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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+)



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



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



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




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)



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



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]