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OCHEM 335 > Quiz 6 > Flashcards

Quiz 6 Flashcards

1
Q

when does a carbon act as a nucleophile vs an electrophile

A

carbon is an electrophile when it is attached to an EWG (ex. halide)

carbon is a nucleophile when it is attached to a metal (ex. Li, MgX)

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2
Q

why does carbon act as an electrophile?

A

carbon is more electronegative than Li or Mg

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3
Q

what is an organometallic compound

A

part organic, part metal. contains a carbon-metal bond

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4
Q

what are two of the most common organometallic compounds

A

organomagnesium and organolithium

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5
Q

explain organolithium compounds

A

require two equivalents of Li metal and one equivalent of alkyl / aryl halide

very strong bases and excellent nucleophiles

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6
Q

explain organomagnesium compounds

A
  • these compounds are called Grignard reagants
  • compounds require one equivalent of Mg metal and one equivalent of alkyl / arenyl / aryl halide
  • they are very strong bases and excellent nucleophiles
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7
Q

describe grignard reagants

A

RMgX
- R can be anything (primary, secondary, or tertiary alkyls, alkenyl, or aryl)
- X can be Cl, Br, or I

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8
Q

what is the solvent for a grignard reaction

A

Mg and ethers (or THF)

ethers are the usual solvents due to their unreactive nature. the solvent provides electrons so magnesium can complete its octet.

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9
Q

how do organomegnesium and organolithium compounds react

A

they react as if they were carbanions
- carbon is more electronegative than Li or Mg
- carbanions (hence, Li or Mg, are great nucleophiles).

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10
Q

how do organomagnesium and organolithium compounds react with a proton of an acidic group

A
  • when the grignard reagant or any other organometallic compound reacts with a proton source, it forms an alkene.
  • ex. CH3MgBr + H2O –> CH4 + HOMgBr
  • organolithium and organomagnesium compounds react violently with water and alcohols, so you must exclude protic molecules or add them to a reaction very slowly / carefully.
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11
Q

explain organocuprates

A
  • also called gilman reagants
  • R2CuLi
  • undergo coupling rxns to link any two alkyl, aryl, or vinyl groups together
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12
Q

what occurs in an organocuperate rxn

A

a coupling rxn joins two CH containing groups. the alkyl group of an organocuprate replaces a halogen

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13
Q

what happens to the configuration of the double bond for organocuprates

A

the configuration is retained

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14
Q

what R group of an alkyl halide and the organocuprate can be used?

A
  • the R group of the alkyl halide (and thus the organocuprate) can be primary, methyl, aryl, vinylic, or allylic
  • the R group cannot be secondary or tertiary because of steric reasons
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15
Q

describe the suzuki rxn

A
  • palladium-catalyzed cross-coupling reactions
  • both reactions replace the halogen of a vinylic halide or an aryl halide with a carbon-conatining group
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16
Q

describe the first step of the mechanism for the suzuki rxn

A
  • both rxns start by adding palladium between the alkyl group and the halogen
  • ex. R-X + PdL2 –> an X shape with Pd in the middle, two Ls on top, an R on the bottom and an X on the other bottom side.
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17
Q

what are the steps of the suzuki rxn

A
  1. oxidative addition –> palladium inserts between the R group and the halogen (causes Pd to be oxidized from 0 to 2+ (II))
  2. involves hydroxide displacing the halide ion
  3. transmetallation, which is where the R group is transferred from the boron to the palladium.
  4. reductive elimintion, which is where the Pd II is reduced to Pd 0 and a new carbon bond is formed

(cyclic method)

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18
Q

describe oxidation

A

C-H bond broken and C-Cl bond formed

an increase in the number of carbon-heteroatom bonds, and/or a decrease in the number of carbon-hydrogen bonds.

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19
Q

describe reduction

A

C-Cl bond broken and C-H bond formed

decrease in the number of carbon-heteroatom bonds, and/or an increase in the number of carbon-hydrogen bonds.

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20
Q

rank compounds in order of oxidation level

A

low oxidation
CH3CH3
CH3OH
CH2Cl2
HC triple bonded to N
CCl4
high oxidation level

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21
Q

describe ether nomenclature

A

R-O-R
- if both R groups are the same, it is a di(group name)
- if both R groups are different, then name them differently
- if other functional groups are present, the ether part is considered an alkoxy substituent
- ex. dimethoxybenzene

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22
Q

what happens if you use a strong acid to form an ether from a primary alcohol

A

its a bad reaction
- both E2 alkene and Sn2 ether products are obtained
- there is a superior route (the williamson ether synthesis)

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23
Q

explain the williamson ether synthesis

A
  • an Sn2 reaction of an alkyl halide with an alkoxide nucleophile
  • reaction of an alcohol (ROH) and an alkali metal hydride ion (NaH) produces RO- (alkoxide ion) + Na+ + H2
  • the less hindered / smaller alkyl group should come from the alkyl halide
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24
Q

what solvent is used to produce alcohols from alkenes

A

cat. H2SO4 and CH3OH

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25
Q

explain the mechanism for the addition of alcohol to alkenes

A

C-C double bond is split –> one C gets an OR and the other gets an H

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26
Q

explain how strong acids convert a poor leaving group into a good leaving group

A
  • alcohols must be protonated before they can react
  • alcohols and ethers have similarly poor leaving groups (-OH / -OR); thus, ethers must also be protonated before the compounds can undergo reaction
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27
Q

what reagents react to cleave a C-O bond

A

HBr, HI, and CF3CO2H
(they have to be strong acids)

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28
Q

what happens if an ether is attempted to be activated using another method

A
  • an alcohol forms an intermediate that can lose a proton
  • an ether forms and intermediate that CANNOT lose a proton, thus reagents such as PCl3 cannot be used to activate ethers.
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29
Q

describe the mechanism for ether cleavage: SN1 Example

A
  • protonate oxygen (acid protonates most basic atom)
  • methanol departs, forming a carbocation
  • addition of nucleophile
  • if a relatively stable carbocation is formed when ROH leaves, it will be an SN1 reaction if the counter anion is nucleophilic
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30
Q

describe the mechanism for ether cleavage: SN2 Example

A
  • the acid protonates the most basic atom
  • the nucleophile attacks the less sterically hindered carbon (smaller of the groups)
  • if a relatively stable carbocation is not formed when ROH leaves, it will be an SN2 reaction if the counter anion is nucleophilic.
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31
Q

explain the mechanism for ether cleavage: E1 example

A
  • double bond formation
  • reagants: CF3CO2H at 0 degrees celcius
  • if a relatively stable carbocation is formed when ROH leaves and the counter anion is NOT nucleophilic, it will be an E1
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32
Q

why are ethers common solvents

A

they react only with hydrogen halides because of their unreactive nature (easier to perform organic reactions)

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33
Q

describe the synthesis of an epoxide

A
  • see slide show
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34
Q

what is an epoxide

A

an ether with a highly reactive three-membered ring

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35
Q

why are epoxides more reactive than ethers

A
  • more strained, higher energy epoxide has smaller delta G double dagger for reaction (threshold for it to react is less than a normal ether).
36
Q

what happens when an acid-catalyzed epoxide ring reacts with water

A
  • epoxides can ring-open via acid catalyzed reaction with water to give TRANS 1,2 diols
  • under acidic conditions, protonate the oxygene to make it into a good leaving group
  • fast ring opening (even at room temp.) due to strained reactive three-membered ring

H3O+ —> X —-> OH2

37
Q

describe nucleophilic substitution of an unsymmetrical epoxide under acidic conditions (where does Nu attack)

A
  • the nucleophile preferentially attackes the more substituted ring carbon
38
Q

why is the more substituted ring carbon attacked for a nucleophilic substitution of an unsymmetrical epoxide in acidic conditions

A
  • less substituted carbon produces a primary carbocation
  • more substituted carbon produces a secondary carbocation
  • there is a high degree of SN1-like carbocation character in the transition state, which leads to the backside attack of the nucleophile at the tertiary center and to formation of a product isomer that has the -Br and -OH groups
  • essentially roots back to carbocation stability
39
Q

describe nucleophilic substitution of an unsymmetrical epoxide under basic or neutral conditions

A
  • the important factor is sterics, with reaction occurring at the least substituted carbon in the three membered ring
  • under basic conditions, the less substituted carbon in the epoxide reacts
40
Q

why is the more substituted ring carbon attacked for a nucleophilic substitution of an unsymmetrical epoxide in basic conditions

A
  • The more substituted carbon of the epoxide is often more hindered by bulky substituents compared to the less substituted carbon. As a result, the nucleophile finds it easier to approach and attack the less hindered carbon atom.
41
Q

what are epoxide reaction with grignard reagants best used in for synthesis reactions

A
  • the reaction of organometallic nucleophiles with epoxides is a great way to extend a carbon chain by (at least) two carbon atoms.
42
Q

why do we use epoxides in synthesis

A
  • carbocation rearrangements can and often do occur during the acid-catalyzed addition of water (helps with more predictable outcomes.
  • Carbocation rearrangements can occur during these reactions, leading to different products. By using epoxides instead of alkenes, the formation of carbocation intermediates is avoided.
43
Q

define a thiol

A
  • capture and bind to mercury
  • have a pKa of ~10 (stronger acids than alcohols with a pKa of ~15) –> thiols don’t bond to H bonds as easily as alcohols do
  • thiols have lower boiling points than alcohols because thiols do not form strong hydrogen bonds
  • prepared via SN2 chemistry
44
Q

key feature of thiolate anions

A

thiolate ions are very good nucleophiles and are better nucleophiles in a protic solvent than alkoxide ions.

45
Q

does the sulfur atom of a thiolate make a good leaving group

A

the positive charge on the sulfur atom makes it a good leaving group as you regenerate the lone pair

46
Q

carboxylic acid vs. aldehyde and ketone in terms of leaving groups

A

C bonded to R and Z and double bonded to O:
- where Z = R or H (ketone or aldehydes) –> Z-group cannot leave
- where Z = OH, OR, X, or NH2 (carboxylic acid) –> Z-group can leave

47
Q

formaldehyde vs. aldehyde vs. ketone

A

formaldehyde –> two hydrogens
aldehyde –> a hydrogen and an alkyl / aryl group
ketone –> two alkyl / aryl groups

48
Q

describe the nomenclature of aldehydes

A

-al

ex. methanal or hexanedial

49
Q

describe the nomenclature for an aldehyde on a ring

A

-carbaldehyde

ex. cyclohexanecarbaldehyde or benzenecarbaldehyde

50
Q

describe the nomenclature of ketones

A

-one

ex. propanone or hexanone

51
Q

what happens when you mix a ketone and an aldehyde

A

the ketone always wins (the ktone is named “oxo” when a second group has higher priority)

ex. 4-oxopentanal (ketone + aldehyde)

52
Q

describe an acyl group

A

C bonded to an R group and double bonded to oxygen

53
Q

describe an acetyl group

A

C bonded to CH3 and double bonded to O

54
Q

describe a formyl group

A

C bonded to H and double bonded to O

55
Q

describe a benzoyl group

A

C bonded to benzene and double bonded to O

56
Q

describe how carbonyls are electrophiles

A

the carbonyl group in an all carbonyl-containing molecule can be polarized. the partial positive charge on the carbon makes the carbonyl more electrophilic and this the carbonyl can be attacked by nucleophiles. Hence, carbonyl compound has a built in dipole.

57
Q

tetrahedral intermediate vs. carbonyl compound

A

they can interchange between each other with the addition / removal of a nucleophile

tetrahedral intermediate:
- sp^3 hybridized carbon
- if it has a leaving group, it’s a carboxylic acid

carbonyl compound:
- sp^2 hybridized carbon

58
Q

what are the fundamental differences in reactivity between aldehydes / ketones and carboxylic acid derivatives

A

aldehydes and ketones:
- the -R and -H compounds CANT act as leaving groups in the nucleophilic substitution reactions

carboxylic acids:
- the -OH, -X, -OR, -SR, -NH2, -OCOR, and -OPO3^2- in these compounds CAN act as leaving groups in nucleophilic substitution reactions

59
Q

what is the difference in how aldehydes / ketones vs. carboxylic acid derivatives interact

A
  • nucleophilic acyl substitution occurs when Y is a group that can be replaced by another group
    (ex. compound + Y can produce the compound with Y attached)
  • in a carboxylic acid derivative, Y- can leave to reform C=O (literally the opposite of the previous)
60
Q

what is the mechanism for a carboxylic acid derivative

A

carboxylic acid derivative (C double bonded to O and single bonded to Y and R) —Nu—> (C double single bonded to O, Nu, Y, and R) —–> (C double bonded to O and single bonded to Nu and R + Y)

Y = -OR (ester), -Cl, NH2, or -OCOR

61
Q

what is the mechanism for a aldehyde / ketone

A

(C double bonded to O and single bonded to two Rs) —Nu—> (C single bonded to two Rs, O, and Nu) —HA—> (C single bonded to two Rs, OH, and Nu)

62
Q

explain how some additions of nucleophiles to aldehydes and ketones lead to an addition-elimination product

A
  • addition-elimination can occur when the nucleophile has a lone pair on the attacking atom (O-, N-, or S-)
  • water, rather tan the other groups attached to the carbonyl, is eliminated
63
Q

are aldehydes or ketones more reactive

A

aldehydes are, with:
most reactive
- formaldehyde
- aldehyde
- ketone
least reactive

64
Q

why are aldehydes more reactive than ketones

A
  • an aldehyde has a greater partial positive charge on its carbonyl carbon than does a ketone
  • an alkyl is more electron donating than a hydrogen atom and thus the carbonyl of a ketone is less positive
  • ketones have a greater steric crowding in their transition states, so they have high energy transition states (harder to form) than aldehydes do.
    • the carbonyl carbon of an aldehyde is more accessible to a
      nucleophile
65
Q

which are more reactive: aldehydes / ketones or acyl halides / anhydrides

A

acyl halides / anhydrides

66
Q

which are more reactive: aldehydes / ketones or carboxylic acids / amides / carboxylate ions

A

aldehydes / ketones

67
Q

how can a new carbon-carbon bond be formed

A

the grignard reaction
- grignard reagants (organometallics) react with aldehydes, ketones, and carboxylic acid derivatives
- tetrahedral intermediate is produced bu it doesn’t have a good leaving group
- alcohol product is obtained

68
Q

describe how grignard reagants react with carbon dioxide to form a carboxylic acid

A
  • the carboxylic acid has one more carbon than the grignard reagant
  • 1 carbon reactant + 3 carbon reactant –> 4 carbon intermediate —> 4 carbon product
69
Q

why are numbers used in the reactants for grignard reagants

A

the reagants are added stepwise, e.g. aqueous acid is not added until the Grignard reagant has reacted completely with the carbonyl compound

70
Q

when a reaction creates an asymmetric center from achiral reactants, what happens

A

a racemic mixture is produced

71
Q

what happens when esters react with grignard reagants

A

One of the Os (the ester O thats not double bonded) is taken away and replaced with one of the R groups
forms tertiary alcohols
- two equivalents of grignard reagant are consumed which means that two of the R groups in the tertiary alcohol must be the same (CH3)

72
Q

what are the reactants for the williamson ether synthesis

A

alcohol-R
halide-R
**R group on halide must be smaller

73
Q

for williamson ether synthesis, what is the rxn

A

two reactants become one product (O is broken apart)

74
Q

what type of reaction is williamson ether synthesis

A

SN2

75
Q

what are the reactants for the acid-catalyzed reaction and which rxn type goes with which

A

HI, HBr, or CF3CO2H
Sn1 and SN2 –> HI and HBr because of the strong nucleophile
E1 –> CF3CO2H because of the weak nucleophile

76
Q

for acid-catalyzed reaction, what is the rxn

A

one reactant becomes two product (O is split apart)

77
Q

for acid-catalyzed reaction, which product gets the halide and which product gets the hydrogen (in the form of an alcohol)

A

the more stable carbocation gets the halide (both OH and halide attach at the end)

78
Q

SN1 rxn

A
  • 2 step, carbocation
    - stability is important
    - rearrangement can occur
79
Q

SN2 rxn

A
  • 1 step, concerted
  • sneaky
80
Q

E1 rxn

A
  • 2 step, carbocation
    - stability is important
    - rearrangement
    - weak Nu
81
Q

how to differentiate between elimination and substitution

A

elimination: double bond form, weak Nu (highly electronegative atom is a poor nucleophile because it is unwilling to share its electrons)

substitution: strong Nu

82
Q

how to differentiate between SN1 and SN2 (based on structure of compound)

A

if R is tertiary, allylic, or benzylic, then SN1 because a stable carbocation is formed

83
Q

reactants for thiol rxn

A

strong base (ex. NaH) and halide

84
Q

what type of rxn is a thiol rxn

A

SN2

85
Q

what is the configuration of the products for a thiol rxn

A

sneaky rxn (SN2), so changed around and the stereoisomer is produced (ex. if reactant halide is trans, product with the S that attaches where the halide use to be is cis)

86
Q

thiol rxn mechanism

A

R-SH –reactants–> R-S-halide with S replacing the halogen

OCHEM 335 (7 decks)

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