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Chapter 8Lecture

Structure and Synthesisof Alkenes

Alkenes are also called olefins, meaning “oil-forming gas.

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C=C bond is shorter than C—C bonda) sp2 hybrid orbitals have _______ s character than the sp3 hybrid orbitals. They are __________ to the nucleus.b) Pi overlap brings carbon atoms _______

s Bonding in Ethylene

p Bonding requires two p orbitals overlapping __________

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Cis and trans isomers cannot be interconverted without breaking the pi

bond (264 kJ/mole).

No rotation around the carbon–carbon bond is possible giving rise to Cis trans geometric isomers

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Elements (degrees) of Unsaturation. Also called index of hydrogen deficiency.

• Unsaturation: A structural element that decreases the number of hydrogens in the molecule by two.

• Double bonds and rings are elements of unsaturation (can not distinguish with calculation

Calculate the degrees of unsaturation for a compound with formula C5H8 and propose a structure

•Reference (saturated compound CnH2n+2)

•Compound

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• For each Group VA atom increase Hs in reference by one• For each halogens decrease Hs atoms in reference by one• Group of oxygen does not change the C:H ratio, ignore oxygen in the reference formula.

Calculate the IHD for a Compound with Formula C4H7Br.

C6H7N.

C5H4O.

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IUPAC and New IUPAC

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In a ring the double bond is given carbon 1 and carbon 2.

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Polyenes: se di-, tri-, tetra- before the ending -ene to specify how many double bonds are present.

Give the double bonds the lowest numbers possible.

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EZ nomenclature: Use the Cahn–Ingold–Prelog rules to assign priorities to groups attached to each carbon in the double bond.If high-priority groups are on the same side, the name is Z (for zusammen).If high-priority groups are on opposite sides, the name is E (for entgegen).

E-1-bromo-1-chloropropene

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Heat of Hydrogenation can provide valuable data as to the stability of the double bond. The more substituted the double bond, the ___________its heat of hydrogenation.

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Stability: cis-2-butene < iso-butene < trans-2-butene -120 kJ > -117 kJ > -116 kJ

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Cyclopropene is highly strained.Rings that are five-membered or larger can easily accommodate double bonds, and these cycloalkenes react much like straight-chain alkenes.

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• Cis isomer is ______ stable than trans in small cycloalkenes.• Must have at least _________carbons to form a stable trans double

bond. • For cyclodecene (and larger), the trans double bond is almost as stable

as the cis. • All cycloalkenes are assumed to be cis unless otherwise specifically

named trans.

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Which of the following alkenes are stable?

Bredt’s Rule: A bicyclic compound cannot have a double bond at a bridgehead position unless one of the rings contains at least eight carbon atoms.

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Summary stability

• How is it quantitated?• Substitution• Stereochemistry• Ring size • Cis vs trans cyclos• Bredt’s rule

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Alkenes have relatively low boiling points which depend on surface area and polarizability (______________ forces

_______ dense than water _________ on waterSlightly 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.

• Cis alkenes have a greater dipole moment than trans alkenes, so they will be slightly polar and have higher bp

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Alkene SynthesisOverview

1. R-X ----E2 dehydrohalogenation (-HX) Alkene

2. R-X ----E1 dehydrohalogenation (-HX) Alkene

3. Vic dibromide ---Dehalogenation (-X2) Alkene

4. Alcohol ---Dehydration (-H2O) Alkene

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1. Dehydrohalogenation by the E2 Mechanism

• Strong_________ abstracts H+ as double bond forms and X- ___________ from the adjacent carbon.

• __________ and hindered ______________ alkyl halides give alkenes in good yields. Tertiary halides are the best E2 substrates because they cannot undergo __________substitution.

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Bulky Bases for E2 Reactions

• If the substrate is prone to substitution, a bulky base can minimize the amount of substitution.

• Large alkyl groups on a bulky base hinder its approach to attack a carbon atom (substitution), yet it can easily abstract a proton (elimination).

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Hofmann Product Bulky bases, such as potassium tert-butoxide, abstract the

least hindered H+, giving the less substituted alkene as the major product (Hofmann product).

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E2 Reactions Are Stereospecific: The stereochemistry of the product depends on the stereochemistry of the reactant

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Rotating meso-2,3-dibromobutane into a conformation where the bromine atoms are anti and coplanar, we find that the product will be trans-2-butene. A similar conformation of either enantiomer of the (±) diastereomer shows that the product will be cis-2-butene. (Hint: Your models will be helpful.)

Show that the dehalogenation of 2,3-dibromobutane by iodide ion is stereospecific by showing that the two diastereomers of the starting material give different diastereomers of the product.

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Meso-2,3-dibromobutane

(R,R)-2,3-dibromobutane

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E2 Reactions of bromocyclohexane is slow

• An anti-coplanar conformation (180°) can only be achieved when both the hydrogen and the halogen occupy ___________ positions.

• The chair must flip to the conformation with the axial halide in order for the elimination to take place.

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In an E2 elimination, the hydrogen atom and the leaving group must have a trans-diaxial relationship. In this compound, only one hydrogen atom—the deuterium—is trans to the bromine atom. When the bromine atom is axial, the adjacent deuterium is also axial, providing a trans-diaxial arrangement.

Explain why the following deuterated 1-bromo-2-methylcyclohexane undergoes dehydrohalogenation by the E2

mechanism, to give only the indicated product. Two other alkenes are not observed.

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2. Debromination of Vicinal Dibromides: reduction with iodide ion in acetone follows an an E2

mechanism

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3. E1 reaction• Tertiary reacts faster than secondary alkyl halides: • Carbocation intermediate; Rearrangements are possible.• Solvolysis reactions in solvents as water or alcohols• Usually have SN1 products, too, since the solvent can attack

the carbocation directly.

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4. Dehydration of Alcohols

• Use concentrated H2SO4 or H3PO4 and remove low-boiling alkene as it forms to shift the equilibrium and increase the yield of the reaction.

• E1 mechanism.• Carbocation intermediate. Rearrangements can occur.• Reaction obeys Zaitsev’s rule.

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Dehydration Mechanism: E1Step 1: Protonation of the hydroxyl group (fast equilibrium).

Step 2: Ionization to a carbocation (slow; rate limiting).

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Step 3: Deprotonation to give the alkene (fast).

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HINT

In acid-catalyzed mechanisms,the first step is often addition of

H+, and the last step is oftenloss of H+.

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Propose a mechanism for the sulfuric acid–catalyzed dehydration of t-butyl alcohol.

The first step is protonation of the hydroxyl group, which converts it to a good leaving group.

The second step is ionization of the protonated alcohol to give a carbocation.

Abstraction of a proton completes the mechanism.

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Other Industrial methods: a) Catalytic Cracking of Alkanes

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

• Complex mixtures are produced.

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• Dehydrogenation is the removal of H2 from a molecule, forming an alkene (the reverse of hydrogenation).

• This reaction has an unfavorable enthalpy change but a favorable entropy change.

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b) Dehydrogenation of Alkanes

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Methods to synthesize alkenes

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