Organic Chemistry I Structure and Reactivity
Organic Chemistry I Structure and Reactivity CHEM 30A
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Date Created: 09/04/15
Chem 30A Fall 2007 Ch em 3 0A Prof Garrell WEEK 2 SYNOPSIS GOALS AND ASSIGNMENTS FOR OCT 8 THROUGH OCT 12 MO THEORY STRUCTURES 8C CONFORMATIONS 0F ALKANES AND CYCLOALKANES TOPICS Molecular orbital theory Introduction to alkanes and cycloalkanes Nomenclature mostly on your own Conformations of alkanes and cycloalkanes GOALS 0 Understand the following terms sigma and pi orbitals sigma and pi bonds hybrid orbitals sp spz sp3 and their geometries bonding antibonding Practice drawing resonance structures Build skills with arrow notation for depicting one and twoelectron shifts Practice identifying primary secondary tertiary and quaternary carbons 0 Be able to identify alkyl groups by their common and IUPAC names 0 Be able to use the IUPAC rules those in ch2 to name alkanes and cyclic alkanes amp simple bicyclic alkanes 0 Be able to identify constitutional isomers 0 Learn the language of molecular conformations dihedral angles staggered eclipsed gauche anti 0 Be able to draw Newman structures and visualize molecules from Newman structures 0 Understand graphs of potential energy vs dihedral angle and Whythe potential energy depends on dihedral angles Understand the terms torsional strain and steric strain 0 For cyclic alkanes be able to relate the degree of strain to the size of the ring 0 For cyclohexane and substituted cyclohexanes be able to identify aXial and equatorial substituents 0 Be able to visualize and draw the chair and boat conformations of cyclohexane and to identify signi cant nonbonded interactions 0 Understand the importance of nonbonded interactions in determining the relative stability of cyclic alkane conformers 0 Be able to convert between stereorepresentations planar hexagon representations and chair representations ASSIGNMENTS READINGS Brown Foote amp Iverson l718 2126 PROBLEMS m Ch 1 51 52 56 6669 Ch 2 1618 2530 3237 60 61 Modeling Worksheet 1 Will be distributed Wednesday Oct 10 due at the beginning of class by 805 AM Monday Oct 15 UPCOMING DATES amp DEADLINES Manashi Chatterjee PhD Spring 2008 Chem 30A Spring 2008 Week 9 and 10 91 Nucleophilic Substitution in haloalkanes table 91 Home Work problems and example 91 92 Solvents for Nucleophilic substitution reactions 93 Mechanism SN2 94 Experimental evidence for SN2 94A Kinetics SN2 Home Work problems and example 92 94 B Structure of Nucleophile and solvent effect 94 C Stereochemistry for SN2 Home Work problems and example 93 Home work End of Chapter problems 910 916 945 c 945 e 945 g Wednesday after Quiz3 L 93 Mechanism SN1 94 Experimental evidence SNl 94A Kinetics SN1 94 D Structure of haloalkane alkyl halide or leaving group Effect of Branching Steric factors for SN2 Stability of carbocations for SNl Home work End of Chapter problems 917 918 927 935 95 E Special Halides Leaving groups on allylic or benzylic carbons even if they look may look primary they the more stable than a normal primary resonance and can go via SNl or SN2 so take a good look at the Nucleophile to decide mechanism Note the order that is given on page 347 read Nucleophilic substitution 4 on page 353 Substitution only if leaving group on sp3 hybrid carbon vinyl and aryl halides don t undergo nucleophilic substitution Home Work example 9 5a Home work End of Chapter problems 924 94 F Leaving groups Alkyl halides RX XI Br Cl and sulfonates esters of alcohols RSOZOR are common leaving groups since 139 Br Cl39 because of the stability of the anion Manashi Chatterjee PhD Spring 2008 Protonation of OH OR can also convert them to leaving groups ROHf is good leaving as HzO is stable However esters RCOOR alcohols ROH ether ROR amine RNHZ are rarely used as leaving groups since the conjugate acid produced acetate CH3COO hydroxide OH methoxide OCH339 amide NHZ hydride H39 ions are less stable anions Home work End of Chapter problems 952 953 94 G Solvent for SN2 and SN1use of mixed polar protic solvents 94 H Rearrangements W agnerMeerwein rearrangements involving carbocations Note SN2 is free of rearrangements since there is no carbocation intermediate Also read from Weeks page 127 to 135 12hydride shift 12methyl shift 12phenyl shift Ring expansion and ring contraction Home work End of Chapter problems 920 931 932 933 95 Analysis of reactions SNl Vs SN2 and Retrosynthesis Home Work example 9 5b and problem 95 Home work End of Chapter problems 923 925 928 930 934 936 943 948 949 951 Friday week 101 96 elimination called dehydrohalogenation if leaving group is a halogen Home work End of Chapter problems 944 9 7B Elimination in one step E2 Mechanism elimination in one step Formation of ALKENES Elimination uses BASE common bases OR or OH NaOMe NaOEt NaOH These can also act as nucleophiles and do substitution reactions when leaving group is on a primary carbon SN2 Remember NaOH is also used for acid base reactions Primary amines are often used as nucleophiles Tertiary amines are generally used as bases to do acid base reactions Triethyl amine pyridine are used as bases in acid base reaction Also note that the lone pair in pyridine is not in resonance so can be used to pick a proton in an acid base reaction Sterically hindered tertbutoxide OR ion always acts as a base and give elimination products even with leaving group on primary carbon Other stronger bases are N aNHz NHZ39 NaH H They are not used as nucleophiles to do substitution reactions Note acetate ion CH3COO is not like an alkoxide OK it is a nucleophile not a base so it is used for substitution reactions If leaving groups is on the secondary or tertiary carbon and you have any of the above bases you will get a one step elimination For elimination look for betahydrogens If there is no betahydrogens there can t be an elimination at that Carbon If there are 2 or more betahydrogens two or more products can be formed 98 B regioselectivity Zaitsev s rule More substituted alkene is the major product Sterically hindered tertbutoxide Give less substituted alkene as major product Home Work problems and example 96 and examples on page 355 Manashi Chatterjee PhD Spring 2008 Home work End of Chapter problems 937 938 Home work End of Chapter problems 945 Biological connections Sadenosylmethionine SAM is one of the primary methyl donor for most biological reactions note that protonated Sulfur is the leaving group S adenosylhomocysteine SAH is formed after transfer of the methyl group from SAM to various nucleophiles SAH is then converted to homocysteine Elevated homocysteine levels is a risk factor for vascular and neurological diseases Vit B12 is required to convert homocysteine back to methionine which is converted to SAM Deficiency in vitamin B12 leads to an accumulation of homocysteine in the body This will not be asked in exam Home Work Chapter 8 84 A Preparation of haloalkane by Halogenation bromination is MORE regioselective THAN chlorination explained by Hammond s Postulate 85 D not required Regioselectivity Hydrogen replacement in order of tertiarygtsecondarygtprimary 85 Mechanism 85 A Formation of radical homolytic bond cleavage change in position of single electron is shown by fish hook arrows Energy to cause bond cleavage is supplied by light or heat 85 B 3 steps chain initiation propagation and termination 85 E Stereochemistry of Radical halogenation Reactions give racemic mixtures equal amounts of both isomers are formed End of book problems 813 814 815 86 Allylic halogenation Bromine and chloride add to double bonds at room temperature However if halogen is heated to very high temperature the allylic hydrogen is replaced by halogen