CHM 2211C

6th edition Notes

 

Chapter 14

 

Conjugated Dienes

and

Ultraviolet Spectroscopy

 

By

Dr. Andrea Wallace

Coastal Georgia Community College

 

Edited by

John T. Taylor

Florida Community College at Jacksonville
Ch. 14  Conjugated Dienes and Ultraviolet Spectroscopy

 

________________ occurs when multiple bonds alternate with single bonds.

 

__________________ contain two double bonds.

 

Which of the following is a CONJUGATED DIENE?

 

 

 

 

 

1,3-butadiene                           OR                               1, 4-pentadiene

 

_____________ contain many alternating double and single bonds.

 

Examples on p. 465:  lycopene, progesterone, and benzene

 

 

14.1 Preparation and Stability of Conjugated Dienes (p. 465)

 

1)  Base-Induced Elimination of HX from an Allylic Halide  (Common Lab Prep)

 

 

 

 

 

 

 

 

2) Thermal Cracking over a catalyst (Specific Industrial Prep for 1,3-butadiene – used to make polymers.)

 

 

 

 

 

 

3)  Acid Catalyzed Double Dehydration  (Specific Industrial Prep for Isoprene)

 

 

 

 

 

 

Heat of hydrogenation data can be used to predict stability.  A more stable alkene will release less heat when hydrogen is added.

 

More substituted alkenes are ____________ stable than less substituted alkenes.

 

Conjugated alkenes are _____________ than non-conjugated alkenes.

 

See Table 14.1, p. 466

Explain the following results.

 

DH, kJ/mol

1-butene

- 126

1,4-pentadiene

- 253

1,3-butadiene

- 236

2-methyl-1,3-butadiene

- 229

 

 

 

 

Compare expected and observed DH values for 1,3-butadiene.  What is the difference?

 

 

 

 

 

 

Problem 14.1, p. 467

Allene, H2C=C=CH2, has a heat of hydrogenation of   –298 kJ/mole.  Rank a conjugated diene, a nonconjugated diene, and an allene in order of stability.

 

 

 

 

 

 

 

14.2 Molecular Orbital Description of 1,3-Butadiene (p. 467)

 

Why so stable?

 

1)  Hybridization – More sp2-sp2 overlaps provide more s character to the bond.  s electrons are closer to the nucleus and form shorter, stronger bonds that are more sigma like and less pi like.

 

 

 

 

2)      Delocalization of Electrons in Molecular Orbitals – More Pi electron delocalization occurs in conjugated systems leading to lower energy and increased stability.  (See Figure on p. 469)

 

 

 

 

 

 

 

 

14.3 Electrophilic Additions to Conjugated Dienes:  Allylic Carbocations (p. 469)

 

1) Electrophilic addition reactions of regular Alkenes follow Markovnikov’s Regiochemistry to produce the more stable carbocation.  This results in only one product being formed.

 

Example:

 

 

 

 

 

2)      Electrophilic addition reactions of conjugated dienes leads to a mixture of products.

 

Example:

 

 

 

                                    +     HBr    ŕ

 

 

            1,3-butadiene                              1,2-Addition                               1,4-Addition

                                                               (71%)                                         (29%)

                                                               Typical Markovnikov’s               (cis/trans isomers)

 

Mechanism:

 

 

 

 

 

 

 

What type of resonance stabilized intermediate forms that gives rise to a 1,4-addition product?  _____________________________

 

Provide products for the following reaction:

 

 

                                    --Br2ŕ

 

1,3-butadiene

 

 

Problem 14.2 (p. 472)

Give the structures of both 1,2 and 1,4 adducts resulting from reaction of 1 equivalent of HCl with 1,3-pentadiene. 

 

 

 

 

 

 

 

14.4 Kinetic versus Thermodynamic Control of Reactions (p. 472)

 

 

+  HBr   ŕ

 

 

1,3-butadiene                           1,2 adduct                                1,4 adduct

                             

         at 0oC          71%                                          29%

 

         at 40oC        15%                                          85%

 

 

A   ŕ              B                      +                      C

            Forms Faster                                 More Stable

            Kinetic Pdt                                   Thermodynamic Pdt

            Larger % of                                    Larger % of Mixture

            Mixture under                                 under High Temperature

            Low temperature/                            Vigorous conditions

            Mild Conditions                               Reversible Rxn

            Irreversible Rxn           

 

The product of a reversible reaction depends only on thermodynamic stability.

The product of an irreversible reaction depends only on reaction rate (kinetics).

 

1,2 adduct is the kinetic product that predominates at low temperature – forms faster, not as stable.

 

1,4 adduct is the thermodynamic product that predominates at high temperature – forms slower, more stable.

 

*Good example of how changing reaction conditions changes the product ratio.

 

 

Problem 14.6, p.474

Why do you suppose 1,4 adducts of 1,3 butadiene are generally more stable than 1,2 adducts?

 

 

 

 

 

 

 

14.5 The Diels-Alder Cycloaddition Reaction (p. 474)

 

A Diels-Alder Cycloaddition uses a conjugated diene.

Example:

 

 

 

 

 

1,3-Butadiene                           Alkene                         Substituted Cylohexene

(conjugated diene)

 

 

Unique Features:

1)      Forms 2 Carbon bonds in one step

2)      Forms a cyclic molecule

 

Mechanism is pericyclic – in a single step there is a cyclic redistribution of bonding electons.

 

See p. Figure 14.6, p. 475

 

 

 

 

 

Head on sigma overlap of 2p orbitals on C1 and C4 with the alkene p orbitals.  The C’s involved change their hybridization from a sp2 to a sp3.  The sp 2 carbons of the diene form a double bond.

 

14.6 Characteristics of the Diels-Alder Reaction (p. 475)

 

A more rapid and higher yield reaction occurs if the alkene is a dienophile – diene lover – has an electron withdrawing substituent group.  (Close by and conjugated works best.)

 

Thus ethylene is unreactive. 

See List of dienophiles on p. 476

(Alkynes will also work)

 

Diels-Alder reactions are stereospecific.  The beginning stereochemistry is maintained.  Only 1 stereoisomer is formed.

 

See p. 477

Z ŕ cis

E ŕ trans

 

See p. 477

Endo products  result from Diels-Alder reactions.

Figure:

 

 

 

 

 

 

 

 

Reason – The endo position provides maximum orbital overlap.  The electron withdrawing group on the dienophile points in toward the diene.

 

 

Problems 14.7, p. 478

Predict the product of the following Diels-Alder reaction:

 

 

 

 

 

 

 

 

 

The Diene (p. 478)

 The diene must adopt a s-cis conformation (cis-like about the single bond) to achieve a successful overlap.

 

See p. 479

 

 

 

 

s-cis                                         s-trans

                       

Some dienes can not adopt an s-cis conformation and thus can not be Diels-Alder Dienes.

Example:  Figure 14.8, p. 479

 

 

 

 

 

 

Problems – rigid s-trans (locked in place) and too much steric strain

 

In contrast, some dienes are rigidly fixed into the correct s-cis geometery.

See p. 479

 

 

 

 

 

1,3-cyclopentadiene                                         bicyclopentadiene

(Reacts with itself at room temperature to form the dimer product)

 

Problem 14.8, p. 480

Which of the following alkenes would you expect to be good Diels-Alder dienophiles?

 

 

 

 

 

 

 

Problem 14.11, p. 480

Which of the following dienes have an s-cis conformation, and which have and s-trans conformation?  Of the s-trans dienes, which can readily rotate to s-cis?

 

 

 

14.7  Diene Polymers:  Natural and Synthetic Rubbers (p. 480)

 

Reaction is similar to that for forming polyethylene.  However, this reaction is more complex – a double bond remains at every 4th Carbon.  Cis/trans isomers are possible.

Reaction is initiated by either an acid catalyst or radical.

 

See p. 481

 

 

Isoprene           ŕ                    Natural Rubber (Z)       +       Gutta-Percha (E)

                                                                                                (golf ball covering)

                                                        soft                                            hard

 

 

Chloroprene     ŕ                    Neoprene (Z)

                                                (industrial hoses and gloves)

 

 

Natural and synthetic rubber are soft and tacky but can be hardened by vulcanization.   Vulcanization was discovered by _______________.  In this process a small amount of _________________ is added to the rubber (polymer) which causes  _______________ or bridges between the polymer chains.

 

See p. 481

 

Problem 14.12, p. 482

Show the mechanism of the acid-catalyzed polymerization of 1,3-butadiene.

 

 

 

 

 

 

 

 

 

 

 


14.8  Structure Determination in Conjugated Systems: 

                      Ultraviolet Spectroscopy (p. 482)

 

Mass Spectrometry –

 

Infrared Spectroscopy -

 

NMR Spectroscopy –

 

UV Spectroscopy –

(Provides only specialized information measured in the 200 – 400 nm region.)

 

In UV, the energy absorbed corresponds to the amount of energy necessary to promote an electron from one orbital to another.

 

See Figure 14.10, p. 482

 

14.9  Ultraviolet Spectrum of 1,3-Butadiene, p. 483

 

See Figure 14.11, p. 483

Pi electrons are promoted from the HOMO _________________________ to the LUMO_________________________

 

The transition is a __________ŕ ____________

                             (Bonding)           (Antibonding)

 

The energy gap is equivalent to energy of the UV light at 217 nm form 1,3-butadiene (See p. 484)

 

Instrument usually scans through wavelengths until an absorption occurs.

 

Lambda max ____________ is the maximum wavelength of absorption.

 

Absorption is plotted against wavelength in nm.

 

Molar absorptivity (   )  =  __________________

 

 

where A is the absorbance, c is the concentration in mol/L, and l is the path length in cm

 

Molar absorptivity for conjugated dienes is 10 – 25, 000

Usually give a single simple peak.

 

 

 

 

14.10   Interpreting UV Spectra:  The Effect of Conjugation.

 

________________ arise from conjugated double bonds and thus give rise to structural information.

 

                                                            Lambda max

 

 

 

 

 

 

Longer wavelength indicates lower energy.

As conjugation increases ŕ  longer wavelength ŕ  lower energy

 

See Examples on p. 485

 

 

 

Problem 14.16, p. 486

Which of the following compounds would you expect to show ultraviolet absorption in the 200 – 400 nm range?

 

 

 

 

 

 

 

 

 

14.11  Conjugation, Color, and the Chemistry of Vision, p. 486

 

Colored compounds have extended systems of conjugation.  Thus UV absorptions extend into the Visible region.

 

See Beta-Carotene on p. 486.

White light strikes beta-carotene, light at 400 – 500 nm is absorbed.  Other wavelengths are transmitted to our eyes.  Blue is removed, we see yellow-orange.

 

Conjugated compounds also make up the light sensitive molecules which are a part of the visual system of all animals.  The key substance is dietary beta-carotene which is converted to Vitamin A by enzymes in the liver.  Vitamin A is oxidized to the aldehyde, trans-retinal and then isomerized to 11-cis-retinal.   See p. 487.

 

There are two types of light sensitive receptor cells in the retina of the human eye, rods and cones.  Rod cells allow us to see dim light and cone cells allow us to see bright light and bright colors.  In the process of vision, 11-cis-retinal is converted into rhodopsin.   When light strikes the rod cells, rhodopsin (cis) is isomerized into metarhodopsin II (trans).  See p. 487.

 

In the absence of light, this cis-trans isomerization takes approximately 1100 years, in the presence of light, it occurs within 2 x 10-11 seconds.

 

The change in molecular geometry causes a nerve impulse to be sent to the brain where it is perceived as vision.