Alkenes Bonding in alkenes Stereoisomerism Additional reaction of
Alkenes Bonding in alkenes Stereoisomerism Additional reaction of alkenes Electrophilic addition in alkenes Polymers from alkenes
Introduction ● ● ● Application of alkene Ethylene or ethene, the simplest form of alkene is used to produce polyethylene. Polyethylene consumes more than half of the world's ethylene supply. Polyethylene, also called polyethene, is the world's most widely used plastic. It is primarily used to make films in packaging, carrier bags, plastic containers and trash liners. The general formula for an alkene is Cn. H 2 n. The general formula for cycloalkene is Cn. H 2 n 2.
Bonding in alkenes ● ● ● Bonding in alkenes and ethene There are two type of bonds in alkenes: σ-bonds and π-bonds. Modified p-orbitals overlap linearly to form σ-bonds. p-orbitals overlap sideways to form π- bonds. π - bonds are weaker than σ-bonds due to less overlap of p-orbitals in π-bonds. These bonds cannot be rotated. The electron configuration of carbon is 1 s 2 2 p 2. It has four valence electrons: two from s-orbital and two from p-orbitals. Three of these valence electrons are involved in σbonds: two bonds with hydrogen and one with another carbon atom. The fourth electron that occupies a p-orbital, overlaps sideways with the p-orbital of another carbon atom to form a π-bond.
Bonding in alkenes ● ● ● Structure of ethene Ethene is planar in shape to ensure maximum overlapping of p-orbitals in πbond. The H—C—H forms a trigonal planar shape with a bond angle of 117⁰. The bond angle for trigonal planar shapes is 120⁰ and in this case, it is 117⁰. This is because of the position of π-bond that minimises the repulsive forces.
● Stereoisomerism ● ● Stereoisomers of but-2 -ene As unsaturated hydrocarbons cannot rotate freely, they exhibits a different type of isomerism called stereoisomerism. Saturated hydrocarbons (single carbon—carbon bonds only) do not exhibit this type of isomerism as it rotates freely. In E-but-2 -ene, the methyl groups are positioned on both the sides of C=C bond(trans isomer). In Z-but-2 ene, the methyl groups are positioned on the same side of the C=C bond. E-but-2 -ene is also called as trans-but-2 -ene. Z-but-2 -ene is also called as cis-but-2 -ene. Cis and trans are used only if the groups attached to each carbon atom in the C=C bond are same( substituent groups are the same). These two stereoisomers have different physical and chemical properties. Note: two identical groups attached to one end of the restricted double bond i. e. to one C atom means no E -Z isomers will be formed.
Stereoisomerism: Cahn-Ingold-prelog (CIP) priority rules ● ● CIP rules for naming stereoisomers Cahn-Ingold-prelog (CIP) priority rules are used to name stereoisomers: Compare the atomic number of the groups attached to the carbon atoms on each side of the C=C bond. Groups with highest atomic numbers is given the highest priority. In this example, for E-1 -bromo-2 -chloro-ethene, the bromine attached to one of the carbon atoms has the highest priority and chlorine attached to another carbon has highest priority. When both the atoms of higher priorities are positioned on both the sides of C=C bond, it is a stereoisomer of type E. When both the atoms of higher priorities are positioned on same side of C=C bond, it is a stereoisomer of type Z.
Stereoisomerism: Cahn-Ingoldprelog (CIP) priority rules ● ● An example for naming stereoisomers ● ● Left group list of Group atoms CH 3 H-H-H CH 3 CH 2 C-H-H Highest CH 3 CH 2 priority Right group list of Group atoms CH 2 OH O-H-H CHO O-O-H Highest CHO priority ● In case the atoms attached to the C=C bond are same, they get equal priorities. In this molecule, it can be seen that atoms attached to C=C bond are all carbon. Hence, to name this kind of molecule, it is important to look at the atoms attached to these carbon atoms. The list of atoms attached to these carbon atoms are given. The list is in the order of increasing atomic number. In case of CHO group, oxygen in counted twice because of the double bond. Comparing the list of atoms one by one, the atom with highest atomic number is marked in red. The group of highest priority is thus found. In this example, as the groups with highest priorities are on the same side of C=C bond, this is a Z type of isomer.
Additional reactions of alkenes ● ● Alkenes are more reactive than alkanes due to the double bonds and the reactivity of πbonds. In additional reactions of alkenes, one of the two bonds in the C=C bond is broken and new single bonds are formed with each of the carbon atom.
Additional reactions of alkenes
Additional reactions of alkenes ● Addition of hydrogen halides (HX (aq)) Alkene is bubbled through a concentrated solution of hydrogen halide at room temperature to produce halogenoalkane. CH 2=CH 2 + HCl → CH 3 CH 2 Cl In case, the alkene is asymmetric such as propene, there are two possible products: 1 chloropropane and 2 -chloropropane. CH 3 CH=CH 2 + HCl → CH 3 CH 2 Cl and CH 3 CH=CH 2 + HCl → CH 3 CHCl. CH 3 (major product)
Additional reactions of alkenes ● ● ● Types of carbocations ● Addition of hydrogen halides (HX (aq)) Markownikoff’s rule is used to predict the major organic product in case of asymmetric alkenes. It states that the one product in which the halogen is attached to the carbon containing least number of hydrogen atoms is the major product. A carbocation is an ion with positively charged carbon atom. There are three types of carbocations which have different stabilities. In the previous reaction, CH 3 CHCl. CH 3 is the major product as the halogen is attached to the carbon with least number of hydrogen atoms. Moreover, this formed when halogen is attached to a secondary carbocation which is more stable than a primary carbocation.
Additional reactions of alkenes ● Addition of halogens (X 2(g)) When alkene is bubbled through a solution of halogens at room temperature, dihaloalkanes are formed. For example: ethene reacts with bromine to form 1, 2 -dibromoethane. This reaction is used as a test to detect the presence of C=C bond. Chlorine and bromine solutions are pale-green and orange colour respectively. When these solutions react with alkenes, the colour disappears.
Additional reactions of alkenes ● Oxidation by acidified potassium manganate (VII) Alkenes shaken with a dilute solution of acidified potassium magnanate at room temperature are oxidised. Alkene is converted to diol. In this reaction, pale purple solution turns colourless.
Electrophilic addition in alkenes ● ● ● Electrophilic addition in alkenes An electrophile is an acceptor of a pair of electrons. Even though C=C bond is a non-polar molecule, a high electron density is present around the C=C bond region. Thus, alkenes are susceptible to attack by an electrophile. Electrophilic addition takes place using heterolytic fission mechanism. When Br 2, a non-polar molecule, approaches a C=C bond, Br atom gets a slightly positive charge due to the high electron density near the C=C bond. This induces a negative charge in another Br atom. The Br 2 molecule breaks heterolytically. A new bond between a carbon and a bromine atom is formed. The Br- ion bonds with the highly reactive carbocation (an ion with positively charged carbon atom) and 1, 2 -dibromoethane is formed.
Electrophilic addition in alkenes ● ● ● Electrophilic addition of HBr in alkenes Electrophilic addition of HBr to propene is shown in the figure below. It can be seen that two types of products, i. e. , 1 bromopropane and 2 -bromopropane are obtained. It is important to note that during electrophilic addition of ethane, only one type of product is formed.
Polymers from alkenes ● ● Additional polymerisation Unsaturated molecules react with each other under specific conditions to form polymer molecules. The small units that react together to make the polymer is called as monomer. For example: up to 10, 000 molecules of ethene reacts to form the polymer polyethene. This reaction is called as addition polymerisation wherein the π-bond of C=C bond is broken and monomers are linked together.
Polymers from alkenes Additional polymerisation ● Monomers that would produce a given section of an addition polymer can be identified by the following steps: ü Split the polymer chain into its repeating units. ü Insert the C=C bond into the monomer.
Waste polymers and alternative ● a) Polymers are huge chain of alkanes that make it highly resistant to chemical attack. Used polymers are disposed to landfill sites which takes up a lot of valuable space. These polymers also pollute the environment as it does not degrade. Alternative uses of polymers are: Combustion for energy production: ü Polymers are burnt to release energy that is used to produce electricity. ü A disadvantage of this application is that alkanes burn in excess oxygen to produce carbon dioxide and water. Carbon dioxide is a green house gas that increases global warming. ü Moreover, if there is limited amount of oxygen, carbon monoxide is produced during the combustion, which is toxic.
Waste polymers and alternative b) c) Difficulty in recycling polymers: ü Recycling polymers is one way of minimising landfills. ü Recycling might become a difficult process if the materials were not sorted. This is because when halogenated plastics are burnt, hydrogen chloride gas is given off which is highly acidic. This gas has to neutralised at high temperatures before releasing it. Modern incinerators are efficient in burning toxins and removing pollutants while burning the polymers. But, greenhouse gases will be still released. Feedstock for cracking: ü Feedstock is a chemical used to support a large-scale chemical reaction. ü Cracking is the process of breaking long-chain hydrocarbons into simpler molecules. ü Waste polymers are used as feedstock for cracking process so that new plastics and other chemicals can be produced.
Waste polymers and alternative ● ● ● Biodegradable and photodegradable polymers are developed to reduce the harmful effects of polymers. Biodegradable polymers are decomposed by living organisms. These polymers are made from corn starch. For example: isoprene (2 -methyl-1, 3 -butadiene) is made from maize starch and is biodegradable. Photodegradable polymers are disintegrated in the presence of sunlight. Some photodegradable polymers contain functional groups such as carbonyl groups that can trap enough energy from sunlight to break the bonds. The smaller fragments then break down faster than larger fragments.
- Slides: 20