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TYPES OF ORGANIC CHEMICAL REACTIONS Categories of Simple Organic Reactions • Substitution • Addition • Condensation/elimination • Hydrolysis • Mild oxidation & strong oxidation • Combustion • Acid/base reactions Substitution Reactions A substitution reaction occurs whenever one atom of an organic molecule is replaced by another. Substitution reactions often require heat and/or a catalyst in order to occur. Example: Substitution of an alkane H H H C H + Cl2 H C Cl + HCl HEAT H H Substitution of Benzene Due to the strong bonding arrangement between the carbon atoms in a benzene ring, the ring cannot be easily broken during chemical changes. The hydrogen atoms that are bonded to each carbon can be substituted for other atoms and groups. Examples: H Br + Br2 FeBr 3..catalyst + HBr + HNO3 H 2 SO 4 NO2 + H 2O TNT synthesis… Addition Reactions • In addition reactions, new atoms or groups are bonded to the atoms of an organic molecule. The organic molecule must be unsaturated to start with (double or triple bonds!) I’m now ready • New bonds can be made to the carbon atoms to make more involved in the double or triple bond if the part bonds! of the bond is first broken. H H CH2=CH2 breakpibond C C H H Some Simple Addition Reactions H H H H C=C + H2 H-C-C-H (a.k.a. “hydrogenation”) H H HH H H H H C=C + HCl H-C-C-Cl H H HH H H H H C=C + H2O H-C-C-OH (a.k.a. “hydrolysis”) H H HH Testing for unsaturated hydrocarbons • Unsaturated hydrocarbons can be identified by their reaction with aqueous bromine, Br2(aq). Demo! H H H H C=C + Br2 Br-C-C-Br H H HH (deep orange) (colourless) Completely saturated molecules DO NOT react in this manner- thus the orange colour remains. Markovnikov’s Rule • If the double or triple bond is part of a molecule with three or more carbon atoms in the chain, the most likely addition reaction products can be predicted using a rule first formulated by the Russian chemist V.V. Markovnikov. Markovnikov’s Rule …the more electronegative atom will predominantly bond to the carbon atom of the double bond that has fewer hydrogen atoms. Eg.1 Markovnikov’s Rule H H OH C=C-CH3 + H2O H- C - C- CH3 H H H H It should be mentioned that the primary alcohol (1-propanol) is also a possibility, but it would be produced in a much lower proportion than the 2-propanol shown Eg.2 above. H Cl H-C C-CH2-CH3 + 2HCl H-C - C-CH2-CH3 H Cl Most favoured product! Condensation/Elimination Reactions • Organic compounds containing hydroxyl or carboxyl groups can combine by eliminating a molecule of water between them. An ether is created! Example 1- condensation of two alcohols H H H H H H H H H-C - C-O-H H-C - C-O-C–C-H + H-O-C - C-H H 2 SO4 H H H H H H H H Molecule of H O is formed + H2O 2 Condensation/elimination • When an alcohol reacts with a carboxylic acid to eliminate a molecule of water between them, the resulting bond creates an ester linkage as shown below: We’ve created an ester H H O H-C - C-O-H + H-O-C – CH2-CH3 O H H If excess -CH2- O- C-CH2-CH3 CH2 water is added to an ester, the + reaction will reverse itself. H2O Uses of Esters Demo! • Esters often have characteristic flavours and aromas which allows them to be used in food products. • Methanol + butanoic acid methyl butanoate (apples) • Octanol + acetic acid octyl acetate (orange) • Benzyl alcohol + butanoic acid phenyl butanoate (flowers) • Pentanol + acetic acid pentyl acetate Condensation/elimination • If a molecule of water is eliminated from within a single molecule, an unsaturated compound is formed and the molecule is said to be “dehydrated.” H H H OH H H H 2 SO 4 + H2O Oxidation Reactions • In the terminology of organic chemistry, when a molecule is oxidized, the number bonds to oxygen atoms increases within the molecule. • Oxidizing agents are varied: CrO3/H+, KMnO4, K2Cr2O7 • Often, the oxidizing agent is represented simply as “O” over the reaction arrow: O Oxidation Reactions Mild Oxidation ..O 1o alcohol mild aldehyde H O CH3-C-OH CH3-C-H mild ..O H ethanol ethanal Strong Oxidation 1o alcohol Carboxylic acid strong ..O H O CH3-C-OH strong ..O CH3-C-OH H Ethanoic acid Oxidation Reactions Strong Oxidation cont’d… • Secondary alcohols require a strong oxidizing agent and can only be oxidized to ketones. OH O CH3-C-CH2-CH3 strong ..O CH3-C-CH2-CH3 H butanone 2-butanol •Tertiary alcohols DO NOT undergo oxidation. Why not? OH CH3-C-CH3 strong ..O NO REACTION CH3 DEMOS! 2-methyl-2-propanol Combustion • All organic compounds can be combusted or burned. Carbon dioxide gas and water vapour are the products of hydrocarbon combustion. • Combustion is really a type of very extreme oxidation reaction where O2(g) is the oxidizing agent. BOOM! 2C2H2(g) + 5O2(g) 4CO2(g) + 2H2O(g) CH3-CH2-OH(l) + 3O2(g) 2CO2(g) + 3H2O(g) Reactions of Organic Acids • Like inorganic acids such as HCl(aq), organic acids react with bases to produce liquid water and a salt. Example: O O C-O-H C-O1-Na1+ + NaOH + H2O Sodium benzoate Benzoic acid Time to Practice! • p.27 #1 3 • p.30 #4 • p.31 #2, 3, 5 • p.37 #2 • p.44 #7, 8, 9 • p.48 #4, 5, 6 • p.56 #6 • p.57 #3, 4 • p.63 #3, 5, 7, 8 • p.68 #3, 4 Organic chemistry will get your gears going!
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