OCR AS Chemistry A H032 for first assessment in Complete Tutor Notes. Section: Alkenes Boomer Publications

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1 OR AS hemistry A 032 for first assessment in 206 omplete Tutor Notes Section: 4..3 Alkenes E/Z Isomerism Alkenes Addition polymers 205 Boomer Publications page 43 page 45 page 5

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3 E/Z Isomerism Page E/Z isomerism (a form of stereoisomerism) Stereoisomers have the same structural formula but different arrangement of atoms in space. For example,2-dibromoethene = same structural formula = different arrangement of atoms E-,2-dibromoethene or trans,2-dibromoethene hydrogens on opposite sides of the double bond Z-,2-dibromoethene or cis,2-dibromoethene hydrogens on same side of the double bond. 2 different groups attached to carbon 2 (hydrogen and methyl) and... You could remember which is E and which is Z by remembering The s on Z are on ze zame zide Why do some alkenes show E/Z isomerism and some not? In order for alkenes to show E/Z isomerism, they must:. have a carbon-carbon double bond to prevent rotation of the groups. have two different groups on each carbon of the carbon-carbon double bond this compound shows E/Z isomerism because different groups attached to carbon 3 (hydrogen and ethyl) 2. carbon-carbon double bond prevents rotation of the groups

4 Page 44 E/Z Isomerism 37 This compound does not show E/Z isomerism because... although there is a carbon-carbon double bond to prevent rotation of the groups... the two groups on carbon 2 are the same (methyl groups) ahn Ingold Prelog priority rules Inspecting the structures to see whether the hydrogens are on the either the same side, (Z), or on opposite sides, (E), works fine when there are two hydrogens an each carbon on the carbon-carbon double bond. owever, in situations where there is not, you need to assign priority rules to each of the four groups. l In this example, we take one of the carbons, on the carbon-carbon double bond and assign priorities to the two groups attached to the carbon. We give the highest priority,, to the group which has the largest relative mass for the atom that is attached to the carbon. In this case chlorine has a higher relative mass than carbon so is assigned and the methyl group, 2. l We now assign priority to the groups attached to the other carbon in the same way. So the methyl group is assigned and the hydrogen, 2. In this case, the s are on ze zame zide so the molecule is given the prefix of Z, Z-2-chlorobut-2-ene. In situations where there is the same atom attached to the same carbon, for example, say there were methyl, 3, and ethyl, 2 5, groups attached, then you go to atom attached to the carbon. In this case, the ethyl will be assigned the higher priority as the next atom is carbon (-) and on the methyl group it will be hydrogen (-)

5 38 Alkenes Alkenes Page 45 ydrocarbons with one or more carbon-carbon double bond are called alkenes and these are unsaturated since it is possible to fit more hydrogens on to form alkanes. 3 3 Formation of the carbon-carbon double bond Although when we draw the carbon-carbon double bond, the two bonds look identical, in fact the two bonds that make up the double bond are different. One of the bonds is formed by direct overlap of orbitals to make a sigma, σ, bond. orbitals overlap directly to form a sigma, σ, bond The second bond is formed by sideways overlap of p orbitals, above and below the plane of the molecule. This forms the pi, π, bond. pi, π, bond formed by sideways overlap of p orbitals above and below the plane of the molecule The pi bond, because it is formed above and below the plane of the molecule, fixes the groups on each carbon in place and prevents rotation. The pi bond is weaker than the sigma bond and this makes alkenes more reactive than alkanes.

6 Page 46 Alkenes 38 The bond angles around each carbon forming the carbon-carbon double bond are 20 and the shape around each carbon is trigonal planar ethene methane whereas alkanes have a tetrahedral shape around the carbon with bond angles of 09.5 Just as it is possible to have cyclic alkanes, cyclic alkenes are possible with one of the more common molecules of this type being cyclohexene. cyclohexene An unsaturated compound contains one or more carbon-carbon double bonds or carbon to carbon triple bonds. Electrophilic addition Alkenes readily undergo addition reactions since the pi bond is readily attacked by an electrophile. An electrophile is a species that can accept a pair of electrons to form a bond.

7 Alkenes Page For example, the mechanism for electrophilic addition of ethene by hydrogen bromide goes like this: + The polar nature of hydrogen bromide means the hydrogen is electron deficient and can attack the double bond as this is electron rich The hydrogen to bromine bond breaks by heterolytic fission and a new bond forms from the carbon to the hydrogen creating a negatively charged bromide ion and a positively charged carbocation as short lived intermediates The negatively charged bromide ion can then form a covalent bond with the carbocation to form the product, bromoethane.

8 Page 48 Alkenes 38 If this reaction occurs with a molecule that is not symmetrical around the double bond, for example propene, two products are formed; -bromopropane and 2-bromopropane: -bromopropane 2-bromopropane By understanding the electrophilic addition mechanism, we will be able to explain why these two products are formed. The double bond can spring open this way, leaving the positive charge on number 2 carbon which will then form a bond with the bromide ion bromopropane Alternatively, the double bond can spring open like this, leaving the positive charge on carbon on which the bromine can then join bromopropane - 3 2

9 Alkenes Page Markovnikov's Rule and predicting the major product The rule states, When a molecule A attaches itself to an alkene that is not symmetric around the double bond, the hydrogen will attach itself to the carbon already carrying the larger number of hydrogens. 3 3 This isomer will be the more abundant product as the hydrogen has attached itself to the carbon already carrying the greater number of hydrogens. The reason why this is the case is the more abundant isomer is formed from the most stable carbocation intermediate. In the above case: This carbocation intermediate is the more stable and so will go on to produce the abundant isomer The reason why this carbocation is more stable is discussed in the video accompanying these notes.

10 Page Alkenes Electrophilic addition reactions can also take place with bromine as the electrophile. Although there is no permanent dipole across the bromine to bromine bond, the high electron density on the pi bond can induce a dipole and then it behaves in the same manner as hydrogen bromide high electron density of carbon-carbon double bond can induce a dipole in the bromine molecule ,2-dibromopropane (only product) This reaction can be used as a test for unsaturation. If a carbon-carbon double bond is present, bromine water will change from orange to colourless Other addition reactions you need to learn: and 2 2 (or other halogen) 2 O(steam) + 3 PO 4, high temp. & high pressure 3 (this reaction is used in the manufacture of margarine) 2 (g), 50 & Ni (catalyst) 3 3 O and 3 O

11 Addition Polymers Page 5 39 Addition Polymers Alkenes are very useful as they can easily polymerise by addition reaction with themselves. For example, a row of ethene molecules can react to produce polyethene. ethene monomers polyethene Other polymers may be formed in a similar way. For example, polypropene: n 3 3 propene monomer unit this is the repeat unit - the whole polymer can be made by cutting and pasting these units n when drawing repeat units make sure that the 2 end bonds cross the brackets

12 Page 52 Addition Polymers 39 Drawing repeat units of a polymer when given the monomer For example, 3-chloroprop--ene, 2 = 2 l: First, draw the double bond horizontal and identify the four groups on each carbon either side of the double bond: 2 l Next, snap one of the bonds making the double bond and swing it outwards in both directions: 2 l 2 l 2 l then place the brackets around with the n to indicate a large number n If you are asked to produce multiples of the repeat unit, just cut and paste 2 l 2 l 2 l Three repeat units of the polymer poly-3-chloroprop--ene

13 Addition polymers Page Finding the monomer from the polymer This is the polymer methyl cyanoacrylate N N N O O 3 O O 3 O O 3 Identify the repeat unit which will be made from 2 carbons from the polymer chain. N N N O O 3 O O 3 O O 3 Isolate this with the two end bonds: N Swing the two end bonds in to make the second bond of the double bond O O 3 N O O 3 monomer unit Many addition polymers are nonbiodegradable. Their disposal does therefore present problems. Recycling is the ideal solution although they may also be burnt and the energy generated used to make electricity. ombustion of PV can generate the toxic gas l. Polymers may also be used as chemical feedstocks. Designing polymers which are biodegradable is also a solution to the problem of polymer waste.

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