Unit 23Unit 23

Alkanes

What are organic compounds ?What are organic compounds ?

Organic CompoundsOrganic Compounds

Derived from living organisms. Name some organic compounds. Is carbon dioxide an organic compound? ??? Examples: proteins, fats, carbohydrates,

petroleum, etc. ???

Carbon as a unique elementCarbon as a unique element

Uses up its four outermost shell electrons to form 4 covalent bonds..

Electronic diagram of methane: ?

CH H

H

H

xo

xo

xo

xo

Carbon as a unique elementCarbon as a unique element

Able to form long straight chain of carbon atoms, long carbon chain with branches, and cyclic

compounds, and able to form multiple bonds, such as carbon

–carbon double and triple bonds

Carbon as a unique elementCarbon as a unique element

long carbon chain with branches, and cyclic compounds,

Propane, C3H8 Butane, C4H10

Structural formulae Structural formulae

H C H

H

H

methane

H C

H

H

H

C

H

H

ethane

Structural formulaeStructural formulae

H C

H

H

H

H

C

H

H

C

H

propane

Molecular formulaeMolecular formulae

Formula to show the actual numbers of different atoms / elements in one molecule of a compound.

e.g., butane: C4H10

Ethanol: C2H6O Ethanoic acid: C2H4O2

Carbon as a unique elementCarbon as a unique element

Straight carbon chain with branches Start to have branches when there are 4 carbon

atoms.

Carbon as a unique elementCarbon as a unique element

Cyclic compounds

HH

HHHHH

HH

HH H

C6H12 Cycloheane

Carbon as a unique elementCarbon as a unique element

Able to form multiple bonds

Homologous SeriesHomologous Series

A group of organic compounds with the same general formula and with successive members differing by a –CH2- unit.

e.g., Alkanes, Alkenes, Alkanols and alkamoic acid

Characteristics of homologous seriesCharacteristics of homologous series

Have the same general formula. With similar chemical properties. With a gradual change / increase in physical

properties such as boiling and melting points. Have the same functional group. Each successive member differs by a –CH2- unit. Can be prepared by similar methods.

Functional groupFunctional group

An organic compound can be divided into 2 parts – the unreactive saturated hydrocarbon part and the reactive functional group.

CH3CH2CH2CH2 OH

Functional group

unreactive saturated hydrocarbon part

Functional GroupFunctional Group

CH3CH2CH2C

OH

O

Functional group

AlkanesAlkanes General formula: CnH2n+2

Saturated hydrocarbons with no functional group Unreactive, with no reaction with common oxidizi

ng agents, dilute acids and alkalis.

Physical Properties of alklanesPhysical Properties of alklanes Insoluble in water, but soluble in organic solvents. The first four homologues (methane to butane) are gase

s at room temperature and pressure. Why ? The heat of combustion increases roughly by the same

amount between successive members. Why? The heat of combustion if the heated released when 1

mole of the compound is burnt completely in air. There is a grdaulal increase in m.p. or b.p. as the molec

ular sizes (masses) increase. Why?

Chemical properties of alkanesChemical properties of alkanes Saturated hydrocarbons – unreactive Without any carbon-carbon multiple bond(s). Have no reaction with oxidzing agents, dilute aci

ds and alkalis.

Reaction with halogensReaction with halogens No reaction with halogens in dark. Reacts with halogens under diffused sunlight. The hydrogen atoms are substituted (replaced)

by halogen atoms. The reaction is known as substitution. A mixture of substituted products are formed

when a mixture of methane and chlorine is exposed to diffused sunlight.

Reaction of methane with chlorineReaction of methane with chlorine CH4 + Cl2 CH3Cl + HCl CH3Cl + Cl2 CH2Cl2 + HCl CH2Cl2 + Cl2 CHCl3 + HCl CHCl3 + Cl2 CCl4 + HCl Hydrogen chloride gas forms a misty white fume in moist air. Put a glass rod wetted with concentrated ammonia solution

near the the mouth of the gas jar. A dense white fume of ammonium chloride is formed.

NH3(g) + HCl(g) NH4Cl(s)

Reaction with halogensReaction with halogens

Bromine reacts similarly with methane, but with a slower reaction rate.

Iodine - no reaction with methane. Relative reactivity of halogens: Cl2 > Br2 > I2

Combustion of alkanesCombustion of alkanes Complete combustion – with excess oxygen (air)

or small alkanes Burns with a blue flame with yellow tip. Products of combustion ? Carbon dioxide and steam (water vapour)

Combustion of alkanesCombustion of alkanes Incomplete combustion – with limited supply of o

xygen / large alkanes Burns with a yellow (luminous) flame. Carbon soots give a yellow colour to the flame. Products of incomplete combustion: carbon parti

cles, carbon monoxide and water vapour

Systematic Naming of alkanes (IUPAC Naming System)Systematic Naming of alkanes (IUPAC Naming System) Choose the longest carbon chain as the parent chain. Number the parent chain from one end such that the br

ances (substituents) bear the smallest numbers. Name of alkyl groups branches)

CH3- methyl group

CH3CH2- ethyl group

CH3CH2CH2- propyl group

Systematic Naming of alkanes (IUPAC Naming System)Systematic Naming of alkanes (IUPAC Naming System) Construct the name by using the prefix to indicat

e the number of carbon atoms in the parent chain.

The parent chain name ends with –ane. Use mono-, di-, tri- and tetra- to indicate the freq

uency of the substitutent Use 1, 2, 3, …. To indicate the positions of the s

ubstituents.

IUPAC NamingIUPAC Naming

CH3CH2CHCH3

CH3

2-methylbutane

Structural isomerismStructural isomerism

Organic compounds exhibit structural isomerism when they have the same molecular formulae, but with different structural formulae (structures).

Starting from butane (4 carbon atoms), there are structural isomers.

Structural isomers of C4H10Structural isomers of C4H10

C C C CH

H H

H

H

H H

HHH

butane

CH3CHCH3

CH3

methylpropane

Structural isomersStructural isomers

Draw all structual isomers of C5H12. Pentane 2-methylbutane Dimethypropane Ex. P. 29 Bk 2

Functional GroupsFunctional Groups An organic compound – with a unreactive hydroc

arbon part (tail) and the reactive funtional group. Functional group – an atom / groups of atoms th

at determines most of the properties of the compound.

Compounds with the same functional group are put into the same homologous series.