Organic Chemistry Reactions Grade 12 Physical Science PDF

Organic Chemistry REACTIONS Grade 12 Physical Science Mrs KL Faling SUBSTITUTION REACTIONS This is a reaction where an atom or group of atoms is replaced by another atom or group of atoms Substitution only occurs in saturated compounds (single bonds) Substitution reactions offer requir...

Organic Chemistry REACTIONS Grade 12 Physical Science Mrs KL Faling SUBSTITUTION REACTIONS This is a reaction where an atom or group of atoms is replaced by another atom or group of atoms Substitution only occurs in saturated compounds (single bonds) Substitution reactions offer require heating and/or a catalyst Example: One of the H’s from methane is replaced by a chlorine. The H and the other chlorine then form HCℓ TYPE 1:Alkanes + HALOGENS (HALOGENATION) Alkanes are generally non-reactive NON- POLAR nature Can react with non-polar HALOGENS Occurs in presence of sunlight, UV light or heat EXAMPLE: Practice Ethane + Bromine  Butane + Iodine  Methane + Fluorine  Hexane + Chlorine   NOTE: The bromine test for saturation Bromine is used to test what homologous series an unknown hydrocarbon belongs to: If Bromine is added to an alkane (saturated) a non- spontaneous substitution reaction occurs and the solution remains brown. (If the hydrocarbon is unsaturated a spontaneous addition reaction occurs and the water goes clear) SUBSTITUTION IN ALCOHOLS Alcohols react with hydrogen halides to produce halo-alkanes and water (OPPOSITE TO THE HYDROLYSIS of halo- alkanes) The reaction is thus reversible NOTE: Reactions only occur with hydrogen and chlorine, bromine or iodine HF is not reactive Primary and secondary alcohols React slowly React best at high temperature An acid like H2SO4 should be added as a catalyst PRACTICE Methanol + HBr  Ethanol + HI  Propan-1-ol + HF  Butan-2-ol + HCℓ  Tertiary alcohols Occurs easily and fastest The tertiary alcohol is converted to a tertiary halo-alkane Occurs at room temperature Only works with hydrogen bromide and hydrogen chloride. Practice  SUBSTITUTION IN HALO-ALKANES with BASES Halo-alkanes react with an aqueous solution of a DILUTED metal hydroxide to form an alcohol. Concentrated hydroxide solutions will cause a different type of reaction EXAMPLE Chloro methane + KOH  2 Bromo-2,3 dimethyl butane + NaOH  ADDITION REACTIONS Unsaturated compounds can undergo addition Double bonds become single bonds and triple bonds become double bonds and an extra “something” is added. 4 types of addition reactions 1. Hydrohalogenation 2. Halogenation 3. Hydration 4. Hydrogenation 1. HYDROHALOGENATION Addition of a hydrogen halide to an alkene Conditions and process HCℓ, HBr or HI must be added to an alkene (NOT HF) No water must be present During the reaction the double bond breaks and the hydrogen attaches to one side and the halogen to the other side. The hydrogen will always attach to the most “primary” carbon of the two. Practice: Propene + HBr  2 methyl-1-butene + HCℓ  3 ethyl-2-pentene + HI  3,3,4 trimethyl-1-pentene + HBr  2. HALOGENATION The addition of a halogen No catalyst is necessary Only with chlorine and bromine (DIATOMIC MOLECULES!!!!!) One gets added each side of the “broken” double bond. Iodine is to slow and fluorine is to volatile Example Practice Propene + Chlorine gas  1-butene + Bromine gas  2,3 dimethyl-2-hexene + Cℓ2  4,5,6 triethyl-2-octene + Br2  3. Hydration Addition of excess water to an alkene to produce an alcohol. A strong acid is required as a catalyst generally H3PO4 + H2SO4 EASY EXAMPLE: Markovnikov’s rule for asymmetrical alkenes When the H2O splits during the addition reaction the single hydrogen atom will go to the carbon with the most hydrogens and the hydroxyl functional group (O-H) will go to become tertiary over secondary over primary. (worked the same way in hydrohalogenation) Example Practice 2-butane + H2O  1-butane + H2O  2 methyl-2-pentene + H2O  3,4 diethyl-2-heptane + H2O  4. Hydrogenation The addition of hydrogen gas in a hydrogen atmosphere (NO OXYGEN) to an unsaturated hydrocarbon which then results in a more saturated hydrocarbon. The alkene must be dissolved in a non-polar solvent like hexane or benzene Needs a catalyst: Pt, Ni or Pd This is sometimes called catalytic hydrogenation and is used in making margarine. Example Practice propene + hydrogen  2 methyl-propene + hydrogen  3 methyl-but-1-ene + hydrogen  APPLICATIONS OF ADDITION REACTIONS Hydrogenation of plant oils – plant oils are unsaturated and due to multiple bonds are in liquid form. Hydrogen is added in the presence of nickel as a catalyst and the plant oil hardens and produces margarine. ELIMINATION 1. Dehydrohalogenation 2. Dehydration (of alcohols) 3. Thermal cracking and catalytic cracking Dehydrohalogenation A hydrogen halide is eliminated from a haloalkane to for an alkene. The reaction must be strongly heated during reflux – the vapours must condense and return to the reaction vessel during heating. What you need for dehydrohalogenation? A strong base dissolved in pure ethanol This is known as warm ethanolic NaOH (or KOH) The product is an alkene, water and a halide salt. Example Practice 1 Bromo-butane + NaOH  2 Chloro-pentane + KOH  1 Bromo-propane + KOH  3 Chloro-hexane + NaOH  DEHYDRATION (of alcohols) H2O is removed from an alcohol. It is a catalysed acid reaction. Concentrated sulfuric acid is used as a dehydrating agent. (OR phosphoric acid) The product is an alkene and water Example ASYMMETRICAL alcohols We can focus on the main product which is that the “H” will come off the more secondary carbon. AKA The one with the least H’s -> This will form the main product (by- products can also be formed) Practice 1. 2 methyl-1-propanol  2. 3,3 dimethyl-2-hexanol  3. 3 ethyl-2-pentanol  Cracking Hydrocarbons can be cracked to make smaller more useful molecules. Thermal cracking occurs at a high temperature and pressure. Catalytic cracking occurs at a lower temperature but requires a catalyst. EXAMPLE – The product of thermal cracking is multiple short chains – at least ONE must be an alkene Complete Ex 9 Page 122