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Organic Chemistry 2 Unit 2: Conjugated Systems and Diels-Alder

Conjugated π systems spread electrons over several atoms, which makes them more stable, changes how they add electrophiles, lets them absorb UV light, and enables the Diels-Alder reaction, the most important ring-forming reaction in organic chemistry.

Covers: Stability of conjugated dienes, Allylic cations, radicals and NBS bromination, 1,2- versus 1,4-addition: kinetic and thermodynamic control, Molecular orbitals of conjugated π systems, UV-visible spectroscopy, Diels-Alder reaction: requirements and stereochemistry, Diels-Alder regiochemistry and the endo rule

Study guide

A conjugated diene has two C=C bonds separated by exactly one single bond, so its four p orbitals overlap continuously. Isolated dienes have their double bonds separated by two or more sp³ carbons, and cumulated dienes (allenes) share a carbon. Conjugation lowers energy: two moles of H₂ add to 1,3-butadiene releasing about 239 kJ/mol, while two isolated monosubstituted alkenes release about 2 × 127 = 254 kJ/mol, so conjugation is worth roughly 15 kJ/mol. A lower heat of hydrogenation always means a more stable starting compound. The C2–C3 single bond in butadiene is shorter than a normal C–C single bond because of partial π overlap and sp²–sp² σ bonding.

Allylic carbocations and allylic radicals are stabilized by resonance: the charge or unpaired electron is shared by the two end carbons of a three-carbon π system. Protonating 1,3-butadiene with HBr forms an allylic cation with positive character at C2 and C4. Bromide can attack C2 (1,2-addition) or C4 (1,4-addition). At low temperature the 1,2-product dominates because it forms faster; this is kinetic control, and the bromide is closest to the site where the proton just added. At higher temperature the reaction becomes reversible and the more stable 1,4-product, with a more substituted internal alkene, dominates; this is thermodynamic control. N-Bromosuccinimide (NBS) with light or peroxide brominates allylic C–H bonds through an allylic radical, which can give rearranged products when the radical is unsymmetrical.

Molecular orbital theory describes a conjugated system of n p orbitals as n π molecular orbitals. Butadiene has four: ψ1 and ψ2 are bonding and filled, ψ3 and ψ4 are antibonding and empty. ψ2 is the HOMO and ψ3 is the LUMO. Each higher orbital has one more node, so ψ1 has zero nodes and ψ4 has three. As conjugation extends, the HOMO–LUMO gap shrinks, so the π → π* transition needs less energy and absorbs a longer wavelength. Ethylene absorbs near 171 nm, butadiene near 217 nm and 1,3,5-hexatriene near 258 nm, and highly conjugated molecules such as β-carotene absorb visible light and appear colored. UV-visible absorbance follows Beer's law, A = εbc.

The Diels-Alder reaction is a concerted [4+2] cycloaddition between a conjugated diene and a dienophile (an alkene or alkyne) that forms a cyclohexene ring, making two new σ bonds and one new π bond in a single step. The diene must adopt the s-cis conformation; dienes locked s-cis, such as cyclopentadiene, are very reactive, while dienes that cannot reach s-cis do not react. The reaction is fastest with an electron-rich diene and an electron-poor dienophile bearing groups such as CHO, COR, COOR, CN or an anhydride. It is stereospecific: substituents that are cis on the dienophile stay cis in the product, and trans stay trans. The two outside groups at the diene termini end up cis to each other.

Two further rules predict Diels-Alder products. The endo rule says that when the diene is cyclic, the electron-withdrawing group of the dienophile prefers to point toward the newly forming π bond (under the diene) in the transition state, because of favorable secondary orbital overlap; the endo product is the kinetic product, even though the exo product is often more stable. The regiochemistry rule says that a diene with a substituent at C1 gives an 'ortho' (1,2) product and a diene with a substituent at C2 gives a 'para' (1,4) product with an unsymmetrical dienophile. In terms of orbitals, the thermal Diels-Alder is allowed because the diene HOMO and dienophile LUMO overlap in phase at both ends, whereas a thermal [2+2] cycloaddition of two simple alkenes is forbidden.

Key ideas

• Conjugated dienes are more stable than isolated dienes, as shown by lower heats of hydrogenation.
• Allylic cations and radicals are resonance-stabilized and can react at either end.
• Low temperature favors the faster 1,2-adduct; higher temperature favors the more stable 1,4-adduct.
• More conjugation means a smaller HOMO–LUMO gap and a longer UV-vis λmax.
• The Diels-Alder reaction is a concerted, stereospecific [4+2] cycloaddition requiring an s-cis diene.
• Electron-poor dienophiles react fastest, and cyclic dienes give mainly endo products.
• C1-substituted dienes give 1,2 ('ortho') products and C2-substituted dienes give 1,4 ('para') products.

Formulas and rules

Common mistakes

• Drawing the Diels-Alder product with the new double bond between the old diene C1 and C2 instead of C2 and C3.
• Letting cis dienophile substituents become trans in the product; the reaction is stereospecific.
• Forgetting that a diene must reach the s-cis conformation to react.
• Swapping kinetic and thermodynamic products in HBr additions to dienes.
• Thinking a larger HOMO–LUMO gap gives a longer λmax; it gives a shorter one.

Before the test, make sure you can

• Identify conjugated, isolated and cumulated dienes and rank their stabilities.
• Calculate conjugation energy from heats of hydrogenation.
• Predict 1,2- and 1,4-adducts and explain which dominates under kinetic or thermodynamic control.
• Draw allylic resonance structures and predict NBS bromination products.
• Sketch π MOs for butadiene, identify the HOMO and LUMO, and relate conjugation to λmax.
• Draw Diels-Alder products with correct stereochemistry, regiochemistry and endo selectivity.
• Work backward from a cyclohexene to the diene and dienophile that made it.

Words to know

Conjugated diene

A diene whose two C=C bonds are separated by exactly one single bond, allowing continuous p-orbital overlap.

Isolated diene

A diene whose double bonds are separated by two or more sp³ carbons, so they do not interact.

Allylic position

A carbon directly attached to a C=C carbon; allylic cations, radicals and anions are resonance-stabilized.

1,2-Addition

Addition of two groups to adjacent carbons of a conjugated diene, usually the kinetic product.

1,4-Addition

Addition to the two end carbons of a conjugated diene, moving the double bond to the center; usually the thermodynamic product.

Kinetic control

Conditions (often low temperature, irreversible) where the product that forms fastest dominates.

Thermodynamic control

Conditions (often higher temperature, reversible) where the most stable product dominates.

NBS

N-Bromosuccinimide, which supplies a low concentration of Br₂ for radical allylic or benzylic bromination.

HOMO

Highest occupied molecular orbital; in a Diels-Alder reaction the diene's HOMO usually reacts.

LUMO

Lowest unoccupied molecular orbital; in a Diels-Alder reaction the dienophile's LUMO usually reacts.

λmax

The wavelength of maximum UV-vis absorbance; it increases as conjugation increases.

Diels-Alder reaction

A concerted [4+2] cycloaddition of a conjugated diene and a dienophile that forms a cyclohexene.

Dienophile

The 2π component of a Diels-Alder reaction, most reactive when bearing electron-withdrawing groups.

s-cis conformation

The conformation of a diene with both double bonds on the same side of the central single bond, required for Diels-Alder.

Endo product

The Diels-Alder adduct in which the dienophile substituent points toward the new π bond; favored kinetically.

Pericyclic reaction

A concerted reaction proceeding through a cyclic transition state, such as cycloadditions and electrocyclic reactions.

Sample practice questions

30 more questions like these, from level 1 to 5, are in the practice set.