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1.6 Alkanes And 1.7 Alkenes
 

1.6 Alkanes And 1.7 Alkenes

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    1.6 Alkanes And 1.7 Alkenes 1.6 Alkanes And 1.7 Alkenes Document Transcript

    • Chemical properties of the alkanes Explain why the old, non-systematic name for the alkanes was the paraffins: Free radicals Almost all alkane reactions involve the formation of free radicals what are they: Combustion • When alkanes are heated in a plentiful supply of air, combustion occurs • Alkanes are energetically unstable with respect to water and carbon dioxide • They only burn when they are in the gaseous state Explain what happens when a candle burns: Sketch and energy profile for the combustion of propane: Enthalpy Cracking • Cracking happens when alkanes are heated in the absence of air • The products of the cracking of long-chain hydrocarbons are shorter chain molecules • Ethane is cracked industrially to produce ethene What are the conditions for catalytic cracking and what are the benefits of using a catalyst?
    • Write an equation for the cracking of dodecane to form one mole of ethene and an alkane product: Breaking bonds Homolytic fission • Homolytic fission involves the equal sharing out of electrons in a bond • Each atom receives one electron from the bond when it splits • This leaves the atoms with one unpaired electron • The atoms are uncharged • These ‘free radicals’ are very reactive Why are free radicals so reactive? In what type of situation is homolytic fission most likely to occur? Draw a dot-cross and a curly arrow diagram for homolytic fission of chlorine (NB the curly arrow is wrong in your textbook): Heterolytic fission • Heterolytic fission involves the unequal sharing of the electrons in a bond • Both electrons go to one atom • This results in the formation of two charged particles In what type of situation is heterolytic fission usually seen? Draw dot-cross and curly arrow diagrams for heterolytic fission:
    • Reactions with chlorine • Alkanes only react with chlorine when a mixture of the two is exposed to sunlight or ultraviolet light • The light provides the energy required to break the very strong bonds • This is an example of a substitution reaction • There are three steps to the chain-reaction: Initiation: The Cl-Cl bond is easier to break than C-H Propagation: There are several possible propagation steps Termination: There are also several possible termination steps Explain the difference in products produced when: methane reacts with a limited supply of chlorine and with a plentiful supply of chlorine:
    • Chemical and physical properties of alkanes Chemical properties • Alkanes contain atoms of 2 elements: ____________ and __________ • Alkanes are _____________ hydrocarbons • This means that they contain no carbon-carbon ___________ bonds • Alkanes are very _________________ because they contain very __________ bonds • They do burn, and in sufficient oxygen produce _________ and __________ ___________. Physical properties • Alkanes are ________________ in water • In straight-chain alkanes boiling temperature _______________ with chain length; however, this trend is not followed by branched structural ________________. Activity Number these alkanes in order of increasing boiling point: Propane Methane Octane Dotriacontane Heptane 1
    • Activity Label the following pairs of structural isomers with highest boiling point (H) and lowest boiling point (L): Butane 2-methylpropane Hexane 2,2-dimethylbutane 2,2,4-trimethylpentane Octane Activity Briefly explain the choices you have made in the above activities: Structural Isomerism “ Isomers are compounds with the same molecular formula but different structural formula ” Structural isomers have the same molecular _____________ but different molecular ______________. This means that they have the same number of _________ of the same ___________, but that these atoms are arranged differently. Exercise: Determine displayed, skeletal, structural, molecular and empirical formulae for all structural isomers of the 1st five compounds in the alkane homologous series (methane-pentane), you can use the Molymod kits to help you do this.
    • The alkenes Comparison of bonding in ethane and ethene Ethane • The carbon-carbon single bond (σ bond) runs through the central axis of the molecule • The two ends of the ethane molecule are free to rotate relative to each other Ethene • The ethene molecule is flat • Ethene contains a carbon-carbon σ bond and a carbon-carbon double bond (π bond) • The π bond lies above and below the central axis • The π bond prevents rotation around the central axis The carbon-carbon π bond is less than twice as strong as the carbon-carbon π bond, why?
    • What is and electrophile and why do alkanes readily react with them? Naming alkenes • We use the IUPAC naming system to name alkenes • The suffix used is –ene • We must use a number to indicate the position of the double bond Complete the table of alkenes: ethene but-1-ene but-2-ene 2-methylpent-2-ene
    • 2,4-dimethylhex-2-ene Alkenes – Chemical reactions Introduction • Alkenes are far more reactive than alkanes • They readily react with electrophiles • They undergo addition reactions • Addition reactions involve heterolytic fission of the double bond Electrophiles • Species containing and atom with a whole or partial ________________ charge • They are attracted to areas of high _____________ density • They form a ______________ bond using a pair of electrons from the π bond • The most common electrophile is ________________ Addition reactions 1. Reaction with hydrogen • Alkenes do not react with hydrogen under normal conditions… Conditions: 2. Reaction with acidified potassium manganate(VII) • This reaction involves addition across the double bond and oxidation • The products are alkane diols • Can be used as a test for _______________ as alkanes do not react
    • Conditions: Observations: 3. Reactions with halogens • Alkenes react halogens to form halogenoalkanes • Reactions happen at room temperature and unlike the reaction of alkanes with halogens do not involve ______ _____________ • Reactions become __________ vigorous down the halogen group (F > Cl > Br > I) 4. Reaction with bromine water • Bromine water is an _______________ solution of bromine • Addition takes place across the double bond and the bromine water becomes _______________ • There are two different products _____________________ and _____________________ • 2-bromoethanol is formed because _____ ions from water also take part in the reaction
    • 5. Reactions with hydrogen halides • Hydrogen halides are compounds containing ________________ and a halogen e.g. • Hydrogen halides react with alkenes readily at room temperature • In the reactions hydrogen halides with higher alkenes (n > 2) there are two possible products • Both possible products are formed but one is favoured, in this case ___________________ • The major product can be predicted using Markovnikov’s rule Markovnikov’s rule: “When HX adds across an asymmetric double bond, the major product is the molecule in which hydrogen adds to the carbon atom in the double bond with the greater number of hydrogen atoms already attached to it” Reaction mechanisms – “Curly Arrows” Key words • Electrophile – • Nucleophile –
    • • Carbocation – • Curly arrow – Reaction mechanisms • Mechanisms are diagrams that represent how organic chemistry reactions happen in terms of the movement of electrons • A single-headed curly arrow shows… • A double-headed curly arrow shows… Mechanism for the reaction of ethene with hydrogen bromide Mechanism for the reaction of propene with hydrogen bromide Remember Markovnikov? There are two possible products: 1. 2. Both mechanisms involve carbocations (careful there’s a mistake in your book!):
    • • The carbocation in reaction ____ is more stable • This is because the __________ groups donate electrons to stabilise the positive charge • As a result the major product is ___________________ Mechanism for the addition of bromine to ethene • The bromine-bromine bond is partially polarised by the electron rich ___ bond • The first stage of the reaction is ________________ attack • This is followed by _______________ attack on the carbocation by Br- Polymerisation – The most important reaction of the alkenes Key words Polymer – Monomer – Synthesis of polymers • Polymers are widely made in addition reactions
    • • Addition occurs across a double or triple bond Common polymers 1. Poly(ethene) (polythene) • Formed from the polymerisation of ____________________ • Comes in 2 forms __________ or ____________ • Conditions of polymerisation reaction determine which form is produced: 1.1 Low- density polythene (LDPE) Properties Conditions of Diagram of structure polymerisation reaction Uses 1.2 High- density polythene (HDPE) Properties Conditions of Diagram of structure polymerisation reaction Uses 2. Poly(propene) Properties Conditions of Monomer polymerisation reaction Polymer
    • 3. Poly(chloroethene) (PVC) Properties Conditions of Monomer polymerisation reaction Polymer 4. Poly(tetrafluoroethene) (PTFE / “Teflon”) Properties Monomer Polymer Properties of polymers • The physical (e.g. _________________ , __________________ and ________________) and chemical properties of polymers depend on their molecular structure: 1. Average length of polymer chain • Tensile strength • Melting temperature 2. Branching of the chain • Tensile strength • Melting temperature • Density 3. Intermolecular forces between chains • Tensile strength • Melting temperature 4. Cross- linking between chains • Rigidity
    • • Hardness • Flexibility • Melting temperature