ACS (FINAL OCHEM)

Chapter 1: Structure: shape and stability

Molecular Orbital diagrams

Aufbau principle = fill the lowest energy first

Pauli Exclusion principle = only two electrons per orbital

hund’s rule = Orbitals of the same energy receive one electron

1. Fill the lowest energy orbital

2. No orbital may hold more than two electrons, and they must be of opposite spin

3. When filling degenerate orbitals (orbitals of the same energy), each orbital receives one electron, then the degenerate orbitals receive a second electron of the opposite spin

   • AFTER hybridization - only one row (put one spin on each line firs,t then fill the others

   • BEFORE hybridization - 2 or more rows (fill the lowest line before moving to higher lines

   • Look up atom in periodic table - column # = valence electrons

VSEPR Theory

Biggest resonance Contributor

1. All atoms obey the octet rule (especially for second-period elements like C, N, O, F).

Atoms should not exceed or fall short of 8 electrons (unless it's an exception like P or S).

2. Minimal formal charges.

The most stable structure has the fewest formal charges.

A neutral molecule is generally more stable than charged forms.

3. If charges exist, they’re on appropriate atoms:

Negative charges should be on more electronegative atoms (like O or N).

Positive charges should be on less electronegative atoms (like C).

4. More covalent bonds (double bonds) = more stability, if the octet rule is still satisfied.

A structure with more double or delocalized bonds tends to be more stable if it doesn’t cause an octet violation.

5. Charge separation is minimized.

Structures with separated positive and negative charges are generally less stable than ones where charges are closer together or nonexistent.

6. Avoid like charges on adjacent atoms.

Two positive or two negative charges next to each other is unstable.

7. Delocalization of electrons (especially charges and π electrons).

If a structure spreads out electron density over multiple atoms (especially in conjugated systems), it gains stability.

Carbocation + radical Stability

Benzylic > Allylic > 3 prime > 2 prime > 1 prime > anything on a double bond

Resonance

Remember, it is making and breaking bonds

• Negative charge = extra electron pair on the atom

• Positive charge = electron pair missing on the atom

• Double and triple bonds can move

The most stable resonance structure = Resonance hybrid

• All atoms have a full octet, and the most electronegative atom should contain the negative charge

Delocalized VS Localized

Localized

• electrons that participate in resonance

• Electrons that CANNOT move

Delocalized

• electrons that can participate in resonance

• electrons that CAN move

If a heteroatom (N,O) has multiple lone pairs, only ONE of those lone pairs is localize the rest are localized and must stay put.

If a heteroatom (N,O) already has a double bond, then any lone pairs still present are localized.

Intensity of dipole moment

the difference of electronegativity

• the more balanced the less strength of the dipole

Chapter 2: Structure: Nomenclature and Functional Groups

Functional Groups

IUPAC Groups

General IUPAC rules

• want the alcohol and alkene on the lowest number possible

• if it has both alcohol and alkene then the alcohol will be on the lowest carbon

• If debating between two ways to start a number and they have a substituent on the same carbon, pick the one that has more substituents.

cycloalkanes = pick the lowest numbering system possible

• Prefixes that DON'T Count: di,tri,tetra, sec, tert, iso

Boiling point and IMF

The highest boiling point will be the strongest IMF

Ionic bonding (strongest)

H-bonding

Dipole-dipole

London Dispersion Forces (weakest)

The more spread out the molecule, and the more carbons, the higher the boiling point

know the primary, secondary, and tertiary carbons of the functional groups

for alcohols you count starting from the first carbon and see how many carbon that is connected too

for other functional croups you look directly at what the functional group is connected too

Solubility

• Like dissolves in like

• Alkanes usually do not dissolve in water : the smallest chain the more likely soluble in water

• More carbons mean LESS soluble

• <5 carbons = likely SOLUBLE in water

• >5 carbons = likely INSOLUBLE in water

Chapter 3: Structure: Isomers

R and S

When assigning priority for R and S, it goes by ATOMIC NUMBER

If the highest priority is dashed (in the plane) then you have to flip it

Chair confirmation

sold wedge = up (axial or equatorial up)

dashed wedge = down ( axial or equatorial down)

the most stable chair is when the groups are equatorial

what is the purpose of confirmations

• A molecule adopts conformations to reduce its overall energy

• observed conformation should focus on minimizing strain, minimizing overall energy

• CAUSED by TORSIONAL STRAIN

Ring strain arises from bond angles that are not ideal (sp3 or 109.5°)

When is a compound optically active? (achiral vs chiral)

Chiral = optically active = stereogenic center (non-superimposable)

Achiral = not optically active = no stereogenic center

• If the molecule is symmetric then it is optically inactive

E vs Z

When an alkene exists in a structure, E and Z are used to identify the name.

E = Trans - priority group is on opposite sides

Z = Cis - priority groups are on the same side

ATOMIC NUMBER

Enantiomers

All stereogenic centers change confirmation

before R,R,S after S,S,R

Diastereomers

Some but not all stereogenic centers change confirmation

before R,S vs after S,S

Constitutional Isomers

have the same molecular formulas but have different connectivity

conformational isomers

have the same molecular formula, are connected in the same absolute way, but differ in rotation of single bonds.

Newman projections

The further apart the bigger groups are and the further apart they are, the more stable.

Less stable = higher energy

• Staggered

  • The substituents on the front and back carbons are as far apart as possible

• Eclipsed conformation

  • The substituents on the front and back carbons are directly aligned. LEAST STABLE

• Gauche conformation

  • staggered conformation where two bulky groups are adjacent to each other (60° apart)

• Anti Conformation

  • bulky groups are opposite from each other

Ring Strain

• The instability in cyclic compounds due to angular distortion and torsional strain

• results in an increased energy and reactivity

• Ring Strain = bond angles INTERNAL to the ring

Torsional strain

• when atoms or groups of atoms are eclipsed causing increased energy (less stable)

Steric Strain

• strain caused by two bulky groups that are close together

• electron clouds experience a repulsion causing increased energy and destabilization in the molecule.

Fisher Projection

Meso compounds

have chiral centers but have a plane of symmetry, making them achiral

• optically inactive

• R and S or S and R

Chapter 4: Acids and Bases

Bronsted lowry acids and Bronsted Lowry Bases

Bronsted Lowry Acid = proton donor

Bronsted Lowry Base = proton acceptor

• only one starting material contains a hydrogen it must be the acid. If only one starting material has a lone pair or a pi bond it must be the base

• starting material with a positive charge = acid

• starting material with a negative charge = base

Lewis Base VS Lewis Acids

Lewis base = donate a pair of electrons

Lewis acid = accepts a pair of electrons

Radicals cannot accept an electron pair

Nucleophile VS Electrophile

Nucleophile - electron-rich

• generally have a negative charge

Electrophile - electron-poor

• generally have a positive charge

Factors that Determine Acid Strength

1. Element Effects - more electronegative = stronger acid

2. Inductive effects - the closer the halide is to an anion and the more electronegative = the stronger the acid

3. Resonance effects - if more resonance is available = then the stronger the acid

4. Hybridization effects - acid strength sp3, sp2, sp

Ph and pka + equilibrium

Higher value (Ph and pka) = Basic

K - equilibrium constant [products/reactants]

• If the products are more basic than K>1

• If the reactants are more basic than K<1

Acid/ Base Reactions

highest pka will be the most basic - HF

Chapter 5: Nucleophilic Substitution Reactions

Acid/Base Reactions

SN2

• Bimolecular

• One step = second order

• Changing the concentration of either reactant impacts the rate

• Aprotic

• Rate law = K [Alkyl Halide][Nucleophile]

• Do R and S (ONE PRODUCT)

• 1° and 2° (good NU) alkyl halides undergo SN2 reactions with ease

• opposite specific rotation

• Inversion of configuration

• Most reactive → methyl, 1°,2°, and 3°

• PBr3 and SOCl2 ALWAYS SN2 (inversion of stereochemistry)

• HBr 1° = SN2

SN1

• Unimolecular

• two step = first order

• polar protic

• Rate law = K [Alkyl Halide]

• forms a racemic mixture

• 2° and 3° alkyl halides undergo SN1 reactions

• Specific rotation is zero

• Most reactive → 3°, 2°, 1°

• HBr 2° and 3° → SN1

Nucleophile strength

Lewis Bases are nucleophiles = electron-rich

1. Negative charged nucleophiles react the fastest

2. nucleophilic strength in the same period (row), the further left, the stronger the nucleophile

3. nucleophilic strength in the same family (column), the further down the stronger the nucleophile

What Makes a Good Leaving Group

The weakest Bronsted-Lowry base/ strongest conjugate acid is what makes the best leaving group

• Large polarizable anions make good leaving groups

Solvolysis

-Tertiary (3°) > Secondary (2°) > Primary (1°)

-Allylic or Benzylic (resonance-stabilized) structures also favor solvolysis.

SN1: The leaving group departs first, creating a carbocation intermediate. The rate of reaction depends on the stability of the carbocation.

SN2: The nucleophile (solvent) attacks the substrate in one step, so the substrate must be less hindered.

Mechanism of HX reaction with alkyl alcohols

• rearrangements can occur (hydride or alkyl shift) when a more stable carbocation can be formed from the initial carbocation

• Only carbocation rearrangement in SN1 (2° and 3°)

Reaction of Alcohols to alkyl halides with PBr3

• Inversion of Stereochem (SN2)

Reaction of alcohols to alkyl halides with SOCl2 and Pyriedine

• Inversion of Stereochem (SN2)

Transition state for SN2

• One bond is being broken, and one bond is being made, ONE STEP

• Halide is being broken and addition of nucleophile

Transition state for SN1

• Two Step reaction

• first step is the bond being broken and the second step is the addition of the nucleophile

Polar Protic VS. Polar Aprotic

Polar protic: H-bonding- H2O, ethanol, and methanol- a strong base is a weak NuPolar

Aprotic: NO H-bonding- DMSO, DMF, and acetone- a strong base is a strong Nu

Chapter 6: Elimination Reactions

E1 And E2 Chart

E2:

• One-step mechanism

• 2nd order = bimolecular

• Better leaving group = faster reaction (strong conjugate acid/ weaker base)

• Polar aprotic (no H-bonding) increases the rate of the reaction

• E or Z

• If a bulky base, then the less substituted product is preferred

• Rate = [Alkyl Halide][Nucleophile]

E1:

• Two-step reaction mechanism

• First order = Unimolecular

• better leaving groups

• Carbocation Rearrangements can occur for E1

• Polar protic (H-bonding) increases the rate of reaction

• Rate= [Alkyl Halide]

Zaitsev’s Rule

• the most substituted for E1 and E2 are generally favored

• Want conjugation

• EXCEPT when a bulky base is used (CH3)3 OH tert-butoxide

• If CIS = two products form

• if TRANS = less substituted product is preferred

Dehydration of alcohols

• 3° and 2° follow E1 mechanism (Carbocation Rearrangment)

• first step is protonation of the alcohol

E2 Mechanism

E1 Mechanism

Chapter 7: Addition Reactions to Alkenes and Alkynes

Addition of Alkenes - R=R with HX (Hydrohalogenation)

• follows Markonikov’s rule

• Two Steps:

  1) Nucleophilic attack

  • can do shifts, carbocation rearrangements

  • X = Cl, Br, I

Markovnikov’s Rule

• Hydrogen will add to the least substituted carbon of the double bond

Addition of Alkenes - R=R with H2O and H2SO4 or HEt, H+ (Hydration)

• Formation of an Alcohol on the most substituted carbon

• Adding water/acid catalyst

H2O/H+ or H2O/H2SO4+

• Markovnikov’s Rule

• CAN DO SHIFTS/ REARRANGEMENTS

• three steps

Addition of Alkenes - R=R with Br2 or Cl2 (Halogenation)

Addition of Alkenes - R=R reacts with Br2 or Cl2 and H2O or protic solvent (Halohydrin Formation)

• OH adds to the most substituted carbon, X added to the least substituted carbon

• Anti products favored

• Anti-Markovnikov’s addition

Addition of Alkenes - R=R with BH3 and H2O (Hydroboration/Oxidation)

• Anti-Markovnikov’s Rule (H is added to the most substituted)

  • Adds an OH group to the LEAST substituted carbon in the double bond

• Syn addition, the H and OH groups will be on the same plane (both dashed or solid)

• 2 steps - NO REARRANGEMENTS

  • BH3/THF followed by -OH, H2O2, H2O

  • B2H6/THF followed by -OH, H2O2, H2O

Reaction of Ethers with Strong Acids (HI or HBr)

Ozonolysis of Alkenes (R=R with O3/KMnO4 in Me2S or H3O+)

Ozonolysis of alkenes produces a ketone and aldehyde with O3 and Me2S

and with KMnO4 and H3O+ in produces CO2

Reaction of Epoxides

Alkoxymercuration Demercuration

• No Carbocation Rearrangements

• Markov’s Rule - OH on more substituted carbon and H on the less

1) Hg(OAc)2, H2O

2) NaBH4

Addition of Alkynes R≡R excess HX (Hydrohalogenation)

Addition of Alkynes - R≡R with Excess X2 (Halogenation)

Addition of Alkynes - R≡R with H2O, H2SO4, and HgSO4 (Halogenation)

Addition of Alkynes - R≡R with BH3, then H2O2 and OH- (Hydroboration Oxidation)

Reductive Ozonolysis

KMnO4 (heat vs cold) Dihydroxylation

Dihydroxylation OsO4 and NaHSO3, H2O

Hydrogenation of Alkenes to Alkanes

Hydrogenation of Alkynes

Regioselectivity vs Stereospecificity

Regioselectivity: where does it add (Markov’s Rule)

Stereospecificity: how does it add in 3D space (Syn or anti addition)

Hydrogenation of Aldehydes and Ketones

Chapter 9: Spectroscopy

Mass Spec - Molecular Ion Peak

Mass Weight of the Molecule

Mass Spec - Major Fragment

• Find the the Functional Group and Cleave

• Calculate the weight

HNMR - Number of H signals

• the number of identical H’s in a molecule

• Only count the H’s on Carbon

Down field vs Up field

Down field = left (more electronegative)

Up field = right (more electronegative)

Higher chemical shift = more downfield

• more substituted or more electronegative atoms

When looking at the HNMR

• Use n+1 to determine the splitting patterns

• when looking at graph do n-1 to determine number of hydrogens in molecule

• NO, OH and NH splitting = ALWAYS singlets

H NMR PPM - Chemical Shift

C NMR PPM - chart

IR Spectroscopy chart

Chapter 8: Addition Reactions to Alcohols and Ethers

Catalytic Hydrogenation of Aldehydes and Ketones

Reduction of Aldehydes and Ketones

Reduction of Acid Chlorides and Esters

Reduction of Carboxylic acids and Amides

Epoxidation

Oxidation of Alcohols

Protecting OH and NH groups