ORGANIC CHEMISTRY Classification of Reactions and Reagents H N O O N HO O N3 Winston Nxumalo (lab 317) www.gh.wits.ac.za/chemnotes 1 Unique Properties of Carbon Elemental Carbon Carbon atoms are unique in their ability to form strong covalent bonds to each other Elemental carbon has many allotropes (structural forms), including diamond, graphite, polyynes and fullerenes. Buckyballs [5]-fullerene-C20 [6,4]-fullerene-C24 [5,6]-fullerene-C602 Unique Properties of Carbon Elemental Carbon Carbon is also able to form strong covalent bonds with nonmetals e.g. Hydrogen (H) and Oxygen (O) Carbon can form long chains of more than 1000 carbon units, with each carbon holding hydrogen atoms e.g. plolymers Atoms in the same group e.g. Si can only have only have a chain of 8 atoms while holding hydrogen atoms 3 Unique Properties of Carbon Structural and bonding properties 1) Bond strengths • Strong bonds form between like elements e.g. H-H (436 kJ/mol) and C-C (348 kJ/mol). Therefore it is favourable for C atoms to link up into chains (catenation). Note: Homonuclear bonds are rare amongst other elements. • Carbon has the unique ability to form long chains and rings. • Another strong bond is between unlike elements C-H (413 kJ/mol) and is very common in organic compounds 2) Nature of bonds • There are a variety of possible bonds involving carbon e.g. C-C, C=C, C≡C, C-X, C=O, C≡N (X= halogen or OH) • The electron configuration of C: 1s22s22p2 therefore there are 4 valence electrons and 4 valence orbitals. This means carbon must always form 4 bonds. 4 Unique Properties of Carbon Reactivity Let’s look at the simplest organic molecule, methane: H H C H H • There are no unused electrons and no unused orbitals therefore there are no “reactive spots”. • The carbon atom has 4 strong covalent bonds to hydrogen which are not easily broken 5 Unique Properties of Carbon Reactivity • Extend this analogy to all other saturated hydrocarbons, and remembering that C-C and C-H bonds are very strong, we arrive at the conclusion that ALKANES ARE VERY UNREACTIVE. • At room temperature alkanes do not react with acids, bases, or strong oxidizing agents. • Therefore alkanes make great non-polar solvents. 6 Organic Reactions Introduction to Organic Reactions The key idea is to look for “patterns” of reactivity. In order to do this you must recognize 3 key things: 1) Identify the structural features in the molecules 2) Identify the types of reactions they might undergo 3) Identify the types of reagents that will help/cause the reaction to go 7 Organic Reactions Reaction types: 4 basic types 1. Addition (increases saturation at C) H H C C AB H H C Y C C A B XY C A X H B H 2. Elimination (increases unsaturation at C) X H Y H C H C A C H A C + H B C H + AB XY B C 3. Substitution (degree of saturation at C is not changed) H H C R H X Y H C Y + X R 4. Oxidation/Reduction (addition of oxygen/removal of hydrogen) 8 Organic Reactions Example: Identify the type of reaction in each of the following: H H a) H C Cl + H 2O H C H c) H H C C H OH H H H H H C C + H C + H2O H H H C + H Cl R R b) OH HCl H H H C C H Cl H 9 Organic Reactions Link between structural features and reaction type H E H H C C H H A H H C H H S S A H C H H H C C H H E A OH E H H C H O C H 10 Organic Reactions Reaction Pathways Substrates [Intermediates] Products A detailed description of the reaction path is known as a reaction mechanism and usual goes from a substrate (starting material) via an intermediate to a product. There can be several intermediates in a reaction pathway and usually they are too unstable to isolate. This year we will seldom go into detailed mechanisms and will mainly look at the substrate/product relationships. 11 Nucleophiles, electrophiles and curly arrows What are electrophiles and nucleophiles? Electrophile: literally means “electron loving”, therefore it is a fully or partially positively charged site and attracts towards negative electrons. Electronpoor Nucleophile: literally means “nucleus loving”, therefore it is a fully or partially negatively charged site and attracts towards positive nuclei. Electronrich 12 Nucleophiles, electrophiles and curly arrows What are electrophiles and nucleophiles? Character Examples Electron poor Electron rich + or δ+ − or δ− F H3C B X CH2 : NH3 F + F H2C H Cl - H3C M Attracted to Electron rich sites of molecules Electron poor sites of molecules Name Electrophiles Nucleophiles Criterion One or more empty One or more lone orbital pair or π bond 13 Nucleophiles, electrophiles and curly arrows Predicting Organic Reactions Reactions occur between sites of opposite polarity (i.e.+ or δ+ will react with – or δ-) Example: CH3 H3C Br + HC 3 N H CH3 H3C + N HBr CH3 14 Nucleophiles, electrophiles and curly arrows Predicting Organic Reactions Examples Electrophile Nucleophile F B : NH3 F F F O H -B N F H O C H3C F + CH3 Nu - H3C + H - C Nu CH3 15 Nucleophiles, electrophiles and curly arrows Examples: Identify electrophilic and nucleophilic sites in the following molecules O H H C C C Cl H3C CH3 H CH3 R H3C CH2 C HO H CH3 H3C C O CH2 Li H3C CH3 CH3 NH2 CH2 H3C CH2 C CH2 H 16 Nucleophiles, electrophiles and curly arrows Examples Identify the structural type and characteristic reactions of the following: O H H C C C Cl H3C CH3 H CH3 R H3C CH2 C HO H CH3 H3C C O CH2 Li H3C CH3 CH3 NH2 CH2 H3C CH2 C CH2 H 17 Nucleophiles, electrophiles and curly arrows Types of Reagents and Substrates Reagents and substrates can be classified according to how their electrons are arranged; 1) Unpaired electrons (H, Cl) radicals (these undergo homolytic reaction pathways) 2) Paired electrons (electron rich or electron poor) (these undergo heterolytic/polar reaction pathways) 18 Halogenation of Methane Radical Reaction These reactions don’t use nucleophiles or electrophiles, but instead they use “free radicals” which have unpaired electrons. Both reactants donate one electron to form the new bond. E.g. H Cl Substitution e.g. CH4 +Cl2 CH3-Cl + Cl2 CH3-Cl CH2-Cl2 CHCl3 CCl4 These reactions are difficult to control and therefore there are mixtures of products. 19 Halogenation of Methane Radical Reaction Pathway (homolytic) to form a small percentage of free radicals. heat Cl Cl or light H H Cl + H C H HCl + C H Cl C H H Propagation H H Cl Initiation Cl + Cl H H Cl C H + Cl Propagation H Termination (two radicals collide and combine) Both propagation steps alternate until the reactants are completely consumed (termination). This is a chain reaction and the chlorine radical can undergo further reactions. 20 Halogenation of Methane Example: Write the equation for when 1mol CH4 (methane) reacts with 1 mol Cl2 CH4 + Cl2 CH3Cl + HCl Write the equation for when I mol CH4 (methane) reacts with 2 mol Cl2 CH4 + 2Cl2 CH2Cl2 + 2HCl 21 Halogenation of Methane Example: Write the equation for when 1mol CH3CH3 (ethane) reacts with 1 mol Cl2 CH3CH3 + Cl2 CH3CH2Cl + HCl Write the equation for when I mol CH3CH3 (ethane) reacts with 2 mol Cl2 CH3CH3 + 2Cl2 CH3CHCl2 + CH2ClCH2Cl + 2HCl 22 Cracking of alkanes Eliminations from Alkanes H H H C H H H C CH3 H C 800 `C H C + H H CH3 This process is known as “cracking”. thermal cracking: Alkanes are very unreactive, but at extremely high temperatures they can undergo elimination reactions. catalytic cracking: Alkanes will also crack at 400 -600°C when a catalyst such as aluminosilicate is added. This is of industrial importance as long-chain alkanes are broken into more useful shorter chains that can be used for fuels and to make alkenes. 23 Cracking of alkanes Eliminations from Alkanes Where will hydrocarbons crack? H2C CH CH2 CH3 H H3C H2C + CH H3C H H H H C C C C H H H H CH CH CH3 H H H2C CH3 H3C + CH2 CH3 C-C (348 kJ/mol) bonds are weaker than C-H (413 kJ/mol) bonds so the major products are the bottom ones 24 Cracking of alkanes Eliminations from Alkanes The cracking mechanism is a radical mechanism (disproportionation). H H H C C H CH3 H H H H C C H CH3 H H H C H C + H H CH3 NB industrial importance is to make unreactive alkanes into reactive alkenes –synthesize more complex molecules. 25 Cracking of alkanes EXAMPLE Using structural formulae, show all possible compounds formed when propane is cracked H H H H H C C C H H H H H C C C H H H H + C H + H2 H H C H H H C H H 26 Unsaturated hydrocarbons Alkenes and Alkynes σ and π bonds H2C CH2 π, π, and σ bonds HC CH Alkenes and alkynes have bonds which are; 1) Non-polar 2) have no lone pairs 3) have no vacant orbitals but have π electrons which are exposed and therefore accessible. The π component is weaker than the σ C-C (348 kJ/mol) vs C=C (619 kJ/mol) therefore π bonds contribute 271 kJ/mol. This makes unsaturated hydrocarbons far more reactive than saturated hydrocarbons. 27 Unsaturated hydrocarbons Alkenes and Alkynes The typical reaction for unsaturated hydrocarbons is an addition reaction, where the π electrons from the double bond are the nucleophile. H H C H C + H X Y Addn H H H C C X Y H The obvious reagent for an addition reaction? An electrophile. Substitution and elimination reactions are less important for unsaturated systems. 28 Unsaturated hydrocarbons Alkenes and Alkynes Halogenation In this addition reaction: Two atoms (e.g., bromine) add across the double bond. One π-bond and one σ-bond are replaced by two σbonds; therefore, ΔH is negative. Unsaturated hydrocarbons Alkenes and Alkynes Addition of hydrogen halides The mechanism for the addition reaction is two-step: First step is slow, rate-determining step. Second step is fast. Unsaturated hydrocarbons Alkenes and Alkynes Hydrogenation H H C H C + H H H H-H is very strong (436 kJ/mol), needs a catalyst Halogenation H C H C + Cl H H C C H H Pt or Pd (catalyst) H H H Cl H H H C C Cl Cl H H Cl-Cl is much weaker (254 kJ/mol) (also Br2 and I2). 31 Unsaturated hydrocarbons Alkenes and Alkynes Examples H H C C + H H + Cl H C C H Cl easy Cl H H H H H - H H H C C + H Cl H - This reaction goes easily due to the weak H-Cl bond 32 Unsaturated hydrocarbons Alkenes and Alkynes Hydration of alkenes Water is a poor electrophile hence the need for trace amounts of H+ catalyst and heat. H H C + C + H H + H C C H OH H (catalyst) OH H H H H H - HO H H H C C H + H - HO 33 Unsaturated hydrocarbons Alkenes and Alkynes Hydration of alkynes Water is a poor electrophile hence the need for trace amounts of H+ catalyst and heat. H H C C H + - H + H H2SO4/HgSO4 OH H C H HO O C H - HO H C H H OH + C C H H C H 34 Unsaturated hydrocarbons Examples Write equations using structural formulae to represent the reaction of i) propene and water ii) 1-butene and H2 iii) ethene and HCl i) CH CH2 H3C + H2O H3C CH2 + OH CH2 OH H3C HC CH3 ii) iii) CH H2C H2C CH3 CH2 CH2 + + H2 HCl H3C CH2 CH3 CH2 CH2 H3C Cl 35 Unsaturated hydrocarbons Alkenes and Alkynes What happens with unsymmetrical alkenes? CH3 H H3C H C H + H C H Cl C H H + H Minor H Major Cl CH3 H H C C Cl H CH3 H H C + C C CH3 H H H More stabilized H C + C H H Less stabilized 36 Unsaturated hydrocarbons Alkenes and Alkynes Carbocation intermediates (relative stability) CH3 H3C + C H CH3 H3C H + C CH3 H 3C H + C H H + C H Increasing Stability Markovnikov’s rule: In the addition of HX to an alkene, the H attaches to the C with fewer alkyl substituents and the X attaches to the C with more alkyl substituents. I.e. via the more stable carbocation intermediate 37 Unsaturated hydrocarbons Alkenes and Alkynes When there is a halogen already attached to the alkene, the second halide ion will add to the same position (the side with fewer hydrogens). H HC CH H Cl H C H H3C CHCl2 Major C Cl ClH2C CH2Cl Minor Alkyl substituents on alkenes speed up the rate of reaction with an electrophile. 38 Reactions of Alkynes Reactivity of Alkynes • Alkynes undergo many of the same reactions alkenes do. • As with alkenes, the impetus for the reaction is the replacement of π-bonds with σ-bonds. Reactions of Alkynes Example Write equations using structural formulae to represent the following reactions i) 1 mol ethyne with 1 mol Br2 ii) 1 mole ethyne with 1 mol H2O Br HC CH + Br Br CH CH Br OH HC CH + HO CH H H CH Reactions of Alkynes Example Write equations using structural formulae to represent the following reactions iii) I mol propyne with 2 mol HCl (NB. 3 products possible) Cl H3C C CH + HCl H3C + C H3C C H HCl H2 C Cl CH H3C CH2 CH + H3C Cl CH Cl H C H3C CH2 Cl Cl Cl H3C H C + C Cl Cl C H Cl H3C CH3