1 Chapter 10 Alkenes. 2 Introduction—Structure and Bonding.

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Chapter 10Alkenes

Alkenes

Introduction—Structure and Bonding

Alkenes

Strength of the p Bonding

Restricted rotation:

Stereoisomerism:

Stability:

Alkenes

• 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.

Introduction—Structure and Bonding

Alkenes

• 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.

Calculating Degrees of Unsaturation : n-m for CnH2(n-m)

Quick assessment of molecular structure from molecular formula

n-m = # of rings + # of p bonds

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:

(a) C6H11Cl; (b) C5H8O; (c) C8H9N.

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

AlkenesNomenclature 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.

Nomenclature of Alkenes : stereoisomers

Alkenes

Zusammen (together)Entgegen (opposite)

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.

Nomenclature of Alkenes

Alkenes

• 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.

Physical Properties

Alkenes

• 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—Csp

2 bond dipoles reinforce each other, yielding a small net molecular dipole. In the trans isomer, the two bond dipoles cancel.

Physical Properties

Alkenes

Interesting Alkenes

Alkenes

• Triacyl glycerols are hydrolyzed to glycerol and three fatty acids of general structure RCOOH.

Lipids

• As the number of double bonds in the fatty acid increases, the melting point decreases.

Alkenes

Lipids

Alkenes

• 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.

Lipids

Alkenes

• 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.

Lipids

Alkenes

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

Preparation of Alkenes

Alkenes

• elimination reactions are stereoselective and regioselective.

Preparation of Alkenes

Reactions of Alkenes

• The characteristic reaction of alkenes is addition—the bond is broken and two new bonds are formed.

Introduction to Addition Reactions

• 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.

• Stereochemical outcome of addition to alkenes

Introduction to Addition Reactions

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

Alkenes

Hydrohalogenation—Electrophilic Addition of HX

What about the stereochemistry & Regiochemistry of the reaction ?

Alkenes

• The mechanism of electrophilic addition consists of two successive Lewis acid-base reactions.

Mechanism of Hydrohalogenation

CH3 CH3

Carbocation

1st new bond

Step 1 Electrophilic attack of H+ on the bond

Step 2 Nucleophilic attack of Br- on carbocation

2nd new bond

AlkenesHydrohalogenation—Electrophilic Addition of HX

Alkenes

Markovnikov’s Rule : Regioselectivity of addition

Markovnikov’s rule (1869) 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

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

Markovnikov’s Rule

Alkenes

According to the Hammond postulate, Path [2] is faster because formation of the carbocation is an endothermic process.

Hydrohalogenation—Markovnikov’s Rule

• 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.

Alkenes

According to the Hammond postulate, Path [2] is faster because formation of the carbocation is an endothermic process.

Hydrohalogenation—Markovnikov’s Rule

NOT 1-Chloropropane2-Chloropropane ONLY

1-bromo-1-methylcyclohexane ONLY

1-methylcyclohexene

Propene

diethylether

Alkenes

• Recall that trigonal planar atoms react with reagents from two directions with equal probability.

• Achiral starting materials yield achiral products or racemic mixture.

Hydrohalogenation—Reaction Stereochemistry

A racemic mixture

Hydrohalogenation—Reaction Stereochemistry

Alkenes

Hydrohalogenation—Summary

Alkenes

• Hydration forms an alcohol.

Hydration—Electrophilic Addition of Water

- Reaction mechanism is very siminar to hydrohalogenation -

Alkenes

• Alcohols add to alkenes, forming ethers by the same mechanism in presence of acid.

Hydration—Electrophilic Addition of Alcohols

• Note that there are three consequences to the formation of carbocation intermediates:1. Markovnikov’s rule holds.2. Addition of H and OH occurs in both syn and anti

fashion.3. Carbocation rearrangements can occur.

Alkenes

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

Halogenation—Addition of Halogen

Alkenes

• 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 occurOnly anti addition of X2 is observed

These facts suggest that carbocations are not intermediates.

Halogenation—Addition of Halogen

C CH H

CH3 CH3

C CH CH3

Racemic mixture of enantiomers

Achiral meso compound

cis-2-Butene

trans-2-Butene

Addition reaction of Br2 to alkenes : the fact!

How can we explain this outcome ?

AlkenesMechanism of Halogenation—Addition of Halogen

Carbocations are unstable because they have only six electrons around carbon.

Halonium ions are unstable because of ring strain.

Step 2

Step 1

..:X:+

Bridged Halonium ionStrained ring

......: :X X

SO2(l)

CH3 CH3

Non-classical carbocation By George Olah

X X:: ....

+ :X:..

Step 1

Step 2

(Step 3)protontransfer X

Halohydrin

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.

Halohydrin Formation : addition of X-OH

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.

AlkenesHalohydrin 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 results in the addition of H2O to an alkene.

Alkenes

Hydroboration—OxidationBH3 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

BH3 can react with three equivalents of alkene to form a trialkylborane.

Alkenes

9-borabicyclo[3.3.1]nonane (9-BBN)

Hydroboration—Selectivity

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

Alkenes

• 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

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 As above As above

X2 Vicinal (1,2) dihalide

2 step via rate-determining formation of bridged halonium ionNo rearrangements

- Anti additionStereospecific

X2/H2O Halohydrin (2-haloalcohol)

3 steps, but similar to halogenation

Markovnikov: X+ bonds to less substituted C.

Anti addition

BH3 (or equivalent), then H2O2/OH-

Alcohol 2 steps: one-step hydroboration, then oxidation.No rearrangements

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….

AlkenesAlkenes in Organic Synthesis: combination of reactionsSuppose we wish to synthesize 1,2-dibromocyclohexane from cyclohexanol.

To solve this problem we must:

AlkenesAlkenes in Organic SynthesisWorking backwards from the product to determine the starting material from which it is made is called retrosynthetic analysis.

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

Homework

Preview of Chapter 11

Alkynes

Extension of chapter 10

Preparation of alkynes

dehydrohalogenation

Reactions of alkynes

Addition reaction

Reaction of acetylide anions

1 Chapter 10 Alkenes. 2 Introduction—Structure and Bonding.

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