Organic Chemistry Lesson 10 Reactions of Organic Molecules Combusition Reactions Substitution Reactions Physical Sciences Grade 12 1 Instructions Lesson 10 (16 April 2020) 1. Read ALL the slides in this document. 2. Homework: Just understand the theory for now. • If you have any questions, post them onto your WhatsApp group and your teacher will attend to them when (s)he has a chance. • Additional Resources: • Summary on pg.132 of your textbook. Physical Sciences Grade 12 2 Types of Reactions of Organic Compounds 1. Esterification: formation of an ester from an alcohol and a carboxylic acid. 2. Combustion Reactions (aka Oxidation): organic molecule + oxygen. 3. Substitution Reactions: one or more atoms are replaced by other atoms. (i) alkanes to alkyl halides; (ii) alcohols to alkyl halides; (iii) alkyl halides to alcohols 4. Addition Reactions: one or more atoms are added. (i) hydrogenation; (ii) halogenation; (iii) hydrohalogenation; (iv) hydration 5. Elimination Reactions: one or more atoms are removed. (i) dehydrohalogenation; (ii) dehydration; (iii) cracking of alkanes Physical Sciences Grade 12 3 1. Esterification • Already discussed (refer to earlier notes). Physical Sciences Grade 12 4 2. Combustion Reactions, i.e. Oxidation • A combustion reaction is the reaction of an organic molecule with oxygen. • Please note: “combustion” does NOT mean “explosion”. All combustion reactions release energy but most do so WITHOUT any explosion. • Alkanes – in the form of fossil fuels – are currently the main source of energy worldwide as their combustion is highly exothermic. • Examples: Burning wood for a braai. Propane combusting in a gas stove. Petrol combusting in a car engine. Physical Sciences Grade 12 5 2. Combustion Reactions, i.e. Oxidation • Combustion of Alkanes: alkane + 푂2 푔 → 퐶푂2 푔 + 퐻2푂 푔 ∆퐻 < 0 • Steps for Balancing: 1. Balance the carbon and hydrogen atoms first. 2. Balance the oxygen atoms second by dividing the total number of oxygen atoms on the RHS by 2. 3. If step 2 leads to a fraction, multiply every coefficient by 2. Physical Sciences Grade 12 6 Examples (Combustion Reactions) Physical Sciences Grade 12 7 Examples (Combustion Reactions) Physical Sciences Grade 12 8 Examples (Combustion Reactions) Here are two examples for you to attempt: e.g.3 Propane is used as a fuel in some gas stoves. Write out a balanced combustion reaction for propane. e.g.4 Butane is also sometimes used as a fuel in gas stoves. Write out a balanced combustion reaction for butane. Fist pump for the first person to post a correct solution on the WhatsApp group. Physical Sciences Grade 12 9 3. Substitution Reactions • A substitution reaction involves one or more atoms being replaced by another atom or group of atoms in an organic molecule. • Substitution reactions take place in saturated organic compounds like alkanes, alkyl halides and alcohols. • Grade 12 Substitution Reactions: 1. Halogenation of alkanes to produce alkyl halides. 2. Reactions of 퐻푋 (푋 = 퐶푙, 퐵푟) with alcohols to produce alkyl halides. 3. Hydrolysis of alkyl halides to produce alcohols. Physical Sciences Grade 12 10 Substitution Reactions: Alkanes to Alkyl Halides • Alkanes will react with free radicals, i.e. atoms with an unpaired electron. • In the presence of light (ℎ푓) and heat (∆), a halogen atom (푋) will replace a hydrogen atom on an alkane to form an alkyl halide. • This reaction is difficult to stop and additional substitution reactions will occur. ℎ푓 • Reaction: alkane + 푋2 → alkyl halide + 퐻푋 푋 = 퐹, 퐶푙, 퐵푟 or 퐼 ∆ Physical Sciences Grade 12 11 Substitution Reactions: Alkanes to Alkyl Halides Physical Sciences Grade 12 12 Substitution Reactions: Alkanes to Alkyl Halides Physical Sciences Grade 12 13 Organic Chemistry Lesson 11 Substitution Reactions Physical Sciences Grade 12 14 Instructions Lesson 11 (17 April 2020) 1. Read ALL the slides in this document. 2. Homework: Think Tank 10.3, pg.134 Q 8 ; 9 ; 10 • If you have any questions, post them onto your WhatsApp group and your teacher will attend to them when (s)he has a chance. • Additional Resources: • Summary on pg.132 of your textbook. • Substitution & Esterification Reactions: www.youtube.com/watch?v=EcrMHmo-YsY Physical Sciences Grade 12 15 Types of Reactions of Organic Compounds 1. Esterification: formation of an ester from an alcohol and a carboxylic acid. 2. Combustion Reactions (aka Oxidation): organic molecule + oxygen. 3. Substitution Reactions: one or more atoms are replaced by other atoms. (i) alkanes to alkyl halides; (ii) alcohols to alkyl halides; (iii) alkyl halides to alcohols 4. Addition Reactions: one or more atoms are added. (i) hydrogenation; (ii) halogenation; (iii) hydrohalogenation; (iv) hydration 5. Elimination Reactions: one or more atoms are removed. (i) dehydrohalogenation; (ii) dehydration; (iii) cracking of alkanes Physical Sciences Grade 12 16 3. Substitution Reactions • A substitution reaction involves one or more atoms being replaced by another atom or group of atoms in an organic molecule. • Substitution reactions take place in saturated organic compounds like alkanes, alkyl halides and alcohols. • Grade 12 Substitution Reactions: 1. Halogenation of alkanes to produce alkyl halides. 2. Reactions of 퐻푋 (푋 = 퐶푙, 퐵푟) with alcohols to produce alkyl halides. 3. Hydrolysis of alkyl halides to produce alcohols. Physical Sciences Grade 12 17 Substitution Reactions: Alcohols ⇌ Alkyl Halides • Since the hydroxyl group (−푂퐻) in alcohols and the halogen in alkyl halides usually occupy similar positions in the molecule, the two groups can be substituted with each other. • In other words, an interconversion reaction can occur where an alcohol can be reacted to form an alkyl halide and vice versa. • Reaction: alcohol + 퐻푋 → alkyl halide 푋 = 퐹, 퐶푙, 퐵푟 or 퐼 ∆ • Heat is needed for primary and secondary alcohols to react with 퐻푋. • Tertiary alcohols DO NOT require heat and will react at room temperature. Physical Sciences Grade 12 18 Substitution Reactions: Alcohols ⇌ Alkyl Halides Physical Sciences Grade 12 19 Examples (Alcohols ⇌ Alkyl Halides) Here are two examples for you to attempt: Use structural and condensed structural formula to show the reaction between: e.g.1 ethanol and hydrochloric acid e.g.2 3-methylbutan-2-ol and HBr High five for the first person to post a correct solution on the WhatsApp group. Physical Sciences Grade 12 20 Substitution Reactions: Alcohols ⇌ Alkyl Halides ethanol • Reaction: alkyl halide + base → alcohol + 푵풂푋 ; 푋 = 퐹, 퐶푙, 퐵푟 or 퐼 ∆ • 푁푎푂퐻 or 퐾푂퐻 are appropriate bases to use. • The alkyl halide and base must be dissolved in ethanol (as alkyl halides do not mix with water). • The solution also needs to be warmed for the reaction to occur. Physical Sciences Grade 12 21 Substitution Reactions: Alcohols ⇌ Alkyl Halides Physical Sciences Grade 12 22 Examples (Alcohols ⇌ Alkyl Halides) Here are two examples for you to attempt: Use structural and condensed structural formula to show the reaction between: e.g.1 2-chloro-2-methylpropane and NaOH e.g.2 1-fluoro-2,2-dimethylpropane and NaOH Chocolate for the first person to post a correct solution on the WhatsApp group. Physical Sciences Grade 12 23 Substitution Reactions: Hydrolysis • Hydrolysis is the reaction of an alkyl halide with water to form an alcohol and a hydrogen halide. • The alkyl halide must be dissolved in ethanol. • Reaction: alkyl halide + 퐻2푂 → alcohol + 퐻푋 푋 = 퐹, 퐶푙, 퐵푟 or 퐼 Physical Sciences Grade 12 24 Substitution Reactions: Hydrolysis Physical Sciences Grade 12 25 Examples (Hydrolysis) Here are two examples for you to attempt: Use structural and condensed structural formula to show the reaction between: e.g.1 the hydrolysis of 2-chloropropane e.g.2 the hydrolysis of 3-methyl-2-fluorobutane Thumbs up for the first person to post a correct solution on the WhatsApp group. Physical Sciences Grade 12 26 Organic Chemistry Lesson 11 Homework Solutions Physical Sciences Grade 12 27 Physical Sciences Grade 12 28 Physical Sciences Grade 12 29 Substitution Reactions: Alcohols ⇌ Alkyl Halides ethanol • Reaction: alkyl halide + base → alcohol + 푵풂푋 ; 푋 = 퐹, 퐶푙, 퐵푟 or 퐼 ∆ • 푁푎푂퐻 or 퐾푂퐻 are appropriate bases to use. • The alkyl halide and base must be dissolved in ethanol (as alkyl halides do not mix with water). • The solution also needs to be warmed for the reaction to occur. Physical Sciences Grade 12 30 Organic Chemistry Lesson 12 Addition Reactions Physical Sciences Grade 12 31 Instructions Lesson 12 (20 April 2020) 1. Read ALL the slides in this document. 2. Homework: Think Tank 10.3, pg.133-134 Q 1 ; 2 ; 3 ; 11 • If you have any questions, post them onto your WhatsApp group and your teacher will attend to them when (s)he has a chance. • Additional Resources: • Summary on pg.131 of your textbook. • Organic Reactions – Addition: www.youtube.com/watch?v=5AOg7B44wt0 Physical Sciences Grade 12 32 Primary, Secondary and Tertiary Carbons • A primary carbon is a carbon atom bonded to one other carbon atom. • A secondary carbon is a carbon atom bonded to two other carbon atoms. • A tertiary carbon is a carbon atom bonded to three other carbon atoms. • A quaternary carbon is a carbon atom bonded to four other carbon atoms. Physical Sciences Grade 12 33 Types of Reactions of Organic Compounds 1. Esterification: formation of an ester from an alcohol and a carboxylic acid. 2. Combustion Reactions (aka Oxidation): organic molecule + oxygen. 3. Substitution Reactions: one or more atoms are replaced by other atoms. (i) alkanes to alkyl halides; (ii) alcohols to alkyl halides; (iii) alkyl halides to alcohols 4. Addition Reactions: one or more atoms are added. (i) hydrogenation; (ii) halogenation; (iii) hydrohalogenation; (iv) hydration 5. Elimination Reactions: one or more atoms are removed. (i) dehydrohalogenation; (ii) dehydration; (iii) cracking of alkanes Physical Sciences Grade 12 34 4. Addition Reactions • An addition reaction involves one or more atoms being added to an organic molecule, usually over a double/triple bond.