C15 hydrocarbons

• Recall that carbon forms branched and unbranched chains and ring compounds

• Illustrate that carbon atoms can form single and double bonds

• Write formulae to represent simple organic compounds• List the general characteristics of a homologous series• Deduce the homologous series given the fully displayed

formulae of compounds• Define structural isomerism

Chapter 15Hydrocarbons

Learning Outcomes

• Write general and molecular formulae for the members of the alkane homologous series

• Write fully displayed structures and names of branched and unbranched alkanes and their isomers, given their molecular formulae

• Describe the reactions of alkanes• Relate the characteristic reactions of alkanes to

their structures• Relate the properties of alkanes to their uses


Learning Outcomes

• Write general and molecular formulae for the members of the alkene homologous series

• Write fully displayed structures and names of branched and unbranched alkenes and their isomers, given their molecular formulae

• Describe the reactions of alkenes• Relate the characteristic reactions of alkenes to

their structures• Relate the properties of alkenes to their uses


Learning Outcomes

• Hydrocarbons belong to a group of compounds called the organic compounds.

• This ability of carbon to form big molecules has enabled nature to form living matter, from simple cells to complex tissues.

• Gasoline or petrol is a mixture of hydrocarbons and is used primarily as fuel in internal combustion engines of cars.

Chapter 15Hydrocarbons Introduction to Hydrocarbons

• Carbon atoms are able to form a few types of covalent bonds with other carbon atoms or atoms of other elements:

• Single bonds: C-C, C-H, C-O, C-X (X = halogens)• Double bonds: C=C, C=O• Triple bonds: C≡ C, C≡N• Organic compounds can be classified as unbranched,

branched or ring compounds.• There are five types of formulae which can be used to describe

and identify organic compounds:

Chapter 15Hydrocarbons Bonding ability of carbon

1. Molecular formula2. Empirical formula3. Full structural or displayed formula4. Condensed structural formula5. General formula

• Straight-chain molecules that are unbranched may contain single, double or triple bonds. E.g.:

Chapter 15Hydrocarbons Straight (unbranched) Chain Compounds

Single bond Double bonds

Triple bonds

• Branched chain compounds consist of one or more carbons of a straight chain compound forming bonds with more than two carbons.


Branched- Chain Compounds

• A homologous series is a group or family of organic compounds that has the following characteristics:

• A functional group is a group of atoms that is responsible for the characteristic chemical properties of a homologous series.

• Alcohols have the –OH functional group, carboxylic acids have the –COOH functional group and alkenes contain a C=C bond.

Chapter 15Hydrocarbons Homologous Series

- They have the same general formula.- They show similar chemical properties because they

have the same functional group.- Each member differs from the previous one by -CH2.- They show a gradual change in density, viscosity,

flammability, melting point and boiling point.

• Isomers are organic compounds with the same molecular formula but different structural formulae.

• Isomers that are in the same homologous series have similar chemical properties but differ slightly in melting points and boiling points.


Structural Isomerism

Isomers of butane (C4H10)

• Alkanes have the general formula CnH2n+2,

• Their names end with “ane”.• The carbon atoms are joined by single covalent bonds.• Each member in the homologous series differs from the next one by -CH2.

Chapter 15Hydrocarbons Alkane Homologous Series

No. of carbon atoms

Chemical name

Molecular formula

Relative molecular

mass

Boiling point /

°C

1 Methane CH4 16 –162

2 Ethane C2H6 30 –89

3 Propane C3H8 44 –42

4 Butane C4H10 58 –0.5

5 Pentane C5H12 72 36

• Alkanes are made up of covalent molecules. Each carbon atom is joined to four other atoms by single covalent bonds.

• The structure of a methane molecule is drawn as


Structure of Alkanes

Name of alkane Molecular formula Structural formula

Methane CH4

Ethane C2H6

Propane C3H8

Butane C4H10

Pentane C5H12

• The alkanes are generally unreactive due to their saturated carbon-carbon bonds. However, they can undergo two types of chemical reactions.

• Alkanes undergo combustion in air or oxygen to form carbon dioxide and water vapour. The following two equations illustrate this.

CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

2C2H6(g) + 7O2(g) 4CO2(g) + 6H2O(g)

Chapter 15Hydrocarbons Chemical Properties of Alkanes

Combustion of Alkanes

• Alkanes react with halogens, such as fluorine, chlorine and bromine under UV light or heat to form a mixture of halogenoalkanes. E.g. methane reacts with chlorine as follows:

CH4 + Cl2 CH3Cl + HClCH3Cl + Cl2 CH2Cl2 + HClCH2Cl2 + Cl2 CHCl3 + HClCHCl3 + Cl2 CCl4 + HCl

• This type of reaction in which the hydrogen atom of the hydrocarbon is replaced by the halogen atom is called a substitution reaction.

Chapter 15Hydrocarbons Chemical Properties of Alkanes

Substitution Reactions with Halogens

• Combustion of alkanes are highly exothermic, so they are widely used as fuels.

• Natural gas (mainly methane) – used in power stations, factories and more recently as LNG (Liquefied Natural Gas) for running motor vehicles.

• Liquefied petroleum gas (LPG) – used for cooking and running motorcars. • Substituted alkanes such as chlorofluorocarbons or CFCs are non-

flammable and not very toxic – used as refrigerants, propellants for aerosols, making polystyrene or polyurethane foam, solvents for dry cleaning and for general degreasing purposes.

• Unfortunately, CFCs are largely responsible for destroying the ozone layer. • The carbon-chlorine bonds break to generate chlorine free radicals, which

destroy the ozone layer. • CFCs are now being replaced by more environmentally friendly

compounds.


Uses of Alkanes and Their Derivatives

Positive

Negative

Chapter 15Hydrocarbons Quick Check 1

Solution

1. Consider the two isomers of pentane (an alkane after butane in the homologous series) shown below. Are the following molecules also isomers of pentane? Explain why.

2. What is the chemical formula of an alkane with (a) eight carbon atoms;(b) 20 carbon atoms?


Solutions to Quick Check 1

1. Yes, because they have the same molecular formula (C5H12) but different structural formula.

2. (a) C8H18

(b) C20H42

Return

• The alkenes are another family of hydrocarbons, ie. they contain hydrogen and carbon only. All alkenes share the following characteristics:

• They have the general formula CnH2n, where n is an integer.

• Their names end with ‘ene’.• There is a carbon-carbon double bond present in

the molecule.• They are said to be unsaturated as other atoms can

be added to the molecule when the carbon-carbon double bond opens up.

Chapter 15Hydrocarbons Alkene Homologous Series

• There is no organic compound with the formula CH2. Just like the alkanes, as the number of carbon atoms increases, the molecules become bigger and heavier.

• As the number of carbon atoms increases, the density, melting point and boiling point increases.

• The first three alkenes are gases, the next few are liquids and the rest are solids.


Chemical name

Molecular formula

Relative molecular mass

Boiling point / °C

Ethene C2H4 28 –104

Propene C3H6 42 –48

Butene C4H8 56 –6

Pentene C5H10 70 30.1

Alkene Homologous Series

• Alkenes have a carbon-carbon double covalent bond.


Structure of Alkenes

Name of alkene

Molecular Formula

Structural Formula

Ethene C2H4

Propene C3H6

But-1-ene(1-butene)

C4H8

Pent-1-ene(1-pentene)

C5H10

• For propene, if we place the C=C double bond on the third carbon atom, we will get a structure which is exactly the same as the original when it is rotated through 180°. Hence, there is no isomer for propene.

• It is possible for the C=C double bond to be in another position in the carbon chain forming new isomers. For example, butene has three isomers as shown below:


(i) (ii) (iii)

Isomers of Alkenes

• The alkenes are much more reactive compared to alkanes.

• Alkenes have an unsaturated carbon-carbon double bond which can be easily broken and other atoms can be added to form a molecule with single bonds.

• Alkenes take part in many important chemical

reactions and provide a basis for the manufacturing of many important materials and chemicals in the industry.


Chemical Properties of Alkenes

• Alkenes undergo combustion in air or oxygen to form carbon dioxide and water vapor.

• Alkenes do not burn as well as the alkanes and generally produce a more sooty flame.

• Due to an insufficient supply of air, the combustion is incomplete and carbon monoxide and soot are produced.

• The combustion reactions of alkenes:


C2H4(g) + 3O2(g) 2CO2(g) + 2H2O(g)

2C3H6(g) + 9O2(g) 6CO2(g) + 6H2O(g)

Combustion of Alkenes

Chemical Properties of Alkenes

• Alkenes react readily with halogens to form haloalkanes.• The reaction takes place quickly, without the need of UV light or

catalyst. E.g. Ethene reacts with chlorine to form dichloroethane.

• This type of reaction in which the chlorine atoms are added across the C=C double bond is called an addition reaction.


C2H4 + Cl2 C2H4Cl2 ethene chlorine 1,2-dichloroethane

1,2-dichloroethane

Chemical Properties of AlkenesAddition Reactions with Halogens

• Alkenes react with aqueous bromine (bromine water) and cause the bromine solution to turn from brown to colourless.

• This reaction can be used as a test for unsaturation of any organic compound.

• The equation for the reaction is:


C2H4 + Br2 C2H4Br2

ethene bromine 1,2-dibromoethane

Chemical Properties of AlkenesTest for unsaturation

• Alkenes react with water (in the form of steam) and in the presence of a catalyst to form alcohols. E.g.ethene reacts with steam at 300 °C and with phosphoric(V) acid as catalyst, to form ethanol.

• It is a very important reaction used for the manufacture of ethanol in the industry.

• The ethanol produced is usually mixed with a little methanol to make it undrinkable and the mixture is used as ‘industrial alcohol’.


C2H4(g) + H2O(g) C2H5OH(l) ethene water vapour ethanol

Chemical Properties of AlkenesAddition reaction with steam (hydration)

• Alkenes react with hydrogen in the presence of the catalyst nickel at a moderately high temperature to form alkanes.

• E.g.

• The addition of hydrogen to carbon-carbon double bonds is used in the industry to change vegetable oils into fats, which can be used for making margarine.

• When hydrogen is added to the unsaturated vegetable oil, the melting point of the oil increases, and the oil becomes solid (fats).


C2H4(g) + H2(g) C2H6(g)ethene hydrogen ethane

Chemical Properties of AlkenesAddition reaction with hydrogen (hydrogenation)

• Alkenes are very versatile in their uses and are the starting materials for various chemicals. E.g. ethene and propene are the starting materials for polymers like poly(ethene), poly(propene), polyvinylchloride (PVC) and Teflon etc.

• Alkenes are also used for the manufacture of industrial chemicals such as alcohols and aldehydes.

• The lower alkenes can be used as fuel and can be obtained by the cracking of kerosene or petrol.

Uses of Alkenes


Chapter 15Hydrocarbons Quick Check 2

Solution

The diagram below shows the structural formula of an organic compound.

(a) Name the organic compound.(b) Name the homologous series to which this compound belongs.


Solutions to Quick Check 2

(a) Propene

(b) Alkene

Return

• Unsaturated hydrocarbons contain carbon-carbon double bonds. • Polyunsaturated oils are oils that contain two or more C=C bonds in the

molecules.• Due to the geometry of these C=C double bonds, the molecules are

harder to pack closely together and thus are liquids at room temperature.

• Saturated oils which are mostly straight-chain molecules are more easily packed and hence, they exist as solids at room temperature.

Chapter 15Hydrocarbons Polyunsaturated Food

• Saturated fats like animal fats and oils, butter and cream are unhealthy in the diet as they raise the blood cholesterol of a person.

• Polyunsaturated oils and fats are healthier as they do not clot the blood vessels and raise the blood cholesterol level. E.g. polyunsaturated oils are found in sunflower oil, palm oil, fish oil, soya oil, linseed oil and nuts.

Chapter 15Hydrocarbons Polyunsaturated Food

References

• Chemistry for CSEC Examinations by Mike Taylor and Tania Chung

• Longman Chemistry for CSEC by Jim Clark and Ray Oliver

C15 hydrocarbons

Science

Transcript of C15 hydrocarbons