Chapter 4An Introduction to Organic Reactions

Organic ChemistryChapter 4An Introduction to Organic ReactionsGeneral Principles of Organic ReactionsA 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 ReactionsMost organic reactions can fall into one of the following categories:

SubstitutionEliminationAdditionIn 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 MechanismsThe reaction equation tells you WHAT happens in a chemical reaction. The reaction mechanism tells you HOW it happens; its 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 its needed electrons) in the second step by a negative bromide ion.Reaction Mechanisms and Energy DiagramsPotential 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):

If the reaction is endothermic (absorbs heat):

Energy of reactants is higher than productsEnergy of products is higher than reactants

Reaction IntermediatesWhen 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:CarbocationsFree radicalsCarboanions

These species is unstable for one or both of the following:The particle is charged (carbocation, carboanion); orThe particle does not have an octet of electrons in the outer shell (carbocation, free radical).

Carbocation (+)

Carboanion (-)

Free radicalReaction IntermediatesIntermediates can be formed in a variety of ways:Homolytic cleavagecleavage in which shared electrons are evenly divided between the parting atoms (shared custody).Heterolytic cleavagecleavage in which shared electrons are unevenly divided between the parting atoms (sole custody)

Heterolytic cleavage:

Forms a carbocationHomolytic cleavage:

Forms a free radicalHeterolytic cleavage:

Forms a carboanion

Sites of Organic ReactionsWhy 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 BasesA 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 & ProductsOrganize 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 () or light (hv).

General Reaction Equation for Halogenation of Alkanes:

C H+ X2 C X + HX

alkane alkyl halideWhere X2 = Cl2 or Br2; HX = HCl or HBrheatlightChlorination of MethaneThe 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 ReactionsElimination 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 B A

Alkynes: C CC = C + 2 A B

B APreparation of Alkenes and Alkynes: Elimination ReactionsOne 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 ReactionsAnother 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

18Mechanism of Dehydration Reaction

19Reactions of Alkenes and Alkynes: Addition ReactionsWhy are alkenes and alkynes reactive and why is addition the characteristic reaction?Carbon-carbon double & triple bonds are composed of bonds in addition to a bond; the double bond has one bond, and the triple bond has two bonds. bonds are formed by p orbital overlapthey 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 ReactionDouble bonds can undergo addition once while alkynes can undergo twice.

Addition to alkenes:

Addition to alkynes:C = C+ E AC CE AC C+ 2 E AC CE AE AAddition Reactions of AlkenesGeneral Reaction for Addition to Alkenes:1) Addition of hydrogen halides: (E = H, A = X; HX = HCl, HBr, HI)C = C+ E AC CE AC = C+ H ClC CH Cl2) Halogenation: (E = X, A = X; X2 = Cl2, Br2, F2 is too reactive; I2 is not reactive)C = C+ Cl ClC CCl ClAddition Reactions of Alkenes3) 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 OHC CH OH H2SO4C = C+ H HC CH HNi, Pt, PdpressureMechanism of Electrophilic AdditionMechanism 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 & AdditionHow do you know which product of an addition reaction will be the major product? By using MARKOVNIKOVS 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

Unsymmetrical or uneven double bondNotice carbon on right has most hydrogensUnsymmetrical reagentH is E+Cl is Nu-Carbon with most hydrogens gets E+ of reagent