General Organic Reactions and Mechanisms - PDF

General Types of Organic Reactions and their Mechanisms Prepared by: Prof. Myra B. Ambon MAT,LPT  Objectives 1. distinguish the general types of organic reaction from given chemical equations 2. illustrate the types of reactions through examples 3. explain the mechanism...

General Types of Organic Reactions and their Mechanisms Prepared by: Prof. Myra B. Ambon MAT,LPT  Objectives 1. distinguish the general types of organic reaction from given chemical equations 2. illustrate the types of reactions through examples 3. explain the mechanism of each general type of organic reaction Prepared by: Prof. Myra B. Ambon MAT,LPT Organic Reaction Mechanism Prepared by: Prof. Myra B. Ambon MAT,LPT Mechanism Prepared by: Prof. Myra B. Ambon MAT,LPT Nucleophilie Prepared by: Prof. Myra B. Ambon MAT,LPT Electrophiles Prepared by: Prof. Myra B. Ambon MAT,LPT  Types of Organic Reactions Organic reactions can be categorized into several basic types. Some reactions fit into more one category. Example, some substitution reactions follow an addition-elimination pathway. This overview isn’t intended to include every single organic reaction, Rather, it is intended to cover the basic reactions Prepared by: Prof. Myra B. Ambon MAT,LPT  Organic Reactions and Mechanisms Organic reactions are the chemical reactions that are undergone by organic compounds (the chemical compounds containing carbon) The four main reaction classes 1. Additions 2. Eliminations 3. Substitutions, and 4. Rearrangements. Prepared by: Prof. Myra B. Ambon MAT,LPT  Types of Reactions Addition Reactions Substitution Reactions Elimination Reactions Rearrangement Reactions Organic Redox Reactions Prepared by: Prof. Myra B. Ambon MAT,LPT  Four Main Types of Organic Reactions Addition Reaction- the components A and B are added to the carbon-carbon multiple bonds and this is called addition reaction Prepared by: Prof. Myra B. Ambon MAT,LPT  Four main Reaction Classes Although a multiple bond is stronger than a single bond, the π bonds of the multiple bond are weaker than the σ bond. The high electron density located between multiply bonded carbon atoms, however, causes alkenes and alkynes to behave like nucleophiles, where nucleophilic attack occurs from the more weakly bound π electrons. Hence alkenes and alkynes are regarded as functional groups. Nucleophilic attack occurs on the Hδ+ atom of the polar HCl bond, initially producing a species with a carbon that has only three bonds, a carbocation. In a second nucleophilic attack, Cl−, the electrophile in , attacks the carbocation: CH2=CH2 + HCl → CH3CH2Cl Prepared by: Prof. Myra B. Ambon MAT,LPT  Four main Reaction Classes Elimination Reaction - reactions which involve the elimination or removal of the adjacent atoms. After these multiple bonds are formed and there is a release of small molecules as products. CH3CH2Cl → CH2=CH2 + HCl https://saylordotorg.github.io/text_general-chemistry-principles-patterns-and-applications-v1.0/s28-04- common-classes-of-organic-reac.html Prepared by: Prof. Myra B. Ambon MAT,LPT  Four main Reaction Classes Substitution Reaction- are characterized by replacement of an atom or group (Y) by another atom or group (Z). Aside from these groups, the number of bonds does not change. Rearrangement Reaction - generates an isomer, and again the number of bonds normally does not change. Prepared by: Prof. Myra B. Ambon MAT,LPT Types of Reactions Prepared by: Prof. Myra B. Ambon MAT,LPT ADDITION REACTION Electrophilic Addition Prepared by: Prof. Myra B. Ambon MAT,LPT Addition Reactions – Electrophilic Addition Prepared by: Prof. Myra B. Ambon MAT,LPT Addition Reactions – Electrophilic Addition Prepared by: Prof. Myra B. Ambon MAT,LPT  Nucleophilic Addition to carbon-carbon double bonds Prepared by: Prof. Myra B. Ambon MAT,LPT  Addition Reactions of Alkenes Because both electrophile and nucleophile add to the double bond, the electrophile is the first species that adds, this characteristic reaction of alkenes is called electrophilic addition reaction. This reactivity makes alkenes an important class of organic compounds because they can be used to synthesize a wide variety of other compounds. The particular product obtained depends only on the electrophile and the nucleophile used in the addition reaction. Prepared by: Prof. Myra B. Ambon MAT,LPT Addition Reactions of Alkenes Prepared by: Prof. Myra B. Ambon MAT,LPT Addition Reactions of Alkenes A. Addition of a Hydrogen Halide to an Alkene Prepared by: Prof. Myra B. Ambon MAT,LPT  Addition Reactions of Alkenes Addition of a Hydrogen Halide to an Alkene Prepared by: Prof. Myra B. Ambon MAT,LPT  Addition Reactions of Alkenes Addition of HCl to 2-methyl propene Prepared by: Prof. Myra B. Ambon MAT,LPT Addition Reactions of Alkenes CARBOCATION STABILITY ✓ Carbocations are classified according to the number of alkyl substituents that are bonded to the positively charged carbon. ✓ Primary carbocation – has one alkyl substituent. ✓ Secondary carbocation – has two alkyl substituents. ✓ Tertiary carbocation – has three alkyl substituents. ✓ The stability of a carbocation increases as the number of alkyl substituents bonded to the positively charged carbon increases. ✓ Thus, the tertiary carbocations are more stable than secondary, and secondary carbocations are more stable than primary carbocation Prepared by: Prof. Myra B. Ambon MAT,LPT  Addition Reactions of Alkenes CARBOCATION STABILITY The reason for this pattern of stability is that alkyl groups bonded to the positively carbon decrease the concentration of positively charge on the carbon since they can donate electrons through the σ bond better than hydrogen can; decreasing the concentration of positive charge makes the carbocation more stable. Prepared by: Prof. Myra B. Ambon MAT,LPT Addition Reactions of Alkenes For the alkene that does not have the symmetric structure, the double bond carbons have different substituents Example: Addition of HCl to 2-methyl propene Prepared by: Prof. Myra B. Ambon MAT,LPT Electrophilic Addition Reaction A reagent which can accept an electron pair in a reaction Prepared by: Prof. Myra B. Ambon MAT,LPT  Electrophilic Addition Reaction The major product obtained from the addition of HI to 2- methyl-2-butene is 2-iodo-2-methylbutane The major product obtained from the addition of HBr to 1- methylcyclohexane is 1-bromo-1methylcyclohexane. In both cases, more stable tertiary carbocation is formed Prepared by: Prof. Myra B. Ambon more rapidly. MAT,LPT Prepared by: Prof. Myra B. Ambon MAT,LPT B. Addition of Water to an Alkene CH3CH=CH2 + H2O no reaction ✓ If an acid is added to the solution (the acid most often used is H2SO4), then the outcome is much different: a reaction will occur because the acid provides an electrophile (H+). The product of the reaction is an alcohol. The addition of water(H2O) to a molecule is called hydration, so the alkene will be hydrated in the presence of water and acid. Prepared by: Prof. Myra B. Ambon MAT,LPT  Addition of Water to an Alkene The acid is a catalyst- it increases the rate at which a product is formed, but it does not affect the amount of product formed. Because the catalyst in the hydration of an alkene is an acid, hydration is an acid- catalyzed reaction ✓ Notice that the first two steps of the mechanism for the acid- catalyzed addition of water to an alkene are essentially the same as the two steps of the mechanism for the addition of hydrogen halide to an alkene: Prepared by: Prof. Myra B. Ambon MAT,LPT Addition of Water to an Alkene Prepared by: Prof. Myra B. Ambon MAT,LPT  Addition of Water to an Alkene The acid most commonly applied to catalyze this reaction is dilute aqueous solution of sulfuric acid (H2SO4). Sulfuric acid dissociates completely in aqueous solution and the hydronium ion (H3O+) generated participates in the reaction. Strong organic acid, tosyl acid (TsOH), is used sometimes as well. The mechanism for acid-catalyzed hydration of alkene is essentially the same as the mechanism for the addition of hydrogen halide, HX, to alkenes, and the reaction therefore follows Markovnikov’s rule as well in terms of regioselectivity. The hydration of 1-methylcyclohexene and the reaction mechanism are shown below. ✓ Markovnikov’s rule - It states that, in hydrohalogenation of an unsymmetrical alkene, the hydrogen atom in the hydrogen halide forms a bond with the doubly bonded carbon atom in the alkene, bearing the greater number of hydrogen atoms. Prepared by: Prof. Myra B. Ambon MAT,LPT Addition of Water to an Alkene Prepared by: Prof. Myra B. Ambon MAT,LPT C. Addition of Alcohol to an Alkene Prepared by: Prof. Myra B. Ambon MAT,LPT Addition Reactions – Electrophilic Addition Prepared by: Prof. Myra B. Ambon MAT,LPT Addition Reactions – Electrophilic Addition Prepared by: Prof. Myra B. Ambon MAT,LPT Prepared by: Prof. Myra B. Ambon MAT,LPT  Addition of Water to an Alkene ✓ Markovnikov’s rule - It states that, in hydrohalogenation of an unsymmetrical alkene, the hydrogen atom in the hydrogen halide forms a bond with the doubly bonded carbon atom in the alkene, bearing the greater number of hydrogen atoms. Prepared by: Prof. Myra B. Ambon MAT,LPT Prepared by: Prof. Myra B. Ambon MAT,LPT Prepared by: Prof. Myra B. Ambon MAT,LPT Nucleophilic Addition Prepared by: Prof. Myra B. Ambon MAT,LPT  Nucleophilic Addition A nucleophile in its more reactive basic form Nu−in basic medium attacks the electrophilic carbonyl Cδ+. Simultaneously, the π -bond bond break, as shown in step#1 of the mechanism below. The O− either re-establish π -bond expelling the nucleophile, or it is neutralized by an acid-base reaction with any acid group present in the medium in step#2 of the mechanism shown below. Prepared by: Prof. Myra B. Ambon MAT,LPT  Check your Understanding 1. Write down the mechanism for the following reaction and predict which of the two possible products will be formed in the greatest yield. Write down the structures of the two possible products and explain your answer. CH3 CH CH2 + HBr 2. What Alkene would be used to synthesize 3- bromohexane? ___?______ + HBr CH3CH2CHCH2CH2CH3 Br Prepared by: Prof. Myra B. Ambon MAT,LPT Check your Understanding Prepared by: Prof. Myra B. Ambon MAT,LPT  Check Your Understanding Classify each reaction as addition, elimination, substitution, or rearrangement. Prepared by: Prof. Myra B. Ambon MAT,LPT Prepared by: Prof. Myra B. Ambon MAT,LPT