please help with organic chemm ll quiz 3 an hw 4 thank you please do carefully reach question thank youCHM 222 Quiz 3
Name________________________
1. What descriptive term is applied to the type of diene represented by 2,4-hexadiene?
A) conjugated diene
B) cumulated diene
C) isolated diene
D) alkynyl diene
E) none of the above
(1 pt.)
2. What descriptive term is applied to the type of diene represented by 1,5-octadiene?
A) conjugated diene
B) cumulated diene
C) isolated diene
D) alkynyl diene
E) none of the above
(1 pt.)
3. What descriptive term is applied to the type of diene shown?
A) conjugated diene
B) cumulated diene
C) isolated diene
D) alkynyl diene
E) none of the above
(1 pt.)
4. Rank the following compounds in order of increasing heats of hydrogenation. (The smallest
ΔH is first.)
A) A < B < C B) C < B < A C) C < A < B D) A < C < B (1 pt.) 5. Rank the following dienes in order of increasing stability: trans-1,3-pentadiene, cis-1,3pentadiene, 1,4-pentadiene, and 1,2-pentadiene. (4 pts.) 6. When the relative energies of the s-cis and s-trans conformers of 1,3-butadiene are compared, one finds that ________. A) the s-cis conformer is lower in energy than the s-trans B) the s-trans conformer is lower in energy than the s-cis C) the two conformers are of equal energy (1 pt.) 7. The number of π molecular orbitals in a molecule is always equal to the number of ________. A) π bonds B) p orbitals used to construct the π bonds C) electrons in the π system D) hydrogen atoms in the molecule (1 pt.) 8. Draw (E)-1,3-hexadiene in its s-cis conformation. (2 pts.) 9. Which of the following represents the highest occupied molecular orbital for the conjugated pi system in Vitamin D3? A) B) C) D) (1 pt.) 10. When 3-bromobut-1-ene is heated in a good ionizing solvent, reactions occur through a carbocation intermediate. Draw all reasonable resonance contributors of this cation and indicate which is the major contributor. (4 pts.) 11. Provide a detailed, stepwise mechanism for the reaction shown below. (7 pts.) 12. Provide the structure of the major product which results from 1,4-addition of Br2 to the diene shown below. (4 pts.) 13. Provide the structure of the major organic product in the following reaction. (4 pts.) (30 pts.) filename:chem222q321s CHM 222 Homework 4 Conjugated Systems and Ultraviolet Spectroscopy Exercises 1. In Wade Chapter 15 pp. 759-761 #s 15-28 a & b , 15-30 e, 15-31c, and 15-40. (20 pts.) filename: chm222hw421s CHAPTER 14 – CONJUGATED COMPOUNDS AND UV SPECTROSCOPY MH COLE – HCC SPRING 2018 INTRODUCTION Conjugated double bonds are double bonds which are separated by one carbon-carbon single bond. Thus the double bonds in butadiene, CH2=CH−CH=CH2, are conjugated, and this compound is an example of a conjugated diene. Just as the term diene indicates the presence of two carbon-carbon double bonds in a compound, so the term polyene is used to describe compounds containing many carbon-carbon double bonds. An enone is a compound containing a carboncarbon double bond (ene) and a carbonyl group (one). A conjugated enone contains the structural unit: CONJUGATED DIENES AND NAMING DIENES A diene is a hydrocarbon chain that has two double bonds that may or may not be adjacent to each other. This section focuses on the delocalization of π systems by comparing two neighboring double bonds. The arrangements of these double bonds can have varying affects on the compounds reactivity and stability. Naming: First identify the longest chain containing both carbons with double bonds in the compound. Then give the lowest possible number for the location of the carbons with double bonds and any other functional groups present (remember when naming alkenes that some groups take priority such as alcohols). Do not forget stereochemistry or any other orientation of the double bond such as (E/Z,cis or trans). Examples: CONJUGATED VS. NONCONJUGATED VS. CUMULATED DIENES Conjugated dienes are two double bonds separated by a single bond Nonconjugated (Isolated) dienes are two double bonds are separated by more than one single bond. Cumulated dienes are two double bond connected to a similar atom. The reactivity of these molecules is substantially different from that of alkenes which have isolated C=C. These molecules are thus considered a different class of organic molecule. Conjugated dienes, especially butadiene, are very important materials in the production of rubber, and thus for the tires of our cars. STABILITY OF CONJUGATED DIENES: MOLECULAR ORBITAL THEORY Here we have two methods frequently used to synthesize conjugated dienes: dehydrohalogenation of an organohalide dehydration of alcohols Looking at an energy diagram of the hydrogenation of 1, 3 butadiene we see its stability is less than predicted Conjugated dienes are more stable than non conjugated dienes (both isolated and cumulated) due to factors such as delocalization of charge through resonance and hybridization energy. This can also explain why allylic radicals are much more stable than secondary or even tertiary carbocations. This is all due to the positioning of the pi orbitals and ability for overlap to occur to strengthen the single bond between the two double bonds. The resonance structure shown below gives a good understanding of how the charge is delocalized across the four carbons in this conjugated diene. This delocalization of charges stabilizes the conjugated diene: HYBRIDIZATION AND MO THEORY Hybridization: electronic structure electron dot structure 22s22p2 (p 1, p 1, p ) C 1s 6 x y z Carbon has 4 bonds to it at all times. How do we get this to happen? Hybridization sp3 hybridization sp2 hybridization sp hybridization MO Theory: In bonding hybrid orbitals overlap to produce molecular orbitals End to end overlap of orbitals is a sigma σ mo Side by side overlap of orbitals is a pi (π) mo * σ σ Hybridization also effects the stability of a compound sp2 hybridized C-C bond has more “s” character (33%) which draws in more of the pi electrons ↓ shorter and stronger than an alkane C-C bond (1.54 Å ) It is also useful to look at heats of hydrogenation of different arrangements of double bonds cumulated dienes isolated dienes conjugated dienes ALLYLIC CARBOCATION Allylic carbocations are a common conjugated system contained in a p orbital of an sp2 hybridized C – more stable because it is spread over 2 Cs An orbital view Allylic carbocation is stable because of the conjugated π electron system (3 adjacent, overlapping, non-hybridized p orbitals) signs indicate phases (phase changes) 2-propenyl A different MO look These orbitals having 2 e-s lowers the overall energy of each system 1, 3 DIENES ELECTROPHILLIC ADDITIONS TO CONJUGATED DIENES Use 1, 3 butadiene as an example because its reactions are typical of conjugated dienes (catalytic hydrogenation or radical and polar additions) However it does so more readily than most alkenes and non-conjugated dienes Their products are typically 1,2 additions and 1, 4 additions REACTION WITH HALOGENS REACTION WITH HALOGEN HALIDES Keep in mind an important feature of reactions in which 1,2 and 1,4 additions occur in competition with one another is that the ratio of the products can depend on the temperature, the solvent, and also on the total time of reaction. KINETIC VS THERMODYNAMIC CONTROL OF THE REACTION DIELS-ALDER REACTION The Diels-Alder reaction is a cycloaddition reaction. A cycloaddition reaction is the concerted (all of the electron rearrangement takes place at once, with no carbocation intermediates) bonding together of two independent pielectron systems to form a new ring of atoms When this occurs, two pi-bonds are converted to two sigmabonds, the simplest example being the hypothetical combination of two ethene molecules to give cyclobutane. The initial bonding interaction reflects this electron imbalance, with the two new sigma-bonds being formed simultaneously, but not necessarily at equal rates electron rich → rapid equilibrium ↓ (more stable in some dienes due to steric crowding) electron poor due to e- withdrawing groups (CN, C=O, & NO2) ↓ (must be in this conformation) RCHO un-reactive electron poor CN Mechanism: Essentially, this process involves overlap of the 2p orbitals on carbons 1 and 4 of the diene with 2p orbitals on the two sp2-hybridized carbons of the dienophile. Both of these new overlaps end up forming new sigma bonds, and a new pi bond is formed between carbon 2 and 3 of the diene CHARACTERISITICS OF DIELS-ALDER The Diels-Alder reaction is very useful to synthetic organic chemists because: ▪ it forms rings ▪ inherently stereospecific – forms two new stereocenters cis denophile trans denophile cis substitution trans substitution bicyclic structure locked in the s-cis-conformation especially reactive cyclopentadiene favored fast cyclopentadiene maleic anhydride alkyne – denophiles The Diels-Alder reaction is a pericyclic reaction (concerted rearrangements that proceed though cyclic transition states) The Cope rearrangement and the Claisen rearrangement are other examples of pericyclic reactions STEREOCHEMISTRY OF THE DIELS-ALDER (E) The relationship between the substituents on both the diene and the dienophile are preserved ESSENTIAL CHARACTERISITICS OF THE DIELSALDER REACTION       The reaction always creates a new six-membered ring. When intramolecular, another ring may also be formed. The diene component must be able to assume a s-cis conformation. Electron withdrawing groups on the dienophile facilitate reaction. Electron donating groups on the diene facilitate reaction. Steric hindrance at the bonding sites may inhibit or prevent reaction. The reaction is stereospecific with respect to substituent configuration in both the dienophile and the diene. EXAMPLES OF THE CHARACTERISITCS DIENE POLYMERS Polymerization of 1, 3-butadiene (needed to synthesize rubber): new bonds (red) monomer (green) monomer Which product is formed depends whether the rxn is thermally or kinetically controlled Natural rubber is formed from the isoprene monomer and has Z stereochemistry. It is obtained as a milky white fluid known as latex from a tropical rubber tree. The E polymer gutta-percha also occurs naturally, but is more brittle than rubber. Uses of this thermoplastic include dentistry, electrical insulators and the covering on golf balls. Charles Goodyear accidentally discovered that by mixing sulfur and rubber, the properties of the rubber improved in being tougher, resistant to heat and cold, and increased in elasticity. This process was later called vulcanization after the Roman god of fire. Vulcanization causes shorter chains to cross link through the sulfur to longer chains. The development of vulcanized rubber for automobile tires greatly aided this industry. The most important synthetic rubber is Neoprene which is produced by the polymerization of 2chloro-1,3-butadiene. cross-linking occurs between Cl of one chain and the double bond of another chain giving neoprene its elasticity Colored molecules: orange color β – carotene red color lycopene ULTRAVIOLET SPECTROSCOPY Electromagnetic radiation: energy that travels through space as a wave c = λν (2.998 X 108 m/s) λ = c/ν ν = λ/c c = speed of light λ = wavelength of e.r. ν = frequency of e.r. •Violet: 400 - 420 nm •Indigo: 440 - 490 nm •Green: 490 - 570 nm •Yellow : 420 - 440 nm •Blue: 570 - 585 nm •Orange: 585 - 620 nm •Red: 620 - 780 nm Visible spectroscopy only works for colored solutions %T = Io/I A = log Io/I [0 (no absorption) – 2 (99%) A=εxlxc ε – molar absorptivity l – path length c – concentration c = A/ε x l ↑ only these two transitions are achieved By energies available in the 200-800 nm range As a rule energetically favored transitions will occur from the highest occupied mo (HOMO) to the lowest unoccupied mo (LUMO) σ - σ* transition. π - π* transition LUMO π → π* HOMO lycopene absorbs green light and transmits red Another transition that can occur: has a strong uv absorbance @ 256 nm (π - π*) also has the absorbance illustrated (wavelength at maximal light absorbance) (A) nicotinamide adenine dinucleotide (green) Red #3 Purchase answer to see full attachment




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