Exam 2 Study Guide
Exam 2 Study Guide CHEM231
Popular in Organic Chemistry I
Popular in Chemistry
This 13 page Study Guide was uploaded by Udoka Oji on Saturday June 18, 2016. The Study Guide belongs to CHEM231 at Brown University taught by Dr. Bonnie Dixon in Summer 2016. Since its upload, it has received 92 views. For similar materials see Organic Chemistry I in Chemistry at Brown University.
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Date Created: 06/18/16
NMR Monday, June 13, 2016 10:56 AM Key Ideas: • There are two types of NMR graphs ○ 1C NMR ○ H NMR • Three characteristicsof NMRs ○ Shifts ○ Integration ○ Multiplicity • Memorizeor get a general idea of where certain types of bonds are found Useful Tips from Organic Chemistryas a Second Language Overview • When the magnetic momentis aligned with the field it is called the α spin state or against the field it is called the β • There is a difference energy between these two spin states ○ Absorption of light energy by a proton in the α spin state can occur if the energy is equivalent to this energy difference • When proton is spin flipped and in β state, it is said to be in resonance with the external magnetic field • NMR typically requires strong magnetic fields and radio wave frequency electromagneticradiation • All nuclei do not absorb the same frequency of radio wave radiation because electrons density surrounding protons shields them from external magnetic fields with their own local magnetic fields ○ As a result, protons in different electronic environmentswill absorb different frequencies of rf radiation • Proton NMR spectrum signals have three important characteristics ○ Location:indicates the electronic environmentof the protons giving rise to the signal ○ Area under each signal: indicates the number of protons giving rise to the signal ○ Shape: indicates the number of neighboring protons • Number of signals in a proton NMR spectrum indicates the number of different kinds of protons (protons in different electronic environments) ○ Protonsthat occupy identical electronic environmentsare called chemically equivalent, and they will produce only one signal Chemical equivalence is defined by interchangeability by a symmetryoperation Chemical Shifts • Locationcan be called the chemical shift of a signal • NMR spectrum scales are defined relative to the frequency of absorption of a reference compound, tetramethylsilane(TMS) (set at 0 ppm) ○ Rare case that you get signals below 0 ppm • Left side of an NMR is called downfield (typically where signals from deshielded protons lie), and right side of an NMR is called upfield (typically where signals from shielded protons lie) • The inductive effect that deshields protons depends on the electronegativityand the multiplicity of the electronegativeelement As always, inductive effect tapers off with distance ○ As always, inductive effect tapers off with distance • Some types of C-H bonds have benchmark values that help with predicting the chemical shift of protons (remember: can easily be changed) • Typically due to tapered effect, the effect of electron withdrawing groups on beta protons (hydrogen atomssecond closest to them) is about one-fifth of the effect on alpha protons (hydrogen atomsclosest to them) • The presence of nearby π electrons, especially in aromaticmolecules, causes a downfield chemical shift ○ The circular motion of π electrons causes an induced, local magnetic field that adds to the external magnetic field rather than subtracts causing a larger B eff ○ Aromatics generally produce signals in the neighborhood of 7 ppm (sometimesas high as 8, or as low as 6.5) • Different general shifts associatedwith protons near certain functional groups Integration • Integration involvesthe area under each signal, which indicates the number of protons giving rise to a signal • Computers will typically display the integration values underneath each signal ○ Must divide by the smallest of these values to understand the meaning of them ○ If there are decimals numbers that arise from this, multiply each number by 2 and round for a whole number ratio • Integration values can also be represented as step curves ○ The height of each step curve represents the area under the signal • Rememberdigital analysis only gives relative ratios ○ Sometimesa 2:3 ratio, can indicate 4 protons and 6 protons Multiplicity • Multiplicity involves the number of peaks in the signal Screen clipping taken: 6/13/2016 8:40 PM • Multiplicity is the result of the magnetic effects of neighboring protons ○ Net effect can be summarized with the n+1 rule: if n is the number of neighboring protons, then the multiplicity will be n+ 1 A proton with three neighbors will be split into a quartet (3+1 = 4) • Nearby protons do not always split each other ○ Two major factors that determine whether or not splitting occurs: Equivalent protons do not split each other The separation of protons by two or three sigma bonds allows for splitting □ Howeverwhen there is a separation by more than three sigma bonds, splitting is not observed Only observed in rigid moleculeslike bicyclic compounds • When signal splitting occurs, the distance between individual peaks of a signal is called the coupling constant, or J value, and is measured in Hz • The coupling constant can range anywhere from 0 to 20 Hz, depending on the type of protons • Certain patterns for commonlyseen molecules Complex Splitting • If a proton has two different kinds of neighboring protons, sometimesit will experience complex splitting ○ Determinationof this type of splitting is dependent on the coupling constants between these neighbors and the proton • If coupling constants are similar, just add all the neighbors together as if it was on large group • No splitting is observed across the oxygen of an alcohol, generally ○ Proton exchange is a rapid process that is catalyzed by trace amounts of acid or base Hydroxyl protons are labile, because of the rapid rate at which they are exchanged ○ Proton exchange happens at a faster rate than the timescaleof an NMR spectrometer, which causes a blurring that averages out any splitting ○ Removetraces of acid or base and you will observe a triplet across the hydroxyl • Aldehydic protons will often couple weakly with their neighbors ○ Depending on size of J value, splitting may not be noticeable If the J value is too small, then the signal near 10 ppm will appear to be a single • To analyze an NMR spectrum ○ Always begin by inspecting the molecular formula and calculating the IHD ○ Consider the number of signals and integration of each signal ○ Analyze each signal for chemical shift, integration and multiplicity, then draw fragments consistent with each signal ○ Assemble the fragments into a molecular structure C NMR 13 • In C NMR spectroscopy,only one in every hundred carbon atoms will resonate which demands a sensitive receiver coil for C NMR ○ Because C makes up 1.1% of the C isotopes 13 • In C NMR only chemical shift is important • All signals are recorded as singlets, because of the broadband decoupling technique that suppresses all C- H splitting ○ Otherwise there would be a lot of splitting and unreadable spectrum • Chemical shift values range from 0 to 220 ppm • Number of signals represent the number of carbon atoms in different electronic environments Resources • Khan AcademyVideos ○ Proton NMR (https://www.khanacademy.org/science/organic-chemistry/spectroscopy- jay/proton-nmr/v/introduction-to-proton-nmr) • Dr. Dixon's NMR Scale Handout • MSU Pages (VERY IN DEPTH, POTENTIALLYCONFUSING) ○ Proton NMR (http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/nmr1.htm#nmr1 ) • UIUC Practice Problemsand Tutorial Site (http://www.chem.uiuc.edu/weborganic/organictutorials.htm) Mechanisms Wednesday, June 15, 2016 5:32 PM Key Ideas: • A nucleophile attacks an electrophileto initiate are reaction • Certain reactants and reaction conditions result in products with unique configurations Useful Tips from Organic Chemistryas a Second Language • Curved arrows in mechanisms refer to the actual movementof electrons ○ Tail shows where the electronsare coming from ○ Head shows where the electrons are going • Only one commandmentwhen drawing mechanisms:Never exceed an octet for second-row elements • The tail of an arrow can only come from a bond or a lone pair. The head can only be drawn to make a bond or lone pair. ○ Thus only possibilities are Lone pair to bond Bond to lone pair Bond to bond ○ Lone pair to lone pair does not exist because we cannot move lone pairs from one atom to another • Never draw arrows going in opposite directions in the same mechanism step ○ Electrons can only flow in one direction at a time • A nucleophile will always use a region of high electrondensity to attack the electrophile • Some molecules,like hydroxide can act as either a nucleophile or a base ○ The difference between the two is a difference of function • Nucleophilicity measures how quickly a reagent will attack another compound, kinetics • Basicity measures base strength by the position of equilibrium, thermodynamics • Regiochemistryrefers to where the reaction take place ○ Markovnikov:Addition is done at the more substituted carbon ○ Anti-Markovnikov:Addition is done at the less substituted carbon ○ The more "substituted" carbon is connected to more carbon atoms ○ "The regiochemistrywill be determined by the preference for the reaction to proceed via the more stable carbocation intermediate" • Double bonds usually form, and can have 1 -4 groups attached to them Screen clipping taken: 6/15/2016 7:07 PM • Anytime we have two of the same group across a double bond, we do not have to worry about the • Anytime we have two of the same group across a double bond, we do not have to worry about the regiochemistry • If the two groups are added across the double bond through a mechanism that allows syn addition, you will get a set of enantiomersthat places the groups on the same side • If the two groups are added across the double bond through a mechanism that allows anti addition, you will get a set of enantiomersthat places the groups on opposite sides • if the reaction is not stereoselective,then you will get both sets of enantiomers Resources • MSU pages (VERY IN DEPTH,POTENTIALLYCONFUSING) ○ Reaction Mechanisms (http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/react1.htm#rx4) Curved Arrow Practice Problems (http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Questions/MOLEDITOR/ mechprb2.htm) Alkenes Saturday, June 18, 201611:05 AM Key Ideas: • There are three things to consider in Alkene reactions ○ The conditions of the reaction (what is being added) ○ Regiochemistry ○ Stereochemistry • Memorize Reaction Conditions and Products Useful Tips from Organic Chemistryas a Second Language • When adding two of the same group, regiochemistrybecomes irrelevant • When adding two different groups across a symmetricalalkene, regiochemistryalso becomes irrelevant • When no stereocentersare formed, stereochemistryis irrelevant • When there is only one stereocenter,stereochemistryis still irrelevant, because there are only two possible products, rather than four • Be careful to recognize meso compounds • There are particular additions to alkenes that have aspects to take note of: ○ Hydrogenation Variety of metal catalyst can be used, Pt, Pd or Ni Syn reaction ○ Hydrohalogenation Some reactions require a rearrangement to make this true □ This is done through a hydride shift or a methyl shift ○ Hydrohalogenationin the presence of peroxide Anti-Markovnikovbecause of movementof entire hydrogen atom rather than proton Use fishhook arrows to indicate the movementof only one electron Tertiary radicals are morestable than secondary radicals ○ Hydration Requires presence of acid catalyst ○ Hydroboration-oxidation THF works to stabilize the BH 3 ○ Halogenation ○ Halohydrin formation ○ Hydroxylation(anti) MCPBA stands for meta-choloroperbenzoicacid Just think of it as a peroxy acid adding OH to an alkene ○ Hydroxylation(syn) This reaction can also occur with KMnO an4 NaOH • Most syn addition reactions occur as a result of intermediate groups being added in a concerted process, as in added simultaneously,effectivelyadding the desired groups to the same side of the molecule • Cleavage reactions of an alkene involve the splitting of the alkene, across its double bond, into two molecules Oxidative cleavage involves O 3nd DMS (dimethyl sulfide) ○ Oxidative cleavage involves O3and DMS (dimethyl sulfide) Resources • Khan AcademyVideos ○ Alkene Nomenclature(https://www.khanacademy.org/science/organic-chemistry/alkenes- alkynes/naming-alkenes-jay/v/alkene-intro-and-stability) ○ Alkene Reactions (https://www.khanacademy.org/science/organic-chemistry/alkenes- alkynes/alkene-reactions-tutorial/v/hydrogenation) • MSU Pages (VERY IN DEPTH, POTENTIALLYCONFUSING) ○ Addition Reactions of Alkenes (http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/addene2.htm#add3c) • Ohio State Reaction Flash cards ○ Alkenes Deck (https://legacyweb.chemistry.ohio-state.edu/flashcards/) • UIUC Practice Problemsand Tutorial Site (http://www.chem.uiuc.edu/weborganic/organictutorials.htm) Alkynes Saturday, June 18, 201611:05 AM Resources • Khan AcademyVideos ○ Alkyne Nomenclature(https://www.khanacademy.org/science/organic-chemistry/alkenes- alkynes/naming-preparation-alkynes/v/alkyne-nomenclature) ○ Alkyne Reactions (https://www.khanacademy.org/science/organic-chemistry/alkenes- alkynes/alkyne-reactions/v/reduction-of-alkynes) • MSU Pages (VERY IN DEPTH, POTENTIALLYCONFUSING) ○ Addition Reactions of Alkynes (http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/addyne1.htm#add1) • Ohio State Reaction Flashcards ○ Alkynes Deck (https://legacyweb.chemistry.ohio-state.edu/flashcards/) • UIUC Practice Problemsand Tutorial Site (http://www.chem.uiuc.edu/weborganic/organictutorials.htm)
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