups.

Alkanes form a homologous series with the general formula C_nH_2n+2 where n is the

number of carbon atoms in the molecule. The first member of the family has the

molecular formula CH₄ (n=1) and is commonly known as methane and the second

member with molecular formula is C₂H₆ (n=2) is called ethane.

These compounds are also known as saturated hydrocarbons. This name is more

descriptive than the term “alkane’’ because both their composition (carbon and

hydrogen) and the fact that the four single covalent bonds of each carbon in their

molecules are fully satisfied or ‘’saturated’’.

The name alkane is the generic name for this class of compounds in the IUPAC

system of nomenclature. These hydrocarbons are relatively unreactive under

ordinary laboratory conditions, but they can be forced to undergo reactions by

drastic treatment. It is for this reason that they were named paraffins (Latin parum

affinis = little activity).

2.1. Nomenclature of alkanes

IUPAC Rules for the nomenclature of alkanes

a. Find and name the longest continuous carbon chain.

b. Identify and name groups attached to this chain.

c. Number the chain consecutively, starting at the end nearest a substituent

group.

d. Designate the location of each substituent group by an appropriate

number and name.

e. Assemble the name, listing groups in alphabetical order. The saturated

hydrocarbon form homologous series (series in which members have similar

chemical properties and each differs from the preceding by a methylene

group –CH₂-).

The first four members are known by their common names, from C₅ and

above the Roman prefixes indicating the number of carbon atoms is written

followed by the ending “ane” of the alkanes.

Note: Alkyl groups are obtained when one hydrogen atom is removed from

alkanes; therefore their names are deduced from the corresponding alkanes

by replacing “ane” ending with “yl” desinence (Table 2.1)

Prefixes di, tri, tetra, sec, tert, are not considered when alphabetizing.

f. In case of chains of the same length, the priority is given for part where

many branched of alkyl groups appear.

   But in case of ramified cyclanes, the priority is for the ring.

Note: If there are more than one substituent, the numbering is done so that the

sum of the numbers used to locate the locants is minimum. This is the lowest sum

rule.

2.2. Isomerism

Alkanes show structural isomerism. The easiest way to find isomers is to draw the

longest chain of carbon atoms first and then reduce it by one carbon first until

repetition begins to occur.

d. Substitute one hydrogen on carbon (3) by the mathyl group

e.Longest chain reduced further to 4 carbon atoms by cutting 2 methyl group

-Putting the methyl group on position 1 or 5 gives you the same straight chain

isomer.

2.3 Occurrence of Alkanes

2.4. Preparation of alkanes

Note: The reaction is practically used to reduce by one carbon the length of carbon chain. It is referred as decarboxylation of sodium carboxylates.

Other reactions used for the preparation of alkanes are the following:

1. Addition reaction of hydrogen to alkenes and alkynes in the presence of

catalyst like Nickel, Palladium or platinum produces alkanes: this reaction

is called hydrogenation reaction of alkenes and alkynes; it is also called a

reduction reaction of alkenes and alkynes.

2. From halogenoalkanes or Alkyl halides

 On reduction of alkyl halides with Zn and concentrated hydrochloric acid, alkyl

halides are converted to alkanes. 

b) Alkyl halides when heated with sodium metal in ether solution give higher alkanes (alkanes with more carbon atoms) (Wurtz reaction).

c) When Alkyl halides are treated with Zn-Cu couple, in the presence of ethanol,

alkanes are formed.

Note: Zn-Cu couple is obtained by adding Zinc granules in aqueous copper (II)

sulphate solution where copper is deposited on the Zn pieces. 

3. From carbonyl compounds

Reduction of carbonyl compounds, with amalgamated Zinc (alloy made of zinc

and mercury) and HCl. This is the Clemmensen reduction). 

2.5. Physical properties of alkanes

The above Table shows that the boiling and melting points of homologue alkanes

increase with the number of carbon i.e. molecular mass.

Explanation:

The boiling and melting points depend on the magnitude of the Van Der Waal’s

forces that exist between the molecules. These forces increase in magnitude with

molecular mass. 

Note: Branched chain isomers have lower boiling and melting points than their

straight chain isomers, because straight chain isomers are closer packed than the

branched chain isomers.

2.6. Chemical properties of alkanes

Generally, alkanes are quite inert towards common reagents because:

• The C-C bond and C-H bonds are strong and do not break easily.

• Carbon and hydrogen have nearly the same electronegativity valuehence

• C-H bond only slightly polarized; generally C-H bond is considered as covalent.

• They have unshared electrons to offer.

They, however, undergo the following reactions.

1. Reaction with oxygen

Alkanes react with oxygen to produce carbon dioxide (if oxygen is enough to

burn all quantity of hydrocarbons), or carbon monoxide or carbon if oxygen is in

insufficient quantity, and water. This reaction is called “combustion”

Carbon dioxide (CO₂) produced from the burning of alkanes or fossil fuels for heating, transport and electricity generation is the major atmospheric pollutant that increases the green house potential of the atmosphere .Carbon dioxide is the major Green House Effect (GHE) gas.

Burning wood and forests produce also carbon dioxide and lead to the increase of that gas in the atmosphere. Methane as another GHE gas is produced by human activities, agriculture (Rice), and cattle-rearing.

There are many natural ways of reducing atmosphere carbon dioxide:

i. Water in seas dissolves millions of tonnes of gas (but less now than it did in

There are other ways than natural ways of reducing GHE gases and among them

there are the use of technologies that reduce the green house gas emissions, the

recycling of the GHE.

Notice: (i) Br2

 reacts as Cl2

 but slowly while iodine reacts hardly or does not.

Notice: (i) Br2 reacts as Cl2 but slowly while iodine reacts hardly or does not

,Fluorine, the most electronegative element of the periodic table reacts with

alkanes to give coke,

i.e. a decomposition reaction:

A mechanism of a reaction is a description of the course of the reaction which

shows steps of the reaction and the chemical species involved in each step.

The mechanism for the reaction between methane and bromine is the following:

(ii) Due to radical formation involved, the main product of reaction is the one from

the most stable radical, starting with tertiary, secondary, primary and methyl in

decreasing order of stability.

A tertiary free radical is better stabilised by the electron donating methyl groups

than the secondary, primary and methyl ones where the carbon atom is attached

to more hydrogen atoms

3. Dehydrogenation of alkanes gives alkenes under heat and a catalyst like V₂O₅

4. Cracking

On heating or in the presence of a catalyst, large molecules of alkanes are

decomposed into smaller alkanes and alkenes. If the cracking is performed on

heating, it is referred as themocracking.

If the cracking is performed using a catalyst; it is referred as catalytic cracking and

many products result from one reactant as shown below.

2.7. Uses of alkanes

1. Methane (CH₄)

Methane finds many uses:

• It is used as a fuel at homes, ovens, water heaters, kilns and automobiles as it

combusts with oxygen to produce heat.

• Highly refined liquid methane is used as rocket fuel.

• Methane is used as fuel for electricity generation.

• It is used as a vehicle fuel in the form of liquefied natural gas (LNG).

• Methane can be used as raw material in the production of urea, a fertilizer.

In general, methane is more environmental friendly than gasoline/petrol and

diesel.

2. Butane (C₄H₁₀)

• Butane is a key ingredient of synthetic rubber.

• It is used as fuel in cigarette lighters.

• When blended with propane and other hydrocarbons, it may be referred to

commercially as LPG, for liquefied petroleum gas.

• Butane gas cylinders are used in cooking.

• Also used in aerosol spray cans.

3. Propane (C₃H₈)

• Propane is used as a propellant for aerosol sprays such as shaving creams

and air fresheners.Used as fuel for home heat and back up electrical

generation in sparsely populated areas that do not have natural gas

pipelines.

• Propane is commonly used in movies for explosions

4. Ethane (C₂H₆)

• Ethane is used in the preparation of ethene and certain heavier

hydrocarbons.

• Ethane can be used as a refrigerant in cryogenic refrigeration systems.

5. Pentane (C₅H₁₂)

• Pentane is used in the production of polystyrene foams and other foams.

• Used in laboratories as solvents.

• It is also an active ingredients of pesticides.

• Used as solvent in liquid chromatography

6. Hexane (C₆H₁₄)

• It is used in the formulation of glues for shoes, leather products, and roofing.

• It is also used to extract cooking oils such as canola oil or soy oil from seeds.

• Hexane is used in extraction of pyrethrine from pyrethrum; e.g. Horizon

SOPYRWA (a pyrethrum factory in Musanze District).

• Also for cleansing and degreasing a variety of items, and in textile

manufacturing.

7. Heptane (C₇H₁₆)

• Heptane is used as solvent in paints and coatings.

• Pure n-heptane is used for research, development and pharmaceutical

manufacturing

• Also as a minor component of gasoline.

• It is used in laboratories as a non-polar solvent.