Chapter 13: Alkenes

• unsaturated hydrocarbons, with at least 1 C=C double bond • aliphatic alkenes that contain 1 C=C bond have the general formula CnH2n • can be branched, have more than one C=C bond, and be cyclic (last 2 don’t follow general formula)

2 sigma bonds, 1 pi bond in each carbon atom • pi bond formed by sideways overlap of 2 p-orbitals • pi electron density is concentrated above and below the line joining the nuclei of the bonding atoms • pi bond locks 2 carbon atoms in position and prevents free rotation around the double bond

bond formed by the sideways overlap of 2 p-orbitals, with the electron density above and below the plane of the bonding atoms

hydrocarbon homologous series with at least 1 C=C double bond

Trigonal planar becuase: - 3 regions of electron density (electron pairs) aroudn each C atom - which repel each other as far as possible so bond angle around each C atom is 120 degrees - all the atoms are in the same plane

Same structural formula but a different arrangement of atoms in space

Because rotation around the double bond is restricted, and therefore each group around each carbon atom is fixed relative to each other. (because pi bond electron density is above and below the sigma bond)

• A C=C double bond • different groups attached to each carbon atom of the double bond

• must have a double C=C bond • each carbon atom must be attached to 2 different groups • one of the attached groups on each carbon atom of the double bond must be the same

• groups with higher priority on the same side are Z • groups with higher priority that are diagonal across the double bond is an E isomer Priority is assigned by atomic number. The higher the atomic number of the atom attached to the carbon, the higher the priority. If the atom is the same, look at next highest priority (i.e. atoms attached to that atom) - 1st point of diffference

• alkenes have a pi bond • in a pi bond, electron density is concentrated above and below the plane of the sigma bond • as the pi electrons are on the outside of the double bond, they are more exposed than the electrons in the sigma bond, so break more readily

Hydrogenation Halogenation Hydrogen halides addition Hydration of alkenes

alkene + hydrogen –> alkane • nickel catalyst • 423 K

alkene + halogen –> haloalkane halogen is chlorine/bromine room temperature and pressure

alkene + gaseous hydrogen halide –> haloalkane • room temperature and pressure If an unsymmetrical alkene reacts with an unsymmetrical compound, 2 products are possible

alkene + steam (water) –> alcohol • steam (gaseous water needed) • phosphoric acid catalyst (H3PO4)

Add bromine water to sample - If alkene is present, colour of sample will turn from orange to colourless due to addition reaction of alkene - If it’s an alkane (or a saturated hydrocarbon), it will remain orange

addition reaction in which 1st step is attack by an electrophile on a region of high electron density. double bond in alkene is a region of high electron density due to pi electrons high electron density of pi electrons attracts electrophiles

atom or group of atoms that is attracted to an electron-rich centre and accepts an electron pair. (Electron pair receptor). They are usually a positive ion, or a molecule containing an atom with a d+ charge (or electron-deficient molecule i.e. what I just said)

• heterolytic fission • carbocation intermediate • electrophile e.g. HBr Alkenes can also react with non-polar molecules e.g. Br2

pi electrons interact with Br-Br bond (e.g.) when Br2 molecule approaches the alkene The Br-Br bond gets polarized (called an induced dipole)

When isomers are produced in addition reactions, there are major and minor products formed The major product has a more stable carbocation intermdeiate

stability depends on the number of alkyl groups - the greater the number of alkyl groups ,the more stable tit is i.e. tertiary is more stable the secondary, which is more stable than primary

Ion that contains a positively charged carbon atom