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75 Cards in this Set
- Front
- Back
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What does IR Spectroscopy tell you?
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IR Spec gives info on the present functional groups
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What wave number do you stop looking at IR Spec Data for?
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1600 nm
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What does Proton NMR do?
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Detects the different types of hydrogen nuclei in an organic compound
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Hows is chemical shift of a proton on NMR spectrum determined?
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By the nature of nearby groups, farther left if closer to more electronegative atoms
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What information does proton NMR give you?
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1) # of nonequivalent proton sets (# of peaks)
2) potential functional groups(chem shift) 2) # of protons in each set (integral) 3) # of protons in adjacent sets (splitting) |
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What is the effect on chemical shift for increasing or decreasing electron density?
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Increased density equals increased sheilding and smaller shift
Decreased density equals decreased shielding and increased chemical shift |
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What causes increase in chemical shift?
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1) Increasing electroneg of nearby groups
2) Increasing number of electroneg groups 3) Decreasing distance between H & electroneg groups |
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What is the alkyl sub trend of chemical shifts?
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-CH3 low shift
-CH2 medium shift -CH high shift |
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What is the integral in a proton NMR spectrum?
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Integral is the height of the absorbance peak and is proportional to the number of protons of that type
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How does splitting arise?
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Splitting arises from the effect that one set of protons has on the NMR absorption of neighboring protons
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What is the n+1 rule?
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n adjacent protons cause the resonance to be split into n+1 lines
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When is splitting not observed?
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1) Between chemically equivalent Hs
2) Between protons on not adjacent carbons |
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What is the coupling constant? When is it the same & when is it different?
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Coupling constant is the spacing between adjacent peaks
2 coupled protons have same J value Multicative Splitting causes unequal J values |
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What causes a doublet of doublets?
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Successive application of 2 one-protons splittings
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What does substituting deuterium in proton NMR do?
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Eliminates resonance of proton and removes any splitting it causes
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What does carbon NMR tell you?
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Number of chemically nonequivalent carbons in a structure
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What are the basic steps to solving structures with NMR?
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1) Get molecular formula and unsaturation number
2) Use IR & NMR to get functional groups |
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What is the formula for unsaturation number?
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U = (2C + 2 - N - H) / 2
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What is a stereoisomer?
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Compounds with sam atomic connectivity but a different arrangement of atoms in space
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What are enantiomers?
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Noncongruent mirror images
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What makes a molecule chiral?
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Molecules that exist as enantiomers
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What makes a molecule achiral?
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Molecules that are not enantiomers or chiral
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What is an asymmetric carbon?
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Has four different groups attached to a central carbon
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If a molecule only has one asymmetric carbon...
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It is chiral
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What is a stereocenter?
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A carbon or atom where the interchange of 2 atoms gives a stereoisomer
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What is a plane of symmetry? What chirality do these molecules have?
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It divides objects into halves that are exact mirror images
They are achiral |
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How do you determine R & S for enantiomers?
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1) Identify the asymmetric carbon
2) Assign priorities based on molecular weight 3) View with molecule of lowest priority pointing away 4) If 1,2,3 is clockwise than its R 5) If 1,2,3 is counterclockwise than its S |
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What is the difference in physical properties between enantiomers?
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They are the same
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How do you tell enantiomers apart?
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They can be distinguished by their effects on polarized light
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What is the optical activity of enantiomers?
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A pair of enantiomers rotate plane of polarized light equal amounts in opposite directions
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What is an enantiomerically pure mixture?
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Contains only one enantiomer in the solution
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What is a racemic mixture? What is its optical rotation?
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A mixture with equal amounts of the two enantiomers
Optical rotation is zero |
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How does a molecule with more than one asymmetric carbon be an enantiomer?
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It must have a different configuration at every asymmetric carbon
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What are diastereomers?
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Stereoisomers that are not enantiomers
They are not congruent mirror images |
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What is the relationship between physical properties of two diastereomers?
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They differ in all physical properties and can be separated by convential means
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What are meso compounds?
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Achiral compounds with asymmetric carbons that are congruent to their mirror image and not optically active and have internal planes of symmetry
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What is the symmetry of a meso compound?
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It can be diveded into structurally identical halves
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How do you interpret Fisher projections?
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The horizontal lines are bonds pointing towards you
The veritical lines are bonds pointing away from you |
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What is a monocyclic compound?
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A compound that contains a single ring
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What is the most stable cyclic compound?
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Cyclohexane it minimizes steric and torsional strain
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What is angle strain?
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The extra energy that a compound has because of nonideal bond angles
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What are the axial substituents of a cyclohexane?
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They are the substituents perpendicular to the plane of the table
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What are the equitorial substituents of a cyclohexane?
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They lie along the edge of the cyclohexane ring
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What shift occurs when a ring flip is performed on a cyclohexane in chair conformation?
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Axial substituents become equitorial and equitorial substituents become axial
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What shift occurs when a chair flip is performed on a cyclohexane already in chair conformation?
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Up substituents become down and down become up
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Is chair or boat conformation more stable for a cyclohexane?
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Chair conformation
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Where is the most stable place for substituents to be placed?
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Molecule is made the most stable when substituents are placed in the equitorial position minimizing steric strain
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What interaction is mimimized by equitorial positioning of substituents?
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1,3-diaxial interaction
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In a cyclohexane, how is the trans isomer configured?
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Two substituents have an up-down relationship
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In a cyclohexane, how is the cis isomer configured?
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Two substituents have an up-up or a down-down relationship
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What is most stable conformation for cyclopentane?
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Envelope conformation which undergoes rapid conformation change where each carbon alternates as a point on the envelope
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What are the least stable monocyclic alkanes?
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Cyclobutane & cyclopropane
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What is a bicyclic compound?
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Shares two or more common atoms
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What is a spirocyclic compound?
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Two rings sharing a single common atom
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How do you name a bicyclic compound?
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Bicyclo[# of carbons between bridgeheads.?.?]common name for total carbon atoms
Ex. Bicyclo[4.3.3]decane |
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Which is more stable for bicyclic compounds: cis or trans?
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For larger fused rings its trans, but smaller rings is cis
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What is a steroid?
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Compound with structure derived from 4-fused ring structure
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How do enantiomers react with an achiral reagent?
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Enantiomers react at the same rate with an achiral reagent because of their identical free energies
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How do enantiomers react with a chiral pure reagent? Why?
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One of the diastereomeric products is formed in a greater amount than the other because diastereomers have different free energies so one is more favorable
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How do enzymes react with enantiomers?
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Only catalyzes the reaction of one enantiomer because all enzymes are pure chiral molecules
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How do diastereomers react with chiral reagents? With achiral?
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With both chiral and achiral reagents, reaction products are produced at different rates because of different free energies
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What is a syn-addition?
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Two groups breaking double bond add to same side or face (cis)
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What is an anti-addition?
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Two groups add to double bond from opposite sides (trans)
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How do we know if syn or anti addition took place?
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Based on if product is cis or trans. Cis = syn & trans = anti
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When do syn & anti additions yield different products?
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If both carbons of the double bond become stereocenters
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For a substitution reaction, what is retention of configuration?
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If X' replace X then X & X' have same stereochemical positions (cis stays cis & trans stays trans) also R,S doesn't change
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For substitution reaction, what is inversion of configuration?
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If X' replaces X then X & X' have different stereochemical positions (cis becomes trans and trans becomes cis) also R,S switch
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What is a stereoselective reaction?
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Particular stereoisomers of the product are formed in excess of others
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For bromine addition, what are the three possible products formed?
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A pair of enantiomers and a meso compound
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For bromine addition, which configuration forms the racemate and how?
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The cis configuration yields the racemate through an anti-addition
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For bromine addition, which configuration forms the meso compound and how?
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The trans configuration yields the meso compound through a syn addition
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What is the stereoselectivity of the bromine addition?
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Anti-addition is highly stereoselective and the pair of enantiomers is formed most often
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What is the stereoselectivity of a hydroboration oxidation reaction?
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The hydroboration portion is a stereospecific syn-addition so the product is racemic, however the oxidation is a sterospecific substitution reaction with retention of original configuration
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What is the net result of a hydroboration oxidation reaction?
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Syn-addition of OH & H to a double bond
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What is the stereochemistry of a catalytic hydrogenation?
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Syn-addition forming a racemate and meso compound
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