CH223. Objectives. Domino effect. The secret. Energy consideration O. Kay Sandberg, Welcome Ph.D. Organic chemistry: What? C students (at best) Fred

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1 Kay Sandberg, Welcome Ph.D. This lecture s objectives: bjectives ) Review 0, basic concepts ) Introduce arenes & aromaticity Get started on the right foot! Strategy for success Domino effect Understanding lecture work Understanding homework Understanding exams Understanding organic chemistry rganic chemistry: ow? Will A students you make an A? Why? B students What? students (at best) Students who have an A daily average, but fail exams have not progressed past the what. Fred Reactions: Sandberg s analogy: increasing E = increasing discomfort E relatively high energy species igher E reactants 0 review Energy consideration Barney Understand species in nature tend to react to lower the system s free energy: ighest E transition state Lower E products Reactions: The SERET!!!!! The secret Understand oulomb s Law: ppositely charged species are attracted to each other. (and conversely, species of like charge are repelled.) 0 review Find the negative, find the positive, and let the negative grab the positive

2 Secret of your success in Find the negative charge carrier Find the positive Terms charge carrier likes nucleophile likes electrophile Lewis base Lewis acid e - pair e - pair donor neg (electrons) grabs pos acceptor Reactions: The SERET!!!!! The secret in scientific terms 0 review Find the negative, find the positive, and let the negative grab the positive Find the nucleophile, find the electrophile, and let the nucleophile bond w/ the electrophile electrons are now the bond 7 8 Wilma: A) Select the appropriate diagram. B) Select the electrophile. Electron red flow arrows blue hapter : Arenes & Aromaticity benzene Arenes Section. toluene LP σ bond δ δ- F σ bond LP F Up or down in E? 9 naphthalene 0 aspirin N F Drugs N Prozac N tryptophan phenylalanine N N Biomolecules estrone tyrosine N cocaine

3 hapter : Arenes & Aromaticity Aromatic Aromatic vs aliphatic Aliphatic Section. Aromatic Aliphatic Rxn comparison onjugated onjugated cyclic Special stability arenes Aromaticity l l l No Rxn x,y-dibromo-a,b-hexadiene x,y-dibromocyclohexene Section. ydrogenation Section. ydrogenation Section. Excess Pd Pd Substitution products are obtained readily Substitution Aromatic ring is retained. Section. Betty What ybridization is carbon s in benzene hybridization? 7 8

4 Structural features of benzene Bonding in benzene 0 o 0 o 0 o 0 pm sp -sp single bond Section. pm Resonance Resonance in benzene Section. double bond pm 9 Pebbles 0 It is common, however, for chemists to use one of the resonance structures, but you should realize Representations that reality is in of between toluene the resonance structures. Section. Shorter bonds Stronger π interactions Bonding in conjugated aliphatic pi electrons are obvious toluene Resonance toluene Longer bonds Section. Weaker π interactions Bonding in aromatic Same length Same strength π interaction Section. sp hybridized Sigma & pi bonding in benzene σ system π electrons ontinuous π system pure p orbitals π system No alternation

5 ( x 0) ( x 9) = kj Bam Bam eats of hydrogentation Section. ere we have π-e s that are lower in E than π-e s Empirical resonance Empirical energy energy Section. E kj/mol 0 0 = 0 9 kj conjugation stabilization 08 0 x 0 E kj/mol x 0 Stability of benzene ydrogenation: Rh or Pt, or higher pressures, higher temperatures. E kj/mol resonance Stability of benzene 0 Empirical energy 08 Section. 0 x 0 Stability of benzene Large stabilization due to π-e - s being aromatic. 9 E kj/mol Stability of benzene 08 7 Section. non conjugated double bonds Small stabilization due to π-e - s being conjugated, but not aromatic Physical properties of arenes Solubitilty nonpolar water insoluble less dense than water Dino density (g/ml) l.9.00 benzene 0.88 Section.9 Nomenclature of benzene & derivatives Nomenclature N bromobenzene nitrobenzene tert-butylbenzene before mixing after mixing 9 0

6 Nomenclature of benzene & derivatives Special names Nomenclature of benzene & derivatives Special names When the benzene ring is a substituent it is named phenyl ( -) benzaldehyde benzoic acid acetophenone phenol Rubble Nomenclature of benzene & derivatives Special names toluene styrene N Monosubstituted derivatives butylbenzene or less carbons 7 Locant needed heptylbenzene 7 or more carbons anisole George aniline hexylbenzene benzene = parent name -phenylheptane Judy phenyl = branch name Elroy Write Monosubstituted the IUPA name. derivatives Nomenclature of benzene derivatives Disubstituted benzenes Disubstituted derivatives ortho (,) o- meta (,) m- para (,) p-

7 Nomenclature of benzene & derivatives Dimethyl derivatives: xylenes o/m/p regioprefixes Nomenclature of benzene & derivatives Disubstituted benzenes o/m/p regioprefixes ortho -xylene meta-xylene para o-xylene m-xylene p-xylene,-dimethylbenzene,-dimethylbenzene,-dimethylbenzene 7 ortho meta para o m ipso p o m 8 Nomenclature of benzene & derivatives Disubstituted benzenes o/m/p silly memory devices Nomenclature of benzene & derivatives Disubstituted benzenes Practice ortho meta para o m p o m 9 Astro m-bromoisopropylbenzene also acceptable meta-bromoisopropylbenzene 0 When parent is benzene, # to give the lowest locant at the first point of difference (before alphabetizing). N Trisubstituted derivatives N o/m/p only used for disubstituted derivatives cannot use for tri or higher substituion Nomenclature of benzene derivatives. When parent is benzene, # to give the lowest locant at the first Numbering point of difference rule(before alphabetizing). (ighest priority rule) N l N N N N N. When differing numbering schemes are lowest, then use the one which gives the lowest locant to the branch listed first alphabetically. l l l,,-trisubstituted,,-trisubstituted,,-trisubstituted -chloro-,-dinitrobenzene Ignore di, tri, tetra, etc. when alphabetizing 7

8 Nomenclature of benzene derivatives RoseyPractice,,-trinitro with special toluene names F N Understood - means no locant in name -ethyl--fluoroanisole N N N Understood - Understood - amino -ethyl--methyl N N aniline toluene derivative? aniline derivative? Nomenclature of benzene derivatives When the benzene ring is a substituent it is Priorities named phenyl ( -) Alcohol has priority which means it is bonded to the parent. Number from end which gives the hydroxy group the lower locant. -phenylpropan--ol Use name to double check carbon #. More practice Which is (Z)--phenylbut--ene? i (E)- (Z)- Lo zame zide i Lo Benzyl group ( -) benzyl bromide Benzyl vs phenyl methylene kink ring is parent bromine branch bromobenzene phenyl bromide ring is branch bromide is parent Spacely benzylic Benzyl vs allyl benzyl bromide allyl bromide Benzyl is analogous to allyl allylic Benzylic Allylic vs& allylic vs aryl Section. Benzylic & l l Vinylic & l arylic aryl & l 7 8 8

9 Nomenclature of benzene & derivatives The adjective aryl is analogous to alkyl. aryl Aryl vs alkyl alkyl Reactions of arenes. Ring as functional group. Birch Reaction reduction overview Electrophilic aromatic substitution (EAS) Nucleophilic aromatic substitution (NAS) Section.0. Influence of aryl group as a substituent on the reactivity of another functional group to which it is attached. osmo aryl alkyl 9 Benzylic radical Benzylic carbocation alkenylbenzene 0 Free-radical halogenation of alkylbenzenes Radical Δ o formation comparison 97 kj/mol tert-butyl rad 8 kj/mol allyl rad Section. Radical resonance delocalizsation δ. δ. i.e., Resonance delocalization Free-radical halogenation of alkylbenzenes Δ o Benzyl radical formation 97 kj/mol tert-butyl rad 8 kj/mol kj/mol Section. allyl rad benzyl rad Jane Stability of benzyl radical Resonance delocalized: Resonance structures Section. 9

10 Resonance structures Section. Notice where the rad is not! Tom E vinyl radical (least stable) Radical R stability chart R R R R R allyl radical Section 0. radical stabilities benzyl radical (most stable) Stabilization due to overlap of benzylic pure p orbital with the extended π system of ring Radical orbital picture Section. Regioselectivity of abstraction Abstraction of benzylic Benzylic vs aryl abstration Section. Abstraction of aryl sp p Benzyl radical 7 radical to π system less stable 8 E vinyl radical (least stable) Radical R stability chart R R R R R allyl radical radical stabilities Section 0. benzyl radical (most stable) 9 Regioselectivity of abstraction Abstraction of benzylic Benzylic vs aryl radical benzylic Section. Abstraction of aryl vinylic ( arylic ) 0 0

11 Regioselectivity of free-radical halogenation hν Free radical halogenation step l l l l The homolytic cleavage of the l-l bond to create the atomic chlorine atoms. Section. Free radicals: alkylbenzene Step initiation Step propagation l hν Step l l l l benzyl radical Section. atom abstraction from toluene by l atom generating benzyl radical l Free radicals: alkylbenzene Step initiation Step propagation hν Step l l l l l hain propagation l l Section. l Benzyl reacts with molecular l creating atomic l... l l Free radicals: alkylbenzene toluene l light Benzylic l chlorination l l light l l light l Section. l l benzyl chloride (dichloromethyl)benzene (trichloromethyl)benzene Free radicals: alkylbenzene Benzylic bromination Section. l, 80 o light N N p-nitrotoluene p-nitrobenzyl bromide Free radicals: alkylbenzene peroxide N l, 80 o NBS benzylic NBS bromination Section. N ethylbenzene -bromo--phenylethane

12 NBS benzylic bromination N N peroxide N Section. l, 80 o N N Jerry 7