Reactions of Alkanes & Alkenes Unit 7

Introduction Overall reaction: reactants  products Mechanism: Step-by-step pathway. To learn more about a reaction: Thermodynamics Kinetics

Reactions and Their Mechanisms Almost all organic reactions fall into one of four categories: Substitutions – characteristic reactions of saturated compounds—one group replaces another Additions—characteristic of compounds with multiple bonds—all parts of the adding reagent appear in the product; two molecules become one Eliminations—one molecule loses the elements of another small molecule—provide a method for preparing compounds with double and triple bonds Rearrangements—a molecule undergoes a reorganization of its constituent parts

Substitution Reactions

Addition Reactions

Elimination Reactions

Rearrangement

Chlorination of Methane Requires heat or light for initiation. The most effective wavelength is blue, which is absorbed by chlorine gas. Many molecules of product are formed from absorption of only one photon of light (chain reaction).

The Free-Radical Chain Reaction Initiation generates a radical intermediate. Propagation: The intermediate reacts with a stable molecule to produce another reactive intermediate (and a product molecule). Termination: Side reactions that destroy the reactive intermediate.

Initiation Step: Formation of Chlorine Atom A chlorine molecule splits homolytically into chlorine atoms (free radicals).

Propagation Step: Carbon Radical The chlorine atom collides with a methane molecule and abstracts (removes) an H, forming another free radical and one of the products (HCl).

Propagation Step: Product Formation The methyl free radical collides with another chlorine molecule, producing the organic product (methyl chloride) and regenerating the chlorine radical.

Overall Reaction C

Termination Steps A reaction is classified as a termination step when any two free radicals join together producing a nonradical compound. Combination of free radical with contaminant or collision with wall are also termination steps.

Alkene Characteristic Reactions

Alkene Characteristic Reactions

Reaction Mechanisms A reaction mechanism describes how a reaction occurs which bonds are broken and which new ones are formed the order and relative rates of the various bond- breaking and bond-forming steps if in solution, the role of the solvent if there is a catalyst, the role of a catalyst the position of all atoms and energy of the entire system during the reaction

Developing a Reaction Mechanism How it is done design experiments to reveal details of a particular chemical reaction propose a set or sets of steps that might account for the overall transformation a mechanism becomes established when it is shown to be consistent with every test that can be devised this does mean that the mechanism is correct, only that it is the best explanation we are able to devise

Why Mechanisms? they are the framework within which to organize descriptive chemistry they provide an intellectual satisfaction derived from constructing models that accurately reflect the behavior of chemical systems they are tools with which to search for new information and new understanding

Electrophilic Additions hydrohalogenation using HCl, HBr, HI hydration using H2O in the presence of H2SO4 halogenation using Cl2, Br2 halohydrination using HOCl, HOBr oxymercuration using Hg(OAc)2, H2O followed by reduction

Addition of HX Carried out with pure reagents or in a polar solvent such as acetic acid or It is a two-step mechanism: The first step is the slow, rate-determining step. The second step is fast.

Addition of HX Addition is regioselective regioselective reaction: an addition or substitution reaction in which one of two or more possible products is formed in preference to all others that might be formed Markovnikov’s rule: in the addition of HX, H2O, or ROH to an alkene, H adds to the carbon of the double bond having the greater number of hydrogens

Addition of H2O addition of water is called hydration acid-catalyzed hydration of an alkene is regioselective; hydrogen adds preferentially to the less substituted carbon of the double bond HOH adds in accordance with Markovnikov’s rule

Addition of Cl2 and Br2 carried out with either the pure reagents or in an inert solvent such as CH2Cl2 addition of bromine or chlorine to a cycloalkene gives a trans-dihalocycloalkane addition occurs with anti stereoselectivity; halogen atoms add from the opposite face of the double bond

Addition of HOCl and HOBr Treatment of an alkene with Br2 or Cl2 in water forms a halohydrin Halohydrin: a compound containing -OH and -X on adjacent carbons

Oxymercuration/Reduction Oxymercuration followed by reduction results in hydration of a carbon-carbon double bond oxymercuration reduction

Hydroboration/Oxidation Hydroboration: the addition of borane, BH3, to an alkene to form a trialkylborane Borane dimerizes to diborane, B2H6 H C H 2 3 H B + 3 C H 2 = C H 3 2 B H C H 2 3 Borane Triethylborane (a trialkylborane) 2 B H 3 B 2 H 6 Borane D iborane

Oxidation/Reduction Oxidation: the loss of electrons alternatively, the loss of H, the gain of O, or both Reduction: the gain of electrons alternatively, the gain of H, the loss of O, or both Recognize using a balanced half-reaction 1. write a half-reaction showing one reactant and its product(s) 2. complete a material balance; use H2O and H+ in acid solution, use H2O and OH- in basic solution 3. complete a charge balance using electrons, e-

Oxidation/Reduction three balanced half-reactions

Reduction of Alkenes Most alkenes react with H2 in the presence of a transition metal catalyst to give alkanes commonly used catalysts are Pt, Pd, Ru, and Ni the process is called catalytic reduction or, alternatively, catalytic hydrogenation Pd + H 2 25°C, 3 atm Cyclohexene Cyclohexane

Diels–Alder Reaction Named after Otto Diels and Kurt Alder. They received the Nobel prize in 1950. Produces a cyclohexene ring. The reaction is between a diene with an electron-deficient alkene (dienophile). The Diels-Alder is also called a [4+2] cycloaddition because a ring is formed by the interaction of four pi electrons of the alkene with two pi electrons of the alkene or alkyne.

Mechanism of the Diels–Alder Reaction One-step, concerted mechanism. A diene reacts with an electron-poor alkene (dienophile) to give cyclohexene or cyclohexadiene rings.

Examples of Diels–Alder Reactions