Chemistry Question

Science

chem212

Santa Barbara City College

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I need to get all 5 questions correct please!!!!

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The following quiz consists of 5 questions from chapter 14. You will have 18 minutes to complete the quiz.

https://youtu.be/eeaeaDfvbH

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Chapter 16 Conjugation Resonance and Dienes Conjugation • Conjugation – occurs whenever p orbitals can overlap on three or more adjacent atoms Conjugation • Delocalized electron density can occur, lowering the energy of the molecule and making it more stable • Conjugated species are more stable! Conjugation and Delocalized Electrons • d Allylic Carbocations • Allylic carbocations are conjugated systems – Differ in the electron count • P orbital is vacant Practice Questions • Which of the following species are conjugated? Resonance • Recall resonance structures differ only in the placement of pi electrons and nonbonded electrons • 4 types we will see – Allyl System – Conjugated Double Bonds – Cations Adjacent to an Atom with Lone Pair (LP) – Double Bonds With Electronegative Atom Resonance: Allyl System • Must have 3 adjacent sp2 hybridized atoms – Terminal p orbital must have zero, one or two electrons (+, -, ∙, or ∙∙) – 2 possible resonance structures Conjugated Double Bonds • Completely conjugated system – Give multiple “chain like” delocalization of electrons Cations Adjacent to an Atom with LP • When an LP and a positive charge are located on adjacent atoms, two resonance structures can be drawn Double Bonds With Electronegative Atom • A double bond, X=Y, where the electronegativity of Y>X Practice Problems • Draw additional resonance structures for each ion. a. c. b. d. >>>>>> Resonance Hybrids • Hybrids more closely resemble the best resonance structure – “Major contributor” • Fewest charges • Octets fulfilled • Negative charge on more EN atom Practice Problems • Draw a second resonance and the hybrid for each species, and then rank the two resonance structure and the hybrid in order of increasing stability Electron Delocalization, Hybridization, and Geometry In a system X=Y-Z: we will generally view Z as sp2 hybridized, and the nonbonded electron pair occupies a p orbital to make the system conjugated Conjugated Dienes • 1,3-dienes - Two double bonds joined by one sigma bond Conjugated Dienes • s-cis conformation has two double bonds on the same side of the single bond • s-trans conformation has two double bonds on opposite sides of the single bond Interesting Dienes and Polyenes • Isoprene – building block for terpenes and rubber. Responsible for making the Blue Ridge Mountains of Virginia, blue! Isoprene and Terpenes Conjugated Dienes vs Isolated Dienes • With respect to Conjugated Dienes 1. 2. 3. 4. C-C single bond is unusually short Conjugated dienes are more stable Reactivity is different Absorb longer wavelengths of UV light Carbon-Carbon 𝛔 Bong Length in 1,3Butadiene 1. C-C single bond is unusually short Practice Problem • Which CO bond is longer? Stability of Conjugated Dienes 2. Conjugated dienes are more stable Recall heat of hydrogenation gives us relative stability (12.3A) Stability of Conjugated Dienes • If hydrogenation gives the same alkane from two dienes, the more stable diene has a smaller heat of hydrogenation ***Conjugated diene has a smaller heat of hydrogenation and is more stable than isolated diene Reactivity is Different -Electrophilic Addition: 1,2- Versus 1,4- AdditionIsolated dienes give single product (Chapter 10) Conjugated dienes give a mixture of products 1,2-addition and 1,4-addition Kinetic Vs Thermodynamic Product May selectively form 1,2 or 1,4-product by altering the temperature You can even heat the 1,2-product and isolate the 1,4-product!!! Kinetic Vs Thermodynamic Product • 1,2-product is formed faster because it predominates at low temperature and is called the kinetic product – Recall rate of a reaction is determined by Ea • 1,4-product is more stable as it predominates at equilibrium (high temp) and is called the thermodynamic product – Recall the amount of product present at equilibrium is determined by stability Kinetic Vs Thermodynamic Product • Lower activation energy = faster reaction • Lower energy of product = more stable Kinetic Vs Thermodynamic Product • Why is 1,4-product lower in energy? • More substituted alkenes are more stable! – Electron donating inductive effect (Chapter 8?) Kinetic Vs Thermodynamic Product • Why is 1,2-product formed faster? • Proximity effect – Br- is closer to the C2 carbon after the addition of hydrogen to C1 Energy Diagram for Addition of HBr to 1,3-Butadiene https://phet.colorado.edu/en/simulation/gas-properties Diels-Alder • Otto Diels and Kurt Alder won the Nobel Prize in 1950 for their work on understanding the addition reaction between 1,3-dienes and alkenes Alkene = dienophile (“loves dienes”) and acts like an electrophile Diels-Alder • Typically exothermic – break 3 pi bonds, but form 2 new 𝛔 and 1 new pi bond • Form new 6-membered rings • Concerted reaction Practice Problem • Draw the product of the following Diels-Alder reaction. 1. First, identify the diene and the dienophile 2. Arrange to form a 6 membered ring 3. Shuffle electrons to form 2 new 𝛔 bonds and 1 new pi bond Rules for Governing the Diels-Alder Reaction 1. Diene can only react when it adopts the s-cis conformation – If bond rotation about the 𝛔 bond is inhibited and forced in the s-cis conformation, the diene is VERY reactive – If bond rotation about the 𝛔 bond is inhibited and forced in the s-trans conformation, the diene is very UNreactive Practice Problem Which is most reactive in a Diels-Alder reaction with ethene? Which is least reactive? A. B. C. Rules for Governing the Diels-Alder Reaction 2. Electron-withdrawing substituents in the dienophile increase the reaction rate – Create a more electrophilic dienophile and thus more susceptible to nucleophilic attack (by the diene) Rules for Governing the Diels-Alder Reaction 3. Stereochemistry of the dienophile is retained in the product – Cis dienophile forms a cis-substituted cyclohexene – Trans dienophile forms a trans-substituted cyclohexene Rules for Governing the Diels-Alder Reaction 3. Stereochemistry of the dienophile is retained in the product Rules for Governing the Diels-Alder Reaction • Cyclic dienophiles for bicyclic products • Bicyclic products are called fused ring systems Practice Problems • Draw the products of each Diels-Alder reaction, and indicate the stereochemistry. Rules for Governing the Diels-Alder Reaction 4. When endo and exo products are possible, the endo product is preferred. A bicyclic ring system in which the two rings share non-adjacent carbon atoms is called a bridged ring system Rules for Governing the Diels-Alder Reaction 4. When endo and exo products are possible, the endo product is preferred. What are exo and endo? When a dienophile has a substitutent (any alkene other than ethylene) endo and exo indicate the position of that substituent. Rules for Governing the Diels-Alder Reaction • Why is endo preferred? Practice Problems • Draw the product of each Diels-Alder reaction. Other Facts About the Diels-Alder Reaction Retrosynthetic Analysis of a Diels-Alder Product – Retrosynthetic – Working backwards to see what the starting materials are – What’s the diene? What’s the dienophile? • Locate 6 membered ring containing C=C • Draw 3 arrows around the cyclohexene ring creating diene and dienophile • Retain stereochemistry Other Facts About the Diels-Alder Reaction • Retro Diels-Alder Reaction Conjugated Dienes and Ultraviolet Light • UV light (200-400 nm) can excite electrons from ground state to an excited state • Conjugated dienes and polyenes absorb light in the UV region of the electromagnetic spectrum Conjugated Dienes and Ultraviolet Light Conjugated Dienes and Ultraviolet Light • Lycopene absorbs visible light at 470 nm in the blue-green region of the visible spectrum – Since it doesn’t absorb light in the red region, lycopene appears bright red Conjugated Dienes and Ultraviolet Light • UV-C (< 290 nm) – filtered out by the ozone layer • UV-A (320-400 nm) – absorbed by melanin • UV-B (290-320 nm) – absorbed by melanin – Melanin is a highly conjugated colored pigment in the skin that serves as protection against harmful effects of UV ...
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