Chapter 4—An Introduction to Organic Reactions Organic Chemistry

General Principles of Organic Reactions A reaction equation is an equation that shows what happens in a chemical reaction by showing reactants and products. The amount of chemical reactions in organic chemistry is large, but most fall into one of three basic categories.

Types of Organic Reactions Most organic reactions can fall into one of the following categories: Substitution Elimination Addition In a substitution reaction, an atom or group of atoms is replaced by another species. In an elimination reaction, an atom or group of atoms is eliminated from adjacent carbon atoms, usually resulting in the formation of a multiple bond. In an addition reaction, atoms or groups of atoms add to the multiple bond (sort of the reverse of an elimination reaction).

Types of Organic Reactions Substitution Reaction Examples: Elimination Reaction Example: Addition Reaction Example:

Reaction Mechanisms Addition of HBr to ethene: The reaction equation tells you WHAT happens in a chemical reaction. The reaction mechanism tells you HOW it happens; it’s a step-by-step description of how the chemical changes occurs. Addition of HBr to ethene: This reaction proceeds by a two step mechanism: The hydrogen adds first as a positive ion to form a short-lived intermediate called a carbocation. The carbocation is neutralized (or gets it’s needed electrons) in the second step by a negative bromide ion.

Reaction Mechanisms and Energy Diagrams Potential energy diagrams are used to show energy changes during chemical reactions. Energy is required to break bonds and this raises the potential energy as these bonds break in the initial stages of the reaction. As new bonds form, energy is then released. If the reaction is exothermic (gives off heat): Energy of reactants is higher than products If the reaction is endothermic (absorbs heat): Energy of products is higher than reactants

Reaction Intermediates When a organic reaction occurs, bonds must break and new bonds form. As bonds break, unstable, short-lived species called reaction intermediates form. There are three types: Carbocations Free radicals Carboanions These species is unstable for one or both of the following: The particle is charged (carbocation, carboanion); or The particle does not have an octet of electrons in the outer shell (carbocation, free radical). Carbocation (+) Free radical Carboanion (-)

Reaction Intermediates Intermediates can be formed in a variety of ways: Homolytic cleavage— cleavage in which shared electrons are evenly divided between the parting atoms (“shared custody”). Heterolytic cleavage— cleavage in which shared electrons are unevenly divided between the parting atoms (“sole custody”) Heterolytic cleavage: Forms a carbocation Homolytic cleavage: Forms a free radical Heterolytic cleavage: Forms a carboanion

Sites of Organic Reactions Why do organic molecules react? The reactivity of an organic compound is determined by its structure; specifically at places in the molecule where there is an availability of finding electrons or where there is a lack of electrons. Electrophiles (means “electron-loving”) is an electron deficient species that accepts electrons from electron rich species in a chemical reaction. Electrophiles are Lewis acids. Nucleophiles (means “nucleus-loving”) is a electron rich species that donates electrons to electron deficient species in a chemical reaction. Nucleophiles are Lewis bases.

Lewis Acids and Lewis Bases A Lewis base is a species that has a nonbonding pair of valence electrons that is can share in a chemical reaction. A Lewis base is known as an “electron pair donator”. A Lewis acid is a species that can accept a pair of electrons for sharing in a chemical reaction. A Lewis acid is known as an “electron pair acceptor”. Lewis Acid-Base Reactions:

How to Predict Reactions & Products Organize your study of reactions as follows: General reaction equation. Learn it and identify the reaction as substitution, elimination, or addition. Predominant product. Learn to determine which product predominates when more than one is present. Reaction mechanism. Learn the step-by-step mechanism as best you can so you can apply it to other examples. Specific examples and practice problems. Be sure to include as many examples of each type of reaction in your notes and write out entire reactions in your practice problems.

Reactions of Alkanes: Halogenation Halogenation is a type of substitution reaction in which a hydrogen atom is replaced or substituted by a halogen. The reaction occurs when an alkane is combined with chlorine (Cl2) or bromine (Br2) in the presence of heat (Δ) or light (hv). General Reaction Equation for Halogenation of Alkanes: C H + X2 C X + HX alkane alkyl halide Where X2 = Cl2 or Br2; HX = HCl or HBr heat light

Chlorination of Methane The chlorination of methane (and other alkanes) occurs by a free radical chain reaction. A chain reaction is a reaction that sustains itself through repeating chain- propagation steps. Once all of the hydrogens have been replaced, the chain reaction stops.

Mechanism of Halogenation

Preparation of Alkenes and Alkynes: Elimination Reactions Elimination reactions are used to introduce carbon-carbon double or triple bonds into a molecule. To do this, two atoms or groups of atoms from two adjacent carbon atoms must be eliminated. General Equations of Elimination Reactions for Preparing Alkenes & Alkynes Alkenes: C C C = C + A B B A Alkynes: C C C = C + 2 A B

Preparation of Alkenes and Alkynes: Elimination Reactions One type of elimination reaction is called DEHYDROHALOGENATION. In it, a hydrogen halide (HX) is removed to form the double or triple bond: General Equations for Dehydrohalogenation: Alkenes: Alkynes:

Preparation of Alkenes and Alkynes: Elimination Reactions Another type of elimination reaction is called DEHYDRATION. In this reaction, the eliminated product is water, H-OH. General Reaction Equation for Preparation of Alkenes by Dehydration: *Usually H2SO4 is used as the dehydrating agent. This reaction does not work well to form alkynes.

Dehydration of Alcohols

Mechanism of Dehydration Reaction

Reactions of Alkenes and Alkynes: Addition Reactions Why are alkenes and alkynes reactive and why is addition the characteristic reaction? Carbon-carbon double & triple bonds are composed of π bonds in addition to a σ bond; the double bond has one π bond, and the triple bond has two π bonds. Π bonds are formed by p orbital overlap—they are loosely held and susceptible to attack by electrophiles. Alkenes and alkynes are unsaturated. This means the two carbons do not have the maximum possible number of atoms or groups bonded to them.

Addition Reaction Double bonds can undergo addition once while alkynes can undergo twice. Addition to alkenes: Addition to alkynes: C = C + E A C C E A C C + 2 E A C C E A

Addition Reactions of Alkenes General Reaction for Addition to Alkenes: C = C + E A C C E A 1) Addition of hydrogen halides: (E = H, A = X; HX = HCl, HBr, HI) C = C + H Cl C C H Cl 2) Halogenation: (E = X, A = X; X2 = Cl2, Br2, F2 is too reactive; I2 is not reactive) C = C + Cl Cl C C Cl Cl

Addition Reactions of Alkenes 3) Hydration: (E = H, A = OH; H2SO4 is the catalyst) 4) Hydrogenation: (E = H, A = H; metal catatlyst such as Ni, Pt, or Pd with reaction lead under pressure) C = C + H OH C C H OH H2SO4 C = C + H H C C H H Ni, Pt, Pd pressure

Mechanism of Electrophilic Addition Mechanism for Addition of Hydrogen Halogen (HX): Electrophilic Addition is an addition reaction initiated by an electron deficient species or an electrophile. In this case, the electrophile is H and the nucleophile is X.

Unsymmetrical Reagents & Addition How do you know which product of an addition reaction will be the major product? By using MARKOVNIKOV’S RULE! It states that when an unsymmetrical reagent (like H-Cl, H-I, H-Br, H-OH) adds to an unsymmetrical double or triple bond, the positive (E+) portion of the reagent adds to the carbon with the most hydrogens (or fewest alkyl groups). Reactions such as these in which one product predominates is called regioselective. If one product is formed exclusively, the reaction is termed regiospecific.

Unsymmetrical Reagents & Addition H is E+ Cl is Nu- Carbon with most hydrogens gets E+ of reagent Unsymmetrical or “uneven” double bond Notice carbon on right has most hydrogens