Physical and chemical properties
Alkenes are unsaturated hydrocarbons General formula is CnH2n Alkenes contain a carbon- carbon double bond somewhere in their structure C C H H H H Ethene C C H H H C H H H Propene Numbers need to be added to the name when positional isomers can occur C H H C C C H H H H H H But-1-ene But-2-ene C=C double covalent bond consists of one sigma (σ) bond and one pi (π) bond. π bonds are exposed and have high electron density. They are therefore vulnerable to attack by species which ‘like’ electrons: these species are called electrophiles. C C C C H H H H H H H H N Goalby chemrevise.org 1 The π bond is formed by sideways overlap of two p orbitals on each carbon atom forming a π-bond above and below the plane of molecule. The π bond is weaker than the σ bond. p orbitals C-C sigma bond C-C pi bond C-C sigma bond C-C pi bond The arrangement of bonds around the >C=C< is planar and has the bond angle 120o Formation of π bond C C H H H H
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4.1.3 Alkenes
Properties of alkenes (a) alkenes as unsaturated hydrocarbons containing a C=C bond comprising a π-bond (sideways overlap of adjacent p-orbitals above and below the bonding C atoms) and a σ-bond (overlap of orbitals directly between the bonding atoms) (see also 4.1.2 a); restricted rotation of the π-bond Hybridisation is not required. HSW1 Use of model of orbital overlap to explain covalent bonding in organic compounds. (b) explanation of the trigonal planar shape and bond angle around each carbon in the C=C of alkenes in terms of electron pair repulsion (see also 2.2.2 g–h, 4.1.2 b) M4.1, M4.2
Stereoisomerism
Alkenes are unsaturated hydrocarbons General formula is CnH2n Alkenes contain a carbon- carbon double bond somewhere in their structure C C H H H H Ethene C C H H H C H H H Propene Numbers need to be added to the name when positional isomers can occur C H H C C C H H H H H H But-1-ene But-2-ene C=C double covalent bond consists of one sigma (σ) bond and one pi (π) bond. π bonds are exposed and have high electron density. They are therefore vulnerable to attack by species which ‘like’ electrons: these species are called electrophiles. C C C C H H H H H H H H N Goalby chemrevise.org 1 The π bond is formed by sideways overlap of two p orbitals on each carbon atom forming a π-bond above and below the plane of molecule. The π bond is weaker than the σ bond. p orbitals C-C sigma bond C-C pi bond C-C sigma bond C-C pi bond The arrangement of bonds around the >C=C< is planar and has the bond angle 120o Formation of π bond C C H H H H
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4.1.3 Alkenes
Stereoisomerism in alkenes (c) (i) explanation of the terms: • stereoisomers (compounds with the same structural formula but with a different arrangement in space) • E/Z isomerism (an example of stereoisomerism, in terms of restricted rotation about a double bond and the requirement for two different groups to be attached to each carbon atom of the C=C group) • cis–trans isomerism (a special case of E/Z isomerism in which two of the substituent groups attached to each carbon atom of the C=C group are the same) (ii) use of Cahn–Ingold–Prelog (CIP) priority rules to identify the E and Z stereoisomers M4.2, M4.3 C C H H E-but-2-ene (trans) M4.2, M4.3 Z-but-2-ene (cis) CH3 H3C H3C C C H CH3 H Use of E as equivalent to trans and Z as equivalent to cis is only consistently correct when there is an H on each carbon atom of the C=C bond. Assigning CIP priorities to double or triple bonds within R groups is not required: C C R” R’ M4.2, M4.3 R”’ R (d) determination of possible E/Z or cis–trans stereoisomers of an organic molecule, given its structural formula M4.2, M4.3
Addition reactions / PAG 7
Addition reactions of alkenes Addition reaction: a reaction where two molecules react together to produce one Change in functional group: alkene alkane Reagent: hydrogen Conditions: Nickel Catalyst Type of reaction: Addition/Reduction 1. Reaction of Alkenes with Hydrogen C C H H H H + H2 ethane C C H H H H H H ethene Electrophilic Addition Reactions of Alkenes Definition Electrophile: an electron pair acceptor The double bonds in alkenes are areas with high electron density. This attracts electrophiles and the alkenes undergo addition reactions 2. Reaction of alkenes with bromine/chlorine Change in functional group: alkene dihalogenoalkane Reagent: Bromine Conditions: Room temperature (not in UV light) Mechanism: Electrophilic Addition Type of reagent: Electrophile, Br+ Type of Bond Fission: Heterolytic C C H H Br Br H H C C H H H H + Br2 1,2-dibromoethane As the Br2 molecule approaches the alkene, the pi bond electrons repel the electron pair in the Br-Br bond. This INDUCES a DIPOLE. Br2 becomes polar and ELECTROPHILIC (Brδ+ ). The INTERMEDIATE formed, which has a positive charge on a carbon atom is called a CARBOCATION C C H H H H C C H H Br Br H H C + C Br H H H H Br Br δ + δ – :Br – N Goalby chemrevise.org 4 3. Reaction of Hydrogen Bromide with Alkenes Change in functional group: alkenehalogenoalkane Reagent: HCl or HBr Conditions: Room temperature Mechanism: Electrophilic Addition Type of reagent: Electrophile, H+ C + HBr H H C C C H H H H H H C C H C H Br C H H H H H H H But-2-ene 2-bromobutane HBr is a polar molecule because Br is more electronegative than H. The H δ + is attracted to the electron-rich pi bond. This reaction can lead to two products when the alkene is unsymmetrical Major product 90% Minor product 10% If the alkene is unsymmetrical, addition of hydrogen bromide can lead to two isomeric products. ‘Markownikoff’s Rule’ In most cases, bromine will be added to the carbon with the fewest hydrogens attached to it C C H H H C H H H H + This carbocation intermediate is more stable because the methyl groups on either side of the positive carbon are electron releasing and reduce the charge on the ion which stabilises it. WHY? The order of stability for carbocations is tertiary > secondary >primary In electrophilic addition to alkenes, the major product is formed via the more stable carbocation intermediate. In exam answers •Draw out both carbocations and identify as primary, secondary and tertiary •State which is the more stable carbocation e.g. secondary more stable than primary •State that the more stable carbocation is stabilised because the methyl groups on either (or one) side of the positive carbon are electron releasing and reduce the charge on the ion. •(If both carbocations are secondary then both will be equally stable and a 50/50 split will be achieved) C + C H CH3 H H H3C C C H CH3 H H H3C Br C C CH3 H H H3C δ +δ – :Br – H Br H2C CH CH2 CH3 H Br H3C C + CH2 CH3 H C + C CH2 CH3 H H H H :Br – :Br δ – + δ – CH2 CH2 CH2 CH3 Br C C H C H Br C H H H H H H H 4. Reaction of alkenes with steam to form alcohols Industrially alkenes are converted to alcohols in one step. They are reacted with steam in the presence of an acid catalyst. CH2=CH2 (g) + H2O (g) CH3CH2OH (l) This reaction can be called hydration: a reaction where water is added to a molecule Reagent : steam Essential Conditions High temperature 300 to 600°C High pressure 70 atm Catalyst of concentrated H3PO4 The high pressures needed mean this cannot be done in the laboratory. It is preferred industrially, however, as there are no waste products and so has a high atom economy. It would also mean separation of products is easier (and cheaper) to carry out. The alkenes are relatively reactive because of the relatively low bond enthalpy of the π-bond.
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4.1.3 Alkenes
Addition reactions of alkenes (e) the reactivity of alkenes in terms of the relatively low bond enthalpy of the π-bond (f) addition reactions of alkenes with: (i) hydrogen in the presence of a suitable catalyst, e.g. Ni, to form alkanes (ii) halogens to form dihaloalkanes, including the use of bromine to detect the presence of a double C=C bond as a test for unsaturation in a carbon chain (iii) hydrogen halides to form haloalkanes (iv) steam in the presence of an acid catalyst, e.g. H3PO4, to form alcohols PAG7 (see also 6.3.1 c) (g) definition and use of the term electrophile (an electron pair acceptor) (h) the mechanism of electrophilic addition in alkenes by heterolytic fission (see also 4.1.1 h–i) For the reaction with halogens, either a carbocation or a halonium ion intermediate is acceptable. HSW1,2,8 Use of reaction mechanisms to explain organic reactions. (i) use of Markownikoff’s rule to predict formation of a major organic product in addition reactions of H–X to unsymmetrical alkenes, e.g. H–Br to propene, in terms of the relative stabilities of carbocation intermediates in the mechanism Limited to stabilities of primary, secondary and tertiary carbocations. Explanation for relative stabilities of carbocations not required. HSW1,2,5 Use of stability to explain products of organic reactions.
Addition polymers
Addition Polymers Addition polymers are formed from alkenes Poly(alkenes) like alkanes are unreactive due to the strong C-C and C-H bonds C C H H CH3 H propene n poly(propene) C C C C C C CH3 H H H CH3 H H H CH3 H H H be able to recognise the repeating unit in a poly(alkene) Poly(propene) is recycled This is called addition polymerisation Add the n’s if writing an equation showing the reaction where ‘n’ monomers become ‘n’ repeating units n C C H CH3 H H H3C CH CH CH3 H3C C C CH3 H You should be able H to draw the polymer repeating unit for any alkene It is best to first draw out the monomer with groups of atoms arranged around the double bond e.g. For but-2-ene C C CH3 H H CH3 Industrial importance of alkenes The formation of polymers from ethene based monomers is a major use of alkenes. The manufacture of margarine by catalytic hydrogenation of unsaturated vegetable oils using hydrogen and a nickel catalyst is another important industrial process. Liquid vegetable oils are generally polyunsaturated alkenes. Hydrogenation by the reaction of hydrogen using a nickel catalyst converts the double bonds to saturated single bonds. This increases the melting point of the oil making it harder and more solid. Waste polymers can be processed in several ways. Separation and recycling The waste is sorted into each different type of polymer (ie PTFE, PVC, PET) and then each type can be recycled by melting and remoulding. Combustion for energy production Waste polymers can be incinerated and the heat released can be used to generate electricity. Combustion of halogenated plastics (ie PVC) can lead to the formation of toxic, acidic waste products such as HCl. Chemists can minimise the environmental damage of this by removing the HCl fumes formed from the combustion process. Feedstock for Cracking Waste polymers can be used as a feedstock for the cracking process allowing for the new production of plastics and other chemicals. Chemists have also been developing a range of biodegradable polymers, compostable polymers, soluble polymers and photodegradable polymers. Dealing with waste polymers Polymers formed from isoprene (2-methyl-1,3- butadiene), maize and starch are biodegradable
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4.1.3 Alkenes
Polymers from alkenes (j) addition polymerisation of alkenes and substituted alkenes, including: (i) the repeat unit of an addition polymer deduced from a given monomer (ii) identification of the monomer that would produce a given section of an addition polymer. Waste polymers and alternatives (k) the benefits for sustainability of processing waste polymers by: (i) combustion for energy production (ii) use as an organic feedstock for the production of plastics and other organic chemicals (iii) removal of toxic waste products, e.g. removal of HCl formed during disposal by combustion of halogenated plastics (e.g. PVC) HSW9,10 Benefits of cheap oil-derived plastics counteracted by problems for environment of landfill; the move to re-using waste, improving use of resources. (l) the benefits to the environment of development of biodegradable and photodegradable polymers. HSW9,10 Benefits of reduced dependency on finite resources and alleviating problems from disposal of persistent plastic waste.
Credits: Neil Goalby