Chapter 4: Organic Reactions Flashcards
Lewis acid (4)
any species (molecule or ion) that can accept a pair of electrons
(an electron acceptor in the formation of a covalent bond)
tend to be electrophiles
often positively charged atoms
Lewis base (4)
any species (molecule or ion) that can donate a pair of electrons
(an electron donor in the formation of a covalent bond)
tend to be nucleophiles
often anions (negative charge)
coordinate covalent bonds
covalent bonds in which both electrons in the bond came from the same starting atom (the Lewis base)
formed when Lewis acids and bases interact
Bronsted-Lowry acid
any species that can donate a proton (H+) to another molecule
Bronsted Lowry base
any species that can accept a proton (H+) from another molecule
amphoteric molecules
can act as either a Bronsted-Lowry acid or base, depending on reaction conditions
(ex. H2O can donate H+ to become OH- or accept H+ to become H3O+)
acid dissociation constant (Ka)
measures the strength of an acid in solution
the equilibrium constant corresponding to the dissociation of and acid (HA) into H + A
(larger Ka = stronger acid)
pka
pKa = - log (Ka)
small or negative pKa = ______
acidic molecules
large pKa = ______
basic molecules
strong acids have a pKA value of:
less than -2
weak acids have a pKA value of:
between -2 and 20
strong acids __________ in aqueous solution
dissociate completely
pKa trends in the periodic table
pka values for common functional groups:
list the acids from weakest to strongest
as bond strength decreases, acidity _______
increases
note: bond strength increases down the periodic table
the more electronegative an atom, the _______ the acidity
higher
doe bond strength or electronegativity take precedence when the 2 trends oppose each other?
bond strength
functional groups that act as acids
alcohols, aldehydes and ketones, carboxylic acids and most carboxylic acid derivatives
functional groups that act as bases
amines and amides
an acid-base reaction will proceed if the conjugate products are _______ than the reactants
weaker (less reactive, more stable)
nucleophiles
“nucleus-loving” species (nucleus is + charge so these species have a region of full or partial negative charge)
contain lone pairs or pi bonds
have a region of high electron density
often carry a negative charge
good nucleophiles tend to be good ______ (acids or bases)
bases
nucleophilicity is determined by 4 major factors
charge: ⇡ electron density (more negative charge) = ⇡ nucleophilicity
electronegativity = ⇡ electronegativity = ⇣ nucleophilicity
steric hindrance: bulkier molecules = less nucleophilic
solvent: protic solvens hinder nucleophilicity
polar protic solvents
contain at least one hydrogen atom connected directly to an electronegative atom
in polar protic solvents, Nucleophilicity increases ______ (up/down) the periodic table
down
in protic solvents, nucleophilicity decreases in the following order:
(4 halogens)
I- > Br- > Cl- > F-
in protic solvents, nucleophilicity decreases in the following order:
(4 halogens)
F- > Cl- > Br- > I-
polar aprotic solvents
have a medium range of polarity
can have hydrogen atoms somewhere in their structures, but no hydrogen atom is directly connected to an electronegative atom
in polar aprotic solvents, Nucleophilicity increases ______ (up/down) the periodic table
up
electrophiles
“electron-loving”
have a region of low electron density (positive or partial positive charge)
contain a positive charge or are positively polarized
more positive compounds are more electrophilic
accept an electron pair when forming new bonds with a nucleophile
electrophiles almost always act as Lewis _____ (acids/bases)
acids
list 4 functional groups that can act as electrophiles
alcohols, aldehydes, ketones, carboxylic acids (and their derivatives)
leaving groups
molecules fragments that retain electrons after heterolysis
a species that leaves an electrophile with an electron pair
the leaving group must be able to stabilize the electrons it leaves with
the best leaving groups can:
best stabilize the extra electrons
_________ are good leaving groups
weak bases
heterolytic reactions
a bond is broken and both electrons are given to one of the two products
the opposite of coordinate covalent bond formation
what makes a good leaving group?
weak bases (the conjugate bases of strong acids)
nucleophilic substitution reactions
a class of reactions in which one group is exchanged for another
a nucleophile forms a bond with a substrate carbon and a leaving group leaves
SN1 reactions
unimolecular nucleophilic substitution reactions
step 1: loss of a leaving groups (rate determining step); a carbocation is formed
step 2: the nucleophile attacks the carbocation
attack from either side of the cation
SN1 reaction rate
rate = k [substrate]
dependent upon the rate at which the leaving groups leaves
SN2 rate
rate = K [substrate] [nucleophile]
note: substrate = R—leaving group
SN2 reactions
bimolecular nucleophilic substitution reactions
step 1: loss of a leaving groups AND nucleophilic attack
“concerted” reaction
backside attack
“stereospecific” reaction: inversion causes a flip from R to S configuration or vice versa
effect of degree of substitution on SN2 reaction rates
less sterically hindered electrophiles react more readily under SN2 conditions
effect of degree of substitution on SN1 reaction rates
tertiary carbocations are more stable than secondary carbocations, which are more stable than primary carbocations
how to determine SN1 vs sN2 based on substrate substitution
how to determine SN1 vs sN2 based on the nucleophile (strong vs weak)
how to determine SN1 vs sN2 based on the leaving group (good vs excellent)
how to determine SN1 vs sN2 based on the solvent (polar aprotic vs protic)
what does SN2 stand for?
S = substitution
N = nucleophilic
2 = bimolecular (number of species involved in the rate determining step)
why are SN2 reactions called bimolecular
they have ONE step which involves TWO chemical entities
redox reactions
the oxidation states of the reactants change
oxidation state
an indicator of the hypothetical charge that an atom would have if all bonds were completely ionic
(note: don’t need to know how to assign oxidation states, just know the definition of oxidation/reduction)
oxidation
an increase in oxidation state (loss of electrons)
usually involves decreasing bonds to hydrogen and increasing bonds to oxygen (or other heteroatoms)
heteroatoms
atoms besides carbon and hydrogen
oxidizing agent
the oxidizing agent accepts electrons and is itself reduced
tend to contain metals bonded to a large number of oxygen atoms
reduction
a decrease in oxidation state (gain in electrons) assisted by reducing agents
usually means increasing bonds to hydrogen and decreasing bonds to other atoms
reducing agents
donate electrons (are themselves oxidized)
have low electronegativity and ionization energy
often contain metals bonded to a large number of hydrogens
chemoselectivity
the preferential reaction of one functional group in the presence of other functional groups
(a key skill is recognizing which reactions will occur by recognizing the reactive regions within a molecule!)
steric protection
what do nucleophiles do?
they provide a pair of electrons to form a new covalent bond
imine
a compound with a nitrogen atom double bonded to a carbon atom
enamines
contain both a double bond and a nitrogen-containing group
condensation reaction
2 molecules are combined to form a single molecule, usually with the loss of a small molecule such as water
if water is lost, the reaction is also known as a dehydration synthesis
imine formation
cyanohydrins
a functional group in which a cyano and a hydroxy group are attached to the same carbon atom
form when hydrogen cyanide (HCN) reacts with aldehydes or ketones
cyanide functions as a nucleophile, attacking the carbonyl carbon
alkyl halides
an alkane with one or more halogens attached (fluorine, chlorine, bromine, iodine)
