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166 Cards in this Set
- Front
- Back
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what is the energy source in NMR? |
radio waves
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what are the requirements for a nucleus to give rise to NMR?
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a nucleus MUST have either an odd atomic number or an odd mass number
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if a nucleus aligns in the same direction as the external magnetic field is a higher or lower energy state assumed?
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lower
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if a nucleus aligns in the opposite direction as the external magnetic field is a higher or lower energy state assumed?
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higher
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when is a nucleus in resonance?
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when it absorbs RF radiation and "spin flips" to a higher energy state
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what two variables characterize NMR?
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-an applied magnetic feild (B sub 0)
-the frequency (v) of radiation used/needed for resonance |
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what relationship do the frequency needed for resonance and the applied magnetic field have?
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proportional
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the _______ the magnetic field, the ________ the energy difference between the two nuclear spin states, and the _______ the v needed for resonance
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the stronger the magnetic field, the larger the energy difference between the two nuclear spin states, and the higher the v (frequency) needed for resonance
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do all protons absorb at the same frequency?
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no, protons in different environments absorb at slightly different frequencies, making them distinguishable my NMR
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what determines the frequency at which a particular proton absorbs?
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the proton's electronic environment
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what does an NMR plot?
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intensity of a peak against chemical shift
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in what direction is increasing chemical shift plotted?
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right to left
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what direction do 'upfield' and 'down-field' refer to respectively?
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upfield = right
down-field = left |
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the number of NMR signals equals what?
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the number of different types of protons in a compound.
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protons in different environments give the same or different NMR signals?
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different (different environment, different signal)
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do equivalent protons give same or different NMR signals?
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same
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what's the most important thing to remember when determining number of signals for alkenes or cyloalkanes?
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they have restricted bond rotation!
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for two protons to be equivalent in a double bond or ring what must be true?
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they must both be cis (or trans) to the SAME groups (pg.498)
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where a particular proton absorbs depends on what?
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it's electronic environment
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the small magnetic field created by electrons around any given nucleus goes in the same or opposite direction of the applied magnetic field?
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opposite
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the inherent small magnetic field that opposes the applied magnetic field is said to do what?
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"shield" the nucleus from the applied field (B sub o)
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what direction does shielding shift an absorption?
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upfield (to the right)
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does a less shielded nucleus "feel" more or less of the applied magnetic field?
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it feels more of the applied magnetic field...and is said to be "deshielded"
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does a deshielded nucleus feel more or less of the applied magnetic field?
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more
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what is the result of a deshielded nucleus experiencing a higher magnetic field?
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it needs a higher frequency to achieve resonance.
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in which direction is higher frequency?
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to the left, toward higher chemical shift (they're proportional-remember?)
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what direction does deshielding shift an absorption?
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downfield ( to the left= to higher chemical shift= to higher frequency)
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in what direction do protons that are near electronegative elements absorb?
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downfield..because they are deshielded!
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decreased electron density _____ a nucleus and an absorption moves _______.
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decreased electron density deshields a nuclus and an absorption moves downfield.
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does the chemical shift value of C-H decrease or increase with increasing alkyl substitution?
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increase
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what's TMS used for?
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TMS is a denatured solvent used as an internal standard in NMR
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what is the area under an NMR signal proportional to?
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the number of absorbing protons
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how do you calculate the number of protons giving rise to each signal?
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1)add up total number of integration units and divide by the number of protons in the formula
2) individually divide each integration number by the number achieved by sum divided by protons in step 1 (pg. 506) |
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what order do sp3, sp2 and sp appear in an NMR?
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sp3 is furthest right (upfield) then sp in the middle then sp2 is furthest downfield (left)
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if, on a H-NMR spectrum there is are nine SP3 furthest upfield, what is understood?
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it represents a tert-butyl group!
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does chemical shift increase/decrease with increasing chemical shift?
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increase
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when does spin-spin splitting occur?
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only between nonequivalent protons on the same or adjacent carbons.
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one adjacent H splits an NMR signal into what?
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a doublet (n+1)
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two adjacent H's splits an NMR signal into what?
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a triplet (n+1)
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when two protons split each others NMR signal, they are said to be what?
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coupled
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do equivalent protons split each other?
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noo
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splitting is not generally observed between protons separated by more than ____ sigma bonds
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3
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what are the two steps in predicting splitting?
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1) determine if two protons are equivalent or different
2) determine if two nonequivalent protons are close enough to split each others signals |
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do OH or NH split NMR signals of adjacent protons?
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no
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are OH or NH protons split by other adjacent protons?
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no
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how does the NMR spec. handle cyclohexane chair formations?
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the chair flip happens so rapidly that the NMR takes an average formation and uses that in it's spectrum
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how many different types of protons does a monosubstituted benzene ring have?
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3
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what is the scale of C-NMR?
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200-0
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what is the approximate scale or H-NMR?
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about 10-0
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what is true of every peak in a C-NMR spec?
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all signals are singlets because no splitting occurs due to the low abundance of carbon 13
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in contrast to H-NMR, what is true about the peak intensity of C-NMR?
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in C-NMR peak intensity is not proportional to the number of absorbing carbons
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are C-NMR signals integrated?
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NO, only H-NMR signals are
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what occurs during oxidation??
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increase in number of C-O bonds
decrease in number of C-H bonds |
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what occurs during reduction??
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increase in number of C-H bonds
decrease in number of C-O bonds |
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what's the most reduced form of carbon?
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CH4
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what's the most oxidized form of carbon?
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O=C=O
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what are the three main types of reducing agents?
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1) molecular H2
2) Na in NH3 3) metal hydride (NaBH4 or LiAlH4) |
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what does reduction of an alkene form?
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an alkane by the addition of H2
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the addition of H2 only occurs in the presence of a metal catalyst and thus the reaction is called what?
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catalytic hydrogenation
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in what fashion does H2 add?
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syn!
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what does oxidation result in?
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increse in C-O bonds
decrease in C-H bonds |
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what does reduction result in?
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increase in C-H bonds
decrease in C-O bonds |
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the conversion of an alkyne to an alkene and an alkene to an alkane are examples of what?
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reduction! because each process adds two new C-H bonds to the starting material.
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what is the simplest reducing agent?
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molecular H2 ... carried out in the presence of a metal catalyst
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what occurs during hydrogenation?
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the weak pi bond in broken and h2 is added (pg. 429)
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when type of hyrdrogenation is said to be carried out when the reaction occurs in the presence of a metal catalyst?
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catalytic hydrogenation
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what metal catalyst are usually used in catalytic hydrogenation?
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Pd, Pt, Ni
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in catalytic hydrogenation in what fashion do the 2 H's add?
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syn addition
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by what process are alkenes reduced to alkanes?
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catalytic hydrogenation
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what is catalytic hydrogenation?
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the additions of hydrogens to double or triple bonds in the presence of a metal catalyst. (Pd, Pt, Ni) in powder form= greater surface area
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is catalytic hydrogenation syn or anti addition?
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syn
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is hydrogenation exothermic or endothermic?
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exothermic
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what is delta H for hydrogenation called?
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heat of hydrogenation
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what can delta H (heat of hydrogenation) be used to measure?
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stability of different alkenes when they have the same hydrogenated products
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are cis or trans alkenes more stable after undergoing catalytic hydrogenation?
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trans.. because the heat of hydrogenation is less* meaning less heat is released, making the alkene more stable.
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when cis and trans alkenes both undergo catalytic hydrogenation under the same conditions, which will be more stable?
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trans, even though the products are the same!
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cis and trans alkenes both undergo catalytic hydrogenation under the same conditions.... are the the products the same or different?
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the same
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more stable alkene has a large or small delta H (heat of hydrogenation) ??
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more stable alkene, small delta H
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does a less stable alkene have a smaller or later delta H?
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larger delta h, less stable
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a more stable alkene has the ______ heat of hydrogenation.
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smaller
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the less stable alkene has the _____ heat of hydrogenation.
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larger
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when can you use heat of hydrogenation data to evaluate relative stability of starting materials?
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only when the hydrogenation products are the SAME
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the mechanism by which hydrogenation occurs implies what two facts?
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1) rapid, sequential addition of H2 occurs from the side of the alkene complexed to the metal surface
2) less crowded double bonds complex more readily to the catalyst surface, resulting in faster rxn |
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do less crowded or more crowded alkenes have a faster rate of hydrogenation ?
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less crowded! the fewer R groups, the faster the rxn. the more R groups, the slower the reaction
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what does the degrees of unsaturation tell you about a molecule?
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the total number of rings and bonds in the molecule
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what are the steps and the formula for calculating degrees of unsaturation?
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-CnH2n+2 tells maximum number of hydrogens
- you then substract actual from the maximum - then divide that by 2 |
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the number of degrees of unsaturation that remain in the product AFTER H2 addition equals what?
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the number of rings in the starting material
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the number of degrees of unsaturation that react with H2 equals what?
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the number of pi bonds
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what other double bonded functional group reacts with h2 and a metal catalyst??
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compounds with carbonyl groups
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what are aldehydes reduced to?
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primary alcohols
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what are ketones reduced to?
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secondary alcohols
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what occurs during reduction of an alkyne?
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h2 is added to one or both of the pi bonds
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what is formed when 2 eq. of h2 are reacted with an alkyne ?
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an alkane
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what is formed when 1 eq. of h2 is reacted with an alkyne? (and adds in a syn fashion)
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a cis alkene
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what is formed when 1 eq. of h2 is reacted with an alkyne? (and adds in a anti fashion)
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a trans alkene
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when an alkyne is treated with 2 eq. of h2 and a Pd catalyst, what 2 things happen?
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reduction of both pi bonds occurs 1) syn addition of one equivalent of h2 forms a cis alkene
2) a second eq. of h2 is then added to form and alkane |
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when an alkyne is treated with 2 eq. of h2 and a Pd catalyst how many new c-h bonds are formed?
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4
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are internal alkynes more or less stable than terminal alkynes?
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more stable
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when and why is a lindlar catalyst used?
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the lindlar catalyst is used when reducing an alkyne to CIS alkene because regular pd is to active to allow the reaction to stop after just 1 eq of h2 --> it will go all the way to an alkane if just used Pd
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what is a lindlar catalyst?
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Pd absorbed onto CaCO3 + lead acetate + quinoline
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does use of Lindlar catalyst result in cis or trans alkenes?
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cis!
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Pd will always reduce alkynes and alkenes to what?
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alkanes
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Lindlar catalyst will reduce what to what?
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alkynes to cis alkenes ONLY!
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what catalyst is used in the reduction of an alkyne to a trans alkene?
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Na + NH3
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what catalyst is used in the reduction of an alkyne to a cis alkene?
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Lindlar catalyst: Pd/CaCO3 + lead acetate + quinoline
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can catalytic hydrogenation be used to prepare trans alkenes from alkynes ?
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no
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can catalytic hydrogenation be used to prepare cis alkenes from alkynes?
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yes
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when reducing an alkyne to a trans alkene, how are the elements of H2 added?
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in an anti fashion, forming a trans-alkene
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what is the use of NH3 + Na in reduction of a alkyne to a trans alkene called?
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dissolving metal reduction AKA birch reduction
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alkyl halides and epoxides contain ______ that makes them react with strong nucleophiles and undergo reduction via _______ reagents.
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1) polar C-X bonds
2) metal hydride |
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when epoxides are reduced using metal hydride reagents, what is the product?
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alcohol (epoxide rings are opened to form alcohols
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when alkyl halides are reduced using metal hydride reagents what is the product?
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alkane - with X as the leaving group
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the reduction of the polar C-X bonds found in epoxides and alkyl halides is an example of what?
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nucleophilic substitution - SN2 (where H- is the strong nucleophile)
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because the reduction of epoxides and alkyl halides with metal hydride reagents is a SN2 reaction, what is true about the primary and unhindered alkyl halides?
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they react FASTEST! unhindered and primary alkyl halides react fastest via SN2 mechanism (are more easily reduced)
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in umsymmetrical epoxides, the nucleophillic attack of H- occurs at the more or less substituted carbon?
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less**
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in reduction of epoxides and alkyl halides, what is the metal hydride reagent most typically used?
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LiAlH4
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what is epoxidation?
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the addition of a single oxygen atom to an alkene to form an epoxide
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with what oxidizing agent is epoxidation usually carried out?
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peroxyacid ..but also mcpba
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what three oxidative reactions alkenes undergo?
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1)epoxidation
2)dihydroxylation 3)double bond cleavage (oxidative cleavage) |
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what oxidative reaction do alkynes undergo?
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1)triple bond cleavage (oxidative cleavage)
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a cis alkene gives an epoxide with ___ substituents. a trans alkene gives an epoxide with ____ substituents.
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cis alkene= cis epoxide
trans alkene= trans epoxide |
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are epoxidation reactions stereospecific?
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yes, because cis and trans alkenes yeild different stereoisomers as products. position of substituents is retained in epoxidation rxns
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cis-2-butene treated with mcpba = ? (epoxidation rxn)
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achiral meso compound with 2 stereogenic centers
(in which the oxygen can be added either to the top of the plane or below the plane) |
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trans-2-butene treated with mcpba =? (epoxidation)
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pair of enantiomers (racemic mixture )
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who is responsible for retrosynthetic analysis?
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EJ Corey
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what is retrosynthetic analysis?
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working backwards...
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what is dihydoxylation?
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the addition of two hydroxy groups to a double bond forming a 1,2-diol or gycol. (can be syn or anti addition depending on reagent used)
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when does syn-dihydroxylation occur?
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when an alkene is treated with either:
1.) KMnO4 + H20 + OH- 2.) [1]OsO4 + [2] NaHSO3 + H20 |
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what is NMO?
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N-methyl morpholine N-oxide: used to "dilute" OsO4 because OsO4 is expensive and toxic (used in syn-dihydroxylation reactions!)
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what is the disadvantage of using KMn04 in syn-dihydroxylation reactions?
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it's use is limited by its insolubility in organic solvents.
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what is the disadvantage of using OsO4 in sny-dihydroxylation reactions?
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it's toxic and expensive - so NMO is added to it
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what is the "cycle" that occurs with the use of OsO4 in syn-dihydroxylation?
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one of the products formed (OsO3) can then be used for dihydroxylation once again, and the cycle continues
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which dihydroxylation occurs in two steps? (syn or anti)
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anti
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what are the 2 steps in which an alkene is converted to a diol via anti-dihydroxylation?
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alkene--->epoxide--->diol
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in anti dihydroxylation what is used to go from alkene to epoxide?
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mcpba
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what is RCO3H?
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peroxyacid!!!!
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what are the reagents primarily used in anti dihydroxylation?
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[1] peroxyacid
[2] hydroxide |
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in general...what type of reaction is the opening of an epoxide?
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sn2 reaction --in which the least hindered C is "attacked"
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when a symmetrical epoxide is treated with peroxyacid and hydroxide what happens and what is the end result?
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a sym. epoxide has equal chance of being attacked at either carbon, therefore a pair of enantiomers is the final product
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anti-dihydroxylation gives what as prodcut?
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enantiomers
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dihydroxylation with peroxyacid gives what?
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ANTI addition. TRANS products
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dihydroxylation with KMnO4 or OsO4 gives what?
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SYN addition. CIS products
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what is oxidative cleavage of an alkene?
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both the sigma and the pi bond of the alkene are broken to form two carbonyl groups. depending on the number of R groups bonded to the double bond, you either get KETONES or ALDEHYDES.
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what are always the products of oxidative cleavage of alkenes?
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ketones and/or aldehydes
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wwhat's the easiest way to draw the products of oxidative cleavage in alkenes?
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split the double bond down the middle and add an 'O' to each end of the break
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what reagent is commonly used in oxidative cleavage??
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[1]ozone (O3)
[2] Zn, H20 ///or/// CH3SCH3 |
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what occurs during oxidatitive cleavage?
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both pi and sigma bonds are broken and two carbonyl groups are formed= either ketones or aldehydes
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what are always the products of oxidative cleavage of alkenes?
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ketone
aldehyde |
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what is typically used in oxidative cleavage of alkenes?
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ozone
Zn, h20 (or CH3SCH3) |
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what are the steps in drawing the products of any oxidative cleavage?
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break the double bonds, then add 'O's at the breaks
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what are internal alkynes oxidized to (oxidative cleavage) ?
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carboxylic acids
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what are terminal alkynes oxidized to (oxidative cleavage) ?
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a carboxylic acid and CO2
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what is commonly used in the oxidative cleavage of alkynes?
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ozone followed by h20
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in hydrogenation (reduction) reactions, what is the relationship between number of R groups and rate of hydrogenation?
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fewer R groups, faster RXN
more R groups, slower RXN |
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more stable alkene= what in terms of delta H?
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more stable alkene = smaller heat of hydrogenation
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less stable alkene = what in terms of delta H?
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less stable alkene = higher heat of hydrogenation
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what are primary alcohols oxidized to?
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aldehydes or carboxylic acid
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what are secondary alcohols oxidized to?
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ketones
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what are tertiary alcohols oxidized to?
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they do not react!
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in oxidation of alcohols, what is considered a mild oxidizing agent?
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PCC [[in CH2Cl2]]
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in oxidation of alcohols what is considered a strong oxidizing agent?
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CrO3, Na2CrO7, K2Cr2O7 [[in h20 and H2SO4]]
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in oxidation of alcohols, what is the product when alcohol is treated with PCC?
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pcc is mild so you get aldehyde
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what kind of oxidizing agent is need to oxidize secondary alcohols?
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either a strong or weak (pcc) will oxidize 2* alcohols... into what you ask???????
a ketone |
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2* alcohols are oxidized to what?
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ketones - with either strong or weak [o] agent
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when a weak [o] agent is used, what are 1* ROH oxidized to?
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aldehydes
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when strong [o] agent is used, what are 1* ROH oxidized to?
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carboxylic acid
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