Orgo I Review

Question 1

Type of Carbon Bonds

Answer 1

Carbon forms four bonds:

sp³ (four single bonds)
sp² (double bond)
sp (triple bond)

Question 2

%s Character

Answer 2

The greater the %s character, the more electronegative is the atom.

Question 3

Alkyl Groups

Answer 3

Alkyl groups are electron-releasing (ERGs) both inductively (sp³ hypridized and less
electronegative) and via hyperconjugation.

Question 4

Acid vs Base: H

Answer 4

Every H is potentially acidic.

Question 5

Base Make-Up

Answer 5

A base has a nonbonding pair of electrons: a negative ion, a nitrogen atom, an oxygen atom
(and atoms in the same group).

Question 6

Strong Acid vs. Weak Base

Answer 6

The stronger acid has the weaker conjugate base.

Question 7

Weaker Base Stability

Answer 7

The weaker base is the more stable base.

Stabilizing factors include: electronegativity
(across a row), size (down a column), resonance (charge delocalization), and an EWG
stabilizing the negative charge.

Question 8

Nucleophile

Answer 8

A nucleophile has a nonbonding pair of electrons.

Question 9

Nucleophilicity

Answer 9

Nucleophilicity increases with basicity for the same nucleophilic atom and atoms in the same
row.

Nucleophilicity increases with size because of increasing polarizability and
decreasing solvation.

The better leaving group is the weaker base.

Question 10

Predicting Reactions

Answer 10

An alkyl halide or tosylate and protonated alcohols contain a leaving group.

Consider the reaction conditions and ask:

1) will the leaving group leave (will a carbocation form)?
2) is a strong base present (E₂)?
3) is a good nucleophile present (S_N²)?

If a carbocation forms it can:
1) react with a nucleophile (S_N¹)
2) lose a β-H to form an alkene (E₁)
3) rearrange to a more stable one
Alkenes undergo electrophilic addition. The electrophile adds to give the most stable
carbocation.

Question 11

Alkenes & HBr

Answer 11

Alkenes undergo free radical addition with HBr with peroxides or light. The attacking atom is a
bromine that adds to give the most stable free radical.

Question 12

Hydroboration-oxidation

Answer 12

syn, anti-Markovnikov addition of water

Question 13

Epoxidation

Answer 13

syn

Question 14

Ozonolysis-reductive workup

Answer 14

cleaves C=C to give aldehydes and ketones

Question 15

Partial reduction of alkynes

Answer 15

Lindlar Pd, Na/NH3

Question 16

Free radical halogenation of alkanes

Answer 16

photochemical chlorination/bromination; both statistical
and relative reactivity effects must be considered

Question 17

Stereochemistry

Answer 17

stereoisomers, E/Z, cis/trans, enantiomers, R/S, diastereomers, meso
compounds, optical activity, specific rotation, optical purity = enantiomeric excess (ee)

Question 18

Substituent effects

Answer 18

inductive and resonance effects; EWGs and ERGs; an atom with a nonbonding
pair of electrons is strongly electron-releasing via resonance

Question 19

Acid-Base Rxns: Strong Base

Answer 19

Since acid-base reactions are rapid and reversible, always check to see if a strong base is present. If there is a base, look for an acidic H.

Question 20

Relative Acidity

Answer 20

In discussing relative acidity, look at the conjugate base and try to assess the stability.

Question 21

Stability of a Negative Charge

Answer 21

The stability of a negative charge is usually governed by electronegativity when comparing atoms in a row and size when comparing atoms in a column.

Question 22

E₂ Rxn of Alkyl Halide with a Strong Base

Answer 22

The E₂ reaction of an alkyl halide with a strong base occurs through an anti-periplanar transition state (the leaving groups are anti) and the more stable alkene (Zaitsev) predominates.

In a cyclohexane chair conformation, the leaving groups must be axial.

Question 23

S_N²: Steric Hindrance

Answer 23

The S_N² reaction is subject to steric hindrance and occurs with inversion of configuration.

Question 24

Carbocation Stability: Solvent Polarity

Answer 24

Carbocation stability is particularly sensitive to solvent polarity. Water is the best of the common solvents for

separating and solvating ions.
Stabilizing influences include resonance delocalization, inductive effects, and electronegativity.

Destabilizing factors include steric hindrance, inductive effects, and electronegativity.

Question 25

Neclophilicity Trends

Answer 25

In comparing atoms in a row, nucleophilicity increases with basicity; in a column, nucleophilicity increases with size.

Question 26

Better Leaving Group

Answer 26

The better leaving group is the weaker base.

Question 27

ERG & EWG

Answer 27

Groups of atoms (functional groups) can be characterized as electron-releasing (ERG) or electron-withdr.awing (EWG). Alkyl groups are ERGs by both the inductive (less EN carbon due to lower %-s character) and resonance (hyperconjugation) effects. Oxygen (and N!) substituents are strongly ERG since the resonance effect outweighs the inductive effect.

Question 28

Heat of Formation

Answer 28

The heat of formation is the energy involved in forming one mole of a compound from the elements in their most stable form at 25°C. The heat of formation of a compound is the energy evolved when one mole of the compound is burned to CO₂ and water.

Question 29

Branching

Answer 29

Branching is stabilizing probably due to intramolecular dispersion forces.

Question 30

Ring Strain

Answer 30

Ring strain is the difference between the calculated ΔH_f or ΔH_c and the actual ΔH_f (or ΔH_c). Contributors to ring strain include: angle strain, torsional strain, and steric hindrance.

Question 31

Axial Substituents

Answer 31

Axial substituents are destabilized by 1,3-diaxial interactions.

Question 32

Cyclohexane Chair Conformations

Answer 32

There are two chair conformations for any cyclohexane species and the most stable one has the fewest axial substituents or has the largest substituent equatorial.

Question 33

Conformations

Answer 33

Conformations are spatial orientations that can be interconverted by rotation around single bonds.

Question 34

Enantiomers

Answer 34

Enantiomers are nonsuperimposable mirror images.

Question 35

Diastereomers

Answer 35

Diastereomers are stereoisomers that are not enantiomers.

Question 36

Meso Compound

Answer 36

A meso compound is an optically inactive (achiral-plane of symmetry) form of a compound that can be optically active.

Question 37

Most Stable Conformation of Cyclohexanes

Answer 37

The most stable conformation of cyclohexanes is the chair and the most stable conformation of substituted cyclohexanes is the one with the largest substituent in the equatorial position. Analyze the cis and trans stereoisomers using the relative orientations of “up” and “down.”

Question 38

Carbocations can...

Answer 38

Carbocations can 1) react with nucleophiles (S_N¹); 2) lose β-hydrogens to give alkenes; and 3) rearrange to a more stable one.

Question 39

Solvolysis

Answer 39

Solvolysis reactions are unimolecular and give carbocations.

Question 40

Hydroboration-oxidation

Answer 40

Hydroboration-oxidation results in anti-Markovnikov addition of water to a double bond; the addition is syn.

Question 41

Addition of Halogens to Alkenes

Answer 41

Addition of halogens to alkenes gives a vicinal dihalide and the addition is anti via a halonium ion.

Question 42

Solvation of a Nucleophile

Answer 42

Solvation of a nucleophile reduces its nucleophilicty. Smaller ions are better solvated than larger ones.

Question 43

Lewis Bases, Bronsted Bases & Nucleophiles

Answer 43

Lewis bases, Bronsted bases, and nucleophiles are terms for the same species: species that contain nonbonding electrons.

Question 44

E₂ Rxns: Size of Base

Answer 44

E₂ reactions are affected by the size of the base. Typical NaOR bases give Zaitsev product distributions; bulky bases give Hofmann product distributions.

Question 45

Nucleophiles & Carbonyl Carbon

Answer 45

Nucleophiles attack the carbonyl carbon leading to a tetrahedral intermediate. With aldehydes and ketones, protonation of the alkoxide leads to nucleophilic addition. With carboxylic acid derivatives, the tetrahedral intermediate collapses with the loss of the best leaving group to give a more stabilized carboxylic acid derivative. Hemiacetals and hemiaminals undergo loss of water to give stabilized carbocations, which react further (alcohols give acetals; primary amines give imines; secondary amines give enamines).