ch 4 - Analyzing Organic Reactions Flashcards
Lewis acids and bases
focus on formation of coordinate covalent bonds
Bronsted-Lowry acids and bases
focus on proton transfer
Lewis acid
an electron acceptor in the formation of a covalent bond; tend to be electrophiles; vacant p-orbitals into which they can accept an electron pair, or are positively polarized atoms
Lewis base
an electron donor in the formation of a covalent bond; tend to be nucleophiles; have a lone pair of electrons that can be donated and are often anions carrying a negative charge
coordinate covalent bonds
covalent bonds in which both electrons in the bond came from the same starting atom
Bronsted-Lowry acid
species that can donate a proton (H+)
Bronsted-Lowry base
species that can accept a proton (H+)
amphoteric
species that are able to act as either Bronsted-Lowry acids or bases; examples are water, Al(OH)3, (HCO3)-, (HSO4)-
acid dissociation constant (K sub a)
measures strength of an acid in solution given by K sub a = ([H+][A-])/[HA] acid = HA
pK sub a
pKa = -log Ka; acids will have a smaller or even negative pKa, bases will have larger. Acids with pKa under -2 are considered strong acids; weak acids range from about -2 to 20
alpha-hydrogens
those connected to the alpha-carbon, which is the carbon adjacent to the carbonyl; because the enol form of carbonyl-containing carbanions is stabilized by resonance, these are acidic and are easily lsot
common functional group acids
alcohols, aldehydes and ketones, carboxylic acids, most carboxylic acid derivatives
common functional group bases
amines and amides
nucleophiles
nucleus-loving species with either lone pairs or pi bonds that can form new bonds to electrophiles; good ones tend to be good bases but strength of these is based on relative rates of reaction with a common electrophile - and is therefore a kinetic property; look for carbon, hydrogen, oxygen or nitrogen (CHON) with a minus sign or lone pair
four factors that determine nucleophilicity
charge (increases with increasing electron density - more neg charge); electronegativity (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
nucleophilicity in protic solvents
I- > Br- > Cl- > F- in polar protic solvents, nucleophilicity increases down the periodic table; protons in solution will be attracted to the nucleophile; I- is conjugate base of strong acid HI
nucleophilicity in aprotic solvents
F- > Cl- > Br- > I- ; there are no protons to get in the way of the attacking nucleophile in these solvents, nucleophilicity relates directly to basicity; increases up the periodic table
functional group that makes good nucleophile
amine
electrophiles
electron-loving species with positive charge or positively polarized atom that accepts an electron pair when forming new bonds with a nucleophile; a kinetic property while acidity (and basicity) are thermodynamic properties but these almost always act as Lewis acids in reactions
carboxylic acid derivatives ranked by electrophilicity
Anhydrides > carboxylic acids and esters > amides
leaving groups
molecular fragments that retain the electrons after heterolysis
heterolytic reactions
the opposite of coordinate covalent bond formation; bond is broken and both electrons are given to one of the two products; best leaving groups are able to stabilize the extra electrons (weak bases are a good example and conjugate bases of strong acids)
substitution
leaving groups and nucleophiles serve opposite functions, the weaker base (the leaving group) is replaced by the stronger base (the nucleophile)
Nucleophilic substitution reactions
in both SN1 and SN2 nucleophile forms a bond with a substrate carbon and a leaving group leaves
SN1 reactions
unimolecular nucleophilic substitution reactions contain two steps; first step is rate-limiting step in which the leaving group leaves, generating a positively charged carbocation; nucleophile then attacks carbocation, resulting in substitution product
rate of SN1 reactions
depends only on concentration of the substrate: rate = k[R-L]; where R-L = alkyl group containing a leaving group
SN2 reactions
bimolecular nucleophilic substitution reactions contain only one step, in which the nucleophile attacks the compound at the same time as the leaving group leaves; called bimolecular because the single rate-limiting step involves two molecules
concerted
reactions that involve only one step
pattern of sn2 reactions
nucleophile must be strong to actively displace the leaving group in a backside attack. Substrate cannot be sterically hindered so the less substituted the carbon, the more reactive it is in these reactions which is opposite SN1.
rate of sn2 reactions
single step involves two reacting species: substrate (often an alkyl halide, tosylate or mesylate) and a nucleophile and both have a role in determining rate: rate = k[Nu:][R-L]
configuration of sn2 reactions
inverted. if nucleophile and leaving have same priority in their respective molecules, inversion will also correspond to a change in absolute configuration from (R) to (S) or (S) to (R)
difference between Lewis acids and bases, and electrophilicity and nucleophilicity
nucleophilicity and electrophilicity are based on relative rates of reactions and are kinetic properties; acidity and basicity are measured by the position of equilibrium in a protonation or deprotonation reaction and are thermodynamic properties
oxidation-reduction (redox) reactions
the state of oxidation of reactants changes
oxidation state
an indicator of the hypothetical charge that an atom would have if all bonds were completely ionic; calculated from molecular formula. example: CH4 has oxidation state of -4 because each hydrogen has a +1 charge. CO2 has an oxidation state of +4 because each oxygen has a -2 charge
oxidation
increase in oxidation state, decrease in electrons, increasing number of bonds to oxygen or other heteroatoms (atoms besides carbon and hydrogen). this occurs with a carbon atom 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
reduction
decrease in oxidation state, gaining electrons, increasing number of bonds to hydrogen. when involving a carbon, this means that 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
oxidizing agent
the element or compound in an oxidation-reduction reaction that accepts an electron from another species; said to be reduced because it is gaining electrons
examples of good oxidizing agents
O2, O3, Cl2, permanganate (MnO4)-, chromate (CrO4)-, dichromate (CrO7)2-, and PCC. Often contain metal and a large number of oxygen atoms
examples of good reducing agents
have low electronegativities and ionization energies or contain a hydride ion (H-). sodium, magnesium, aluminum, zinc, sodium hydride: NaH, calcium dihydride: CaH2), lithium aluminum hydride (LiAlH4), and sodium borohydride (NaBH4); often contain a metal and a large number of hydrides
chemoselectivity
the preferential reaction of one functional group in the presence of other functional groups
functional groups targeted by nucleophiles in order of priority
carboxylic acids and their derivatives, aldehyde or ketone (with aldehydes generally being more reactive because of less steric hindrance), alcohol or amine
preference carbon for SN1 reactions
prefer tertiary to secondary carbons as reactive sites, and prefer secondary to primary
preference carbon for SN2 reactions
methyl and primary carbons preferred over secondary. Tertiary won’t react
steric hindrance
prevention of reactions at a particular location within a molecule due to size of substituent groups
steric protection
bulky groups make it impossible for nucleophile to reach the more reactive electrophile, making nucleophile less likely to attack another region. useful tool in synthesis of desired molecules and prevention of formation of alternative products
protecting group
a group that temporarily masks the leaving group; it is an aldehyde or ketone that is first converted to a nonreactive acetal or ketal
steps to solve organic chemistry reactions
- know nomenclature; 2. Identify the functional groups; 3. Identify other reagents; 4. Identify the most reactive functional groups; 5. Identify the First Step of the Reaction (with acid or base, first step is usually protonation or deprotonation; if involving a nucleophile, first step is usually nucleophile attacking the electrophile); 6. Consider stereoselectivity
