Chapter 13: Reactions of Alkenes

• The pi bond has relatively low bond enthalpy (not much energy is required to break it) as the electrons in a pi bond are more exposed than those in a sigma bond

• Hydrogen • Halogens • Hydrogen halides • Steam

• Alkene and hydrogen • Nickel catalyst • Alkane

• Alkene and halogen • Room temperature • No catalyst • Dihaloalkane

• It can be used as a test for saturation • This is because when bromine water is added to an alkene, it goes from orange to colourless • However, when this is done to an alkane, there is no addition reaction, and therefore no colour change, so the bromine water stays orange

• Room temperature, no catalyst • Concentrated hydrochloric or hydrobromic acid can be used • Haloalkanes

• They form a mixture of 2 different products

• Alkene and steam • Phosphoric acid (H3PO4) catalyst (or concentrated sulfuric acid) • > 100°C (already indicated if you said steam/ wrote the gas state symbol) • Alcohol (however, positional isomerism is possible again)

• An electron pair acceptor • (They’re usually a positive ion or a molecule containing an atom with a partial positive charge)

• An addition reaction in which the first step is attack by an electrophile on a region of high electron density

• Electrophiles are atoms/ groups of atoms that are attracted to an electron rich-centre (in this case the high electron density of the pi-electrons) and accept an electron pair

• The reaction between alkenes and halogens/ hydrogen halides

• The hydrogen halide molecule would be polar as halogens are more electronegative than hydrogen • The electrons in the pi-bond are attracted to the H atom due to its partial positive charge, and shares its electrons with it, so the double bond breaks • The H atom then bonds to one of the carbon atoms that was in the double bond • The bond between the H atom and halogen atom breaks by heterolytic fission, and the electrons go to the halogen atom • A halide ion and carbocation (C atom with a positive charge as it has only 3 bonds) are formed • The halide ion reacts with the carbocation to form the final product

• From the bond/ lone pair to the atom the electrons are being transferred to (always going form high electron density to low electron density)

• The hydrogen halide (more specifically the hydrogen atom)

• Halogen molecules aren’t polar, but as they approach the pi-bond, electrons are repelled, and a dipole is formed across the halogen molecule

• According to Markownikoff’s rule, most of the time the halide will attach itself to the carbon atom bonded to more carbon atoms than the other one • The product formed in this situation is the major product, while the other is the minor product

• There are 3 types of carbocations; primary, secondary and tertiary • Tertiary carbocations are bonded to 3 carbon atoms, secondary to 2 carbon atoms and primary carbocations to 1 carbon atom • Tertiary carbocations are therefore the most stable, while primary carbocations are the least stable • This is because alkyl groups can donate electrons

• A small molecule which combines with many other monomers to form a polymer

• A large molecule formed from many thousands of repeat units of smaller molecules (monomers)

• The simplest unit of a polymer that is repeated many times to form the full polymer (drawn with brackets and n)

• They undergo addition polymerisation to produce long saturated chains that contain no double bonds

• The monomer is the reactant, and has an n next to it • The product is the repeat unit drawn inside square brackets with bonds extending beyond the brackets, and with an n outside the bracket

• It consists of 2 carbons from the main chain, and the other atoms they are vertically bonded to (make sure to check to see which 2 carbons are being repeated)