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Review and Preview

•  We have studied many compounds with pi bonds. •  When a compound has more than one multiple

bond in its structure, the relative positions of those multiple bonds with respect to one another can dictate the molecule’s properties.

•  The ability to delocalize electrons in p orbitals across a pi system leads to many of the interesting properties discussed in this chapter.

Coursepack: pp. 131-164 (14.6 is FYI only)

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Review and Preview (cont.)

•  We have seen how pi bonds can increase stability of radicals due to delocalization:

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As You Study…

As you study, keep the following questions in mind: •  What effect does the relative spacing of pi bonds

in a molecule have on its chemical reactivity, stability, and other properties?

•  What information can ultraviolet spectroscopy provide about a compound?

•  What is meant by kinetic and thermodynamic control, and how can one use kinetic and thermodynamic principles to control the product of a reaction?

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Definitions

•  Pi bonds are said to be conjugated if there is exactly one single bond separating them. Unconjugated (isolated) pi bonds are separated by more than one single bond:

Bimolecular Elimination: E2 (cont.)

•  The mechanism of the E2 reaction is thought to be:

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Oenanthotoxin: Oenanthotoxin is a toxin extracted from Oenanthe crocata; it is a central nervous system poison that causes facial muscles spasm resulting in a grinning appearance, a trait that is anciently known as risus sardonicus – the “sardonic smile”. This plant was used in ritual killings in ancient pre-Rome times in Sardinia. Elderly citizens who were unable to support themselves were seen as a burden to society. Hence, they were intoxicated with this sardonic herb prior to being killed by beating or dropping from a high rock, so they would have a defiant grin in the face of death. Researchers later discovered that the plant toxins oenanthotoxin and dihydrooenanthotoxin block the receptors to the neurotransmitter -aminobutyric acid (GABA), which is directly involved in muscle tone regulation. The sardonic smile could be explained on the basis of this observation.

Organic Chemistry In Our Everyday Life Professor Michael J. Krische

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Conjugated Dienes

•  Conjugated dienes derive their special properties from the continuous overlapping system of p orbitals, something not present for isolated pi bonds:

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Conjugated Dienes (cont.)

•  A variety of resonance structures can be drawn for 1,3-butadiene, demonstrating the ability to spread electrons over the molecule:

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Physical Consequences of Conjugation •  Delocalization in a conjugated pi system leads

to greater stability versus two isolated pi bonds. Heats of hydrogenation (ΔH) can be used to measure double bond stability (Chapter 10):

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Physical Consequences of Conjugation (cont.) •  Conjugation gives ~4 kcal/mol greater stability:

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Ultraviolet Spectroscopy

•  How do scientists determine the structure of a molecule? Currently, much structural information is derived from spectroscopy.

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Ultraviolet Spectroscopy (cont.)

•  Ultraviolet (UV) spectroscopy or electronic spectroscopy measures the absorption of energy (provided by UV or visible light) by electrons. It provides the difference in energy between the HOMO and LUMO:

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Ultraviolet Spectroscopy (cont.)

•  The spectrometer works by measuring the wavelength (inversely proportional to energy) and intensity of light absorbed by the sample:

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Ultraviolet Spectroscopy (cont.)

•  The absorbance (A) of the light depends on the path length of the sample (b), the concentration of the sample in the solution (c), and a proportionality constant (ε) called the extinction coefficient. A = log I0 / I = εbc

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Ultraviolet Spectroscopy (cont.)

•  The value of the extinction coefficient is characteristic of a particular molecule and can tell you something about the type of electrons that are absorbing the energy.

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Ultraviolet Spectroscopy (cont.)

•  In conjugated materials, pi electrons typically absorb the energy.

•  The more pi bonds that are conjugated together, the longer the wavelength of absorbed light:

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Ultraviolet Spectroscopy (cont.)

•  Substituents/pi bonds can be used to predict the wavelength:

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Chemical Consequences of Conjugation •  When a simple conjugated diene is reacted

with HCl, the 1,2-addition product is observed (as for alkenes in Chapter 10), but there is also a 1,4-addition product:

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Chemical Consequences of Conjugation (cont.) •  To see why this may be, consider the mechanism:

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Chemical Consequences of Conjugation (cont.) •  The 1,4-addition product results because the

intermediate cation has another resonance contributor that places partial positive charge on the 4 position:

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Chemical Consequences of Conjugation (cont.) •  X2 (X = Cl or Br) can also add to conjugated dienes.

Interestingly, the reaction temperature greatly impacts product distribution:

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Thermodynamic and Kinetic Control of Addition Reactions •  Another puzzling observation is that addition of

Cl2 to a conjugated diene is reversible; even if one of the pure products is subjected to the reaction conditions, the equilibrium distribution of products is observed!

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Thermodynamic and Kinetic Control of Addition Reactions (cont.) •  Questions:

–  Why would the 1,4-product be favored at higher temperature?

–  How do the products equilibrate? –  Why would the 1,2-product be favored at lower

temperature?

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Thermodynamic and Kinetic Control of Addition Reactions (cont.) •  The 1,4 product is the more substituted, thus

more stable product alkene:

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Thermodynamic and Kinetic Control of Addition Reactions (cont.) •  The products could equilibrate by SN1 reaction

of either product:

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Thermodynamic and Kinetic Control of Addition Reactions (cont.) •  Why the 1,2-product

is formed to a greater extent at lower temperature is easiest to determine from a reaction coordinate diagram:

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Thermodynamic and Kinetic Control of Addition Reactions (cont.) •  The transition state leading to the 1,2-product is

lower in energy, so this path has a lower energy of activation and is faster.

•  At lower temperature, energy is more scarce, making reactions that have lower activation energies predominate. Reactions that run at low temperature are under stronger kinetic control.

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Thermodynamic and Kinetic Control of Addition Reactions (cont.) •  At higher temperature, there is plenty of energy

to overcome activation barriers and equilibrate to the more stable product; such reactions are said to be under thermodynamic control.

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The Allyl System

•  The allyl system has three overlapping p orbitals:

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The Allyl System (cont.)

•  Resonance stabilization of allyl cations facilitates SN1 reactions:

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The Allyl System (cont.)

•  The impact of resonance stabilization of the carbocation intermediate has a striking influence on SN1 reaction rates…

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The Allyl System (cont.)

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The Allyl System (cont.)

•  SN2 reaction at allylic sites is also faster:

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The Allyl System (cont.)

•  This is due to delocalization of charge in the transition state:

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The Allyl System (cont.)

•  In Chapter 11, we saw that allylic radicals are also significantly more stable than radicals lacking resonance stabilization

•  Anions can also be stabilized by resonance in allylic systems:

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The Diels-Alder Reaction

•  A conjugated diene can react with another pi bond–containing molecule known as a dienophile to form a cyclic structure containing three new bonds:

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The Diels-Alder Reaction (cont.)

•  The concerted mechanism of this reaction, known as the Diels-Alder reaction, will be discussed in more detail in Chapter 20.

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The Diels-Alder Reaction (cont.)

•  The reaction is thermodynamically favored because two new sigma bonds are formed and two pi bonds are lost.

•  The diene must be in the s-cis conformation for the reaction to work:

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The Diels-Alder Reaction (cont.)

•  Like the other concerted cyclizations we saw in Chapter 10, the Diels-Alder reaction proceeds with retention of stereochemistry:

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The Diels-Alder Reaction (cont.)

•  The dienophile need not be an alkene; alkynes also work.

•  Electron-withdrawing groups on the dienophile make the reaction faster:

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The Diels-Alder Reaction (cont.)

•  Bicyclic molecules can also be formed by the Diels-Alder reaction:

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The Diels-Alder Reaction (cont.)

•  There can be two isomeric products in some such reactions:

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The Diels-Alder Reaction (cont.)

•  The endo product is favored:

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The Diels-Alder Reaction (cont.)

•  This is due to overlap of p orbitals in the transition state: