ALKANE

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Alkanes

Copyright© 2006, MRSM Kuantan. All rights reserved.

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1: Classification of Hydrocarbons1.

Alkanes are saturated hydrocarbons:

CH HH

HC C CH HH H

H

H

HH

saturated (only single C-C bonds) hydrocarbon (only C and H)


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Carbon and hydrogen atoms can bond to each other to form very extensive and molecular systems.

2: Bondings in hydrocarbon

Carbon atoms has 4 valence electrons need 4 electrons to reach octet electron arrangement. four bonds to each carbon atom

Hydrogen atoms has 1 valence electron need 1 electron to reach duplet electron arrangement.

one bond to each hydrogen atom


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3: Molecular Formulas of Alkanes

Write the molecular formula for each of the alkanes below.

C CH HH

H

H

HC C C CH HH

H

H

H

H

H

H

H

C2H6 C4H10


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C5H12

C6H14


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What is the general molecular formula for alkanes?

Alkanes have the general formula

CnH2n+2

n = no.of carbon atoms in the molecule


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4: Nomenclature of Alkanes


1. The name of alkanes consist of 2 parts : the root & ending.

2. Root- tells the number of carbon atoms in the longest continuous carbon chain.

3. Ending-tells the family of the compound.No. of

carbon

atoms

1 2 3 4 5 6 7 8 9 10

Root name

Meth

Eth Prop

But Pent

Hex Hept

Oct Non Dec

The root names for the first ten straight-chain alkanes

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Naming Alkanes• Example : Butane

Name of alkane

Root Ending

Butane But (no. of carbon atoms = 4)

-ane (the family is alkane)

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Methane CH4

Ethane C2H6

Propane C3H8

CH H

H

H

C CH H

H

H

H

H

C C CH H

H H

H

H

HH

Nomenclature / Molecular formula

No. of C

atoms

Structural Formula

3 C

2 C

1 C


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Butane C4H10

C C C CH H

H

H

H

H

H

H

H

H

Nomenclature / Molecular formula

No. of C

atoms

Structural Formula

Hexane C6H14

4 C

6 C


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Structural FormulaShows how the atoms in

molecule are bonded together and by what types

of bonds.C C C CH H

H

H

H

H

H

H

H

H

Example :

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* Low melting and boiling points because these molecules are held together by weak intermolecular forces.


a) Melting and Boiling Point

5: Physical Properties5: Physical Properties

Alkanes- covalent compounds

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• The boiling point increases between 20-30 °C per -CH2 group.

• Boiling points and melting points increase as number of carbon atoms per molecule increases.

• Then, size of molecules increases (molecular mass increases)

• Forces between molecules (van der Waals forces) are stronger

• More heat energy is needed to overcome the forces


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b) Physical State at Room Temperature

CH4 - C4H10 : gasC5H12 - C17H38 : liquid C18H38 : solid

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• -Soluble in non polar solvents (organic solvents),

• -Insoluble in water.

* The density of the alkanes increases with increasing number of carbon atoms, BUT is less dense than that of water. Hence, alkanes form the upper layer in an alkane-water mixture.


c) Solubility

d) Density

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• Cannot conduct electricity • because : alkanes are covalent

compounds, do not have ions but are made up of molecules.

e) Electrical Conductivity

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Chemical Properties of alkanes

• Unreactive compounds • Saturated compounds.• The strong C-C and C-H bonds

must be broken for the alkanes to react.

• Although alkanes do not react with most chemicals, they do react with oxygen and halogens.

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Combustion• The reaction with oxygen• Complete combustion of methane / in

the presence of sufficient oxygen :CH4(g) + 2O2(g) CO2(g) + 2H2O(l)

• Incomplete combustion / insufficient oxygen available :2CH4(g) + 3O2(g) CO(g) + 4H2O(l)CH4(g) + O2(g) C(s) + 2H2O(l)

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Halogenation• Reaction with halogen • Takes place readily in sunlight.• C-H bonds-broken, formed C-halogens

bonds• Halogenation - example of a

substitution reaction• Substitution reaction – when one atom

or a group of atoms in a molecule is replaced by another atom or group of atoms.

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Halogenation• Example-Mixture of methane

and chlorine is exposed to ultravolet light-varying amounts of four different products are formed.

• CH4 CH3Cl chloromethane CH2Cl2 dichloromethane CHCl3 trichloromethane

CCl4 tetrachloromethaneCH4(g) + Cl2(g) CH3Cl(g) + HCl(g)

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