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Orgo > Ch. 4: Analyzing Organic Reactions > Flashcards

Ch. 4: Analyzing Organic Reactions Flashcards

1
Q

summary: what happens in an acid-base reaction? (3)

A
  1. an acid and a base react
  2. resulting in the formation of the conjugate base of the acid and the conjugate acid of the base
  3. this reaction proceeds so long as the reactants are more reactive, or stronger, than the products that they form
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2
Q

in the MCAT, we will focus on Lewis and Bronsted-Lowry definitions of acids and bases, summarize each of these definitions focus on

A

Lewis concerns itself with the transfer of electrons in the formation of coordinate covalent bonds

Bronsted Lowry focuses on proton transfer

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3
Q

defn: Lewis acid

what is going on with their p-orbitals? are they positive or negative?

are they electrophiles or nucleophiles?

A

an electron acceptor in the formation of a covalent bond

have vacant p-orbitals into which they can accept an electron pair

are positively polarized atoms

tend to be electrophiles

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4
Q

defn: Lewis base

what is going on with their p-orbitals? are they positive or negative?

are they electrophiles or nucleophiles?

A

an electron donor in the formation of a covalent bond

have a lone pair of electrons that can be donated

are often anions, carrying a negative charge

tend to be nucleophiles

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5
Q

why do coordinate covalent bonds form? + defn

A

they form when Lewis acids and bases interact

these are covalent bonds in which both electrons in the bond came from the same starting atom (the Lewis base)

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6
Q

defn: Bronsted-Lowry acid and base

A

acid: a species that can donate a proton (H+)

base: a species that can accept a proton

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7
Q

defn: amphoteric

A

molecules, like water, that have the ability to act as either Bronsted-Lowry acids or bases

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8
Q

explain how water is amphoteric

A

can act as an ACID: by donating its proton to a base, and thus becoming its conjugate base OH-

can act as a BASE: by accepting a proton from an acid to become its conjugate acid H3O+

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9
Q

the degree to which an amphoteric molecule acts as an acid or base is dependent on what?

A

it depends upon the properties of the solution – water can only act as a base in an acidic solution, and only as an acid in a basic solution

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10
Q

what are 3 common amphoteric molecules other than water?

A

Al(OH)3

HCO3-

HSO4-

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11
Q

func + eqn: acid dissociation constant (Ka)

A

measures the strength of an acid in solution

Dissociation: HA <–> H+ + A-

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12
Q

eqn + intepretation: pKa

A

more acidic molecules: smaller (or negative) pKa

more basic molecules: larger pKa

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13
Q

what range of pKa’s corresponds to strong acids? weak acisd?

A

pKa < -2 = strong acids (almost always dissociate completely in aqueous solution)

-2 < pKa < 20 = weak organic acids

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14
Q

what 2 periodic trends commonly contribute to acidity? describe how they increase or decrease with acidity. which takes precedence when they oppose each other?

A

bond strength decreases down the periodic table –> acidity increases

the more electronegative an atom –> acidity increases

when they oppose each other: low bond strength takes precedence

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15
Q

defn: alpha-hydrogens

A

connected to the alpha-carbon (the carbon adjacent to the carbonyl) in carbonyl compounds

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16
Q

why are alpha-hydrogens easily lost?

A

because the enol form of carbonyl-containing carbanions is stabilized by resonance

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17
Q

what functional groups act as acids? (5)

A
  1. alcohols
  2. aldehydes at the alpha-carbon
  3. ketones at the alpha-carbon
  4. carboxylic acids
  5. most carboxylic acid derivatives
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18
Q

what type of reactants are these acidic functional groups easy targets of? why?

A

these compounds are easier to target with basic (or nucleophilic) reactants because they readily accept a lone pair

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19
Q

what are the two main functional groups that act as bases?

where should we keep an eye out for these compounds?

A
  1. amines
  2. amides

keep an eye out for them in the formation of peptide bonds

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20
Q

how can amines form coordinate covalent bonds?

A

the nitrogen atom of an amine can form coordinate covalent bonds by donating a lone pair to a Lewis acid

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21
Q

what two groups can almost all reactions in orgo be divided into?

A
  1. redox reactions
  2. nucleophile-electrophile reactions
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22
Q

nucleophiles, electrophiles, and leaving groups are particularly important to the reactions of what 2 types of compounds?

A
  1. alcohols
  2. carbonyl-containing compounds
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23
Q

defn: nucleophiles

A

nucleus-loving species with either lone pairs or pi bonds that can form new bonds to electrophiles

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24
Q

good nucleophiles tend to be good bases, however what is the distinction between the two?

A

nucleophile strength is based on relative rates of reaction with a common electrophile (and is thus a kinetic property)

base strength is related to the equilibrium position of a reaction (and is thus a thermodynamic property)

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25
Q

what 3 groups are common examples of nucleophiles?

A
  1. anions
  2. pi bonds
  3. atoms with lone pairs
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26
Q

as long as the nucleophilic atom is the same, the more basic the nucleophile, the more or less reactive it is?

does this hold when comparing atoms within the same row of the periodic table? what about down a column?

A

the more reactive it is

holds across a row, does not hold down a column

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27
Q

what 4 major factors if nucleophilicity determined by? how does nucleophilicity increase or decrease in relation to these?

A
  1. CHARGE = nucleophilicity increases with increasing electron density (more negative charge)
  2. ELECTRONEGATIVITY = nucleophilicity decreases as electronegativity increases because these atoms are less likely to share electron density
  3. STERIC HINDRANCE = bulkier molecules are less nucleophilic
  4. SOLVENT = protic solvents can hinder nucleophilicity by protonating the nucleophile or through hydrogen bonding
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28
Q

summarize: the solvent effect on nucleophilicity

A

in polar protic solvents, nucleophilicity increases DOWN the periodic table

in polar aprotic solvents, nucleophilicity increases UP the periodic table

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29
Q

what is the main difference in the functionality of protic solvents and aprotic solvents?

A

protic solvents can hydrogen bond, aprotic solvents can’t hydrogen bond

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30
Q

what are 2 common groups of protic solvents?

what are 3 common aprotic solvents?

A

PROTIC: carboxylic acids, water/alcohols

APROTIC: DMF, DMSO, acetone

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31
Q

if a solvent is not given on test day, should you assume that the reaction occurs in a polar or nonpolar solvent? why?

A

polar

polar solvents, whether protic or aprotic, can dissolve nucleophiles and assist in any reaction in which electrons are moved

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32
Q

the halogens are good examples of the effects of the solvent on nucleophilicity. explain.

A

in PROTIC solvents, nucleophilicity DECREASES in the order: I- > Br- > Cl- > F-

    because the protons in solution will be attracted to the nucleophile
    F- is the conjugate base of HF, a weak acid, so it will form bonds with the protons in solution and be less able to access the electrophile to react
    I- is the conjugate base of HI, a strong acid, so it is less affected by the protons in solution and can react with the electrophile

in APROTIC solvents, nucleophilicity DECREASES in the order: F- > Cl- > Br- > I-

      because there are no protons to get in the way of the attacking nucleophile
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33
Q

nucleophilicity relates directly to basicity in protic or aprotic solvents?

A

aprotic solvents

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34
Q

what are 4 examples of strong nucleophiles? 2 fair? and 3 weak or very weak?

A

STRONG: HO-, RO-, CN-, N3-

FAIR: NH3, RCO2-

WEAK/VERY WEAK: H2O, ROH, RCOOH

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35
Q

in terms of functional groups, what groups tend to make good nucleophiles?

A

amine groups

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36
Q

defn: electrophiles

A

electron-loving species with a positive charge or positively polarized atom that accepts an electron pair when forming new bonds with a nucleophile

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37
Q

True or false: electrophiles will almost always act as Lewis acids in reactions

A

true

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38
Q

what are 2 factors that increase electrophilicity?

A
  1. a greater degree of positive charge
  2. the nature of the leaving group in species without empty orbitals (better leaving groups make it more likely a reaction will happen)
39
Q

how does the activity of a leaving group change if empty orbitals are present on the electrophile?

A

if empty orbitals are present, an incoming nucleophile can make a bond with the electrophile without displacing the leaving group

40
Q

how do the carboxylic acid derivatives rank in terms of electrophilicity?

A

Anhydrides (most reactive)

carboxylic acids and esters

amides

41
Q

defn: leaving groups

A

the molecular fragments that retain the electrons after heterolysis

42
Q

defn: heterolytic reactions

A

essentially the opposite of coordinate covalent bond formation: a bond is broken and both electrons are given to one of the two products

43
Q

what factors (3) or groups (@) makes good leaving groups?

A
  1. are able to stabilize the extra electrons
  2. weak bases (because they are more stable with an extra set of electrons)
  3. the conjugate bases of strong acids
  4. can be augmented by resonance and by inductive effects from electron-withdrawing groups (which help delocalize and stabilize negative charge)
44
Q

why will alkanes and hydrogen ions almost never serve as leaving groups?

A

because they form very reactive, strongly basic anions

45
Q

explain how we can think of leaving groups and nucleophiles as serving opposite functions

A

in substitution reactions, the weaker base (the leaving group) is replaced by the stronger base (the nucleophile)

46
Q

what is true about both Sn1 and Sn2 reactions?

A

a nucleophile forms a bond with a substrate carbon and a leaving group leaves

47
Q

steps: unimolecular nucleophilic substitution (Sn1) reactions

A
  1. the rate-limiting step in which the leaving group leaves, generating a positively charged carbocation
  2. the nucleophile then attacks the carbocation, resulting in the substitution product
48
Q

why is a more substituted carbocation more stable?

A

because the alkyl groups act as electron donors, stabilizing the positive charge

49
Q

since the rate-limiting step is the formation of the carbocation, the rate of the reaction depends only on what?

+ rate eqn

A

the concentration of the substrate

rate = k[R-L] where R-L is an alkyl group containing a leaving group

50
Q

what order reaction is an Sn1 reaction?

A

first order

51
Q

why are the product of Sn1 reactions usually a racemic mixture? what is the impact of this?

A

because Sn1 reactions pass through a planar intermediate before the nucleophile attacks

impact: the incoming nucleophile can attack the carbocation from either side, resulting in varied stereochemistry

52
Q

steps: bimolecular nucleophilic substitution (Sn2) reactions

A

only one step in which the nucleophile attacks the compound at the same time as the leaving group leaves

53
Q

why are Sn2 reactions referred to as concerted? why are they bimolecular?

A

CONCERTED = the reaction only has one step

BIMOLECULAR = the single rate-limiting step involves 2 molecules

54
Q

defn + requirements for (3): backside attack of Sn2 reactions

A

the nucleophile actively displaces the leaving group in a backside attack

  1. the nucleophile must be strong
  2. the substrate cannot be sterically hindered
  3. the less substituted the carbon, the more reactive it is in Sn2 reactions
55
Q

what are the two reacting species involved in the single step of an Sn2 reaction?

A
  1. the substrate (often an alkyl halide, tosylate, or mesylate)
  2. the nucleophile
56
Q

based on this, the concentrations of both have a role in determining the rate (eqn)

A

rate = k[Nu:][R-L]

57
Q

Sn2 reactions are accompanied by an inversion of relative configuration (explain, 2)

A
  1. the position of substituents around the substrate carbon will be inverted
  2. if the nucleophile and leaving group have the same priority in their respective molecules, this inversion will also correspond to a change in absolute configuration from R to S or vice versa
58
Q

defn: stereospecific

A

a reaction in which the configuration of the reactant determines the configuration of the product due to the reaction mechanism

59
Q

what is the main change that occurs in redox reactions?

A

oxidation states of the reactants change

60
Q

func: oxidation state

A

an indicator of the hypothetical charge that an atom would have if all bonds were completely ionic

61
Q

defn + how will we view in orgo: oxidation vs. reduction

A

OXIDATION = an increase in oxidation state = a loss of electrons = increasing the number of bonds to oxygen or other heteroatoms (atoms besides carbon and hydrogen)

REDUCTION = a decrease in oxidation state = a gain in electrons = increasing the number of bonds to hydrogen

62
Q

when does oxidation occur?

what does this mean in practice?

A

when a bond between a CARBON atom and an atom that is LESS electronegative than carbon is replaced by a bond to an atom that is MORE electronegative than carbon

in practice: decreasing the number of bonds to hydrogen and increasing the number of bonds to other carbons, nitrogen, oxygen, or halides

63
Q

defn: oxidizing agent

A

the element or compound in a redox-reaction that accepts an electron from another species

because it is gaining electrons, it is said to be reduced

64
Q

char (2): good oxidizing agent

A
  1. high affinity for electrons (such as O2, O3, Cl2)

OR

  1. unusually high oxidation states (like Mn7+ in permanganate, MnO4-, and Cr6+ in chromate, CrO42-)
65
Q

organize the different functional groups by “levels” of oxidation:

A

Level 0 = no bonds to heteroatoms: alkenes

Level 1: alcohols, alkyl halides, amines

Level 2: aldehydes, ketones, imines

Level 3: carboxylic acids, anhydrides, esters, amides

Level 4 = 4 bonds to heteroatoms: carbon dioxide

66
Q

what can primary alcohols be oxidized to by one level? by another level?

which is more common when using strong oxidizing agents?

can it be made to stop?

A

primary alcohols –> aldehydes –> carboxylic acids

commonly proceeds all the way to the carboxylic acid when using strong oxidizing agents

can be made to stop at the aldehyde level using specific reagents

67
Q

what are 3 examples of strong oxidizing agents?

A
  1. chromium trioxide (CrO3)
  2. sodium dichromate (Na2Cr2O7)
  3. potassium dichromate (K2Cr2O7)
68
Q

What specific reagent can stop the oxidation of primary alcohol to aldehyde to carboxylic acid at the aldehyd level?

A

pyridinium chlrochromate (PCC)

69
Q

what are secondary alcohols oxidized to?

A

ketones

70
Q

what are the 2 key themes you should remember with oxidation reactions?

A
  1. oxidation reactions tend to feature an increase in the number of bonds to oxygen
  2. oxidizing agents often contain metals bonded to a large number of oxygen atoms
71
Q

when does reduction to a carbon occur?

what does this mean in practice?

A

when a bond between a carbon atom and an atom that is MORE electronegative than carbon is replaced by a bond to an atom that is LESS electronegative than carbon

in practice: this usually means increasing the number of bonds to hydrogen and decreasing the number of bonds to other carbons, nitrogen, oxygen, or halides

72
Q

what are good reducing agents? why? (2 groups each with 4 examples)

A
  1. sodium, magnesium, aluminum, zinc: have low electronegativies and ionization energies
  2. metal hydrides (NaH, CaH2, LiAlH4, NaBH4): contain H- ion
73
Q

what are aldehydes reduced to? what are ketones reduced to?

A

aldehydes –> primary alcohols

ketones –> secondary alcohols

74
Q

LiAlH3 is a common reducing agent, what does it reduce …

amides to?

carboxylic acids to?

esters to?

A

amides –> amines

carboxylic acids –> primary alcohols

esters –> a pair of alcohols

75
Q

what are the 2 key themes you should remember with reduction reactions?

A
  1. tend to feature an increase in the number of bonds to hydrogen
  2. reducing agents often contain metals bonded to a large number of hydrides
76
Q

defn: chemoselectivity

A

the preferential reaction of one functional group in the presence of other functional groups

77
Q

which site is the reactive site of a molecule depends on what?

A

the type of chemistry that’s occurring BUT the more oxidized the functional group, the more reactive it is in BOTH nucleophile-electrophile reactions and redox reactions

78
Q

why are aldehydes generally more reactive toward nucleophiles than ketones?

A

because they have less steric hindrance

79
Q

a nucleophile is looking for a good electrophile, so why are carboxylic acids and their derivatives the first to be targeted by a nucleophile? what follows?

A

the more oxidized the carbon –> the more electronegative groups around it –> the larger partial positive charge it will experience

carboxylic acids + derivatives
aldehyde or ketone
alcohol or amine

80
Q

why is the carbonyl carbon a common reactive site? (3)

A
  1. the carbon of the carbonyl acquires a positive polarity due to the electronegativity of the oxygen
  2. thus, the carbonyl carbon becomes electrophilic and can be a target for nucleophiles
  3. further, the alpha-hydrogens are much more acidic than in a regular C-H bond due to the resonance stabilization of the enol form
81
Q

what happens when the alpha hydrogens of the carbonyl can be deprotonated easily with a strong base? (2)

A
  1. an enolate forms, which can then become a strong nucleophile
  2. alkylation can result if good electrophiles are available
82
Q

consider the potential of the substrate carbon as a reactive site in Sn1 and Sn2 reactions

A

Sn1: prefer tertiary to secondary carbons as reactive sites and secondary to primary due to them having to overcome the barrier of carbocation stability

Sn2 (have a bigger barrier in steric hindrance): methyl and primary carbons are preferred over secondary, tertiary carbons won’t react

83
Q

defn: steric hindrance/steric protection

A

the prevention of reactions at a particular location within a molecule due to the size of substituent groups

84
Q

in what 2 circumstances can steric protection be helpful? how so?

A
  1. in the synthesis of desired molecules
  2. in the prevention of the formation of alternative products

bulky groups make it impossible for the nucleophile to reach the most reactive electrophile, making the nucleophile more likely to attack another region

85
Q

explain how sterics come into play in the protection of leaving groups + when this is helpful to use

main concept + 2 steps

A

main concept: one can temporarily mask a reactive leaving group with a sterically bulky group during synthesis

  1. reduction of a molecule containing both carboxylic acids and aldehydes or ketones can result in reduction of all of the functional groups
  2. to prevent this, the aldehyde or ketone is first converted to a nonreactive acetal or ketal, which serves as a protecting group, and the reaction can proceed
86
Q

what is another example of a protective reaction?

A

the reversible reduction of alcohols to tert-butyl ethers

87
Q

what are the 6 steps to orgo problem solving?

A
  1. Know your nomenclature
  2. Identify the functional groups
  3. Identify the other reagents
  4. Identify the most reactive functional group(s)
  5. Identify the first step of the reaction
  6. Consider stereospecificity/stereoselectivity
88
Q

explain step 1 to orgo problem solving: know your nomenclature

A

know which compounds IUPAC and common names refer to

89
Q

explain step 2 to orgo problem solving: identify the functional groups

A

What functional groups are in the organic molecules? Do they act as acids or bases? How oxidized is the carbon? Are there functional groups that act as good nucleophiles, electrophiles, or leaving groups?

Helps to define a category of reactions that can occur

90
Q

explain step 3 to orgo problem solving: identify the other reagents

A

determine the properties of the other reagents

Are they acidic or basic? Are they suggestive of a particular reaction? Are they good nucleophiles or a specific solvent? Are they good oxidizing or reducing agents?

91
Q

explain step 4 to orgo problem solving: identify the most reactive functional group(s)

A

more oxidized carbons tend to be more reactive to both nucleophile-electrophile reactions and redox reactions

note the presence of protecting groups that exist to prevent a particular functional group from reacting

92
Q

explain step 5 to orgo problem solving: identify the first step of the reaction

involves acid or base? nucleophile? oxidizing or reducing agent?

A

if it involves an acid or base: usually protonation or deprotonation

if it involves a nucleophile: usually for the nucleophile to attack the electrophile, forming a bond with it

if it involves an oxidizing or reducing agent: the most oxidized functional group will be oxidized or reduced, accordingly

once you know what will react, think through how the reaction will go:

did the protonation or deprotonation of a functional group increase its reactivity? when the nucleophile attacks, how does the carbon respond to avoid having 5 bonds? does a leaving group leave, or does a double bond get reduced to a single bond?

93
Q

explain step 6 to orgo problem solving: CONSIDER STEREOSPECIFICITY and STEREOSELECTIVITY (does not apply to all)

A

stereospecificity: consider whether the configuration of the reactant necessarily leads to a specific configuration in the product (i.e. SN2)

stereoselectivity: occurs in reactions where one configuration of product is more readily formed due to product characteristics
- seen often bc different products have different traits that affect relative stability
- if there is more than one product: the major product will most often be determined by differences in strain or stability between the two molecules
- more strained molecules are less like likely to form molecules than less strained
- products conjugation (alternating single and multiple bonds) are more stable than those without

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