alkene = olefin - Yonsei University · alkene = olefin. Ch.6 Alkenes: Structure and Reactivity 6.1...

Ch.6 Alkenes: Structure and Reactivity

H2C CH2

Ethylene

α-Pinene

β-Carotene(orange pigment and vitamin A precursor)

alkene = olefin

6.1 Industrial Preparation and Use of Alkenes

H2C CH2

Ethylene(26 million tons / yr)

CH3CH2OHCH3CHOCH3COOHHOCH2CH2OHClCH2CH2ClH2C=CHCl

EthanolAcetaldehydeAcetic acidEthylene glycolEthylene dichlorideVinyl chloride

OEthylene oxide

Vinyl acetate

Polyethylene

Compounds derived industrially from ethylene

Compounds derived industrially from propylene

CH CH2

Propylene(14 million tons / yr)

Cumene

Polypropylene

H3C CH3

Isopropyl alcohol

Propylene oxide

• Ethylene, propylene, and butene are synthesized industrially by thermal cracking of natural gas (C1-C4 alkanes) and straight-run gasoline (C4-C8

alkanes).

CH3(CH2)nCH3850-900oC

steamH2 + CH4 + H2C=CH2 + CH3CH=CH2

+ CH3CH2CH=CH2

- the exact processes are complex; involve radical process

900oCCH3CH2 CH2CH3 H2C CH

H2 2 H2C=CH2 + H2

• Thermal cracking is an example of a reaction whose energetics are dominated by entropy (∆So) rather than enthalpy (∆Ho) in the free-energy equation (∆Go = ∆Ho - T∆So) .; C-C bond cleavage (positive ∆Ho) ; high T and increased number of molecules → larger T∆So

6.2 Calculating Degree of Unsaturation

unsaturated: formula of alkene CnH2n

; formula of alkane CnH2n+2

degree of unsaturation: the number of rings and/or multiple bonds

in general, each ring or double bond corresponds to a loss of two hydrogens from alkane formula

unknown hydrocarbon with molecular weight 82; C6H10

corresponding alkane; C6H14

H14-H10 = H4 = 2H2

therefore, degree of unsaturation= 2

possible structures:

BrCH2CH=CHCH2Br HCH2CH=CHCH2H

C4H6Br2 = "C4H8" one unsaturation: one double bond or one cycle

degree of unsaturation: containing elements other than just C, H

■Organohalogen compounds (C, H, X, X= F, Cl, Br, I)Add the number of halogens to the number of hydrogens; a halogen is simply a replacement of hydrogen

H2C=CHCH=CHCH2OH

C5H8O= "C5H8" two unsaturation: two double bonds

H2C=CHCH=CHCH2-H

■Organooxygen compounds (C, H, O)Ignore the number of oxygens; oxygen forms two bonds; C-C vs C-O-C or C-H vs C-O-H

C5H9N= "C5H8" two unsaturation: one double bond and one ring

■Organonitrogen compounds (C, H, N)Subtract the number of nitrogens from the number of hydrogens; nitrogen forms three bonds; C-C vs C-NH-C or C-H vs C-NH2

6.3 Naming Alkenes

pentene hexene

Name the parent hydrocarbon: Find the longest carbon chain containing the double bond and name the compound accordingly, using the suffix -ene:

Step 1

Numbering: Begin at the end nearer the double bond or, if the double bond is equivalent from the two ends, begin at the end nearer the first branch point. This rule ensures that the double bond carbons receive the lowest possible numbers:

Step 2

2-Hexene

2-Methyl-3-hexene

2-Ethyl-1-pentene

2-Methyl-1,3-butadiene

Write the full name: list substituents alphabetically; indicate the position of double bond (the number of the first alkene carbon) immediately before the parent name; more than one double bonds: -diene, triene...

Step 3

1-Methylcyclohexene 1,4-Cyclohexadiene

cycloalkanes are named similarly, but double bond is between C1 and C2 and the first substituent has as low a number as possible; it's not necessary to indicate the position of the double bond in the name (always C1 and C2)

1,5-Dimethylcyclopentene

Ethene Ethylene

Propene Propylene

2-Methylpropene Isobutylene

2-Methyl-1,3-butadiene Isoprene

1,3-Pentadiene Piperylene

IUPAC name Common nameCommon names

Substituent Names

H2C H2CHC

A methylene group A vinyl group An allyl group

Vinyl bromide

Allyl bromide

Μethylenecyclopentane

6.4 Electronic Structure of Alkenes

• Rotation around double bond is restricted: The π-bond must break for rotation to take place around a C=C double bond - 268 kJ/mol (64 kcal/mol) is required to break the π-bond- rotational energy barrier for ethane: only 12 kJ/mol

rotation

π-bond(p-orbitals are parallel)

broken π-bond after rotation(p-orbitals are perpendicular)

6.5 Cis-Trans Isomerism in Alkenes

cis-trans isomerism: when both carbons are bonded to two different groups

H3C CH3

cis-2-Butene trans-2-Butene

A Dthese two compounds are identical;they are not cis-trans isomers

these two compounds are not identical;they are cis-trans isomers

6.6 Sequence Rules: The E,Z Designation

cis-trans isomerism: describe the disubstituted double bond geometries; tri-and tetrasubstituted double bonds- a general method is needed

H3C CH3

H CH2CH2CH3

H3C CH2CH2CH3

cis or trans ? cis or trans ?

E, Z isomerism: a more general method for describing double-bond geometry; E (entgegen, "opposite"); Z (zusammen, "together")

High High

Low Low

High Low

Low High

the higher priority groups on each carbon are on the same side of the double bond

the higher priority groups on each carbon are on the oppositeside of the double bond

Sequence Rule (Cahn-Ingold-Prelog rule; CIP rule); priority of substituents

Br > Cl > O > N > C > H

35 17 8 7 6 1

Considering each of the double-bond carbons separately, identify the two atoms directly attached and rank them according to atomic number.

Cl CH3

(Z)-2-Chloro-2-butene

H3C CH3

(E)-2-Chloro-2-butene

Rule 1

If a decision can't be reached by ranking the first atoms in thesubstituents, look at the second, third, or fourth atoms away from the double-bond carbons until the first difference is found.

HCH3< O H O CH3<

HCH3 C

HCH3< C NH2 C Cl<

Rule 2

Multiple-bonded atoms are equivalent to the same number of single-bonded atoms.

Rule 3

C C CC

CH3H3C

(E)-3-Methyl-1,3-pentadiene

(E)-1-Bromo-2-isopropyl-1,3-butadieneBr

(Z)-2-Hydroxymethyl-2-butenoic acid

6.7 Stability of Alkenes

Relative stability from equilibrium constant:- cis-trans isomers interconvert under strong acid condition

H3C CH3

cis (24 %) trans (76%)

catalyst

Erel= + 2.8 kJ/mol (0.66 kcal/mol) Erel= 0.0 kcal/mol

cis trans

HH H H

From heat of combustion

H3C CH3

∆Hocombustion= -2685.5 kJ/mol ∆Ho

combustion= -2682.2 kJ/mol

Erel = +3.3 kJ/mol Erel = +0.0 kJ/mol

From heat of hydrogenation

H3C CH3

∆Hohydro = -120 kJ/mol

H2CH3CH2CH2CH3 Pd

∆Hohydro = -116 kJ/mol

4 kJ/mol difference

Butane

∆Gotrans

∆Gocis

Reaction progress

Energy profile for hydrogenation

Stabilities of alkenes: increasing the degree of substitution leads further stabilization

tetrasubstituted trisubstituted disubstituted monosubstituted

1. Hyperconjugation: a stabilizing interaction between the unfilled antibonding C=C p bond and a filled C-H s bond orbital on a neighboring substituent. The more substutuents that are present, the more opportunities exist for hyperconjugation, and the more stable the alkene.

bonding C-H σ orbital (filled)

antibonding C-C π orbital (unfilled)

Explanations of alkene stabilities

2. Bond strength: sp2-sp3 C-C bond is stronger than sp3-sp3 C-C bond; more highly substituted alkenes always have a higher ratio of sp2-sp3

bonds to sp3-sp3 bonds

CH CH CH3CH3 CH2 CH CH2CH3

sp3-sp2 sp3-sp2 sp3-sp2

sp3-sp3

6.8 Electrophilic Addition of HX to Alkenes

• alkenes: electron rich, nucleophilic

Electrophilic addition reaction: addition of electrophiles to nucleophilic alkenes

H H3CH3C

H3CH3C H

carbocation intermediate

The electrophile HBr is attacked by the p-electrons of the double bond, and a new C-H σ-bond is formed. This leaves the other carbon atom with a + charge and a vacant p orbital

The Br- donates an electron pair to the positively charged carbon atom, forming a C-Br σ-bond and yielding the neutral addition product.

Reaction energy diagram for the two-step electrophilic addition of HBr to 2-methylpropene.

Reaction progress

reactants ∆Go

TS1TS2

H3CH3C

C CH3H3CH3C

carbocation intermediate

∆G1 > ∆G2

The first step is slower than the second step.

Writing Organic Reactions

A + B C

both reactants (A and B) are equally emphasized

R CH3R

Et2O, 25oC

solvents, temperature and other reaction conditions are written either above or belowthe reaction arrow

R CH3R

Cl+ HCl

reactants A is of greater interest than B

Electrophilic addition of HX: HCl, HBr, HI (HI is generated in the reaction mixture)

Cl+ HCl

2-Methylpropene 2-Chloro-2-methylpropane(94 %)

KIH3PO4

1-Pentene 2-Iodopentane

6.9 Orientation of Electrophilic Addition: Markovnikov's Rule

regiospeccific: only one of two possible orietation of additions occurs

Cl+ HCl

NOT formedsole product

regioselective: one of two possible orientation of additions preferred

Markovnikov's rule: In the addition of HX to an alkene, the H attaches to the carbon with fewer alkyl substituents and the X attaches to the carbon with more alkyl substituents.

Cl+ HCl

sole productmore substituted carbon

less substituted carbon

more substituted carbon

When both ends of the double bond have the same degree of substitution, a mixture of products formed.

Interpretation of Markovnikov's rule: In the addition of HX to an alkene, the more highly substituted carbocation is formed as the intermediate

3o carbocation

1o carbocationNOT formed

Why should this be?

3o carbocation

2o carbocation NOT formed

CH3HBr

6.10 Carbocation Structure and Stability

Carbocation: planar, sp2-hybridized, electron deficient

vacant p orbital

Stability of carbocation: measure the amount of energy required to form the carbocation from its alkyl halide: R-X → R+ + :X-

CH3Cl CH3 + Cl

CH3 + e-CH3

Cl-Cl + e-

D = 351 kJ/mol (Bond dissociation energy)

Ei = 948 kJ/mol (Ionization energy)

Eea= -348 kJ/mol (Electron Affinity)

CH3Cl +CH3 Cl- 951 kJ/mol (Dissociation enthalpy)

→ tertiary halides dissociate to give carbocation much more easily than secondary or primary halides; tertiary carbocations are more stable than secondary or primary ones

Dissociation enthalpy:

1o alkyl halide > 2o alkyl halide > 3o alkyl halideMethyl halide >

Carbocation stability:

< <<CH

1o 2o 3o

1. Inductive effect: result from the shifting of electrons in a σ-bond in response to the electronegativity of nearby atom; Electrons from a relatively large and polarizable alkyl group can shift toward a neighboring positive charge more easily than the electron from a hydrogen. ; alkyl grpups donates electrons inductively and stabilize carbocations

inductive effect of alkyl groups:

1o 2o 3o

methyl

2. Hyperconjugation: interaction of nearby C-H σ-rebital with the vacant carbocation p orbital stabilizes the cation and lowers its energy

bonding C-H σ orbital (filled)

empty p orbital (unfilled)

stabilization carbocation through hyperconjugation

Two different C-H bonds in t-Butyl cation:

2 C-H σ orbitals are in plane:perpendicular to the cation p orbital(no hyperconjugation)

(CH3)3C+

6 C-H σ orbitals above and below the plane:nearly parallel to the cation p orbital(hyperconjugation)

staggered conformation eclipsed conformation

stabilizing hyperconjugation

destabilizing torsional strain

6.11 The Hammond Postulate

Summary about electrophilic addition:

■ Electrophilic addition to an unsymmetrically substituted alkene gives the more highly substituted carbocation. A more highly substituted carbocationforms faster than a less highly substituted one and, once formed, rapidly goes to give the final product.

■ A more highly substituted carbocation is more stable than a less highly substituted one.

- How are these two points (stability and rate) related?- Why does the stability of the carbocation intermediate affect the rate at which it's formed and thereby determine the structure of the final product?

slowerreaction

less stableintermediate

more stableintermediate

faster eaction

slowerreaction less stable

intermediate

more stableintermediate

faster eaction

reaction rate ~ activation energy (∆G‡)stability ~ ∆Go

Hammond Postulate

“The geometry of the transition state for a step most closely resembles the side (i.e. reactant or product) to which it is closer in energy.”

reactant

product

transition state

product-like TS

reactant

product

transition state

reactant-like TS

The hypothetical structure of a transition state for alkene protonation.

TS1carbocation intermediate

alkene

The transition state is closer in both energy and structure to the carbocationthan to the alkene. Thus, an increase in carbocation stability (lower ∆Go) also causes an increase in transition state stability (lower ∆G‡), therefore, increase the reaction rate.

product-like transition state

δ+ δ-

R RR R HBr

carbocationalkene

(CH3)2C CH2

Reaction progress

∆Gprim

tertiary TS

(CH3)3C+Cl-

(CH3)2CHCH2+Cl-

(CH3)3CCl

(CH3)2CHCH2Cl

∆Gtert

primary TS

More stable carbocations form faster because their stability is reflected in the transition state leading to them.

6.12 Evidence for the Mechanism of Electrophilic Addition: Carbocation Rearrangement

How do we know that the carbocation mechanism for addition of HX to alkenes is correct?

The answer is we never know the correct mechanism entirely proven. The best we can do is to show that a proposed mechanism is consistent with all known facts.

The evidence of two step, carbocation mechanism: structural rearrangements

H+ HCl H3C

~50 % ~50 %How is it formed ?

It is difficult to explain it with one step mechanism.

• two step mechanism can explain the rearrangements:( F.C. Whitmore, 1930s)

H+ HCl

HH H3C

Hydrideshift

2o carbocation 3o carbocation

Cl- Cl-

- involve hydride shift: rearrangement of adjacent hydride ion (H-) to form more stable carbocation

• alkyl group can also rearrange with its electron pair

CH3+ HCl

H3CCH3

H3CH H3C

methylshift

2o carbocation 3o carbocation

Cl- Cl-

Rearrangements of carbocations are common.a group (:H- or :CH3

-) moves to an adjacent positively charged carbon;- taking its bonding electron pair with it- a less stable carbocation rearranges to a more stable ion

Myrcene(oil of bay)

α-Pinene(turpentine)

Carvone(spearmint oil)

Terpenes: Naturally Occuring AlkenesChemistry @ Work

Essential oils: fragrant mixtures of liquids from plant materialsTerpenes: plant essential oils consist largely of mixtures of compounds

Isoprene rule: head-to-tail joining of five-carbon isoprene

IsopreneHead

Myrecene

Lanosterol, a triterpene (C30)- precusors for steroid hormons

Terpenes: Naturally Occuring AlkenesChemistry @ Work

Monoterpene: 10 carbons (two isoprene units)Sesquiterpene: 15 carbons (three isoprene units)Diterpene: 20 carbons (two isoprene units)Monoterpenes and sesquiterpenes are found primarily in plants, but the higher terpenes occur in both plants and animals.

Isopentenyl diphosphateO

Dimethylallyl diphosphateO

Acetic acid

Ture biological precursor of terpenes is not isoprene itselt but iospreneequivalents made from acetic acid.

alkene = olefin - Yonsei University · alkene = olefin. Ch.6 Alkenes: Structure and Reactivity 6.1...

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