Chem 2030 Week 6 Notes
Chem 2030 Week 6 Notes CHEM 2030 - 01
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CHEM 2030 - 01
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This 11 page Class Notes was uploaded by Shannon Z. on Sunday October 4, 2015. The Class Notes belongs to CHEM 2030 - 01 at University of Missouri - Columbia taught by Rainer Glaser in Fall 2015. Since its upload, it has received 46 views. For similar materials see Survey of Organic Chemistry in Chemistry at University of Missouri - Columbia.
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Date Created: 10/04/15
Chapter 4 Cont 1 Benzene gives rise to a class of compounds called aromatic hydrocarbons which are very stable six carbon rings a Arene i Class of aromatic hydrocarbons ii R 1 Used to represent alkyl groups iii Ar 1 Used to represent aryl aromatic hydrocarbon groups Orbital Model for Benzene a Each C atom in benzene is connected to three other atoms and is thus sp3 hybridized i Two of each carbon s orbitals overlap with another carbon s orbitals to form a 0 bond with the carbon atoms on either side ii The third orbital overlaps with the hydrogen atom attached to each carbon atom to make another 0 bond iii The fourth valance electron of each carbon atom is located in a TC bond orbital that lies perpendicular to the sp3 orbitals The carbon s TE bond orbitals overlap to form a hexagonal ring b The bond angles between the carbon atoms are 120 3 Primary methods for drawing benzene b delocalized pi cloud 4 Nomenclature a Base structure prefix is named after the following EH5 E HSCHEH he mane toluene ctuttene at rene phenol Er Ill a H A 2 39ltimn39nmberlaerie chin minimal at rue n i I when Emil Elll39lj lh EEi iL propylljen time b Assign location numbers minimizing the numbers assigned to carbons with substituents or multiple bonds c Substituents i One substituent 1 If the structure is not pictured in the image above follow normal nomenclature rules If the structure is pictured above the name is written in blue below the Iquot structure ii Two substituents 1 2 If one of the substituents in the image above assign that carbon atoms position X location number one i The X stands for ipso b For the second substituent i Carbon atoms located one bond away from carbon X location numbers 26 have the prefix ortho before that carbon s substituent name 1 Ortho can be abbreviated as 0 ii Carbon atoms located two bonds away from carbon X location numbers 35 have the prefix meta before that carbon s substituent name 1 Meta can be abbreviated as m iii The carbon atom directly across from carbon X location number 4 has the prefix parabefore that carbon s substituent name 1 Para can be abbreviated as p 3 Place a hyphen after listing either the location number or ortho meta para positions of the multiple substituents 4 If both substituents are the same remember to use the prefix di before the substituent name d Using the above chart write the name of the root aromatic molecule i When in doubt use the name benzene but remember to list all of the substituents e Examples H l I H H quotl39l39lI quot39 til llfl39lU 39 k k u it l lrlll Pn l J zx quotxiiL1H l39tiJ uln lN IllJ vi lmm l Br t l 39 39 ill quot2H No a l39 39 H 39 li HaIliiII39lHittin 39I l 3939lquotliuilg lI MNLiv H XML 21 mull MIL R I 5 Aromatic Reactions a Due to all of benzene s double bonds one would expect it to take place in addition reactions but it doesn t The reactions in this chapter are all substitution reactions b Chlorination i Addition of Clz ii Requires a Lewis Acid as a catalyst 1 The Lewis Acid polarizes the ClCl bond making it a strong electrophile iii Process 1 The ClCl is made an electrophile by the catalyst 2 The electrophile bonds to one of the carbon atoms on the benzene a The electrophile uses two TE electrons from benzene s TE cloud to form a 0 bond with the carbon atoms b The benzene acts as a nucleophile 3 Thisjoining forms a benzenonium ion F b Note that the double bond that was located where the Cl joined a carbon has moved to allow the new Cl atom to join the molecule This has left a charge on the other carbon atom that took part in the double bond c This structure is a carbocation The charge is delocalized amongst three other resonance structures i The double bonds are positioned so that they do not touch the carbon atom that has the new chlorine atom ii The positive charge is positioned so that it is touching neither the doublely bonded carbon atoms nor the carbon atom the chlorine atom has been added to 4 However benzenonium is less stable than benzene and the molecule will remove either the hydrogen atom from the carbon that was attached by the electrophile or the new chlorine atom 5 When the hydrogen atom is removed it can join with the second chlorine atom that was not bonded to the benzene to form HCl c Nitration i Addition ofa nitronium ion ii Process 1 A sulfurinc acid catalyst adds H to nitric acid a The nitric acid is broken into a nitronium ion N02 which is a strong electrophile and water d Sulfonation e quottizl in II I z U gr w H I H I A H Ha zu H l NR 1 Hal 1 nitric amid protonMed nitronium i nitric acid ion The electrophile bonds to one of the carbon atoms on the benzene a The electrophile uses two TE electrons from benzene s TE cloud to form a 0 bond with the carbon atoms b The benzene acts as a nucleophile This joining forms a benzenonium ion However benzenonium is less stable than benzene and the molecule will remove either the hydrogen atom from the carbon that was attached by the electrophile or the new nitronium ion i Substitution of 03 to a benzene ii Does not require a catalyst as 03 is already a electrophile iii Process Alkylation 1 The electrophile bonds to one of the carbon atoms on the benzene a The electrophile uses two TE electrons from benzene s TE cloud to form a 0 bond with the carbon atoms b The benzene acts as a nucleophile This joining forms a benzenonium ion However benzenonium is less stable than benzene and the molecule will remove either the hydrogen atom from the carbon that was attached by the electrophile or the new 503 i This reaction is called the FriedelCrafts Reaction ii The electrophile is a carbocation formed by one of two ways 1 A halide ion is removed from an alkyne halide with a Lewis Catalyst 2 A proton is added to an alkene iii Process 1 The electrophile bonds to one of the carbon atoms on the benzene a The electrophile uses two TE electrons from benzene s TE cloud to form a 0 bond with the carbon atoms b The benzene acts as a nucleophile This joining forms a benzenonium ion However benzenonium is less stable than benzene and the molecule will remove either the hydrogen atom from the carbon that was attached by the electrophile or the newly attached substituent iv Limitations 1 Won t work with aromatic rings that already have a nitro or sulfonic acid group already attached f Acylaltion i The electrophile is an acyl cation generated from an acid derivative ii Process 1 The electrophile bonds to one of the carbon atoms on the benzene a The electrophile uses two TE electrons from benzene s TE cloud to form a 0 bond with the carbon atoms b The benzene acts as a nucleophile 2 Thisjoining forms a benzenonium ion 3 However benzenonium is less stable than benzene and the molecule will remove either the hydrogen atom from the carbon that was attached by the electrophile or the newly attached acyl cation 6 RingActivating and RingDeactivating Substituents a Some substituents will speed up or slow down electrophilic aromatic substitution reactions b Substituents that donate electrons to the aromatic ring will speed up reactions c Substituents that take electrons from the aromatic ring will slow down reactions 7 OrthoPara Directing and MetaDirecting Groups a Substituents already present on an aromatic ring can determine the position taken by a new substituent b The new substituent will be added in either the para ortho or meta position based on the existing substituents c There are two main classification groups of these existing substituents i OrthoPara Directing 1 Direct the new substituent to the ortho and para positions ii MetaDirecting 1 Direct the new substituent to the meta position d Resonance structures from ortho para and meta additions onto a molecule can help why certain structures are meta para or ortho directing i OrthoPara Directing 1 Four total resonance structures a The resonance structures always include a form in which the carbocation s charge is on the carbon atom that the original substituent is attached to i This carbon atom is also a tertiary carbon making it very stable ii Having the positive charge on this tertiary carbon contributes greatly to the resonance hybrid of these structures iii Paradirecting groups do not place a positive charge on this tertiary carbon atom b The other two resonance structures place the charge on secondary carbon atoms c In each of the orthoparadirecting groups the atom from the substituent that attaches to the carbon from the aromatic ring has an unshared electron pair i This electron pair can stabilize the carbocation s charge further by delocalizing it into the substituent ii This creates an additional fourth resonance structure 2 Example quotm 6 SH SH 39 a ll I H H ll V Br l m Bl T Hr 01 JUN 61 OH quotrulhl 4 ll Br H ls H Br N Br a One resonance form is made by delocalizing the charge onto the substituent b One resonance form is made by placing the charge onto the tertiary carbocation c Two other resonance forms are made by placing the charge on secondary carbocations d Note that when draws as a resonance structure neither the double bonds nor the charge touch the carbon atom that the new substituent was attached to 3 All alkyl and methyl groups are orthometadirecting ii MetaDirecting Groups 1 The original substituent has a charge on the atom that attaches the substituent to the carbon atom in the aromatic ring 2 Three resonance forms 3 The orthometadirecting groups end up placing the charge on the tertiary carbon atom Since the metadirecting substituents has no unshared electron pairs like orthometadirecting groups do this results in a very bad and unstable structure 4 Metadirecting groups are characterized by the connecting atom between the aromatic ring and substituent being either part of a multiple bond or being partially charged 8 Activating and Deactivating Substituents a Existing substituents on an aromatic ring not only affect where new substituents are added but also how fast that substitution reaction occurs b Activating Substituents i The substitution reaction in which the new substituent is added goes quickly ii All orthoparadirecting groups are activating except halogens which are deactivating c Deactivating i The substitution reaction in which the new substituent is added goes slowly ii All metadirecting groups are deactivating 1 iiiHh i lH i39sil39i393 amino EIH Ema DEE Mummy aim39 Miami ll HHil E my Clam H alkyl SEEN na u 4 quotiH m w a ciHi Ell n gammamimpcamxoalkmy Elm E quot39 E E E 2 II a la 5 H Eulmm and a a i i Ii H he a H ni n m all d 9 Misc a Phenyl a benzene minus a hydrogen ion The spot is left open for something else to connect to Chapter 5 1 Review a Stereoisomers molecules that have the same connectivity but different spatial orientation i The different spatial arrangements can play a huge role in how the compounds acts b You cannot rotate between cis and trans isomers by simply rotating carbon bonds Instead it would have to break the subtituent off of the carbon ring and switch places with one of the H atoms on that particular carbon ring atom i When you have to break bonds to form an isomer those isomers are called configurational isomers ii Isomers created by rotating bonds are called conformers 2 Chiral vs Achiral a A chiral molecule s mirror image when laid next to the actual molecule will have a different spatial orientation Chiral molecule rotuu39 l 39 about the 2 39l homl to superimposv 39cmral carbon H and II lhc positions of the methyl and ethyl groups arc I10 i superimposed b Enantiomers i A pair of molecules related as nonsuperimposable mirror images 1 2 Chiral structures of the same molecule 2 The two structures pictured above are enantiomers c Achiral i An achiral molecule s mirror image when laid next to the actual molecule will have the same spatial spacing 3 Stereogenic Center a Stereogenic Carbon Atom i Carbon atom with four different attached groups ii Also called the stereogenic center iii These carbon atoms are called stereogenic centers because they can give rise to different stereoisomers b Enantiomers differ in the arrangement of groups attached to the stereogenic center 4 Plane of Symmetry a A plane that passes through a molecule in such a way that both sides of the plane are exact reflections of one another b Molecules with a plane of symmetry are achiral 5 CahnIngold Prelog System CIP a A system for ranking substituents and determining the relative positions of substituents b Process i Rank substituents according to atomic number 1 The higher the atomic number the higher the ranking 2 For two directly attached atoms a Write the first atom in the substituent that together atoms are attached to quotOpen that atom s list i List that atom s abbreviated form ii In parenthesis list up to three atoms that are in the same substituent and directly attached to that atom according to atomic number ranking iii List these atoms according to decreasing priority iv If there are multiple bonds each multiple bond counts as a single bond 1 Example EEK CCCC l Equot H I In CHEEHE CHECHEEHS EEHHI CEHHI ElHHHl EHHI iHHHj v Example 3 You can then compare the substituents to one another a Example In the CH2CH2CH3 substituent above the first two CH2 fragments outrank the CH3 at the end but if you were to compare both of the whole substituents branching from the stereogenic center the propyl substituent would win 1HH1 EEHHI CHHH1I EHHI lHHHj I ii The propyl s CCCC outranks the ethyl s CCCH shown in circles ii Orientations for Drawing 1 Primary way of drawing 3D structures these instances a Two bonds in the plane one behind the plane and one in front of the plane 2 LPS a Lowest Priority Substituent 3 Process a Identify the LPS has the lowest atomic number ranking i Place this substituent behind the plane b You have ranked all of the other substituents by atomic number i Now note whether the rankings of these substituents around the stereogenic center goes clockwise or counterclockwise not including the LPS 1 Clockwise Caled a R molecule 2 Counterclockwise Called a L molecule Images 0 httpwwwpersonaluneeduauquot39sgloverCHEM120Topic20Asld026htm 0 Class PowerPoint slides 0 Text
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