Alkanes-and-Alkenes：烷烴和烯烴課件

1、KS4 Chemistry Alkanes and Alkenes KS4 Chemistry Alkanes and AlkeAlkanes and AlkenesAlkenesAlkanesSummary activitiesCombustion of alkanes ContentsCracking and polymerization Alkanes and AlkenesAlkenesAlkaBecause the main use of hydrocarbons is as a fuel there is no point in going to the effort to sep

2、arate them into individual hydrocarbons. It is, however, possible to obtain pure hydrocarbons by very careful distillation.This section is about pure hydrocarbons. Pure hydrocarbonsBecause the main use of hydrocCarbon is an unusual atom in that it is able to form very strong covalent bonds with othe

3、r carbon atoms.When we then include its ability to also bond with other elements we open up the possibility of the highly diverse and complex molecules (like DNA) that have led to the possibility of life. Because of this, the chemistry of carbon containing compounds is often called organic chemistry

4、. Organic chemistry: carbonCarbon is an unusual atom in tThe simplest hydrocarbons form a series of compounds known as alkanes.These all consist of carbon and hydrogen only and every carbon has four single covalent bonds.HydrocarbonFormulaStructureMethaneCH4Ethane C2H6PropaneC3H8ButaneC4H10hydrogenc

5、arbon AlkanesThe simplest hydrocarbons formThe names of the 4 simplest alkanes are methane, ethane, propane and butane.After that the names are systematic (like the words used to describe geometric shapes). E.g.5 carbons = pentane C5H126 carbons = hexaneC6H147 carbons = heptaneC7H168 carbons = octan

6、eC8H189 carbons = nonaneC9H2010 carbons = decaneC10H22carbonhydrogenpentane Names of alkanesThe names of the 4 simplest alAlkanes all have very similar structures.They all consist of carbon and hydrogen only and every carbon has four single covalent bonds.They have a CH3 at each end of the molecule.

7、What differs is the number of CH2 groups between the two ends.It is possible to build up a series by simply adding an extra CH2 groupThis leads to a general formula of CnH2n+2hydrogencarbonn= 1 n=2 n=3 n=4 Structure of alkanesAlkanes all have very similar What will be the formula for alkanes contain

8、ing the following numbers of carbons?C12H26C16H34C31H64C19H40Number of carbonsFormula1216 3119 Whats the formula?What will be the formula for aAlthough normal chemical formula - like C5H12 - are used to describe alkanes, they do not convey which atom is joined to which other atom.To get around this

9、we often used displayed formulae to describe organic molecules.Displayed formulae show which 4 atoms each carbon is bonded to, but even these do not show the actual 3D shapes. For that we use models.methane, CH4CHHHH Formulae and models of alkanesAlthough normal chemical formuCHHHHmethane, CH4CHHHCH

10、HHethane, C2H6CHHHCHHCHHHpropane, C3H8 Formulae and models of alkanesCHHHHmethane, CH4CHHHCHHHethabutane, C4H10CHHHCHHCHHCHHHpentane, C5H12CHHHCHHCHHCHHCHHH Formulae and models of alkanesbutane, C4H10CHHHCHHCHHCHHHpenCHHHCHHCHHCHHCHHCHHHhexane, C6H14and so onNotice the carbon chainis not really stra

11、ight Formulae and models of alkanesCHHHCHHCHHCHHCHHCHHHhexane, C6Alkanes of the same formula can have different arrangements of atoms. Such different arrangements are known as isomers.Two isomers of C4H10 are shown:Isomers of butane IsomerismAlkanes of the same formula caCHHHHCHHHHAlkanes contain at

12、oms held together by single covalent bonds.In the displayed formula we show these bonds as a single line.Each line is really a pair of shared electrons. Bonding in alkanes: methaneCHHHHCHHHHAlkanes contain atomEthane is the simplest alkane containing a C-C single covalent bond.CHHHCHHHCHHCHHHH Bondi

13、ng in alkanes: ethaneEthane is the simplest alkane Complete the diagram below including its electrons.CHHCHHHHCHHCarbon electronHydrogen electron Bonding in alkanes: propaneComplete the diagram below incAlkanes and AlkenesAlkenesAlkanesSummary activitiesCombustion of alkanesCracking and polymerizati

14、on ContentsAlkanes and AlkenesAlkenesAlkaAlkanes are not especially reactive but they do undergo one very important reaction: combustion.With an adequate supply of air they react to form carbon dioxide and water. methane + oxygen water + carbon dioxideCH4 + 2O2 2H2O + CO2 Combustion of alkanesAlkane

15、s are not especially reaIn the absence of an adequate supply of air, alkanes may react to form carbon monoxide and water. Carbon monoxide is highly poisonous and this is one reason why gas boilers must be serviced regularly.methane + oxygen water + carbon monoxide2CH4 + 3O2 4H2O + 2COA carbon monoxi

16、de detector Incomplete combustion of alkanesIn the absence of an adequate Complete the equations below assuming an adequate supply of oxygen for complete combustion. (These are quite tricky!) 2C2H6 + 7O2 C3H8 + 5O2 2C4H10 + 13O2 2C2H6 + 7O2 4CO2 + 6H2O C3H8 + 5O2 3CO2 + 4H2O2C4H10 + 13O2 8CO2 + 10H2

17、O Complete the equationComplete the equations below aAlkanes and AlkenesAlkenesAlkanesSummary activitiesCombustion of alkanesCracking and polymerization ContentsAlkanes and AlkenesAlkenesAlkaWhen carbon forms compounds each carbon atom always forms four bonds.This does not, however, mean that each c

18、arbon is joined to four other atoms.It is possible to have bonds grouped into pairs. These are called double bonds.Alkenes contain carbon atoms joined by double covalent bonds.Single covalent bondCCDouble covalent bondCC AlkenesWhen carbon forms compounds eaA series of alkanes exist, differing only

19、in the number of CH2 groups.The same is true for alkenes.This leads to a homologous series with the general formula CnH2n. hydrogencarbonn= 1 n=2 n=3 n=4Alkenesnone= Structure of alkenesA series of alkanes exist, difWhat will be the formula for alkenes containing the following numbers of carbons?Num

20、ber of carbonsFormula1113 3221C11H22C13H26C32H64C21H42 Whats the formula?What will be the formula for aThe simplest alkene is ethene.It has the formula C2H4The carbon atoms are joined together by a double bond.Its displayed formula may be drawn in slightly different forms but should always clearly s

21、how the double bond.CHHCHHor The formula of etheneThe simplest alkene is ethene.In all alkenes there are two carbon atoms that are joined by two pairs of electrons.This is the double bond.CHHCHH The electron structure of etheneIn all alkenes there are two cDisplayed formula for propene and butene ar

22、e shown.Draw a displayed structure for pentene.CCCHHHHHHCCHHHCCHHHHHCCHHHCCHHHHHHHC The structure of penteneDisplayed formula for propene Saturated means “full up”.They have a double bond that could instead become two single bonds. This means that other atoms can be added. It is not “full up”.Alkene

23、s are unsaturated.Every carbon atom has already used all four of its bonds to join to four other atoms. No other atoms can be added.Alkanes are saturated. Saturated or unsaturated?Saturated means “full up”.TheyAlkenes are unsaturated and so extra atoms can be added to alkene molecules.This forms the

24、 basis of a test to distinguish between alkanes and alkenes.When bromine water is added to an alkane nothing happens, but when bromine is added to an alkene the red colour of the bromine disappears.+Br2 gas red colourlessBromine loses this red colour Testing for alkanes and alkenesAlkenes are unsatu

25、rated and soCopy the table and complete the empty boxes.Number of carbonsNameAlkane or alkeneFormula5pentene8octane4butene10decanealkeneC5H10alkaneC8H18alkeneC4H8alkaneC10H22 Identify the compoundCopy the table and complete thAlkanes and AlkenesAlkenesCracking and polymerization AlkanesSummary activ

26、itiesCombustion of alkanes ContentsAlkanes and AlkenesAlkenesCracCrude oil contains many large molecules.If these are to be used as fuels or feedstock for the chemical industry then they have to be broken down (or cracked) into smaller molecules.Big moleculesSmall moleculesMedium molecules Source of

27、 alkenesCrude oil contains many large Large hydrocarbons are broken down into smaller molecules using heat and a catalyst.This process is known as catalytic cracking.The small molecules produced are then separated by distillation.CatalyticcrackerHeat tovaporizeDistillationtowerpressureBig moleculesS

28、maller moleculesMolecules break up Catalytic crackingLarge hydrocarbons are broken In the catalytic cracker long chain molecules are split apart or cracked. An example of such a reaction is:C8H18 C6H14 + C2H4+etheneoctanehexaneUsedto makeplasticsheat & pressurecatalystUsed as a fuel Catalytic cracki

29、ngIn the catalytic cracker long Draw out displayed formulae of a pair of products formed by cracking decane.Heat & pressurecatalystHHCCHHHHHCCHHHHCCHHHHCCHHHHCCHHHdecane Cracking decane+etheneHHCCHHHHHCCHHHHCCHHHCHHCHHoctaneDraw out displayed formulae of Making polymersHow do monomers become polymer

30、s? Making polymersHow do moOne important reaction of alkenes involves the joining together of alkene molecules.HHCCHHHHHCCHHHHCCHHHHCCHHHHCCHHAndlotsmore.This is called addition polymerization and is written as:pressurehigh temperaturecatalystnnethenepoly(e)thene1 2 3 4 5 thousands Production of pol

31、ytheneOne important reaction of alkeEthene is only one alkene. Other unsaturated molecules such as propene, vinyl chloride and styrene can also be polymerized to produce a range of plastics. E.g. propene:poly(propene)nnpropene PolypropeneEthene is only one alkene. OthTetrafluoroethene is another alkene that is made into an important plastic used to coat non-stick pans: polytetrafluoroethene or PTFE.ntetrafluoroethenepoly(tetrafluoroethene)or PTFEn PTFETetrafluoroethene is another apvcCCHHHClnnVinyl chlorideCCClHHHFill in the products th