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Org Chem 1 > Exam 3 > Flashcards

Exam 3 Flashcards

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1
Q

What is chirality?

A

It is the attribute of handedness as seen in amino acids and sugars

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2
Q

What does it mean to be chiral?

A

To have a non-superimposable mirror image

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3
Q

What does it mean to be achiral?

A

To have a superimposable mirror image

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4
Q

What is an enantiomer?

A

A pair of non-superimposable mirror images
Know how to draw a molecules’ enantiomer

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5
Q

What is a chiral center?

A

A central atom (ie Carbon) that has four different groups attatched

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6
Q

What is a chiral compound?

A

It is a compound that has a non-superimposable mirror image
no internal plane of symmetry

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7
Q

What are the ways to determine a chiral molecule?

A

Internal plane of symmetry or draw mirror image
* If there is only one chrial carbon, compound will b e chiral
* If there is more than one then it may or may not be chiral

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8
Q

What is a steriocenter?

A

Any center that can give sterioism
Includes chiral center (not vice versa)

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9
Q

What designates which enantiomer you have?

A

R or S

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10
Q

What are the rules to determine R, S chirality?

A

1) Establish a priority based on atomic mass (higher atomic mass = higher priority)
-tie: use atom next along in chain
-double & triple bonds: treat as if bonded to separate atoms
2) Place fourth priority back and draw arrow from 1 to 3 (through 2)
-Clockwise: R
-Counterclockwise: S
-Write: treat like cis/trans out front. more than one write location (2S, 4R)

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11
Q

What are the rules of Fischer projections?

A

-Intersection of lines is a chiral carbon
-Vertical lines always point away (dashes)
-Horizontal lines always point toward (wedges)
-Put Carbon backbone on vertical line with C#1 at top

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12
Q

How do you determine the orientation of a Fischer projection chiral carbon?

A

Number priority then draw line from 1 to 3.
If 4 is horizontal then flip arrow

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13
Q

What happens if you turn a Fischer projection (on the paper) 90? 180?
What happens if you flip a Fischer projection?

A

90: different molecule
180: same molecule
flip: different molecule

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14
Q

What is a diastereomer? What are the examples of diastereomers?

A

A stereoisomer that are not mirror images
-term can only be used to determine a relationship (there must be a pair)
-cis/trans double bonds
-cis/trans on rings
-2+ chiral centers

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15
Q

What is a stereoisomer?

A

Compounds whose atoms are connected in the same order but have different orientation in space.

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16
Q

What is the rule for chiral centers and stereoisomers?

A

Compound with n chiral centers has up to 2^n stereoisomers

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17
Q

What are examples of diastereomers that have 2+ chiral centers?

A

The relationship between enantiomer pairs. if A and B and C and D are enantiomers, A and C, A and D, B and C, and B and D are diastereomers

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18
Q

What is a meso compound? How do you identify them?

A

Achiral compounds with chiral centers.
Fischer projection will have internal symmetry.

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19
Q

When drawing Fischer projections from line angle form, remember…

A

Carbon chain must be pointing away from you. Reorientate molecule so that they are.

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20
Q

What is the isomer chart?

A

Isomers: Constitutional or Stereoisomers (enantiomers or diastereomers {cis/trans or others})

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21
Q

What must you remember with drawing Fischer projections?

A

CHO is a aldehyde (C=O -H)
CO2H is carboxylic acid (C=O -OH)
Make sure lines connect the atoms that actually connect with each other.

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22
Q

What was the Fischer-Rosanoff Convention?

A

It determined the D and L conventions
D is like D-glyceraldehyde turns plane polarized light positive direction OH group on right
L is like L-glyceraldehyde that turns plane polarized light in the negative direction OH group on the left

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23
Q

What does enantiomers do with plane polarized light?

A

One by itself will bend the light
Both together will not

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24
Q

What is the Fischer-Rosanoff Convention of amino acids?

A

All naturally occurring amino acids in the human body with acid on top has NH2 on Left making it “L”

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25
Q

What is the Fischer-Rosanoff Convention of sugars?

A

Sugars are all D - look at bottom of sugar OH on right.

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26
Q

When do chiral molecules not have a chiral center?

A

When there is locked conformation (very rare)
Two phenol groups bonded w/ bulky groups on them cannot rotate around sigma bond: non-superimposable mirror images
Allene: Two double bonds –> non-superimposable mirror images

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27
Q

What about the chirality of mobile systems?

A

Cannot superimpose until there is a chair flip.
If enantiomers are in equilibrium it is an achrial molecule

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28
Q

How is chirality determined?

A

Polarimetry: “optical activity” because enantiomers have all the same physical properties but will rotate the plane of polarized light differently

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28
Q

What is a plane of polarized light?

A

A light that goes through a filter that only allows one plan of light though (light goes in direction in all planes of light at once)
enantiomers rotates plane of light when go through tube of chiral compound
-how far rotated determined by second polarizing filter-rotate until don’t block light
-wavelength dependent usually use Na D line

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29
Q

What is opitcally active?

A

Any compound that rotates planes of polarized lights

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30
Q

How can the direction an enantiomer rotates light be determined?

A

Only experimentally - it cannot be predicted

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31
Q

What are the ways plane polarized light can be rotated?

A

clockwise (+) dextrarotatory
counterclockwise (-) levrorotory

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32
Q

How is the specific rotation caclulated?

A

[alpha]^T wavelength = alpha observed/(concentration (g/mL) * Cell length (dm))

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33
Q

What are racemates?

A

A racemic mixture: 50/50 mix of enantiomers (+_) (d,l)
- what you get when you form a chiral product
Has zero rotation of planar polarized light - not optically active

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34
Q

What is the rule of producing optically active products?

A

Cannot come form optically inactive reactants.

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35
Q

What is homochirality?

A

The idea that life supposedly came from one chiral molecules (from a meteor)

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36
Q

What is enantiomer excess?

A

Optical activity/ purity, e.e. = o.p.
=alpha observed / alpha pure x 100
= |d-l|/ |d+l|

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37
Q

What is absolute stereochemistry?

A

do we know if it is R or S

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38
Q

What is relative stereochemistry?

A

Know it is related
-relative stereochemistry has not changed in a reaction when the reaction did not affect the chiral center

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39
Q

What is hallogenation?

A

It is a reaction were a organic molecule receives a hallogen

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40
Q

What are the two categories of reactions of hallogens?

A

1) Nucleophilic Substitution
- substitute halogen “x” for nucleophile
2) Elimination
- use base to remove H and halogen produces double bond in organic molecule

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41
Q

What do you need to know about NBS?

A

It can be a replacement for Br2 in a halogenation reaction so that two Br are not added to the organic molecule (contains low concentration of Br2)

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42
Q

What is the orientation of the nucleophile to the leaving group?

A

The nucleophile attacks from the opposite side because there is then not a lot of steric hindrance

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42
Q

What is SN2?

A

substitution nucleophilic bimolecular (rate- 2nd order)
Nucleophile replaces leaving group and leaving group is by itself with a negative charge
**The nucleophile needs to be in a certain orientation to Lg

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43
Q

What happens to the chiral carbon when a nucleophil attacks?

A

Inversion and there is a change in stereochemistry

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44
Q

What makes a good nucleophile?

A

1) Base is more nucleophilic than its conjugate acid (negative species more often stronger than same neutral species)
2) Larger and more polarizable
3) Less steric hindrance
4) Solvent effect

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45
Q

What is a base and what is a nucleophile?

A

Base: Ability to abstract a proton (Bronsted-Lowry Definition)
Nucleophile: ability to donate e- to electrophile
(OH- is a good base and a good nucleophile)

46
Q

How do size and polarizability affect the nucleophile?

A

The more polarizable the easier the nucleophile can “squish” into the Carbon’s orbit.
Opposite trend of electronegativity: More electronegative want to hold on the e- and don’t squish well.
More polarizable to the left and down
HO < HS
Larger the nucleophile the farther the e- are from the nucleus and the easier it is for the e- to escape
I > Br > Cl > F

47
Q

KNOW HOW TO RANK NEUTROPHILES

A

FOR THE EXAM

48
Q

How does steric hindrance affect neutrophils?

A

must be able to approach molecule, more steric hindrance, harder
Ethyloxide < Isopropyloxide
Tertbutyloxide is not a nucleophile in most cases

49
Q

What are reactions almost always done in?

A

Some kind of solvent

50
Q

What affect do solvents have on a reaction? 18-crown-6?

A

A big effect
18-crown-6 “crown ether” makes less soluble salts more soluble - a catalyst to bring CN into solution

51
Q

What are the categories of solvents?

A

Protic (has easily removable protons: H2O, R-OH, R-NH2, R-SH, H-bond and H-bond donor easily)
Aprotic (not H-bond easily, no H-bond donors)
-Non Polar (toluene, hexane, pretty much just hydrocarbon)
-Polar (Ethyl acetate, acetonitrile (H3C-CN), DMF dimethylformamide, dichloromethane, acetone)

52
Q

What kind of solvents do Sn2 reactions like?

A

Really like aprotic solvents
Goes faster in non-polar aprotic solvents but less likely to get Nu into solution
-more likely to see polar aprotic solvents

53
Q

What is the function of solvents?

A

To get reaction going and to determine between Sn2, Sn1, E1, E2 reactions
Not likely to be in product

54
Q

What are the purposes of a leaving group?

A

1) polarize the bond - want to leave with electron, make C want new e-
High electronegativity
2) Weak base are happy to keep e- (halogens)

55
Q

What makes a good leaving group?

A

1) e- withdrawing (takes e-) often means highly electronegative
2) Stable with negative charge (weak base)
3) Want bond to be polarizable (bigger groups tend to be better)
I is good with all three while F only good with first two because I is a larger atom and more polarizable “longer arm”

56
Q

When the leaving group and neutrophile could both be either, what do you do to get the product you want?

A

Le Chatelier’s principle - add more reactant to force equilibrium toward products side

57
Q

Is OH a good leaving group?

A

No because it does not want to be on own, add a H and it is much happier: two part mechanism, must make good leaving group first - Protonate good LG hogs e- making C want new e- (nucleophile)

58
Q

What type of Carbon is best for a leaving group?

A

methyl > 1* > 2* > > 3* does not happen
because mechanism goes through back side attack, if there is too much hindrance this cannot happen
If there is a lg on a 3* C there will not be a reaction
Inversion of stereochemistry - Lg wedge, Nu will be dashed etc.

59
Q

What groups make good bases?

A

OH- OR (alkoxides), NR3 (amines)

60
Q

What groups are only nucleophiles?

A

CN, PR3, dihalide

61
Q

Reminder for Sn2 reactions.

A

Add nucleophile, note number of bonds, remove extra bond if necessary with another nucleophile.

Leaving group tends to not be a good base so it will not pick up the extra bond (H)

62
Q

What are good bases to pick up extra nucleophile bond?

A

NR3, H2O, OH-
Not usually Leaving group

63
Q

What is a Sn1 reaction, how is it differentiated from Sn2?

A

“first order” reaction - rate depends on substrate (that which has Lg) not on nucleophile
Look for Carbon priority, Nu strength, solvent (protic/aprotic), is heat added
Sn1: towards tertiary, weak Nu, protic, heat is added

64
Q

What is the difference between the energy digrams of Sn1 and Sn2?

A

Sn1: two step reaction, first hump is the RDS as the Lg pops off, intermediate: carbocation, second hump: nucleophile strength not matter, only whatever closer (nucleophile or lg- and there tends to be nucleophile in excess)

Sn2: one step reaction, hump includes substrate and nucleophile

65
Q

What are the steps of a Sn1?

A

Lg is very electronegative and adding heat allows it to pop off
Lg does not reattach because there is lots of nucleophiles that meet the substrate first therefore weak nucleophiles work just as good as strong nucleophiles
Nucleophile on substrate has extra bond, must use another nucleophile to get rid of extra bon

66
Q

How is the _______ charge on the carbocation balanced?

A

Positive
Resonance > 3* > 2* > > > > 1* (1* not happen without resonance)

66
Q

What is solvolysis?

A

If the nucleophile is the solvent

67
Q

What is allylic position and allyl carbocation?

A

The position adjacent to the double bond
Carbocation adjacent to the double bond

67
Q

What is the solvent effect of Sn1?

A

Need good ionizing solvent so Sn1 is often in protic solvents (provides H bonding to stabilize the ion)

68
Q

What is the stereochemistry of the carbocation?

A

Sp2

69
Q

What sort of compounds will be produced by Sn1, why?

A

Because the stereochemistry is Sp2 the nucleophile can attach above and below (to free pi bond) therefore the products will be 50/50 of top and bottom: a racemic mixture, optically inactive.
draw both DASHES and Wedges

70
Q

What are the rearrangements of carbocations?

A

Protons (H) and methyl groups can shift to move positive charge from lower priority carbon,
“1,2-hydride shift” “1,2-methyl shift”

71
Q

Can there be a Sn1 reaction on a primary carbon?

A

Only if shift is possible and that happens at same time Concerted step MUST SHOW ELECTRONS MOVING IN SAME STEP

72
Q

What are common nucleophils?

A

X, OH, OR, SH, SR, NH3, N3, CCR, CN, OCOR, PPh3
negative species more often stronger than same neutral species

73
Q

What are commonly good leaving groups?

A

Halogens, oxygen, nitrogen, sulfur
sulfate ions, sulfonate ions, phosphate ions
& Neutral molecules, water, alcohols, amines, sulfides often in positively charged species- need to protonate alcohol
NOT STRONG BASES (-OH, -OR, -NH2)

74
Q

What kind of carbons do Sn2 react with?

A

CH3X > I* > 2* > > > > 3*
less steric hindrance on substrate

75
Q

What kinds of carbons do Sn1 preferably react with?

A

3* > 2* > 1* > +CH3
Enhance carbocation stabilization

76
Q

What are intermolecular and intramolecular attacks?

A

Intermolecular: Nu/Lg groups between molecules
Intramolecular: Within molecule
-if a 5 or 6 membered ring can form then it will through intramolecular attack do not forget possibility of stereochemistry inversion

77
Q

What are the elements of unsaturation?

A

They tell how many double bonds or rings there are
ie 1 EU = 1 bond OR 1 ring

78
Q

What is the equation for elements of unsaturation?

A

EU = 1/2 (2C+2+N - (H+Hal)

79
Q

What are the common alkene groups?

A

Methylene (double bond coming off of ring or chain, can be used as prefix)
Vinyl (something directly attached to double bond, vinyl chloride)
Allyl (Vinyl + a carbon in between, allyl chloride)
Phenyl (benzyl ring attached directly to chain/ring/atom) phenyl chloride

80
Q

What is E/Z?

A

E includes trans, highest priority groups are on opposite side
Z includes cis, highest priority groups are on zame zide
-can be used for more than one double bond, must have location if more than one double bond

81
Q

What determines the stability of alkenes?

A

Heats of hydrogenation
Use hydrogen and plantinum to replace double bond with two hydrogens
+ heat stored = - heat released
*More heat released, less stable

82
Q

Zaitsev’s rule:

A

Alkenes that have more substituents are more stable
When making alkenes the more stable alkene will be made

83
Q

What is the chart of alkene energies?

A

(Greatest energy, least stable) methene (0) > 1substituents > cis 2 subs > geminal 2 subs > trans 2 subs > 3 subs > 4 subs

84
Q

What is the rule for E/Z cycloalkenes?

A

Must have 8 or more carbons to have E

85
Q

Bredt’s Rule:

A

For bridged bicyclo molecules cannot have double bond on bridge head atom unless one loop has 8+ molecules

86
Q

What is the resonance energy of alkenes?

A

Conjugated double bonds (separated by one sigma bond) adds stability via resonance energy
Isolated (separated by 2+ sigma bonds) do not have resonance energy

87
Q

How are the energy savings of a cyclohexane?

A

1 bond: x energy used
2 isolated bonds : x energy used for both
2 conjugated bonds: x - 8 energy used for both
3 conjugate bonds x - 151 energy used for each

88
Q

What is dehydrohalogenation?

A

It is elimination: getting rid o a hydrogen and halogen to make an alkene

89
Q

What are the two methods of elimination?

A

E1: Elimination unimolecular
1) Lg pops off
2) Base takes H
E2: Elimination bimolecular
1) Strong base takes H, electrons shift, and leaving group pops off (concerted)

90
Q

What are the things to keep in mind about E1 reactions?

A

RDS happens with just the substrate,
SN1 and E1 compete usually forming a mixture of both
ASK: is it a nucleophile or a base (if both they compete)

91
Q

What is the Zaitsev product?

A

The major product will be the one that is the most substituted

92
Q

What is dehydration?

A

The leaving group is water
-Often H2SO4 used (write Lewis)
-Protonates molecule
-Water leaves
-Forms carbocation

93
Q

What is a symbol used in dehydration reactions?

A

Equilibrium arrows because this reaction can be reversed
Le Chatelier’s principle keeps it forward
-H2SO4 is very concentrated and sucks water
-HSO4 not a good nucleophile
Very Good E1 Reaction

94
Q

What can happen with a four-membered ring?

A

Ring expansion to release ring strain.

95
Q

When can a leaving group leave a primary carbocation?

A

If the 1* becomes a 3* simultaneously CONSERTED SHIFT ONLY

96
Q

When does rearrangement occur?

A

When there is a carbocation and a possibility for the positive charge to be on a higher priority carbon or for there to be resonance.

There will NOT be rearrangement if base/nucleophile is NEGATIVE

97
Q

What about aromatic resonance?

A

Aromatics provide powerful resonance but they do not want to break their ring.

98
Q

What is the basis of E2?

A

Both the base and the substrate effect the RDS
There is a concerted movement of base taking H, H electrons moving to C, and LG leaving
LG wants to leave but cannot by substitution, must leave by electrons pushing it off
*pay attention to all carbond that could be deprotonated

99
Q

What is the reactivity of E2?

A

The higher the priority the LG the better
3>2>1*,

100
Q

What are the products of E2?

A

Zaitsev product: more substituted, often major
Hofman product: less substituted

101
Q

How do E2 and Sn2 overlap?

A

3: E2
2: E2 & Sn2
1*: Sn2
to get only E must have good base that is a poor nucleophile
Bulky Bases

102
Q

What are the bulky bases?

A

Tert-butoxide, Diisopropyl amine, triethyl amine (not as bulky as it could be), 2,6-dimethylpyridine

103
Q

Which H will a bulky base pull off?

A

The easiest proton
1* > 2* > 3*
resulting in the Hofman more than Zaitsev product

104
Q

What is the stereochemistry of E2?

A

H and LG must be coplanar
- syn-coplanar: H & LG are eclipsed
- anti-coplanar: H & LG are anti *will predominate when possible

*Not possible when molecules are locked in place

105
Q

What must you keep in mind with drawing double bonds?

A

When there is one double bond lines and wedges/dashes cannot mix
Either: Two forward & Two Back OR all in plane of paper.

106
Q

What must you do when H and LG are not coplanar?

A

Rotate sigma bonds to make them coplanar (preferably anti-coplanar)
DRAW rotation

107
Q

When there is more than one H for an E reaction

A

Pick H that will lead to most stable bond E more stable than Z

108
Q

What must you ask with an E2 reaction?

A

Base bulky or not, what is the LG priority
PAY ATTENTION TO STEREOCHEMISTRY

109
Q

What do bulky bases do when they choose a H?

A

Lowest priority H leading to Hofman product

110
Q

What is Duterium?

A

Hydrogen with extra nucleus - will get pulled off like hydrogen

111
Q

Cyclohexane and H LG to form bond

A

H and LG will not be both forward or both back. Must be able to chair flip to get anti coplanar
-IF THE FORMATION IS LOCKED NO DOUBLE BOND CAN BE FORMED-

112
Q

Alcohol priorities:

A

Primary alcohol: OH2 cannot popoff so it must be Sn2
Secondary &Tertiary alcohol: OH2 can popoff NOTE REARRANGEMENT

113
Q

What are Lucas reagents?

A

HCl/ZnCl2, O attracts entire molecule then OH+lucas reagent pops off.

Org Chem 1 (4 decks)

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