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CHM 255 Exam 2
| Question | Answer |
|---|---|
| Define chiral | An object is not super imposable on its mirror image |
| What constitutes a stereocenter | A carbon with 4 different groups attached to it |
| What are enantiomers | Non-super imposable mirror images with one chiral center |
| How do you determine between pairs of enantiomers in nomenclature | R (clockwise) & S (counterclockwise) |
| What is the purpose of the Cahn/Inglod/Prelog rules? | Indicate the chirality or handedness of a stereocenter |
| How to determine R or S enantiomerism (3 steps) | 1. Order substituents by atomic weight of the 1st atom attached to the sterocenter 2. Put the lowest order substituent away from you 3. Draw an arrow connecting substituents 1,2,3: clockwise(R) & counterclockwise (S) |
| Where do you put the R/S in nomenclature? | At the very beginning of the name |
| What do you do if the 1st atoms attached to the stereocenter are the same? | Go to the next attached atoms and the one with the highest atomic weight wins |
| How do you determine R/ S priority with double/triple bonds | Double-Count as an extra carbon atom on each carbon in the double bond Triple - count as 2 extra carbon atoms on each carbon in the triple bond |
| If one enantiomer is in an R arrangement can you determine the other? | Yes; S |
| How do you determine R/S if H is in the plane of the paper? | Put your eye on the other side of the molecule and use the reversal rule |
| What are fischer projections good for | Acyclic molecules with more than one chiral center |
| What do the vertical and horizontal lines in a fischer projection represent? | Vertical: bonds going into the plane Horizontal: bonds coming out of the plane |
| When checking to see if molecules are superimposable you should rotate the fischer projection ____degrees ___the plane of the paper. | 180; INTO |
| For an acyclic molecules with n stereocenters, what is the maximum number of steroisomers possible? | 2^n |
| What are diastereomers | Stereoisomers that are not mirror images |
| What is a meso compound | An achiral molecule having 2 or more stereocenters |
| What are the 3 properties of enantiomer pairs | 1. Identical physical properties 2. Interact with and rotate plane polarized light 3. May interact with biological systems differently |
| _____________ molecules do not rotate plane-polarized light; __________ molecules do. | Achiral; chiral |
| Rotation of plane polarized light to the left is (?), right is (?) | (-); (+) |
| Enantiomers rotate plane polarized light how much and in what relative direction? | The same amount; opposite directions |
| What is a racemic mixture | 1:1 mixture (equimolar) of a pair of enantiomers |
| Is a racemic mixture optically active and what is its specific rotation | Optically inactive; [a]^20D=0 deg |
| Optical Purity | Enantiomeric excess =ee=([a]Dimpure/[a]Dpure)*100 |
| What are 3 characteristics of diastereomers | 1. Unique molecules with unique physical properties 2. Interact with plane polarized light differently 3. May interact with biological targets differently |
| What is an alkene? | Organic molecule that contains a double bond |
| ____ alkenes are less stable because of ______. | cis; strain in the interaction between CH3 groups |
| 3 Rules for nomenclature of alkenes | 1. Use the infix -en instead of -an 2. The parent chain is the longest containing the double bond 3. # the parent chain so that the double bond has the lowest # possible |
| How to use the E-Z system (3 steps) | 1. Use R-S priority rules 2. Divide molecules in two and compare priority of 2 substituents on each side 3. Z: high/high or low/low; E: high/low or low/high |
| Bond length: 1. Alkane 2. Alkene | 1. 1.54 A 2. 1.34 A |
| Bond dissociation energy: 1. Alkane 2. Alkene | 1. 90kcal/mol 2. 172 kcal/mol |
| Bond rotational barrier: 1. Alkane 2. Alkene | 1. Relatively free rotation around C-C (3kcal/mol) 2. p-orbitals& pi bond restrict rotation (63kcal/mol) |
| Many organic reactions have an energy of activation of _________________? | 10-35 kcal/mol 10-20 can happen at room temp |
| What kinds of molecules are nucleophiles (3)? | 1. Compounds with a site of (-) charge 2. Compounds with atoms with love pairs of electrons 3. Compounds with pi bonds |
| What kinds of molecules are electrophiles (2)? | 1. Compounds with a site of positive charge 2. Compounds that have an electron deficient site |
| Nucleophile vs electrophiles | Nucleophiles: donate e-; electrophiles: accept e- |
| A bond in the _________ may need to break in order to maintain the octet rule | electrophile |
| Alkenes will be a _____________ that can be treated with many different ________ | nucleophile; electrophiles |
| What is electrophilic addition | Adding an electrophyle to an alkene |
| What is a carbocation? | A cation of carbon |
| What is the order of carbocation stability? | methy cation < 1(o) < 2(o) < 3(o) |
| In what two ways can alkyl groups stabilize carbocations | 1. inductive effect: neighboring carbons help share the positive charge (donate e- density) 2. hyperconjugation: adjacent pi bonds overlap with the vacant p orbital |
| Why would a rearrangement occur | Hydrogen can shift to form the more stable carbocation |
| T/F: Alkyl groups and hydrogens can shift to any carbon on the molecule to form the most stable carbocation | FALSE: Can only shift to the C next door |
| What does it mean to be stereoselective? | One stereoisomer is formed in preference to all others; occurs for reactions with a bridged ion intermediate |
| What is anti-addition? | Addition from opposite sides of the double bond |
| Cyclic addition of alkenes always yields a molecule that is a. cis, b.trans, c. both, d. neither | b. Trans |
| Markovnikov vs. Anti-Markovnikov addition | M: Substituent ends up on the more substituted end of the double bond A-M: Substituent ends up on the lest substituted |
| What are conjugated double bonds | When two or more double bonds are separated by one single bond |
| What are unconjugated double bonds | When two or more double bonds are separated by more than one single bond |
| Which is more stable and why between a conjugated and unconjugated double bond? | Conjugated because e- in the pi system can be delocalized over both double bonds |
| A reaction is an oxidation if the starting material 1.___OR 2.___. | 1. Has lost H2 2. Has gained O |
| A reaction is an reduction if the starting material 1.___OR 2.___. | 1. Has gained H2 2. Has lost O |
| What is syn addition | Stereoselectivity: Addition of H2 on the same side of the double bond; results in a cis product |
| How do you form a radical | Homolytic cleavage: Take a bond with 2e- and send on in each direction |
| How do you indicate that you are moving only 1 e-? | 1/2 an arrow head |
| A reaction of radicals will proceed until all radicals are consumed. A process known as _______________ | Chain termination |
| What is the result of non-productive chain termination when forming alkyl halides from alkanes | The reactant or the reagent reforming |
| What is the relative stability of allylic radicals? | Allylic radicals>3(o)>2(o)>1(o) |
| When is electrophillic addition possible | ONLY at room temperature |
| How can we avoid the problem of unwanted electrophilic addition reaction at room temp with alkenes and Br | Using a different reagent: NBS |
| What does halogenation of alkanes | Heat/Light and Cl2 or Br2 or NBS |
| What is an allylic carbon | Adjacent C to a C=C... C1-C2=C3 & C1 is allylic |
| What is allylic substitution | A substitution a the allylic C |
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jkmccord11
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