Organic Reactions SCH4U – Unit B Types of organic reactions • • • • Addition Elimination Substitution Condensation (dehydration synthesis) ▫ Esterfication • • • • Hydrolysis Oxidation Reduction Combustion ▫ Complete ▫ Incomplete Addition • Reaction in which atoms are added to a carbon-carbon double or triple bond • LOOK FOR: check if C atoms in product(s) are bonded to more atoms than C atoms in reactant • Alkenes and alkynes generally react with one of four reactants: ▫ ▫ ▫ ▫ H2, H-H example #1 H2O, H-OH example #2 Acid Halides, H-X (X = F, Cl, Br, I) example #3 Diatomic Halogens, X-X example #4 Addition • Example #1 2-butene butane Addition • Example #2 2-butanol 2-butene Addition • Example #3 2-fluoro butane Addition • Example #4 2,3-difluoro butane Addition • When adding halogens to an alkyne, a total of four atoms can be added • The amounts of the halogen reactant must be considered: ▫ Excess example #5 ▫ Limited example #6 Addition • Example #5 (excess halogen) 2,2,3,3-tetrafluoro butane Addition • Example #6 (limited halogen) 2,3-difluoro 2-butene Addition • If the alkene/alkyne contains many C atoms and is reacting with a small molecule, isomers may form example #7 • If the small molecule reacts with an asymmetrical alkene, can use Markinnikov’s rule to predict the more abundant isomer example #8 • Markovnikov’s rule: H atom of a small molecule will attach to C atom of double bond that is already bonded to the most H atoms Addition • Example #7 • 50% 2-bromo pentane • 50% 3-bromo pentane Addition • Example #8 2-bromo pentane Elimination • Reaction in which atoms are removed from an organic molecule to form a double bond • LOOK FOR: check if C atoms in product(s) are bonded to less atoms than C atoms in reactant • Basically, this is the reverse of an addition reaction • Methods of undergoing elimination reactions: ▫ Heat and strong acid (catalyst) used for alcohols example #1 ▫ Heat and strong base (catalyst) used for haloalkanes example #2 Elimination • Example #1 Elimination • Example #2 Elimination • If an asymmetrical molecule undergoes an elimination reaction, constitutional isomers can form example #3 • General rule: H atom most likely to be removed from C atom with most C-C bonds • “The poor get poorer!” ▫ opposite of Markovnikov’s Rule ▫ Called Zaitsev’s rule Elimination • Example #3 (major product) (minor product) Substitution • Reaction in which a hydrogen atom or functional group is replaced by a different atom or functional group • LOOK FOR: ▫ Two compounds react to form two different compounds ▫ Carbon atoms are bonded to the same number of atoms in product and reactant Substitution • Alcohols and haloalkanes undergo substitution reactions relatively easily ▫ Alcohol reacts with acids containing a halogen (HCl, HBr, etc.) to produce a haloalkane example #1 ▫ Haloalkane reacts with hydroxide to produce an alcohol example #2 • Haloalkanes also react with bases to undergo elimination reactions ▫ Thus, hard to control reactions of haloalkanes with bases • For the purposes of our course: ▫ OH– = substitution reaction ▫ NaOCH2CH3 = elimination reaction Substitution • Example #1 ethanol chloroethane Substitution • Example #2 chloroethane ethanol Substitution • Alkanes also undergo substitution reactions • Alkanes are relatively unreactive, thus a lot of energy is required (UV light) to catalyze rxn • Alkanes react with chlorine and bromine to form haloalkanes ▫ If enough of the halogen is present, a mix of organic compounds forms example #3 • Ultimately, because of the mix of products, this process is not used to produce haloalkanes Substitution • Example #3 Substitution • Aromatic hydrocarbons (benzene derivatives) are also stable • Require a catalyst to react with chlorine and bromine example #4 Substitution • Example #4 benzene bromobenzene Condensation • Reaction in which two molecules combine to form a larger molecule, producing a small, stable molecule, usually water, as a second product or functional group • LOOK FOR: Hydroxyl group from one molecule and a hydrogen atom from a second molecule being removed, and water being produced • EXTREMELY IMPORTANT IN BIOLOGY! • Generally forms an amide bond when it occurs between ~COOH and ~NH2 Condensation • Example #1 O R2 C R1 O + H carboxylic acid H O N R2 C R3 amine R1 amide N + H R3 water H O Condensation • Example #2 H H N H C R1 O C O + H H H N H C O R2 H H N H C O C + H H H N H C O C O R3 H O H C N R1 H C H2O O H C N R2 H C R3 + H2O O C O H Esterification (condensation) • Reaction of a carboxylic acid with an alcohol to form an ester and water ▫ Specific type of condensation reaction example #3 ▫ Catalyzed by a strong acid H2SO4 • Flavours and smells of fruits and spices are due to ester compounds • Can be duplicated in a lab ▫ Production of synthesized ester compounds used to flavour juices, candies, etc. example #4 (cherry flavour) Esterification (condensation) • Example #3 O O C R1 OH + carboxylic acid HO R2 H2SO4 C R1 alcohol ester R2 O + H water O H Esterification (condensation) • Example #4 O CH O C HC C OH HC CH + CH benzoic acid CH2 CH3 HO CH H2SO4 HC C C HC O CH2 CH3 CH + CH ethanol ethyl benzoate water H2O Hydrolysis • Reaction in which a molecule is broken apart by adding hydroxyl group from a water molecule to one side of a bond and hydrogen atom of same water molecule to other side of bond • Basically, this is the reverse of a condensation reaction • LOOK FOR: a large molecule containing an ester or amide reacting with water to produce to smaller molecules example #1 Hydrolysis • Example #1 O O C R1 R2 + H O O ester C H R1 water carboxylic acid H + R2 H O O alcohol Hydrolysis • Both the condensation reaction and hydrolysis reaction are catalyzed by acid example #2 • The double arrow indicates the reaction is reversible • How can we control the direction of a reversible reaction to favour one side of the equation??? ▫ EQUILIBRIUM! (Unit #4) Hydrolysis • Example #2 O O C R1 OH + carboxylic acid HO R2 H2SO4 C R1 alcohol ester R2 + H O water H O Oxidation • Reaction in which a carbon atom forms more bonds to oxygen atoms or fewer bonds to hydrogen atoms (orgo) • Always occurs along with a reduction reaction • For organic chemistry, focus only on the organic compound • Some oxidation reactions can also be classified as elimination reactions Oxidation • Occurs when organic compound reacts with an oxidizing agent ▫ KMnO4 = potassium permanganate ▫ K2Cr2O7 = acidified potassium dichromate ▫ O3 = ozone • Redox reactions are often left unbalanced examples #1-3 Oxidation • Example #1 H O R2 C O H + C [O] R1 • C atom has lost H atoms R2 R1 Oxidation • Example #2 O C R1 H O + [O] • C atom has gained an O atom C R1 OH Oxidation • Example #3 H H H H C C H O H + [O] H C H H ethanol ethanal O C H Reduction • Reaction in which a carbon atom forms fewer bonds to oxygen atoms or more bonds to hydrogen atoms (orgo) • Always occurs along with an oxidation reaction • For organic chemistry, focus only on the organic compound • Some reduction reactions can also be classified as addition reactions Reduction • Occurs when organic compound reacts with an reducing agent ▫ LiAlH4 = lithium aluminum hydride ▫ H2/Pt = hydrogen gas over a platinum catalyst • Redox reactions are often left unbalanced examples #1-3 • Redox reactions will be covered in greater depth during ELECTROCHEMISTRY (Unit #5)! Reduction • Example #1 H O O + C R1 [H] R1 C R2 H aldehyde or ketone alcohol • C atom has less bonds to O atom R2 Reduction • Example #2 H H C H + C H [H] H H H C C H H alkene alkane • C atoms have more bonds to H atoms H Reduction • Example #3 H O O C H3C + [H] H3C C CH3 CH3 H propanone 2-propanol • C atom has less bonds to O atom Combustion • Type of reaction in which a compound reacts with oxygen to produce the oxides of elements that make up the compound • 2 types: 1) Complete combustion: an excess of oxygen reacts with a hydrocarbon and produces carbon dioxide and water vapour, and releases energy 2) Incomplete combustion: reaction that occurs when insufficient oxygen is present; all elements in the fuel will not combine with oxygen to the greatest extent possible Combustion • Example #1 HC + O2(g) CO2(g) + H2O(g) + energy • Example #2 HC + O2(g) C(s) + CO(g) + CO2(g) + H2O(g) + energy