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AFB QO I 2007/08 1

Química Orgânica I

Ciências Farmacêuticas

Bioquímica

Química

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Hidrocarbonetos insaturados

Terpenes: antiviral, antiseptic, anti-inflammatory.

OLEFINS“oil-forming gas”

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� first algal pheromone (Müller DG et al., Science 1971, 171, 815).

� 117 y

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Adaptado de:

� Organic Chemistry, 6th Edition; L. G. Wade, Jr.

� Organic Chemistry, 6th edition; McMurry’s

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Functional Group

� Pi bond is the functional group.

� More reactive than sigma bond.

� Bond dissociation energies:

� C=C BDE 146 kcal/mol

� C-C BDE -83 kcal/mol

� Pi bond 63 kcal/mol

=>

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Bond Lengths and Angles

� Hybrid orbitals have more s character.

� Pi overlap brings carbon atoms closer.

� Bond angle with pi orbital increases.� Angle C=C-H is 121.7°

� Angle H-C-H is 116.6° =>

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Pi Bond

� Sideways overlap of parallel p orbitals.

� No rotation is possible without breaking the pi bond (264 kJ/mole).

� Cis isomer cannot become trans without a chemical reaction occurring.

=>

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Elements of Unsaturation

� A saturated hydrocarbon: CnH2n+2

� Each pi bond (and each ring) decreases the number of H’s by two.

� Each of these is an element of unsaturation.

� To calculate: � find number of H’s if it were saturated,

� subtract the actual number of H’s,

� then divide by 2.

=>

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6.2 Degree of Unsaturation

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Propose a Structure:

� First calculate the number of elements of unsaturation.

� Remember:� A double bond is one element of unsaturation.

� A ring is also one element of unsaturation.

� A triple bond is two elements of unsaturation.

� =>

for C5H8

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Propose a Structure:

� Isopreno� nH “saturado”, CnH2n+2; neste caso é 12

� 12-8 = 4

� elementos de insaturação 4/2 =2

for C5H8

http://physics.nist.gov/PhysRefData/MolSpec/Hydro/Html/Tables/C5H8.html

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Heteroatoms

� Halogens take the place of hydrogens, so add their number to the number of H’s.

� Oxygen doesn’t change the C:H ratio, so ignore oxygen in the formula.

� Nitrogen is trivalent, so it acts like half a carbon.

C

H

H

C

H

H

N C

H

HH

=>

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Structure for C6H7N?

� Since nitrogen counts as half a carbon, the number of H’s if saturated is 2(6.5) + 2 = 15.

� Number of missing H’s is 15 – 7 = 8.

� Element of unsaturation is 8 ÷ 2 = 4.

e.g. anilina=>

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Cycloalkene nomenclature

CH3 CH3CH31

2

1

2

3

1

2

4

1-methylcyclohexene 3-methylcyclohexene 4-methylcyclohexene

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Name These Alkenes

CH2 CH CH2 CH3

CH3 C

CH3

CH CH3

CH3

CHCH2CH3

H3C

1-butene

but-1-ene

2-methyl-2-butene

2-methylbut-2-ene

3-methylcyclopentene

2-sec-butyl-1,3-cyclohexadiene

2-sec-butylcyclohexa-1,3-diene

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6-vinyltridecane

4-isopropyldecane

4-vinyloctane

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Alkene Substituents

= CH2

methylene

(methylidene)

- CH = CH2

vinyl

(ethenyl)

- CH2 - CH = CH2

allyl

(2-propenyl)

Name: =>

methylenecyclohexane ethenylbenzene

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In a ring, the double bond is assumed to be

between carbon 1 and carbon 2.

Br

(Z)-7-bromo-4-ethyl-3-methylcyclohept-1-ene

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Common Names

� Usually used for small molecules.

� Examples:

CH2 CH2

ethylene

CH2 CH CH3

propylene

CH2 C CH3

CH3

isobutylene

=>

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Name these:

C C

CH3

H

H

CH3CH2

C C

Br

H

Br

H

trans-2-pentene

trans-pent-2-ene

cis-1,2-dibromoethene

=>

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Example, E-Z

C C

H3C

H

Cl

CH2

C C

H

H

CH CH3

Cl1

2

1

2

2Z

2

1

1

2

5E

3,7-dichloro-(2Z, 5E)-2,5-octadiene

3,7-dichloro-(2Z, 5E)-octa-2,5-diene

=>

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Commercial Uses: Ethylene

=>

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Commercial Uses: Propylene

=>

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Other Polymers

=>

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Stability of Alkenes

� Measured by heat of hydrogenation:

Alkene + H2 → Alkane + energy

� More heat released, higher energy alkene.

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Substituent Effects� More substituted alkenes are more stable.

H2C=CH2 < R-CH=CH2 < R-CH=CH-R < R-CH=CR2 < R2C=CR2 unsub. < monosub. < disub. < trisub. < tetrasub.

� Alkyl group stabilizes the double bond.

� Alkene less sterically hindered.

=>

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Disubstituted Isomers

� Stability: cis < geminal < trans isomer

� Less stable isomer is higher in energy, has a more exothermic heat of hydrogenation.

-116 kJTrans-2-butene

-117 kJ(CH3)2C=CH2Isobutylene

-120 kJCis-2-butene CH3C C

CH3

H H

HC C

CH3

CH3 H=>

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Relative Stabilities

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Hyperconjugation

� Electrons in neighboring filled σ orbital stabilize vacant antibonding π orbital – net positive interaction

� Alkyl groups are more stabilizing than H

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Bond strengths/hybridization effects

� sp3-sp3 bond is weaker than sp3-sp2, sp2-sp2

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Cycloalkene Stability

� Cis isomer more stable than trans.

� Small rings have additional ring strain.

� Must have at least 8 carbons to form a stable trans double bond.

� For cyclodecene (and larger) trans double bond is almost as stable as the cis.

=>

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Bredt’s Rule

� A bridged bicyclic compound cannot have a double bond at a bridgehead position unless one of the rings contains at least eight carbon atoms.

� Examples:

Unstable.Violates Bredt’s rule Stable. Double bond

in 8-membered ring.=>

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Physical Properties

� Low boiling points, increasing with mass.

� Branched alkenes have lower boiling points.

� Less dense than water.

� Slightly polar� Pi bond is polarizable, so instantaneous dipole-

dipole interactions occur.

� Alkyl groups are electron-donating toward the pi bond, so may have a small dipole moment.

=>

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Polarity Examples

µ = 0.33 D µ = 0

=>

cis-2-butene, bp 4°C

C C

H

H3C

H

CH3

trans-2-butene, bp 1°C

C C

H

H

H3C

CH3

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Alkene SynthesisOverview

� E2 dehydrohalogenation (-HX)

� E1 dehydrohalogenation (-HX)

� Dehalogenation of vicinal dibromides (-X2)

� Dehydration of alcohols (-H2O)

=>

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Removing HX via E2

� Strong base abstracts H+ as X-

leaves from the adjacent carbon.

� Tertiary and hindered secondary alkyl halides give good yields.

� Use a bulky base if the alkyl halide usually forms substitution products.

=>

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Some Bulky Bases

C

CH3

H3C

CH3

O_

tert-butoxide

(CH3CH2)3N :triethylamine

=>

N

H

CH(CH3)2

CH(CH3)2

diisopropylamine

N CH3H3C

2,6-dimethylpyridine

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Hofmann Product

� Bulky bases abstract the least hindered H+

� Least substituted alkene is major product.

=>

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E2: Diastereomers

Stereospecific reaction: (S, R) produces only trans product, (R, R) produces only cis.

Ph

Br H

H CH3

Ph

≡≡≡≡

H

Ph CH3Br

PhHH

Ph

CH3

Ph

HBr

CH3Ph

PhH =>

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E2: Cyclohexanes

Leaving groups must be trans diaxial. =>

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E2: Vicinal Dibromides� Remove Br2 from adjacent carbons.

� Bromines must be anti-coplanar (E2).

� Use NaI in acetone, or Zn in acetic acid.

I-

Br

CH3H

Br

CH3H

C CCH3

H

H

H3C =>

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Removing HX via E1

� Secondary or tertiary halides

� Formation of carbocation intermediate

� May get rearrangement

� Weak nucleophile

� Usually have substitution products too

=>

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Dehydration of Alcohols� Reversible reaction

� Use concentrated sulfuric or phosphoric acid, remove low-boiling alkene as it forms.

� Protonation of OH converts it to a good leaving group, HOH

� Carbocation intermediate, like E1

� Protic solvent removes adjacent H+

=>

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Dehydration Mechanism

=>

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Industrial Methods� Catalytic cracking of petroleum

� Long-chain alkane is heated with a catalyst to produce an alkene and shorter alkane.

� Complex mixtures are produced.

� Dehydrogenation of alkanes� Hydrogen (H2) is removed with heat, catalyst.

� Reaction is endothermic, but entropy-favored.

� Neither method is suitable for lab synthesis

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