Chapter 10 Alkenes

Alkenes Introduction—Structure and Bonding

Alkenes Strength of the p Bonding Restricted rotation: Stereoisomerism: Stability:

Alkenes Introduction—Structure and Bonding Cycloalkenes having fewer than eight carbon atoms have a cis geometry. trans-Cyclooctene is the smallest isolable trans cycloalkene less stable than cis-cyclooctene, making it one of the few alkenes having a higher energy trans isomer.

Alkenes Calculating Degrees of Unsaturation : n-m for CnH2(n-m) n-m = # of rings + # of p bonds Quick assessment of molecular structure from molecular formula general structural formula CnH2n : acyclic alkenes, Cycloalkanes Each  bond or ring removes two hydrogen atoms from a molecule, and this introduces one degree of unsaturation. The number of degrees of unsaturation for a given molecular formula can be calculated by comparing the actual number of H atoms in a compound to the maximum number of H atoms possible for the number of carbons present if the molecule were a straight chain alkane. This procedure gives the total number of rings and/or  bonds in a molecule.

Alkenes Calculating Degrees of Unsaturation w/ hetero atoms Halogens (F, Cl, Br, I) - Add the number of halogens to the number of hydrogens in the formula. Oxygen – Ignore the number of oxygens in the fomula. Nitrogen – Subtract the number of nitrogens from the number of hydrogens in the formula. Examples. Deduce the number of degrees of unsaturation (d.u.) in the following molecular formulas and suggest one possible structure for each: C6H11Cl; (b) C5H8O; (c) C8H9N.

Alkenes Nomenclature of Alkenes: alkenes are identified by the suffix –ene.

Alkenes Nomenclature of Alkenes Compounds with two double bonds : the suffix “–adiene”. three double bonds trienes, and so forth. Always choose the longest chain that contains both atoms of the double bond. In cycloalkenes, the double bond is located between C1 and C2, and the “1” is usually omitted in the name. The ring is numbered clockwise or counterclockwise to give the first substituent the lower number. Compounds that contain both a double bond and a hydroxy group are named as alkenols and the chain (or ring) is numbered to give the OH group the lower number.

Alkenes Nomenclature of Alkenes : stereoisomers Entgegen (opposite) Zusammen (together)

Alkenes Nomenclature of Alkenes Some alkene or alkenyl substituents have common names. The simplest alkene, CH2=CH2, named in the IUPAC system as ethene, is often called ethylene.

Alkenes Physical Properties physical properties are similar to alkanes of comparable molecular weight. Alkenes have low melting points and boiling points. Melting and boiling points increase as the number of carbons increases because of increased surface area. Alkenes are soluble in organic solvents and insoluble in water. The C—C single bond between an alkyl group and one of the double bond carbons of an alkene is slightly polar because the sp3 hybridized alkyl carbon donates electron density to the sp2 hybridized alkenyl carbon.

Alkenes Physical Properties A consequence of this dipole is that cis and trans isomeric alkenes often have somewhat different physical properties. cis-2-Butene has a higher boiling point (4°C) than trans-2-butene (1°C). In the cis isomer, the two Csp3—Csp2 bond dipoles reinforce each other, yielding a small net molecular dipole. In the trans isomer, the two bond dipoles cancel.

Alkenes Interesting Alkenes

Alkenes Lipids Triacyl glycerols are hydrolyzed to glycerol and three fatty acids of general structure RCOOH. As the number of double bonds in the fatty acid increases, the melting point decreases.

Alkenes Lipids

Alkenes Lipids Fats and oils are triglycerols with different physical properties. Fats have higher melting points—they are solids at room temperature. Usually from animal sources Oils have lower melting points—they are liquids at room temperature. Usually from vegitable sources The identity of the three fatty acids in the triacylglycerol determines whether it is a fat or an oil. An exception to this generalization is coconut oil, which is largely composed of saturated alkyl side chains.

Alkenes Lipids Increasing the number of double bonds in the fatty acid side chains decreases the melting point of the triacylglycerol. Fats are derived from fatty acids having few or no double bonds. Oils are derived from fatty acids having a larger number of double bonds. An exception to this generalization is coconut oil, which is largely composed of saturated alkyl side chains.

Alkenes Preparation of Alkenes alkenes can be prepared from alkyl halides and alcohols via elimination reactions.

Alkenes Preparation of Alkenes elimination reactions are stereoselective and regioselective.

Reactions of Alkenes Introduction to Addition Reactions The characteristic reaction of alkenes is addition—the  bond is broken and two new  bonds are formed. Alkenes are electron rich. Because alkenes are electron rich, simple alkenes do not react with nucleophiles or bases, reagents that are themselves electron rich. Alkenes react with electrophiles.

Introduction to Addition Reactions Stereochemical outcome of addition to alkenes When the addition is selective, only one set of enantiomers forms.

Addition Reactions to Alkenes

Alkenes Hydrohalogenation—Electrophilic Addition of HX Addition reactions are exothermic

What about the stereochemistry & Regiochemistry Alkenes Hydrohalogenation—Electrophilic Addition of HX What about the stereochemistry & Regiochemistry of the reaction ?

Alkenes Mechanism of Hydrohalogenation The mechanism of electrophilic addition consists of two successive Lewis acid-base reactions.

Alkenes Hydrohalogenation—Electrophilic Addition of HX

Alkenes Markovnikov’s Rule : Regioselectivity of addition in the addition of HX to an unsymmetrical alkene, the H atom adds to the less substituted carbon atom—that is, the carbon that has the greater number of H atoms to begin with.

Alkenes Markovnikov’s Rule The basis of Markovnikov’s rule : the formation of a carbocation in the rate-determining step of the mechanism.

Alkenes Hydrohalogenation—Markovnikov’s Rule According to the Hammond postulate, Path [2] is faster because formation of the carbocation is an endothermic process.

in an endothermic step, TS resembles the products, The Hammond postulate relates reaction rate to stability. It provides a quantitative estimate of the energy of a transition state. The Hammond postulate : the transition state of a reaction resembles the structure of the species (reactant or product) to which it is closer in energy. in an endothermic step, TS resembles the products, in an exothermic step, TS resembles the reactants. 30

Alkenes Hydrohalogenation—Markovnikov’s Rule According to the Hammond postulate, Path [2] is faster because formation of the carbocation is an endothermic process.

Alkenes Hydrohalogenation—Reaction Stereochemistry Recall that trigonal planar atoms react with reagents from two directions with equal probability. Achiral starting materials yield achiral products or racemic mixture. A racemic mixture 

Hydrohalogenation—Reaction Stereochemistry

Alkenes Hydrohalogenation—Summary

Alkenes Hydration—Electrophilic Addition of Water Hydration forms an alcohol. - Reaction mechanism is very siminar to hydrohalogenation -

Alkenes Hydration—Electrophilic Addition of Alcohols Alcohols add to alkenes, forming ethers by the same mechanism in presence of acid. Note that there are three consequences to the formation of carbocation intermediates: Markovnikov’s rule holds. Addition of H and OH occurs in both syn and anti fashion. Carbocation rearrangements can occur.

Alkenes Halogenation—Addition of Halogen Halogenation is the addition of X2 (X = Cl or Br) to an alkene to form a vicinal dihalide.

Alkenes Halogenation—Addition of Halogen Halogens add to  bonds because halogens are polarizable. Two facts demonstrate that halogenation follows a different mechanism from that of hydrohalogenation or hydration. No rearrangements occur Only anti addition of X2 is observed These facts suggest that carbocations are not intermediates.

Addition reaction of Br2 to alkenes : the fact! How can we explain this outcome ?

Alkenes Mechanism of Halogenation—Addition of Halogen Carbocations are unstable because they have only six electrons around carbon. Halonium ions are unstable because of ring strain.

Non-classical carbocation By George Olah

Halohydrin Formation : addition of X-OH Even though X¯ is formed in step [1] of the mechanism, its concentration is small compared to H2O (often the solvent), so H2O and not X¯ is the nucleophile.

Alkenes Halohydrin Formation Bromohydrins are also formed with N-bromosuccinimide (NBS) in aqueous DMSO [(CH3)2S=O]. In H2O, NBS decomposes to form Br2, which then goes on to form a bromohydrin by the same reaction mechanism.

Alkenes Halohydrin Formation : Selectivity

Halohydrin Formation application

Alkenes Halohydrin Formation

Alkenes Hydroboration—Oxidation Hydroboration—oxidation is a two-step reaction sequence that converts an alkene into an alcohol.

Alkenes Hydroboration—Oxidation Hydroboration—oxidation results in the addition of H2O to an alkene.

Alkenes Hydroboration—Oxidation BH3 is a reactive gas that exists mostly as a dimer, diborane (B2H6). Borane is a strong Lewis acid that reacts readily with Lewis bases. The first step in hydroboration

Alkenes Hydroboration—Oxidation The proposed mechanism involves concerted addition of H and BH2 from the same side of the planar double bond:

Alkenes Hydroboration—Oxidation BH3 can react with three equivalents of alkene to form a trialkylborane.

Alkenes Hydroboration—Oxidation 9-borabicyclo[3.3.1]nonane (9-BBN)

Hydroboration—Selectivity With unsymmetrical alkenes, the boron atom bonds to the less substituted carbon atom.

Alkenes Hydroboration—Oxidation

Alkenes Hydroboration—Oxidation Oxidation replaces the C—B bond with a C—O bond, forming a new OH group with retention of configuration. The overall result of this two-step sequence is syn addition of the elements of H and OH to a double bond in an “anti-Markovnikov” fashion.

Alkenes Hydroboration—Oxidation

Summary of Addition Reactions of Alkenes Reagent Product Mechanism Regioselectivity Stereochemistry HX Alkyl halide 2 step via rate-determining formation of R+. Rearrangements possible Markovnikov Syn and anti addition H2O Alcohol As above X2 Vicinal (1,2) dihalide 2 step via rate-determining formation of bridged halonium ion No rearrangements - Anti addition Stereospecific X2/H2O Halohydrin (2-haloalcohol) 3 steps, but similar to halogenation Markovnikov: X+ bonds to less substituted C. BH3 (or equivalent), then H2O2/OH- 2 steps: one-step hydroboration, then oxidation. Anti-Markovnikov Syn addition in hydroboration step; retention of configuration in oxidation step

keeping track of all the reactions? Alkyl halides – Substitution and elimination (they have good leaving groups). Alcohols – As alkyl halides, but only if OH group has been converted to a good leaving group. Alkenes – Addition ( bond is easily broken) Firstly, check the basic reaction types for a functional group. This provides an overall organization of reactions. Then, learn the specific reagents for each reaction. This helps to classify the reagent according to its major properties. Is it basic or acidic? Is it electrophilic or nucleophilic? Is it an oxidizing agent or a reducing agent? Think mechanism ! – reasonable ones….

Alkenes Alkenes in Organic Synthesis: combination of reactions Suppose we wish to synthesize 1,2-dibromocyclohexane from cyclohexanol. To solve this problem we must:

Alkenes Alkenes in Organic Synthesis Working backwards from the product to determine the starting material from which it is made is called retrosynthetic analysis.

Homework 10.40, 10.46, 10.51, 10.52, 10.53, 10.57, 10.61, 10.63, 10.64

Preview of Chapter 11 Alkynes Extension of chapter 10 Preparation of alkynes dehydrohalogenation Reactions of alkynes Addition reaction Reaction of acetylide anions