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Alkenes, Alkynes and Conjugated Dienes

Alkenes

• Alkenes are also called olefins.

• Alkenes contain a carbon—carbon double bond.

• Terminal alkenes have the double bond at the end of the carbon chain.

• Internal alkenes have at least one carbon atom bonded to each end of the double bond.

• Cycloalkenes contain a double bond in a ring.

Introduction—Structure and Bonding

Alkenes

• Recall that the double bond consists of a bond and a bond.

• Each carbon is sp2 hybridized and trigonal planar, with bond angles of approximately 1200.


Alkenes

Introduction—Structure and Bonding• Bond dissociation energies of the C—C bonds in ethane

(a bond only) and ethylene (one and one bond) can be used to estimate the strength of the component of the double bond.

Alkenes

• Cycloalkenes having fewer than eight carbon atoms have a cis geometry. A trans cycloalkene must have a carbon chain long enough to connect the ends of the double bond without introducing too much strain.

• trans-Cyclooctene is the smallest isolable trans cycloalkene, but it is considerably less stable than cis-cyclooctene, making it one of the few alkenes having a higher energy trans isomer.


Alkenes

• An acyclic alkene has the general structural formula CnH2n

• Alkenes are unsaturated hydrocarbons because they have fewer than the maximum number of hydrogen atoms per carbon.

• Cycloalkanes also have the general formula CnH2n.

• 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 and the maximum number of H atoms possible.

• This procedure gives the total number of rings and/or bonds in a molecule.

Calculating Degrees of Unsaturation

AlkenesNomenclature of Alkenes

Nomenclature of Alkenes

Alkenes

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

• Most alkenes exhibit only weak van der Waals interactions, so their 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 0C) than trans-2-butene (1 0C).

• 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

AlkenesInteresting 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

• 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.

• Saturated fats are typically obtained from animal sources, whereas unsaturated oils are common in vegetable sources. An exception to this generalization is coconut oil, which is largely composed of saturated alkyl side chains.

Lipids

Alkenes

Lipids

Alkenes

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

Preparation of Alkenes

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, with the electron density of the bond concentrated above and below the plane of the molecule.

• Because alkenes are electron rich, simple alkenes do not react with nucleophiles or bases, reagents that are themselves electron rich. Alkenes react with electrophiles.

Alkenes

• Because the carbon atoms of a double bond are both trigonal planar, the elements of X and Y can be added to them from the same side or from opposite sides.


Alkenes


Alkenes

• Two bonds are broken in this reaction—the weak bond of the alkene and the HX bond—and two new bonds are formed—one to H and one to X.

• Recall that the H—X bond is polarized, with a partial positive charge on H. Because the electrophilic H end of HX is attracted to the electron-rich double bond, these reactions are called electrophilic additions.

Hydrohalogenation—Electrophilic Addition of HX

Alkenes

• Addition reactions are exothermic because the two bonds formed in the product are stronger than the and bonds broken in the reactants. For example, H0 for the addition of HBr to ethylene is –14 kcal/mol, as illustrated below.


Alkenes

• The mechanism of electrophilic addition consists of two successive Lewis acid-base reactions. In step 1, the alkene is the Lewis base that donates an electron pair to H—Br, the Lewis acid, while in step 2, Br¯ is the Lewis base that donates an electron pair to the carbocation, the Lewis acid.


Alkenes

• In the representative energy diagram below, each step has its own energy barrier with a transition state energy maximum. Since step 1 has a higher energy transition state, it is rate-determining. H0 for step 1 is positive because more bonds are broken than formed, whereas H0 for step 2 is negative because only bond making occurs.


Alkenes

• With an unsymmetrical alkene, HX can add to the double bond to give two constitutional isomers, but only one is actually formed:

Hydrohalogenation—Markovnikov’s Rule

• This is a specific example of a general trend called Markovnikov’s rule.

• Markovnikov’s rule states that in the addition of HX to an unsymmetrical alkene, the H atom bonds 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 is the formation of a carbocation in the rate-determining step of the mechanism.

• In the addition of HX to an unsymmetrical alkene, the H atom is added to the less substituted carbon to form the more stable, more substituted carbocation.


Alkenes

According to the Hammond postulate, Path [2] is preferred because formation of the carbocation is an endothermic process, so the more stable intermediate is formed faster. Path [2] is much faster than Path [1] because the transition state to form the more stable 20 carbocation is lower in energy.


Alkenes

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

• Achiral starting materials yield achiral products.• Sometimes new stereogenic centers are formed from

hydrohalogenation:

Hydrohalogenation—Reaction Stereochemistry

A racemic mixture

Alkenes

• The mechanism of hydrohalogenation illustrates why two enantiomers are formed. Initial addition of H+ occurs from either side of the planar double bond.

• Both modes of addition generate the same achiral carbocation. Either representation of this carbocation can be used to draw the second step of the mechanism.


Alkenes

• Nucleophilic attack of Cl¯ on the trigonal planar carbocation also occurs from two different directions, forming two products, A and B, having a new stereogenic center.

• A and B are enantiomers. Since attack from either direction occurs with equal probability, a racemic mixture of A and B is formed.


Alkenes

• Hydrohalogenation occurs with syn and anti addition of HX.

• The terms cis and trans refer to the arrangement of groups in a particular compound, usually an alkene or disubstituted cycloalkene.

• The terms syn and anti describe stereochemistry of a process—for example, how two groups are added to a double bond.


Alkenes


Alkenes

Hydrohalogenation—Summary

Alkenes

• Hydration is the addition of water to an alkene to form an alcohol.

Hydration—Electrophilic Addition of Water

Alkenes


Alkenes

• Alcohols add to alkenes, forming ethers by the same mechanism. For example, addition of CH3OH to 2-methylpropene, forms tert-butyl methyl ether (MTBE), a high octane fuel additive.


• Note that there are two 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.

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.

• The electron rich double bond induces a dipole in an approaching halogen molecule, making one halogen atom electron deficient and the other electron rich.

• The electrophilic halogen atom is then attracted to the nucleophilic double bond, making addition possible.

Halogenation—Addition of Halogen

AlkenesHalogenation—Addition of Halogen

Carbocations are unstable because they have only six electrons around carbon. Halonium ions are unstable because of ring strain.

Alkenes

Halogenation—Reaction Stereochemistry• Consider the chlorination of cyclopentene to afford both

enantiomers of trans-1,2-dichlorocyclopentene, with no cis products.

• Initial addition of the electrophile Cl+ from (Cl2) occurs from either side of the planar double bond to form a bridged chloronium ion.

AlkenesHalogenation—Reaction Stereochemistry• In the second step, nucleophilic attack of Cl¯ must occur from

the backside.• Since the nucleophile attacks from below and the leaving group

departs from above, the two Cl atoms in the product are oriented trans to each other.

• Backside attack occurs with equal probability at either carbon of the three-membered ring to yield a racemic mixture.

Alkenes

Halohydrin FormationTreatment of an alkene with a halogen X2 and H2O forms a halohydrin by addition of the elements of X and OH to the double bond.

AlkenesHalohydrin Formation

Even through 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.

• Oxidations of Alkenes: Syn 1,2-Dihydroxylation• Either OsO4 or KMnO4 will give 1,2 diols (glycols)

– Mechanism for Syn Hydroxylation of Alkenes• Cyclic intermediates result from reaction of the oxidized

metals

• The initial syn addition of the oxygens is preserved when the oxygen-metal bonds are cleaved and the products are syn diols

• Oxidative Cleavage of Alkenes• Reaction of an alkene with hot KMnO4 results in

cleavage of the double bond and formation of highly oxidized carbons

– Unsubstituted carbons become CO2, monosubstituted carbons become carboxylates and disubstituted carbons become ketones

• This be used as a chemical test for alkenes in which the purple color of the KMnO4 disappears and forms brown MnO2 residue if alkene (or alkyne) is present

AlkynesAn alkyne is a hydrocarbon with one triple bond

Acetylene is the simplest alkyne CH CH

The IUPAC nomenclature is analogous to alkenes, the suffix for an alkyne is –yne. In the trivial

nomenclature acetylene is the parent compound and the groups attached to the sp Carbons are named as

substituens on acetylene

CH3C CCH2CH3 2-pentyne or ethylmethylacetylene

The Structure of Alkynes

A triple bond is composed of a s bond and two p bonds

Acetylene is a weak acid, but not nearlyAcetylene is a weak acid, but not nearlyas weak as alkanes or alkenes.as weak as alkanes or alkenes.

•Compound pKa

• HF 3.2

• H2O 16

• NH3 36

• 45

• CH4 60

HH22CC CHCH22

HCHC CHCH 26

Acetylene

The stronger the acid, the weaker its conjugate base

top 252

RCHRCH22CHCH22RR''

•Another way to convert alkynes to alkenes isby reduction with sodium (or lithium or potassium)in ammonia.

•trans-Alkenes are formed.

RCRC CR'CR' RCHRCH CHR'CHR'

Partial Reduction

HBrHBr

BrBr

(60%)(60%)

•Alkynes are slightly less reactive than alkenes

CHCH33(CH(CH22))33CC CHCH33 CHCH33(CH(CH22))33CC CHCH22

Follows Markovnikov's Rule

•expected reaction:

enolenolobservedobserved reactionreaction::

RCHRCH22CR'CR'

OO

HH++

RCRC CR'CR' HH22OO++

HH++

RCRC RCRC'' HH22OO++

OHOH

RCHRCH CR'CR'

ketoneketone

Hydration of Alkynes

•enols are regioisomers of ketones, and exist in equilibrium with them

•keto-enol equilibration is rapid in acidic media

•ketones are more stable than enols andpredominate at equilibrium

enolenol

OHOH

RCHRCH CR'CR' RCHRCH22CR'CR'

OO

ketoneketone

Enols

Conjugation, Resonance and Dienes

• Conjugation occurs whenever p orbitals are located on three or more adjacent atoms.

Conjugation

• The four p orbitals on adjacent atoms make a 1,3-diene a conjugated system.


• Having three or more p orbitals on adjacent atoms allows p orbitals to overlap and electrons to delocalize.

Conjugation


• 1,4-pentadiene is an isolated diene. • The bonds in 1,4-pentadiene are too far apart to be

conjugated.

Conjugation


• The allyl carbocation is another example of a conjugated system.

Conjugation

• Conjugation stabilizes the allyl carbocation.


• Drawing resonance structures for the allyl carbocation is a way to see how to use Lewis structures to illustrate how conjugation delocalizes electrons.

Conjugation

• The true allyl cation is a hybrid of the two resonance forms.

• In the hybrid, the positive charge is delocalized over the two terminal carbons.

• Delocalizing electron density lowers the energy of the hybrid, thus stabilizing the allyl carbocation and making it more stable than a normal 10 carbocation.


• Conjugated dienes are compounds having two double bonds joined by one bond.

• Conjugated dienes are also called 1,3-dienes.

• 1,3-Butadiene (CH2=CH-CH=CH2) is the simplest conjugated diene.

• Three stereoisomers are possible for 1,3-dienes with alkyl groups bonded to each end carbon of the diene.

Conjugated Dienes


Four features distinguish conjugated dienes from isolated dienes.

The Carbon—Carbon Bond Length in 1,3-Butadiene

1. The C—C single bond joining the two double bonds in unusually short.

2. Conjugated dienes are more stable than similar isolated dienes.

3. Some reactions of conjugated dienes are different than reactions of isolated double bonds.

4. Conjugated dienes absorb longer wavelengths of ultraviolet light.


The observed bond distances can be explained by looking at hybridization.


• Each carbon atom in 1,3-butadiene is sp2 hybridized, so the central C—C single bond is formed by the overlap of two sp2 hybridized orbitals, rather than the sp3 hybridized orbitals used to form the C—C bond in CH3CH3.


A resonance argument can also be used to explain the shorter C—C bond length in 1,3-butadiene.


• Based on resonance, the central C—C bond in 1,3-butadiene is shorter because it has partial double bond character.


• Finally, 1,3-butadiene is a conjugated molecule with four overlapping p orbitals on adjacent atoms.

• Consequently, the electrons are not localized between the carbon atoms of the double bonds, but rather delocalized over four atoms.

• This places more electron density between the central two carbon atoms of 1,3-butadiene than would normally be present.

• This shortens the bond.



When hydrogenation gives the same alkane from two dienes, the more stable diene has the smaller heat of hydrogenation.

Stability of Conjugated Dienes


A conjugated diene has a smaller heat of hydrogenation and is more stable than a similar isolated diene.



• A conjugated diene is more stable than an isolated diene because a conjugated diene has overlapping p orbitals on four adjacent atoms. Thus, its electrons are not localized between the carbon atoms of the double bond, but rather delocalized over four atoms.

• This delocalization which adds stability to the diene, cannot occur in an isolated diene. For example, no resonance structures can be drawn for 1,4-pentadiene, but three can be drawn for (3E)-1,3-pentadiene (or any other conjugated diene).



The bonds in conjugated dienes undergo addition reactions that differ in two ways from the addition reactions of isolated double bonds.

Electrophilic Addition: 1,2- Versus 1,4-Addition

1. Electrophilic addition in conjugated dienes gives a mixture of products.

2. Conjugated dienes undergo a unique addition reaction not seen in alkenes or isolated dienes.

Recall that electrophilic addition of one equivalent of HBr to an isolated diene yields one product and Markovnikov’s rule is followed.


With a conjugated diene, electrophilic addition of one equivalent of HBr affords two products.


The 1,2-addition product results from Markovnikov addition of HBr across two adjacent carbon atoms (C1 and C2) of the diene.

The 1,4-addition product results from addition of HBr to the two end carbons (C1 and C4) of the diene. 1,4-Addition is also called conjugate addition.



Addition of HX to a conjugated diene forms 1,2- and 1,4-products because of the resonance-stabilized allylic carbocation intermediate.


Kinetic Versus Thermodynamic Products

The amount of 1,2- and 1,4-addition products formed in electrophilic addition reactions of conjugated dienes depends greatly on the reaction conditions.



When a mixture containing predominantly the 1,2-product is heated, the 1,4-addition product becomes the major product at equilibrium.



In the reactions we have learned thus far, the more stable product is formed faster—i.e. the kinetic and thermodynamic products are the same.

The electrophilic addition of HBr to 1,3-butadiene is different in that the kinetic and thermodynamic products are different—i.e the more stable product is formed more slowly.

Why is the more stable product formed more slowly in this case?


Kinetic Versus Thermodynamic Products• Recall that the rate of a reaction is determined by its

energy of activation (Ea), whereas the amount of product present at equilibrium is determined by its stability.


Kinetic Versus Thermodynamic ProductsThe overall two-step mechanism for addition of HBr to 1,3-butadiene to form both 1,2- and 1,4 addition products is illustrated in the energy diagram below.



Why is the ratio of products temperature dependent?

• At low temperature, the energy of activation is the more important factor. Since most molecules do not have enough kinetic energy to overcome the higher energy barrier at lower temperature, they react by the faster pathway, forming the kinetic product.

• At higher temperature, most molecules have enough kinetic energy to reach either transition state. The two products are in equilibrium with each other, and the more stable compound—which is lower in energy, becomes the major product.


Conjugated Dienes and Ultraviolet Light

• The absorption of ultraviolet (UV) light by a molecule can promote an electron from a lower electronic state to a higher one.

• Ultraviolet light has a slightly shorter wavelength (and thus higher frequency) than visible light.

• The most useful region of UV light for this purpose is 200-400 nm.



• When electrons in a lower energy state (the ground state) absorb light having the appropriate energy, an electron is promoted to a higher electronic state (excited state).

• The energy difference between the two states depends on the location of the electron.


Conjugated Dienes and Ultraviolet Light• The promotion of electrons in bonds and unconjugated

bonds requires light having a wavelength of < 200 nm; that is, a shorter wavelength and higher energy than light in the UV region of the electromagnetic spectrum.

• With conjugated dienes, the energy difference between the ground and excited states decreases, so longer wavelengths of light can be used to promote electrons.

• The wavelength of UV light absorbed by a compound is often referred to as its max.


Conjugated Dienes and Ultraviolet Light• As the number of conjugated bonds increases, the

energy difference between the ground and excited state decreases, shifting the absorption to longer wavelengths.

• With molecules having eight or more conjugated bonds, the absorption shifts from the UV to the visible region, and the compound takes on the color of the light it does not absorb.



• Lycopene absorbs visible light at max = 470 nm, in the blue-green region of the visible spectrum. Because it does not absorb light in the red region, lycopene appears bright red.

Porphyrine ring


Sunscreens

• UV radiation from the sun is high enough in energy to cleave bonds, forming radicals that can prematurely age skin and cause cancer.

• However, since much of this radiation is filtered out by the ozone layer, only UV light having wavelengths > 290 nm reaches the skin’s surface.

• Much of this UV light is absorbed by melanin, the highly conjugated colored pigment in the skin that serves as the body’s natural protection against the harmful effects of UV radiation.


Sunscreens • Prolonged exposure to the sun can allow more UV radiation to

reach your skin than melanin can absorb.• Commercial sunscreens can offer some protection, because they

contain conjugated compounds that absorb UV light, thus shielding the skin (for a time) from the harmful effects of UV radiation.

• Commercial sunscreens are given an SPF rating (sun protection factor), according to the amount of sunscreen present. The higher the number, the greater the protection.

• Two sunscreens that have been used for this purpose are para-aminobenzoic acid (PABA) and padimate O.

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