© D.M. Collard, 2005D.M. Collard 2007TOPIC 10. REVIEWMOLECULAR STRUCTURECONCEPTS, MODELS, RULES AND THEORIESConcept PredictionOctet rule Valency, presence of lone pairsLewis Dot Structures Formal charges electronegativityElectronegativity Polar bonds, molecular dipolesBonding Covalent bonds between atoms of similar electronegativityIonic bonds between atoms of different electronegativityVSEPR Molecular geometryHybridization Molecular geometryResonance Electron distribution, hybrid structuresHyperconjugation σ-bonds as electron donors© D.M. Collard, 2005D.M. Collard 2007RESONANCE THEORYXYZResonance theory helps to explain:Structurei.e., Reactivityi.e.,CORNH2CORO© D.M. Collard, 2005D.M. Collard 2007CCCHHHCCHHHCCHHHHyperconjugation accounts for the enhanced stability of cations, radicals and alkenes with high degrees of substitution.HYPERCONJUGATIONSYSTEMATIC IUPAC NOMENCLATURELinear (Unbranched) AlkanesCH4CH3CH3CH3CH2CH3CH3(CH2)2CH3CH3(CH2)3CH3CH3(CH2)4CH3CH3(CH2)5CH3..C12345678910C11121314151617181920methaneethanepropanebutanepentaneSubstituents-CH3methyl -CH2CH3ethyl -CH(CH3)3isopropyl -CH(CH3)CH2CH3sec-butyl -CH2CH(CH3)2isobutyl -C(CH3)3tert-butyl-C6H5phenyl -CH2C6H5benzylBranched AlkanesLongest chain numbered from the end that has the substituents at the lowest possible number. Substituents listed alphabetically (ignoring di, tri, sec, tert)© D.M. Collard, 2005D.M. Collard 2007Alkyl HalidesClasses of alkyl halidesRHalAlcoholsClasses of alcoholsROH88 kcal/mol1.43Å110 kcal/mol0.97 ÅEthersROR'AminesClasses of AminesRNHHRiNRiiHRiNRiiRiiiRiNAldehydes and KetonesaldehydeketoneCORHCORRi177 kcal/mol1.22 Å© D.M. Collard, 2005D.M. Collard 2007Carboxylic acids Carboxylic estersAmides NitrilesCOR(H) OHCOR(H) ORiCOR(H)NRii(H)Ri(H)CRNFunctional MoleculesAlcohols, R-OH: Alkyl halides, R-Hal:OH at lowest possible position named as halo-substituted alkaneAlkenes (and alkynes):Longest chain containing C=C (C≡C), numbered to keep C=C (C≡C) as low as possibleAldehydes: Ketones:Numbered from CHO (C1) Numbered to keep C=O as low as possibleEne-Ols Ene- Ynes#-alken-#-ol (OH at lower position) #-alken-#-yne (C=C at lower position)Alkenes: Compounds with Stereocenters:(E) or (Z) (R) or (S)© D.M. Collard, 2005D.M. Collard 2007CHIRALITY: ENANTIOMERSAn object which has a non-superimposable mirror image is chiral (the opposite of chiral is “achiral”).Another test for chirality is to assess whether the object itself has a mirror plane of symmetry or point of symmetry (point of inversion).EnantiomersMolecules can be chiral. Pairs of molecules which are non-superimposablemirror images of one another are called enantiomers. Enantiomers are examples of stereoisomers: molecules which differ only in the spatial arrangement of atoms.Molecules with a single carbon atom bearing four different substituents can exist as a pair of enantiomers which differ in the arrangement (“configuration”) of these substituents.The carbon is stereogenicThe carbon is a stereocenterYou must be able to recognize when pairs of molecules are identical (superimposable) or entiomers (non-superimposable mirror images) ADBCADBC© D.M. Collard, 2005D.M. Collard 2007Designating ConfigurationStereocenters are designated as having either R-or S-configurations….- Assign priorities to the substituents using the Cahn-Ingold-Prelog system (briefly, atoms are ranked in order of atomic weight; if two atoms are identical, the next set of attached atoms is considered).- View the molecule with the lowest priority (4) substituent pointing away from you.- Trace from highest priority (1) to second priority (2), to third (3)….Clockwise = RCounterclockwise = Se.g., HClBrFOptical RotationThe observed rotation is αThe observed specific rotation is [α] = a / c·lwhere c = concentration in g/mL and l = pathlength in dm (10-1m)The observed rotation, α or [α ] depends on solvent, temperature and wavelength of the polarized light. Generally the sodium D line is used for the light source and the experiment is done at room temperture, 25 °C.The specific rotation is then noted asThe specific rotation of an optical pure chiral compound is a “property” like melting point or boiling pointThe specific rotation of a given sample depends on it “optical purity”[α] (conc./solvent)25D© D.M. Collard, 2005D.M. Collard 2007DiastereomersProblem: Draw all the stereoisomers of the molecule below.STEREOISOMERS WITH MORE THAN ONE STEREOCENTERABCXYZFor a molecule with n stereocenters, there are a maximum of 2nstereoisomers.H3CCH CHCO2HOH OHCO2HCO2HCO2HCO2HMeso CompoundsIf the sets of substituents on stereogenic centers are identical there will be fewer than 2nstereoisomers. Compounds with stereogenic centers which are not chiral are called mesocompounds.Meso compounds possess a point or plane of symmetryABCABCAABCBCAABBCC© D.M. Collard, 2005D.M. Collard 2007CLASSIFYING REACTIONSReactions are conveniently classified as substitutions, additions, eliminations and rearrangements. These terms describe the overall process, simply comparing the structure of starting materials and products. They do not indicate anything about the pathway (“mechanism”) by which the reaction proceeds. SubstitutionsAdditionsEliminationsRearrangements(often in combinationwith another type of reaction) TWO CLASSES OF NUCLEOPHILIC SUBSTITUTIONS• Substitution reactions can be performed under different conditions which give rise to dramatically different outcomes. Nucleophilic substitution reactions can be classified as one of two types, based on these experimental observations. • Characteristics which allow this classification are listed on the next slide, and will be studied in greater detail in the next two sections.• In order to develop predictive tools, we need to understand reasons why these observations are important. That is, we need to develop proposals for two different mechanisms which are consistent with the two sets of data and which we can use to predict the outcome of other reactions.CLNuCNu L© D.M. Collard, 2005D.M. Collard 2007Rate = k[R-L][Nu]ChiralityChiral R-L forms R-Nu with opposite stereochemistry (inversion of stereochemistry, “Walden Inversion”)Effect of Nucleophile Rate: I–> OH–> Br–> Cl–> F–> H2OEffect of Leaving Group Rate: -I > -Br > -Cl >> -FSubstitution At 1° Substrates: Bimolecular Nucleophilic SubstitutionsEffect of substrateRate: methyl > 1° > 2° ( 3° unreactive)Adjacent groups slow the