Ch. 4: Analyzing Organic Reactions Flashcards
summary: what happens in an acid-base reaction? (3)
- an acid and a base react
- resulting in the formation of the conjugate base of the acid and the conjugate acid of the base
- this reaction proceeds so long as the reactants are more reactive, or stronger, than the products that they form
in the MCAT, we will focus on Lewis and Bronsted-Lowry definitions of acids and bases, summarize each of these definitions focus on
Lewis concerns itself with the transfer of electrons in the formation of coordinate covalent bonds
Bronsted Lowry focuses on proton transfer
defn: Lewis acid
what is going on with their p-orbitals? are they positive or negative?
are they electrophiles or nucleophiles?
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
defn: Lewis base
what is going on with their p-orbitals? are they positive or negative?
are they electrophiles or nucleophiles?
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
why do coordinate covalent bonds form? + defn
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)
defn: Bronsted-Lowry acid and base
acid: a species that can donate a proton (H+)
base: a species that can accept a proton
defn: amphoteric
molecules, like water, that have the ability to act as either Bronsted-Lowry acids or bases
explain how water is amphoteric
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+
the degree to which an amphoteric molecule acts as an acid or base is dependent on what?
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
what are 3 common amphoteric molecules other than water?
Al(OH)3
HCO3-
HSO4-
func + eqn: acid dissociation constant (Ka)
measures the strength of an acid in solution
Dissociation: HA <–> H+ + A-
eqn + intepretation: pKa
more acidic molecules: smaller (or negative) pKa
more basic molecules: larger pKa
what range of pKa’s corresponds to strong acids? weak acisd?
pKa < -2 = strong acids (almost always dissociate completely in aqueous solution)
-2 < pKa < 20 = weak organic acids
what 2 periodic trends commonly contribute to acidity? describe how they increase or decrease with acidity. which takes precedence when they oppose each other?
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
defn: alpha-hydrogens
connected to the alpha-carbon (the carbon adjacent to the carbonyl) in carbonyl compounds
why are alpha-hydrogens easily lost?
because the enol form of carbonyl-containing carbanions is stabilized by resonance
what functional groups act as acids? (5)
- alcohols
- aldehydes at the alpha-carbon
- ketones at the alpha-carbon
- carboxylic acids
- most carboxylic acid derivatives
what type of reactants are these acidic functional groups easy targets of? why?
these compounds are easier to target with basic (or nucleophilic) reactants because they readily accept a lone pair
what are the two main functional groups that act as bases?
where should we keep an eye out for these compounds?
- amines
- amides
keep an eye out for them in the formation of peptide bonds
how can amines form coordinate covalent bonds?
the nitrogen atom of an amine can form coordinate covalent bonds by donating a lone pair to a Lewis acid
what two groups can almost all reactions in orgo be divided into?
- redox reactions
- nucleophile-electrophile reactions
nucleophiles, electrophiles, and leaving groups are particularly important to the reactions of what 2 types of compounds?
- alcohols
- carbonyl-containing compounds
defn: nucleophiles
nucleus-loving species with either lone pairs or pi bonds that can form new bonds to electrophiles
good nucleophiles tend to be good bases, however what is the distinction between the two?
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)
what 3 groups are common examples of nucleophiles?
- anions
- pi bonds
- atoms with lone pairs
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?
the more reactive it is
holds across a row, does not hold down a column
what 4 major factors if nucleophilicity determined by? how does nucleophilicity increase or decrease in relation to these?
- CHARGE = nucleophilicity increases with increasing electron density (more negative charge)
- ELECTRONEGATIVITY = nucleophilicity decreases as electronegativity increases because these atoms are less likely to share electron density
- STERIC HINDRANCE = bulkier molecules are less nucleophilic
- SOLVENT = protic solvents can hinder nucleophilicity by protonating the nucleophile or through hydrogen bonding
summarize: the solvent effect on nucleophilicity
in polar protic solvents, nucleophilicity increases DOWN the periodic table
in polar aprotic solvents, nucleophilicity increases UP the periodic table
what is the main difference in the functionality of protic solvents and aprotic solvents?
protic solvents can hydrogen bond, aprotic solvents can’t hydrogen bond
what are 2 common groups of protic solvents?
what are 3 common aprotic solvents?
PROTIC: carboxylic acids, water/alcohols
APROTIC: DMF, DMSO, acetone
if a solvent is not given on test day, should you assume that the reaction occurs in a polar or nonpolar solvent? why?
polar
polar solvents, whether protic or aprotic, can dissolve nucleophiles and assist in any reaction in which electrons are moved
the halogens are good examples of the effects of the solvent on nucleophilicity. explain.
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
nucleophilicity relates directly to basicity in protic or aprotic solvents?
aprotic solvents
what are 4 examples of strong nucleophiles? 2 fair? and 3 weak or very weak?
STRONG: HO-, RO-, CN-, N3-
FAIR: NH3, RCO2-
WEAK/VERY WEAK: H2O, ROH, RCOOH
in terms of functional groups, what groups tend to make good nucleophiles?
amine groups
defn: electrophiles
electron-loving species with a positive charge or positively polarized atom that accepts an electron pair when forming new bonds with a nucleophile
True or false: electrophiles will almost always act as Lewis acids in reactions
true