Exam 1 Summary of Concepts
Exam 1 Summary of Concepts CH 231
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This 13 page Study Guide was uploaded by Mallory Ivy on Sunday September 13, 2015. The Study Guide belongs to CH 231 at University of Alabama - Tuscaloosa taught by Silas Blackstock in Summer 2015. Since its upload, it has received 104 views. For similar materials see Elem Organic Chemistry I in Chemistry at University of Alabama - Tuscaloosa.
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Date Created: 09/13/15
Dr Silas Blackstock CH 231Elementary Organic Chemistry 1 Exam 1 Study Outline CHAPTER 1 l Electronic Structure of Atoms a Atoms consist of a small dense nucleus and electrons distributed about the nucleus in regions of space called shells i Each shell can contain as many as 2n2 electrons n number of the shell ii Each principal energy level is subdivided into regions of space called orbitals The valence shell is the outermost occupied shell and it contains the valence electrons that participate in chemical bonding b The Lewis dot structure of an atom shows the symbol of the atom surrounded by dots that equal the number of valence electrons ll Lewis Model of Bonding a According to the Lewis model of covalent bonding atoms bond together in such a way that each atom participating in a chemical bond acquires a completed valenceshell electron con guration resembling that of the noble gas nearest it in atomic number i Anions and cations attract each other but do not form bonds with de ned directionality ionic bonds ii Covalent bond chemical bond formed by sharing of electron pairs between adjacent atoms iii Octet rule tendency of elements Group 1A7A to achieve an outer shell of eight valence electrons b Electronegativity measure of the force of attraction by an atom for electrons it shares in a chemical bond with another atom i Nonpolar covalent bond AENltO5 ii Polar covalent bond 055 AEN519 1 In a polar covalent bond the more electronegative EN atom hogs the electrons and thus has a partial negative charge 6 and the less EN atom bears a partial positive 6 charge 2 A polar bond has a dipole moment equal to the product of the absolute value of the partial charge times the bond length between the two charged atoms iii An acceptable Lewis structure for a molecule or an ion must show 1 The correct connectivity of atoms 2 The correct number of valence electrons Dr Silas Blackstock CH 231Elementary Organic Chemistry 1 Exam 1 Study Outline 3 No more than two electrons in the outer shell of H eight electrons in any 2nOI period element and 12 electrons in 3rd period elements 4 And all formal charges 5 Note neutral compounds of boron and aluminum can only have 6 valence electrons lll Functional Groups a Functional groups are characteristic structural units by which we divide organic compounds into classes and that serve as a basis for nomenclature i Also sites of chemical reactivity ll magma 1 END L atc l l bt39 DH E P TW KTHET HwE angmttlc A i mm x All mil3mm E111 rim1c M1413 l n l a 39l j Ti l n C 3 Eliquot 1 swam E l 1 HETMLETWWEPETE t gt l PittFWDng anth L Ellial1 Pl 113 lierammingin wgwl lnm t WF l LFIMl 3E IV Bond Angles and Shapes of Molecules a Bond angles of molecules and polyatomic ions can be predicted using Lewis structures and valenceshell electronpair repulsion VSEPR i 4 regions of electron density sp3 tetrahedral 1 Angles of 1095 ii 3 regions of electron density sp2 trigonal planar VI WI Dr Silas Blackstock CH 231Elementary Organic Chemistry 1 Exam 1 Study Outline 1 Angles of 120 2 regions of electron density sp linear 1 Angles of 180 Polar and Nonpolar Molecules a Dipole moment of a molecule is the vector sum of its bond dipoles Quantum or Wave Mechanics a Quantum mechanics branch of science that studies particles and their associated waves b Provides a way to determine the shapes of atomic orbitals and to quantify the energetics of covalent bond formation Combining Valence Bond and Molecular Orbital Theory to Approach Covalent Bonding a According to the molecular orbital MO theory combination of n atomic orbitals gives 17 molecular orbitals Molecular orbitals are divided into 0 and TI bonding and antiboding denoted with molecular orbitals These orbitals are arranged in order of increasing energy and their order of lling with electrons is governed by the same rules as for lling atomic orbitals Although useful for quantitative calculations on computers molecular orbital theory on does not provide an intuitive understanding of Sigma bonds in complex molecules b For an intuitive understanding of bonding in molecules we use molecular orbital theory concepts in phase and out of face in addition of overlapping orbitals to give bonding and antibonding orbitals in combination with valence bond VB theory Valence bond theory involves the combination of atomic orbitals on each item and a process called hybridization and the resulting atomic orbitals are called hybrid orbitals 1 In combination of one 25 atomic orbital and three 2p atomic orbitals produces for a equivalent sp3 hybrid orbitals each directed toward a corner of a regular tetrahedron at angles of 1095 2 The combination of one 25 atomic orbital and two 2p atomic orbitals produces three equivalent sp2 hybrid orbitals Angles 1200 3 The combination of one 25 atomic orbital and one 2p atomic orbital produces two equivlent sp hybrid orbitals Angles 180 V Resonance Dr Silas Blackstock CH 231Elementary Organic Chemistry 1 Exam 1 Study Outline S and P atoms are commonly depicted with more than eight valence electrons invoking the participation of 3d electrons 1 Recent calculations reveal that in many cases The S and P items of best thought of as sp3 hybridized it with a formal charge rather than involving 3d orbitals In the combined molecular orbital theoryvalence bond theory approach bonding in organic molecules is thought of as the inphase addition overlapping to create bonding orbitals and outof phase addition also referred to as subtraction to create antibonding orbitals print the seas of the hybridized parentheses and possibly any unhybridized 2p atomic orbitals on adjacent atoms 1 All CC C0 and CN single bonds are sigma 0 bonds formed by the overlapping of hybrid orbitals 2 All CH OH NH single bonds are sigma bonds formed by overlapping hybrid orbitals on C O or N with the ls orbital of H 3 All CC CO CN NN and NO double bonds are a combination of one sigma bond formed by overlapping hybrid orbitals and one pi TI bond formed by overlapping parallel unhybridized 2p orbitals 4 All CEC and CEN triple bonds are a combination of one sigma bond formed by the overlap of sp hybrid orbitals and two pi bonds formed by the overlap of twosets of parallel unhybridized 2p orbitals a According to the theory of resonance molecules and ions for which no single Lewis structure is adequate are best described by writing two or more contributingresonance structures The actual molecules are a hybrid of the various contributing structures are drawn between contributing structures to describe the hybrid The most important contributing structures have Filled valence shells A maximum number of covalent bonds The least separation of unlike charges And any negative charge on the more EN atom andor any positive charge on the less EN atom Doubleheaded arro s wNI i Dr Silas Blackstock CH 231Elementary Organic Chemistry 1 Exam 1 Study Outline iii Curved arrows show the manner in which valence electrons are redistributed from one contributing structure to the next Use of curved arrows in this way is commonly referred to as electron pushing iv Electron Pushing Patterns and Common Mistakes 1 ONLY electron pairs are moved Never atoms and never positive charge 2 When a lone pair is adjacent to a pi bonding system the lone pair can be pushed into a pi bond and the pi bond will become a lone pair on the other atom 3 Make sure there are not an excessive amount of valence electrons that the atom can hold 4 Charge must be conserved IX Molecular Orbitals for Delocalized Systems a Most examples of molecules described by more than one resonance contributing structure have charge andor electron delocalization a stabilizing effect in which charge andor electrons are spread over more than two atoms i Delocalization occurs in molecules that have conjugation A pi bond and a lone pair of electrons without an intervening atom are conjugated ii The delocalized electrons are in molecular orbitals formed from overlapping 2p orbitals on three or more adjacent atoms 1 If atoms are taking part in delocalization as described by resonance contributing structures they must have 2p orbitals so they must be sp2 hybridized or in rare cases sp hybridized X Bond Lengths and Bond Strengths in Alkanes Alkenes and Alkynes a The greater the number of bonds between two atoms the shorter the bond lengths and the greater the bond strength i Carbon carbon triple bonds are shorter and stronger than Carbon carbon double bonds which are shorter and stronger than Carbon carbon single bonds b The more scharacter the hybridized orbital taking part in a bon the shorter and stronger it is CHAPTER 2 All About Alkanes lThe Structure of Alkanes a Hydrocarbon compound composed of only carbon and hydrogen iSaturated hydrocarbons contain only CC single bonds and are completely lled iiGeneral formula CnH2n2 ll Constitutional lsomers Dr Silas Blackstock CH 231Elementary Organic Chemistry 1 Exam 1 Study Outline aConstitutional isomers same molecular formula but different connectivity of atoms lll Nomenclature and IUPAC system a IUPAC International Union of Pure and Applied Chemistry declared a universal system of naming alkanes iFind the longest carbon chain parent chain and identify with the name ending in ane iiThe parent chain is numbered to give the rst substituent encountered the lowest number and each substituent is assigned a number from the main chain 1The alkyl group substituents are then identi ed and named each on ending in y aCH3 methyl bCH2CH3 ethyl LCHzCHzCH3 propyl dCH2CH2CH2CH3 eCH2CH2CH2CH2CH3 pentyl 2 Use di tri tetra etc pre xes for multiple identical substituents 3 LThe name is constructed by listing the substituents with their numbers in alphabetical order followed by the parent chain name 1 Di tri tetra etc are not included in the alphabetizing process but iso is ivThere are still some common names that don t correlate with the IUPAC nomenclature 1 C6H6 benzene 2 isobutane 2methylbutane C Do you need to know this Probs not alkane with cyclo V 3 Dr Silas Blackstock CH 231Elementary Organic Chemistry 1 Exam 1 Study Outline Secondary suf xes indicate the functional group present in the compound aAlcohols 0 bAdehydes a LKetones 0ne d Carboxylic acids 0ic acid e Esters 0ate fAmines 1 methyl amine methanamine A carbon is classi ed as primary 1 secondary 2 tertiary 3 or quaternary 4 depending on the number of carbon atoms bonded to it IV Cycloalkanes aA saturated hydrocarbon that contains carbon atoms bonded to form a ring is called a cycloalkane bTo name a cycloalkane name and locate each substituent on the ring and pre x the name of the analogous open chain Say it s a 6C ringthe name is cyclohexane Fivemembered rings and sixmembered rings are especially abundant in the biological world dA conformation is any 3D arrangement of the atoms of a molecule resulting from rotations about the single bonds One convention for showing conformations is the Newman projection A dihedral angle is the angle created by two intersecting planes 1 7 1 2 For ethane staggered conformations occur at 60 180 and 300 Eclipsed conformations occur at dihedral angles of0 120 and 240 For butane viewed along the C2C3 bond the staggered conformation of dihedral angle 180 is called an anti conformation the staggered conformation of dihedral angle 60 and 300 are called gauche conformations 3The anti conformation of butane is lower in energy than the gauche conformations by approximately 28 k 09 kcalmol lntramolecular strain 1Torsional strain arises when nonbonded atoms separated by 3 bonds are forced from a staggered conformation to an eclipsed conformation 2Angle strain arises from creation of either abnormally large or abnormally large or abnormally small bond angles Dr Silas Blackstock CH 231Elementary Organic Chemistry 1 Exam 1 Study Outline 3Steric strain also called nonbonded interaction or van der Waals strain arises when nonbonded atoms separated by four or more bonds are forced abnormally close to each other ivThe relationship between the change in Gibbs free energy temperature in kelvins and an equilibrium constant is given by the equation AG RTnKeq vn all cycloalkanes larger than cyclopropane nonplanar conformations are favored 1The lowestenergy conformation of cyclopentane is an envelope conformation 2The lowestenergy conformations of cyclohexane are two interconvertible chair conformations a In a chair conformation six bonds are axial and six bonds are equatorial b Bonds axial in one chair are equatorial in the alternative chair LBoat and twistboat conformation are higher in energy than chair conformations dGroups larger than H are less stable in the axial position of chair conformations because of 13diaxial steric interactions eThe more stable chair conformation of a substituted cyclohexane is the one that minimizes diaxial steric interactions and therefore has large groups equatorial V Stereoisomers are compounds that have the same connectivity but different orientation of their atoms in space a A stereocenter is an atom most commonly a carbon atom about which exchange of two groups produces a different stereoisomer b Con guration refers to the arrangement of atoms or groups of atoms bonded to a stereocenter c A cis isomer is whose substituents are on the same side of the ring d A trans isomer is one whose substituents are on opposite sides of the ring e Most cycloalkanes with substituents on two or more carbons show cis trans isomerism Vl Physical Properties of Alkanes and Cycloalkanes a Alkanes are nonpolar compounds and the only forces of attraction between their molecules are dispersion forces weak electrostatic interaction between temporary induced dipoles of adjacent atoms or molecules Dr Silas Blackstock CH 231Elementary Organic Chemistry 1 Exam 1 Study Outline b The greater the molecular weight of the alkanes the more compact the alkane is gasljliquidljsolid at room temperature and atmospheric pressure c Among a set of alkane constitutional isomers the least branched isomer generally has the highest boiling point the most branched isomer generally has the lowest boiling point Vll Reactions of Alkanes a As determined by heats of combustion strain in cycloalkanes varies with ring size b Cyclohexane which has the most common ring size among organic compounds is strainfree Vlll Sources and Importance of Alkanes a Natural gas b Petroleum c Synthesis gas IX Key Reaction a Oxidation of Alkanes i Oxidation of alkanes to carbon dioxide and water is the basis for their use as energy sources of heat power ii CH3CH2CH3 502D 3C02 4H20 AH 2220 kl 5306 kcalmol CHAPTER 3 l Chirality The Handedness of Molecule a Molecules that are not superposable on their mirror images are said to be chiral Describes an object of a whole not a certain of an atom i An achiral object lacks chirality no duh that is it s an object with a superposable mirror image Almost all achiral objects possess at least one plane or center of symmetry 1 A plane of symmetry is an imaginary plane passing through an object dividing it such that one half is the re ection of the other half 2 A center of symmetry is a point so situated that identical components of the object are located on opposite sides and equidistant from the point along any axis passing through that point ll Stereoisomerism a lsomers are molecules that have the same molecular formula but possess different structures because of different connectivity of atoms andor spatial arrangement of atoms b Constitutional isomers molecules that have the same molecular formula but have a different connectivity of atoms Dr Silas Blackstock CH 231Elementary Organic Chemistry 1 Exam 1 Study Outline c Stereoisomers isomers that have the same connectivity of atoms but have a different spatial arrangement of their atoms i Conformation isomers are stereoisomers that interconvert by rotation along single bonds ii Con gurational isomers stereoisomers that do not interconvert at room temperature by virtue of having to break either single or double bonds to interconvert iii Enantiomers stereoisomers that are nonsuperposable mirror images iv Diastereomers stereoisomers that are not mirror images v Atropisomers enantiomers or diastereomers that differ because of hindered rotation along a single bond d A stereocenter is an atom about which exchange of two groups produces a stereoisomer e A chiral center is a tetrahedral atom with four different groups bonded to it lll Naming Chiral Centers The R S System a The absolute con guration at any chiral center can be speci ed by the R S i To apply this convention 1 Each atom or group of atoms bonded to the chiral center is assigned a priority and is numbered from highest priority to lowest priority 2 The molecule is oriented in space so that the group of lowest priority is directed away from the observer 3 The remaining three groups are read in order from highest priority to lowest priority If the order of groups is clockwise the con guration is R Latin rectus right correct If the order is counterclockwise the con guration is 5 Latin sinister left ii To invert the con guration at a chiral center R to Sor vice versa switch the location of any two groups bonded to the chiral center IV Acyclic Molecules with Two or More Stereocenters a For a molecule with n chiral centers the maximum number of stereoisomers possible is 2n i Certain molecules have special symmetry properties that reduce the number of stereoisomers to fewer than that predicted by the 2n rule Dr Silas Blackstock CH 231Elementary Organic Chemistry 1 Exam 1 Study Outline ii Two stereoisomers are enantiomers if all of the chiral centers have the opposite con guration R versus 5 or vice versa iii If even one chiral center is the same between two stereoisomers they are diastereomers iv A meso compound contains two or more chiral centers bonded in such away that it is achiral Meso compounds usually have an internal plane of symmetry b Fischer projections are 2D projections of chiral molecules that are useful when evaluating stereochemistry i In Fischer projections groups on the right and the left are by convention in font ii Groups on the top and bottom are by convention behind c The type of stereoisomer relationship dictates the relationships of properties between stereoisomers i Enantiomers have identical physical and chemical properties in achiral environments but different properties in chiral environments such as in the presence of planepolarized light They also have different properties in the presence of chiral reagents and enzymes as chiral catalysts ii Diastereomers have different physical and chemical properties even in achiral environments V Cyclic Molecules with Two or More Chiral Centers a When evaluating symmetry in ring structures with chiral centers such as cyclohexane derivatives it is helpful to evaluate the planar representations Vl Tying All the Terminology Together a There are two overarching classes of isomers constitutional isomers and stereoisomers i Stereoisomerism can arise either from con gurational or conformational isomerism 1 Con gurational isomers are either diastereomers or enantiomers if the compound is chiral 2 Conformational isomers are the same way 3 Enantiomers or diastereomers that do not readily interconvert by bond rotations are atropisomers Vll Optical Activity How Chirality is Detected in the Laboratory a Planepolarized light is used to evaluate stereoisomers i Light that oscillates in parallel planes is said to be planepolarized ii Planepolarized light contains equal components of left and right circularly polarized light Dr Silas Blackstock CH 231Elementary Organic Chemistry 1 Exam 1 Study Outline b A compound is said to be optically active if it rotates the plane of the planepolarized light vi V Observed rotation is the number of degrees the plane of the planepolarized light is rotated Speci c rotation is the observed rotation measured in a cell 1 dm long and at a sample concentration of 1 M 1 A compound is dextrorotary if the analyzing lter must be turned clockwise to restore the zero point 2 A compound is levorotatory if the analyzing lter must be turned counterclockwise to restore the zero point Each member of a pair of enantiomers rotates the plane of the planepolarized light an equal number of degrees but opposite in direction A racemic mixture is a mature of equal amounts of two enantiomers and has a speci c rotation of zero Percent optical purity identical to enantiomeric excess is de ned as the speci c rotation of a mixture of enantiomers divided by the speci c rotation of the pure enantiomer times 100 Meso compounds are NOT chiral so they are optically inac ve The Signi cance of Chirality in the Biological World a Biological molecules are chiral and are usually found as single enantiomers in living systems Enzymes are made of chiral amino acids and they catalyze biological reactions in a chiral way by rst positioning the molecules or molecules at binding sites and holding them there by a combination of Hbonds electrostatic attractions dispersion forces and sometimes covalent bonds An enzyme with speci c binding sites for three of the four groups on a chiral center can distinguish between a molecule and its enantiomer Almost all enzymecatalyzed reactions are stereoselective IX Separation of Enantiomers Resolution a Resolution is the experimental process of separating a mixture of enantiomers into the two pure enantiomers A common chemical means of doing this is to treat the racemic mixture with achiral resolving agent that converts the mixture of enantiomers into a pair of diastereomers The diastereomers are separated based on differences in their physical properties each Dr Silas Blackstock CH 231Elementary Organic Chemistry 1 Exam 1 Study Outline diastereomers is then converted to a pure stereoisomer uncontaminated by its enantiomers Enzymes are also used as resolving agents because of their ability to catalyze a reaction of one enantiomer but not that of its mirror image Chromatography on a chiral substrate is also and effective separation method
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