Exam 1 Study Guide Part 1
Exam 1 Study Guide Part 1 CHEM231
Popular in Organic Chemistry I
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This 9 page Study Guide was uploaded by Udoka Oji on Sunday June 5, 2016. The Study Guide belongs to CHEM231 at Brown University taught by Dr. Bonnie Dixon in Summer 2016. Since its upload, it has received 86 views. For similar materials see Organic Chemistry I in Chemistry at Brown University.
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Date Created: 06/05/16
General Chemistry Review, Structure Sunday, June 5, 2016 6:35 PM Key Ideas: • Lewis Dot Structures ○ See class notes for how to steps for drawing • Bond-line drawings provide a simplified representationfor analyzing complexorganic molecules and the reactions they undergo ○ Each apex of the bond line structure represents a carbon atom Don't forget: the ends of the lines represent carbon atoms as well ○ Assume the octetaround carbon atoms is filled by hydrogen atoms • Resonance Useful Tips from Organic Chemistryas a Second Language: • Bond-line drawings ○ Double bonds are shown with two lines, triple bonds are show with three lines Draw triple bonds in a straight line rather than a zig-zag, because these are typically rigid bonds When drawing double bonds try to draw the other bonds as far away from the double bond as possible ○ When drawing zigzags it does not matter in which direction you start drawing However,once again, try to draw the bonds as far apart as possible Screen clipping taken: 6/5/2016 6:55 PM ○ Hydrogen atomsconnected to atoms other than carbon must be drawn ○ All second-rowelements must have full octets ○ Either show all of the C's and the H's connected to them, or do not show either at all • Formal charge is a charge associated with an atom that does not exhibit the expected number of valence electrons ○ Why is it important? Understanding structure stability Understanding charged molecules Understanding bonds: If carbon has a formal charge, don't assume that it has a formal charge don't assume that it has four bonds ○ Conventionis to always show formal charges and to leave out the lone pairs • Resonance is a method of drawing multiple structures for a single molecule because electrons movearound resulting in different configurations for one molecule ○ Concept: no single drawing adequately describes the nature of a the electron density spread out over the molecule out over the molecule ○ However,at the same time, you cannot simply say the molecule is flipping back and forth between different resonance structures ○ Rules Never break a single bond Do not exceed an octet for a second row element □ More specifically do not move electrons to atoms with sp3 hydridization □ But could have fewer ○ 3 Rules of significant resonance structures Minimize charges Electronegativeatoms can have positivecharges if they have an octet Avoid drawing a resonance structure in which two carbon atoms bear opposite charges Resources • Dr. Dixon's Resonance Hand Out • Khan AcademyVideos ○ Bond-Line Structures (https://www.khanacademy.org/science/organic-chemistry/gen- chem-review/bond-line-structures/v/condensed-structures) ○ Resonance Structures (https://www.khanacademy.org/science/organic-chemistry/organic- structures/formal-charge-resonance/v/resonance-intro-jay) Acid-Base Chemistry Sunday, June 5, 2016 6:35 PM Key ideas: Acid Base Arrhenius Produce hydrogen ions Produce hydroxide ions • Bronsted - Lowry Proton Donor Proton Acceptor Lewis Electron pair acceptor Electron pair donor (Electrophile) (Nucleophile) • Amphoteric:A substance that can either serve as an acid or a base ○ Example: water • Equilibrium position is determined by stability of products vs stability of reactants Useful Tips from Organic Chemistryas a Second Language • Factors determining strength (in order of importance) ○ Electronegativityand Size Same Row Example: Negative charge on Oxygen is morestable than a negative charge on Carbon, thus acid with Oxygen is stronger Same Column Example: Negative charge on large Iodine is more stable than a negative charge on smaller Fluorine, thus acid with Iodine is stronger ○ Resonance Delocalized charge is more stable than localize charge, thus acids producing resonance structures are stronger ○ Induction Acids with electronegativeatoms on one end are stronger than acids with Carbon atoms on one end because resulting atom can more easily spread the negative charge ○ Orbitals Negative charge on an sp hybridized Carbon is more stable than a negative charge on 3 2 an sp or sp hybridized Carbon because it is smaller and closer to nucleus, thus can more tightly hold negative charge • Predicting the Positionof Equilibrium ○ If two moleculesare competing for the proton, the direction of equilibrium will be a depicted by a larger arrow pointing in the direction of the competing moleculethat is more stable without the proton When drawing these arrows take into consideration the fact that the top arrow represents the forward reaction and the bottom arrow represents the reverse reaction Resources • Khan AcademyVideos ○ Organic Acid-Base Chemistry (https://www.khanacademy.org/science/organic- chemistry/organic-structures/acid-base-review/v/acid-base-lewis-bronsted-lowry) • My Quizlet Flashcards ○ pKa values (https://quizlet.com/_2cqf8w) Thermodynamics; Reaction Coordinate Diagrams; Radical Reactions Thursday, June 2, 2016 8:35 PM Key Ideas: • Rate determining step has the largest ∆G (a ) (Transition state) and is the slowest step • With halogenation, Bromination is the mostsynthetically useful form of halogenation ○ Overall Exothermic ○ Largest ∆G, thus most selective • Hammond's Postulate(Reactivity - SelectivityPostulate): The slower reaction is the most selective in it's products • See class notes for how to draw Reaction Coordinate Diagrams Related Textbook Readings: • To develop an understanding of reaction mechanisms,chemists propose several sets of steps, or mechanisms,and test each proposal by carrying out experimentsthat provide experimental observationsthat will allow them to exclude mechanisms that are not consistent with facts • Reasons for doing this ○ Mechanisms provide a frameworkwithin which to organize a great deal of descriptive chemistry ○ The intellectual satisfaction derived from constructing models that accurately reflect the behavior of chemical systems ○ A mechanismis a tool to be used in the search for new informationand new understanding IR, Functional Groups, Nomenclature Sunday, June 5, 2016 6:35 PM Key Ideas: • Constitutional isomer: same formula, different backbones • Absorbance ~ % Transmittance x 100% A = ɛbc A α c ○ If the frequency of light matches the frequency of motion of the molecules,some of the light will be absorbed • In % Transmittance vs. Wavenumbergraphs, dips indicate absorption ○ Particular Functional Groups show particular characteristicsin these graphs See class notes • Degrees of Saturation are measured with the Index of Hydrogen Deficiency(IHD) C: Carbon H: Hydrogen X: Halogen N: Nitrogen • The parent chain of a moleculehas: ○ Longest C-C ○ Chain reunited ○ End-to-end connection Prefix Infex Suffix Useful Tips from Organic Chemistryas a Second Language: • Five parts to every name: ○ Stereoismerism:indicates whether double bonds are cistrans, and indicates stereocenters (R, S), which we will cover in the chapter on configuration ○ Substituents are groups coming off the main chain ○ Parent is the main chain ○ Unsaturation identifies if there are any double or triple bonds ○ Functional group: this is the group after which the compound is named • Functional group refers to specific arrangements of atoms that have certain characteristicsfor reactivity ○ When naming compoundswith two functional groups, one will go in the suffix and the other will be a prefix in the substituent part The listing above shows the hierarchy of how you choose as the suffix • Unsaturation (presence of double or triple bond) is indicated with an "en" for a double bond, and "yn" for a triple bond ○ For multiple double and triple bonds, use: ○ Double bonds always get listed before triple bonds • The Parent chain will be named by the number of carbons in it Screen clipping taken: 6/5/2016 11:04 PM ○ If carbon atom is a ring, we add the term cyclo Example: ring of six carbons is called cyclohex- ○ Parent chain should be as long as possible and include the following groups (in order of importance) Functional group (Highest ranking) Double bond Triple bond ○ Thus if there are no functional groups, no double bonds, and no triple bonds, then we simply choosethe longest chain possible simply choosethe longest chain possible • Everything else connected to the parent chain is called a substituent ○ Alkyl substituents follow same terminologyused for parent chains, but add "yl" to the end ○ Branched substituents have their own prefixes Important to be familiar with tert-butyl (4 carbons) □ Three methyl groups connected to one carbon which itself is attached to the parent chain ○ Functional groups that are called substituents OH - hydroxy NH - amino 2 Ketone - keto Aldehyde - aldo ○ Halogens as substituents F - fluoro Cl - chloro Br - Bromo I - iodo ○ Multiple substituents use the same prefixes used to classify the number of double and triple bonds • Stereoisomerismis the first part of every name ○ Identifies the configuration of any double bond or sterocenters ○ Two ways to arrange double bond atomsin space: cis and trans Can only be used if we have two identical groups ○ When all four groups around double bond are different, have to use different method for naming double bonds To be seen later in stereochemistry Superior method • To number the parent chain properly, we begin with the same hierarchy that we used when choosing the parent in the first place: ○ If there is a functional group, number the chain so the functional group gets the lowest possible number ○ If there is no functional group, number the chain so the double bond gets the lowest possible number ○ If there is no double bond or functional group, number the chain so that the triple bond gets the lowest possible number ○ If there is no functional group, double bond, or triple bond, number so that the substituent gets the lowest possible number If there is more than one substituent, number so the substituents get the lowest average number • Once we know the numbers we typically place them in front of each part of the moleculename to indicate which carbon each part is attached to ○ Unless the part appears on the number 1, then no need to place the number ○ For functional groups, it is ok to place the number at the beginning of the parent 2-hexanol is the same as hexan-2-ol ○ For unsaturation, use the lower number in the bond Double bond between C2 and C3, use C2 ○ If there are multiple substituents, then every substituent must be number Example, 2,3-dicholohexane Example, 2,3-dicholohexane ○ If there are multiple substituents of different types, alphabetize them ○ Two numbers are always separated by commas • If a photon of light possesses exactly the amount of energy between energy levels determined by the nature of a bond, the bond can absorb the photon to promote a vibrational excitation • Bonds can stretch or bend to store vibrational energy • Different types of bonds will absorb characteristicfrequencies allowing us to determine which types of bonds are present in a compound • The frequency related unit is called the wavenumber Screen clipping taken: 6/5/2016 11:39 PM -1 -1 ○ Range from 400 cm to 4000cm ○ Higher wavenumbermeans higher energy • Signals in an IR spectrum have the following three characteristics ○ Wavenumber ○ Intensity (Strong vs. weak) ○ Shape (Broad vs. narrow) • Wavenumberis dependent on two factors ○ Bond strength: increases with bond strength ○ Atomic mass: Decreaseswith atomic mass Screen clipping taken: 6/5/2016 11:43 PM • Two regions of IR spectra ○ Diagnostic region: fewer peaks and provides the most information ○ Fingerprint region: Mostly bending vibrations, as well as stretching of single bonds. More difficult to analyze Each compound has a unique pattern of signals in this region • Intensity of signal depends on the dipole momenton the bond ○ Larger dipole momentshave larger signals • Carbonyl groups often produce the strongest signals, while C=C often produce fairly weak signals • Intensity of signal derives from number of bonds contributing to the signal as well • Shape of signal depends on the uniformity in strength of bonds contributing to the signal ○ Characteristic bonds with a large distribution of bond strength will have a broad signal • Symmetricstretching and asymmetricstretching also contribute signal shape Resources: • Dr. Dixon's NomenclatureBasics Handout • Dr. Dixon's Functional Groups Handout • Khan AcademyVideos: ○ Naming Alkanes (https://www.khanacademy.org/science/organic-chemistry/bond-line- structures-alkanes-cycloalkanes/naming-alkanes/v/representing-structures-of-organic- molecules) ○ Naming Alkanes, Cycloalkanes,and bicyclic compounds (https://www.khanacademy.org/science/organic-chemistry/bond-line-structures-alkanes- cycloalkanes/naming-alkanes-cycloalkanes-bicyclic/v/alkane-and-cycloalkane-nomenclature- i) • My Quizlet Cards ○ Carbon Parent Chain Names
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