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MCAT Ochem > Ch 4 - Analyzing Organic Reactions > Flashcards

Ch 4 - Analyzing Organic Reactions Flashcards

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MCAT flashcards
1
Q

What are Lewis acids and bases?

A
  • acids: electron acceptors; have vacant orbitals or positively polarized atoms
  • bases: electron donors; have a lone pair of electrons and are often anions
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2
Q

What are Bronsted-Lowry acids and bases?

A
  • acid: proton donors

- bases: proton acceptors

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3
Q

What are amphoteric molecules?

A
  • can act as either acids or bases, depending on reaction conditions
  • water is a common example as well as bicarbonate and dihydrogen phosphate
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4
Q

What is the acid dissociation constant, Ka?

A
  • a measure of the acidity
  • the equilibrium constant corresponding to the dissociation of an acid, HA, into a proton (H+) and its conjugate base (A-)
    Ka = [H+][A-]/[HA]
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5
Q

What is pKa and its periodic trend?

A
  • the negative logarithm of Ka (pKa = -logKa)
  • a lower (or even negative) pKa indicates a stronger acid
  • decreases down the periodic table and increases with electronegativities
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6
Q

What are common acidic functional groups?

A
  • alcohols, aldehydes, ketones, carboxylic acids and their derivatives
  • an alpha-hydrogen (hydrogens connected to an alpha-carbon, a carbon adjacent to a carbonyl) are acidic
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7
Q

What are common basic functional groups?

A

amines and amides

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8
Q

What are nucleophiles?

A
  • nucleus loving and contain lone pairs or pi bonds

- have increased electron density and often carry a negative charge

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9
Q

How is nucleophilicity relate to basicity?

A

it is similar however, nucleophilicity is a kinetic property, while basicity is thermodynamic

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10
Q

What can affect nucleophilicity?

A
  • charge: n increases with increasing electron density (more negative charge)
  • electronegativity: n decreases as EN increases because these atoms are less likely to share electron density
  • steric hindrance: bulkier molecules are less N
  • solvent: protic solvents can hinder N by protonating the nucleophile through H bonding
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11
Q

What are common organic nucleophiles?

A

amino groups

  • anions: want to form bonds within nearby positive charge
  • double/triple bonds: have extra electron within pi bond and increased electron density
  • lone pairs
  • good base
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12
Q

What are electrophiles?

A
  • electron loving and contain a positive charge or positively polarized
  • more positive compounds are electrophilic
  • can accept electrons to make bonds
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13
Q

What are common electrophiles?

A

alcohols, aldehydes, ketones, carboxylic acids and their derivatives

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14
Q

What are leaving groups and makes a good leaving group?

A
  • molecular fragments that retain the electrons after heterolysis
  • the best-leaving groups can stabilize additional charge through resonance or induction: will dissociate along with the electron in its bond and be more stable than nucleophile before it reacted
  • weak bases (the conjugate bases of strong acids) make good leaving groups, halogens (especially further down group), inorganic esters, water, alcohol, ethers, tosylate ion
  • alkanes and hydrogen ions are almost never leaving groups because they form reactive anions (strong bases, carbanions)
  • nucleophile must be stronger base than leaving group
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15
Q

What are the steps followed for unimolecular nucleophilic substitution (Sn1) reactions to proceed?

A
  • in the first step, the leaving group leaves, forming a carbocation, an anion with a positively charged carbon atom
  • in the second step, the nucleophile attacks the planar carbocation from either side, leading to a racemic mixture of products
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16
Q

What do Sn1 reactions prefer?

A

more substituted carbons because the alkyl groups can donate electron density and stabilize the positive charge of the carbocation

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17
Q

What does the rate of Sn1 reactions depend on?

A

only on the concentration of the substrate: rate = k[R-L]

18
Q

What are the steps followed for biomolecular nucleophilic substitution (Sn2) reactions to proceed?

A
  • the nucleophile attacks at the same time as the leaving group leaves
  • the nucleophile must perform a backside attack, which leads to an inversion of stereochemistry
  • the absolute configurations is changed - R to S and vice versa - if the incoming nucleophile and the leaving group have the same priority in the molecule
19
Q

What do Sn2 reactions prefer?

A

less substituted carbons because the alkyl groups create steric hindrance and inhibit the nucleophile from accessing the electrophilic substrate carbon

20
Q

What does the rate of Sn2 reactions depend on?

A

the concentration of both the substrate and the nucleophile: rate = k[Nu:][R-L]

21
Q

What is the oxidation state and which have the highest and lowest states?

A
  • the charge an atom would have if all its bonds were completely ionic
  • carboxylic acids and their derivatives are the most oxidized functional groups; followed by aldehydes, ketones, and imines; followed by alcohols, alkyl halides, and amines
22
Q

What compounds have the lowest and highest oxidation state of carbon?

A

CH4 is the lowest oxidation state of carbon (most reduced); CO2 is the highest (most oxidized)

23
Q

What is oxidation?

A

an increase in oxidation state and is assisted by oxidizing agents

24
Q

What are oxidizing agents?

A
  • accept electrons and are reduced in the process
  • have a high affinity for electrons or an unusually high oxidation state
  • often contain a metal and a large number of oxygens
25
Q

What can primary alcohols be oxidized to?

A
  • aldehydes by pyridinium chlorochromate (PCC)

- carboxylic acids by strong oxidizing agents like chromium trioxide (CrO3) or sodium or potassium dichromate

26
Q

What can secondary alcohols be oxidized to?

A

ketones by most oxidizing agents

27
Q

What can aldehydes be oxidized to?

A

carboxylic acids by most oxidizing agents

28
Q

What is reduction?

A

a decrease in oxidation state and is assisted by reducing agents

29
Q

What are reducing agents?

A
  • donat electrons and are oxidized in the process
  • have low electronegativity and ionization energy
  • often contain a metal and a large number of hydrides
30
Q

What can aldehydes, ketones, and carboxylic acids be reduced to?

A

alcohols by lithium aluminum hydride

31
Q

What can amides and esters be reduced to?

A
  • amides –> amines by LiAlH4

- esters –> a pair of alcohols by LiAlH4

32
Q

When will an acid-base reaction proceed, based on the strength of the reactants and products?

A

acid-base reactions will proceed when the acid and base react to form conjugates that are weaker than the reactants

33
Q

How do the definitions of nucleophile and electrophile differ from those of Lewis base and acid?

A
  • N and E are based on relative rates of reactions and are therefore kinetic properties
  • A and B are measured by the position and equilibrium in a protonation or deprotonation reaction and are therefore thermodynamic properties
34
Q

How must the nucleophile and leaving group be related in order for a substitution to proceed?

A

a substitution reaction will proceed when the nucleophile is a stronger base (more reactive) than the leaving grou

35
Q

Will an Sn1 reaction occur faster in tertiary or secondary carbons?

A
  • tertiary where a carbocation can be most easily stabilized
36
Q

What are the 6 steps for solving organic chemistry reactions?

A
  • know nomenclature
  • identify functional groups
  • identify other reagents
  • identify most reactive functional groups
  • identify the first step of the reaction
  • consider stereoselectivity
37
Q

If there are no reaction conditions listed, what determines how the reaction will proceed?

A

the properties of the functional groups on the reactants themselves

38
Q

Why do Sn1 reactions show first-order kinetics?

A

because the rate-limiting step involves only one molecule

39
Q

Why are aldehydes generally more reactive than equivalent ketones to nucleophiles?

A
  • difference in steric hindrance

- aldehydes have one alkyl group connected to the carbonyl carbon, wheras ketones have 2 creating more steric hindrance

40
Q

What is the hierarchy to the reactivity of carboxylic acid derivatives toward nucleophiles?

A

anhydrides > carboxylic acids and esters > amides

- higher can form lower, but not vice versa

MCAT Ochem (12 decks)

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