ch 4 - Analyzing Organic Reactions

Question 1

Lewis acids and bases

Answer 1

focus on formation of coordinate covalent bonds

Question 2

Bronsted-Lowry acids and bases

Answer 2

focus on proton transfer

Question 3

Lewis acid

Answer 3

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

Question 4

Lewis base

Answer 4

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

Question 5

coordinate covalent bonds

Answer 5

covalent bonds in which both electrons in the bond came from the same starting atom

Question 6

Bronsted-Lowry acid

Answer 6

species that can donate a proton (H+)

Question 7

Bronsted-Lowry base

Answer 7

species that can accept a proton (H+)

Question 8

amphoteric

Answer 8

species that are able to act as either Bronsted-Lowry acids or bases; examples are water, Al(OH)3, (HCO3)-, (HSO4)-

Question 9

acid dissociation constant (K sub a)

Answer 9

measures strength of an acid in solution given by K sub a = ([H+][A-])/[HA] acid = HA

Question 10

pK sub a

Answer 10

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

Question 11

alpha-hydrogens

Answer 11

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

Question 12

common functional group acids

Answer 12

alcohols, aldehydes and ketones, carboxylic acids, most carboxylic acid derivatives

Question 13

common functional group bases

Answer 13

amines and amides

Question 14

nucleophiles

Answer 14

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

Question 15

four factors that determine nucleophilicity

Answer 15

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

Question 16

nucleophilicity in protic solvents

Answer 16

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

Question 17

nucleophilicity in aprotic solvents

Answer 17

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

Question 18

functional group that makes good nucleophile

Answer 18

amine

Question 19

electrophiles

Answer 19

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

Question 20

carboxylic acid derivatives ranked by electrophilicity

Answer 20

Anhydrides > carboxylic acids and esters > amides

Question 21

leaving groups

Answer 21

molecular fragments that retain the electrons after heterolysis

Question 22

heterolytic reactions

Answer 22

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)

Question 23

substitution

Answer 23

leaving groups and nucleophiles serve opposite functions, the weaker base (the leaving group) is replaced by the stronger base (the nucleophile)

Question 24

Nucleophilic substitution reactions

Answer 24

in both SN1 and SN2 nucleophile forms a bond with a substrate carbon and a leaving group leaves

Question 25

SN1 reactions

Answer 25

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

Question 26

rate of SN1 reactions

Answer 26

depends only on concentration of the substrate: rate = k[R-L]; where R-L = alkyl group containing a leaving group

Question 27

SN2 reactions

Answer 27

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

Question 28

concerted

Answer 28

reactions that involve only one step

Question 29

pattern of sn2 reactions

Answer 29

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.

Question 30

rate of sn2 reactions

Answer 30

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]

Question 31

configuration of sn2 reactions

Answer 31

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)

Question 32

difference between Lewis acids and bases, and electrophilicity and nucleophilicity

Answer 32

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

Question 33

oxidation-reduction (redox) reactions

Answer 33

the state of oxidation of reactants changes

Question 34

oxidation state

Answer 34

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

Question 35

oxidation

Answer 35

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

Question 36

reduction

Answer 36

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

Question 37

oxidizing agent

Answer 37

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

Question 38

examples of good oxidizing agents

Answer 38

O2, O3, Cl2, permanganate (MnO4)-, chromate (CrO4)-, dichromate (CrO7)2-, and PCC. Often contain metal and a large number of oxygen atoms

Question 39

examples of good reducing agents

Answer 39

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

Question 40

chemoselectivity

Answer 40

the preferential reaction of one functional group in the presence of other functional groups

Question 41

functional groups targeted by nucleophiles in order of priority

Answer 41

carboxylic acids and their derivatives, aldehyde or ketone (with aldehydes generally being more reactive because of less steric hindrance), alcohol or amine

Question 42

preference carbon for SN1 reactions

Answer 42

prefer tertiary to secondary carbons as reactive sites, and prefer secondary to primary

Question 43

preference carbon for SN2 reactions

Answer 43

methyl and primary carbons preferred over secondary. Tertiary won’t react

Question 44

steric hindrance

Answer 44

prevention of reactions at a particular location within a molecule due to size of substituent groups

Question 45

steric protection

Answer 45

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

Question 46

protecting group

Answer 46

a group that temporarily masks the leaving group; it is an aldehyde or ketone that is first converted to a nonreactive acetal or ketal

Question 47

steps to solve organic chemistry reactions

Answer 47

1. 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