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CHM 255 Exam 2

QuestionAnswer
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
Created by: jkmccord11