Chapter 4 - Analyzing Organic Reactions

Question 1

Lewis acid:

1. definition
2. tend to be what kind of molecule
3. orbital vacancy and how that helps

Answer 1

1. electron acceptor in the formation of a covalent bond
2. Tend to be electrophiles
3. Have vacant p-orbitals into which they can accept an electron pair, or have positively polarized atoms

Question 2

Lewis base:

1. definition
2. tend to be what kind of molecules
3. electron behavior
4. anions or cations?

Answer 2

1. electron donor in the formation of a covalent bond
2. Tend to be nucleophiles
3. Have a lone pair of electrons that can be donated
4. Often are anions that carry a negative charge

Question 3

coordinate covalent bonds:

Answer 3

When Lewis acids and bases interact

covalent bonds in which both electrons in the bond came from the same starting atom (the Lewis base)

Question 4

​Bronsted-Lowry definition:

1. acid
2. base

Answer 4

1. Acid is a species that can donate a proton
2. Base is a species that can accept a proton

Question 5

Acid Dissociation constant (Ka):

1. what it measures
2. equation

Answer 5

measures the strength of an acid in solution

Question 6

pKa:

1. in acidic molecules

• electronegativity
• bond strength

2. in basic molecules
3. equation relating it to Ka

Answer 6

1. More acidic molecules will have a smaller (or even negative) pKa

• The more electronegative at atom, the higher the acidity
• Less bond strength, the higher the acidity

2. More basic molecules will have a larger pKa
3. pKa = –log Ka

Question 7

α-hydrogens and the the α-carbon:

Answer 7

α-hydrogens are connected to the α-carbon, which is a carbon adjacent to the carbonyl

Question 8

Common Functional Groups:

1. Groups that act like acids
2. Groups that act like bases

Answer 8

1. Functional groups that act as acids: alcohols, aldehydes, and ketones (at the α-carbon), carboxylic acids, and most carboxylic acid derivatives
2. Amines and amides are the main functional groups that act as bases - form peptide bonds

Question 9

Nucleophiles:

1. definition
2. look out for what molecules
3. good nucleophiles tend to be what
4. what makes them more reactive

Answer 9

1. “nucleus-loving” species with either lone pairs or pi bonds that can form new bonds to electrophiles
2. Look out for carbon, hydrogen, oxygen or nitrogen (CHON) with a minus sign or lone pair to identify most nucleophiles
3. Good nucleophiles tend to be good bases
4. The more basic the nucleophile, the more reactive it is

Question 10

Factors that determine Nucleophilicity: (4 - just list)

Answer 10

Charge

Electronegativity

Steric Hindrance

Solvent

Question 11

Factors that determine Nucelophilicity - Charge:

Answer 11

nucleophilicity increases with increasing electron density (more negative charge)

Question 12

Factors that determine Nucleophilicity - Electronegativity:

Answer 12

nucleophilicity decreases as electronegativity increases because these atoms are less likely to share electron density

Question 13

Factors that determine Nucleophilicity - Steric Hindrance:

Answer 13

bulkier molecules are less nucleophilic

Question 14

Factors that determine Nucleophillicity - Solvent:

Answer 14

protic solvents can hinder nucleophilicity by protonating the nucleophile or through hydrogen bonding

Question 15

Solvent effects with Nucleophilicity:

1. in polar protic solvents
2. in polar aprotic solvents
3. in protic solvents
4. in aprotic solvents

Answer 15

1. In polar protic solvents, nucleophilicity increases down the periodic table
2. In polar aprotic solvents, nucleophilicity increases up the periodic table
3. In protic solvents, nucleophilicity decreases in the order: I- > Br- > Cl- > F-
4. In aprotic solvents, nucleophilicity decreases in the order: F- > Cl- > Br- > F (Because there are no protons to get in the way of the attacking nucleophile)

Question 16

Protic vs. Aprotic Solvents;

Answer 16

protic solvent: solvent that has a hydrogen atom bound to an oxygen (as in a hydroxyl group), a nitrogen (as in an amine group) or a fluorine (as in hydrogen fluoride).

In general terms, any solvent that contains a labile H+ is called a protic solvent. The molecules of such solvents readily donate protons (H+) to reagents.

aprotic solvent: cannot donate hydrogen.

Question 17

Common Strong Nucleophiles:

Answer 17

HO-, RO-, CN- and N3-

Question 18

Common Weak Nucleophiles:

Answer 18

H2O, ROH, RCOOH

Question 19

Electrophiles:

1. what they are
2. what makes it more likely a reaction will happen?

Answer 19

1. “electron-loving” species with a positive charge or positively polarized atom that accepts an electron pair when forming new bonds with a nucleophile
2. Better leaving groups make it more likely that a reaction will happen - If empty orbitals are present, an incoming nucleophile can make a bond with the electrophile without displacing the leaving group

Question 20

Electrophilicity and Aciditiy:

1. their relationship to reactivity
2. what is most reactive

Answer 20

1. Electrophilicity and acidity are identical properties when it comes to reactivity

Alcohols, aldehydes and ketones, carboxylic acids and their derivatives

2. Anhydrides are most reactive, followed by carboxylic acids and esters, and then amides

Question 21

Leaving Groups:

Answer 21

molecular fragments that retain the electrons after heterolysis

Question 22

Heterolytic reactions:

Answer 22

essentially the opposite of coordinate covalent bond formation; a bond is broken and both electrons are given to one of the two products

Question 23

Qualities of Leaving Groups:

1. the best leaving groups will be able to do what
2. what kind of bases make good leaving groups + why?
3. following #2, what assumption can be made
4. what are almost never leaving groups
5. another way to think of leaving groups

Answer 23

1. The best leaving groups will be able to stabilize the extra electrons
2. Weak bases are more stable with an extra set of electrons and therefore make good leaving groups
3. Following that logic - the conjugate bases of strong acids (like I-, Br-, and Cl-) make good leaving groups
4. Alkanes and hydrogen ions will almost never serve as leaving groups because they form very reactive, strongly basic anions
5. Leaving groups are nucleophiles as serving opposite functions

Question 24

Nucleophilic substitution reactions:

Answer 24

a nucleophile forms a bond with a substrate carbon and a leaving group leaves

Question 25

Unimolecular nucleophilic substitution (Sn1) reactions:

1. two steps
2. rate of reaction

Answer 25

1. Rate-limiting step in which the leaving group leaves, generating a positively charged carbocation
2. Nucleophile attacks the carbocation resulting in the substitution product

The rate of reaction depends only on the concentration of the substrate: rate = k[R-L], where R-L is an alkyl group containing a leaving group

Question 26

Unimolecular nucleophilic substitution (Sn1) reactions characteristics:

1. the more substituted the carbocation…
2. what order reaction?
3. what does the product consist of

Answer 26

1. The more substituted the carbocation, the more stable it is because the alkyl groups act as electron donors, stabilizing the positive charge
2. First order reaction: anything that accelerates the formation of the carbocation will increase the rate of an Sn1 reaction
3. Product will usually be a racemic mixture - resulting in varied stereochemistry

Question 27

Bimolecular nucleophilic substitution (Sn2) reactions:

1. how many steps + description
2. also called what kind of reaction
3. what needs to be strong?
4. what characteristic can the substrate not have?
5. rule with carbons, substitutions + reactivity

Answer 27

1. contain only one step; nucleophile attacks the compound at the same time as the leaving group leaves - Nucleophile actively displaces the leaving group in a backside attack
2. Called a concerted reaction b/c it only has 1 step
3. Nucleophile must be strong and the substrate cannot be sterically hindered
4. The less substituted the carbon, the more reactive it is to Sn2 reactions

Question 28

Bimolecular nucleophilic substitution (Sn2) reaction characteristics:

1. involves what reacting species
2. rate equation
3. what inverts?

Answer 28

1. Involves two reacting species:

• The substrate (often an alkyl halide, tosylate or mesylate)
• The nucleophile

2. Rate = k[Nu:][R-L]
3. Also have an inversion of relative configuration - Position of substituents around the substrate carbon will be inverted

Question 29

Oxidation-reduction (redox) reactions:

Answer 29

reactions in which the oxidation states of the reactants change

Question 30

Oxidation state:

1. definition
2. how it is determined
3. for an ion

Answer 30

1. indicator of the hypothetical charge that an atom would have if all bonds were completely ionic
2. Can be calculated from the molecular formula for a molecule
   * Ex: methane (CH4) carbon has an oxidation state of -4 because the hydrogens each have an oxidation state of +1 - Most reduced form of carbon*
   * Ex: carbon dioxide (CO2) each of the oxygen atoms has an oxidation state of -2 and the carbon has an oxidation state of +4 - Most oxidized form of carbon*
3. For an ion: the oxidation state is the charge

Ex: Na+ → oxidation state of +1
Ex: S2- → oxidation state of -2

Question 31

Oxidation:

Answer 31

increase in oxidation state, which means a loss of electrons

Increasing the number of bonds to oxygen or other heteroatoms (atoms besides carbon and hydrogen)

Question 32

Reduction:

Answer 32

decrease in oxidation state, or a gain in electrons

Increasing the number of bonds to hydrogen