4.1.3 Alkenes Alkenes contain a carbon- General formula is 2n Alkenes are unsaturated hydrocarbons CnH carbon double bond somewhere in their structure H H H H C C C H C C Ethene Propene H H H H H H H H H Numbers need to be H added to the name when H C C C C H But-2-ene C C C C H But-1-ene positional isomers can occur H H H H H H H H π bonds are exposed and have high C=C double covalent bond electron density. consists of one sigma (σ) bond and one pi (π) bond. They are therefore vulnerable to attack by species which ‘like’ electrons: these C-C sigma bond C-C pi bond species are called electrophiles. Formation of σ bond in alkenes C C two sp2 orbitals (one from each carbon) overlap to sigma σ bond form a single C-C bond called a sigma σ bond Formation of π bond in alkenes p orbitals Rotation can occur around a sigma bond 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. C-C sigma bond The π bond is weaker than C-C pi bond the σ bond. H H H H The arrangement of bonds around C C C C the >C=C< is planar and has the bond angle 120o H CH3 H H N Goalby chemrevise.org 1 Stereoisomerism Stereoisomers have the same structural formulae but Alkenes can exhibit a type of isomerism have a different spatial arrangement of atoms. called E-Z stereoisomerism E-Z isomers exist due to restricted E-Z stereoisomers arise when: rotation about the C=C bond (a) There is restricted rotation around the C=C double bond. (b) There are two different groups/atoms attached both ends of the Single carbon-carbon covalent double bond. bonds can easily rotate But-1-ene is a structural isomer of But-2-ene H H two different groups but does not show E-Z isomerism. H H H C C H attached either end of the restricted double bond- H C C H But-1-ene C C leads to EZ isomers C C H H H H H H H Z- but-2-ene H two identical groups attached to one end of the restricted double bond – H no E-Z isomers H These are two isomers as C H the lack of rotation H Skeletal formulae can also represent E-Z isomerism C C around the double bonds H means one cannot be Z-but-2-ene H C switched to the other. H H E-but-2-ene E -but-2-ene Naming E-Z stereoisomers Priority Group: The atom with the bigger First determine the priority groups on both sides of the double bond atomic number is classed as the priority atom Priority Priority group Cl Cl group Cl H side 1 side 2 C C C C Z-1,2-dichloroethene H H H Cl E-1,2-dichloroethene If the priority atom is on the same side of the If the priority atom is on the opposite side of the double bond it is labelled Z from the german double bond it is labelled E from the german zusammen (The Zame Zide!) entgegen (The Epposite side!) Cahn–Ingold–Prelog (CIP) priority rules. 1. Compare the atomic number of the atoms directly attached to each side of the Cl Br priority priority double bond; the atom of higher atomic number is given priority. C C 2. If the atoms are the same, consider the H Cl atoms at distance 2 from the double bond. Make a list of each atom bonded to the one directly attached to the double bond. H C Arrange list in order of decreasing atomic 3 CH3 priority number. Compare the lists atom by atom; at C C the earliest difference, the group containing priority the atom of higher atomic number is given CH H priority 2 H3C N Goalby chemrevise.org 2 cis-trans isomerism is a special case of EIZ isomerism in which two of the substituent groups are the same. H H H H H C C H C H H C C C C H H H H H H C H H Z- but-2-ene E- but-2-ene Can also be called Can also be called Cis- but-2-ene trans- but-2-ene The effect of EZ stereoisomerism on physical properties E-Z stereoisomers can have differing melting and boiling points. δ- δ- Cl δ- Cl Cl H δ+ C C δ+ δ+ C C δ+ H H H Cl δ- Z-1,2-dichloroethene Boiling point =60oC E-1,2-dichloroethene o This molecule is polar. The polar C-Cl bonds are on Boiling point =48 C the same side of the molecule. One side of the This molecule is non- polar. The polar C-Cl molecule is slightly negative. bonds are on opposite sides of the molecule. The intermolecular forces are both London forces The dipoles cancel out. and permanent dipole-dipole attractions. The intermolecular forces are is only London forces so lower boiling point. N Goalby chemrevise.org 3 Addition reactions of alkenes The alkenes are relatively reactive because of the relatively low bond enthalpy of the π-bond. Addition reaction: a reaction where two molecules react 1. Reaction of Alkenes with Hydrogen together to produce one Change in functional group: alkene alkane H H H H Reagent: hydrogen H C C H Conditions: nickel catalyst C C + H2 Type of reaction: Addition/reduction H H H H ethene ethane Electrophilic Addition Reactions of Alkenes The double bonds in alkenes are areas with high Definition Electrophile: an electron pair acceptor electron density. This attracts electrophiles and the alkenes undergo addition reactions 2. Reaction of alkenes with bromine/chlorine H H Change in functional group: alkene dihaloalkane H H Reagent: Bromine H C C H C C + Br Conditions: Room temperature (not in UV light) 2 Mechanism: Electrophilic Addition H H Br Br + Type of reagent: Electrophile, Br 1,2-dibromoethane Type of Bond Fission: Heterolytic As the Br molecule H H 2 H H H H approaches the alkene, the pi bond electrons + The INTERMEDIATE C C H C C H H C C H repel the electron pair in formed, which has a the Br-Br bond. This δ+ positive charge on a H H Br carbon atom is called a INDUCES a DIPOLE. Br2 Br Br Br becomes polar and :Br - CARBOCATION δ+ ELECTROPHILIC (Br ). Brδ- N Goalby chemrevise.org 4 3. Reaction of Hydrogen Bromide with Alkenes Change in functional group: alkenehaloalkane H H H H H H Reagent: HCl or HBr H C C C C H + HBr H C C C C H Conditions: Room temperature Mechanism: Electrophilic Addition H H H H H H Br H Type of reagent: Electrophile, H+ But-2-ene 2-bromobutane HBr is a polar H H H molecule because H H H This reaction can Br is more lead to two H3C C C CH3 + products when the electronegative H3C C C CH3 H3C C C CH3 than H. The H δ + is δ+ alkene is attracted to the H H Br H unsymmetrical - electron-rich pi Brδ :Br - bond. - ‘Markownikoff’s Rule’ H :Br In most cases, bromine will be added to the + H H H H carbon with the fewest hydrogens attached to it C CH2 Major H C C C C H If the alkene is H3C CH3 product unsymmetrical, δ+ δ- :Br - 90% addition of hydrogen H Br H H Br H bromide can lead to H H + two isomeric CH2 CH2 CH2 CH3 Minor H2C CH CH2 CH3 C C CH2 CH3 products. product Br H H 10% WHY? In electrophilic addition to alkenes, the major product is This carbocation intermediate H H H formed via the more stable carbocation intermediate. is more stable because the methyl groups on either side H C C C H In exam answers + of the positive carbon are •Draw out both carbocations and identify as primary, secondary electron releasing and reduce H H and tertiary the charge on the ion which •State which is the more stable carbocation e.g. secondary more stabilises it. stable than primary •State that the more stable carbocation is stabilised because the The order of stability for carbocations is methyl groups on either (or one) side of the positive carbon are tertiary > secondary >primary 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) 4. Reaction of alkenes with steam to form alcohols Industrially alkenes are converted to alcohols in one step. They are This reaction can be called reacted with steam in the presence of an acid catalyst. hydration: a reaction where water is added to a molecule CH2=CH2 (g) + H2O (g) CH3CH2OH (l) Reagent : steam Essential Conditions The high pressures needed mean this cannot be High temperature 300 to 600°C done in the laboratory. It is preferred industrially, however, as there are no waste products and so has High pressure 70 atm a high atom economy. It would also mean separation Catalyst of concentrated H PO of products is easier (and cheaper) to carry out. 3 4 N Goalby chemrevise.org 5 Addition Polymers Addition polymers are formed from alkenes Poly(alkenes) like alkanes are unreactive due to the strong C-C and C-H bonds This is called addition polymerisation be able to recognise the repeating unit in a poly(alkene) n Monomer H H H H H H Polymer Ethene polyethene C C C C C C H H H H CH3 H CH3 H CH3 H n C C C C H CH3 H CH3 n propene poly(propene) Add the n’s if writing an equation showing the Poly(propene) is recycled reaction where ‘n’ monomers become ‘n’ repeating units It is best to first draw out H CH3 You should be able e.g.