CHEM 2202 FINAl - theory from all chapters

Constitutional isomers

Compounds with same molecular formula but different structures

What forces keep the bond at optimal length

• attractive forces between positively charged nuclei and negatively charged electrons

• Repulsive forces between 2 positive charged nuclei

• Repulsive forces between 2 negatively charged electrons

Electronegativity difference between the three types of bonds

1. Covalent - less than 0.5

2. Polar covalent - betweeen 0.5 and 1.7

3. Ionic bond - more than 1.7

The greater difference in electronegativity, what happens to polarity

Greater difference in electronegativity the more polar the bond

What orbital are electrons most stable

1s orbital (lowest in energy)

3 rules for the placement of electrons (know how to apply them when making electron configuration)

2 types of molecular orbitals

1. HOMO - highest occupied molecular orbitals

2. LUMO - lowest unoccupied orbital

Sp3 hybrid orbital

• single bonds (sigma)

• Overlap of 4, 1s

• 25% s character and 75% p character

Sp2 hybrid orbital

• 1 sigma 1 pi bond

• 3 equal energy sp2 orbitals and one unhybridized p orbital

• 33% s character and 67% p character

Sp atomic orbitals

• 1 sigma bond 2 pi bond

• Overlap head on to form sigma bonds while unhybridized P orbitals overlap side by side to form pi bonds

• 50% s character , 50% p character

What is stronger, a pi or sigma bond and why

Sigma is stronger as it takes almost twice the bond energy of a pi bond to break it

Order the hybrid orbitals from longest to shortest

Sp3 > sp2 > sp

What steric numbers associate with each hybridization

Steric number:

4 -sp3

3 - sp2

2 - sp

3 types of intermolecular forces (know how to idetnfiy them)

1. Dipole dipole interactions

2. Hydrogen bonding

3. Dispersion forces

Hydrogen bonding

• strong type of dipole attraction

• Is the attractive force between an H bonded to an electronegative atom and a lone pair on another electronegative atom

Attractive forces and their effects on MP and BP

• less dipole = lower polarity = lower boiling point

• More branching = less surface area = lower boiling point

• More hydrogen bonding = greater boiling point

• Greater London dispersion = greater mass = greater boiling point

Oxygen bonding patterns

-ve charge - 3 lone pairs, 1 bond

Neutral - 2 bonds, 2 lone

+ve- 3 bonds, 1 lone pair

Nitrogen bonding patterns

-ve - 2 bonds, 2 lone pairs

Neutral - 3 bonds, 1 lone pair

+ve - 4 bonds, no lone pairs

Carbon bonding patterns

-ve - 3 bonds , 1 lone pair

Neutral - 4 bonds

+ve - 1-3 bonds, no lone pairs

When does resonance occur

Only in overlapping p orbitals (thus cant occur in single bonds)

2 types of carbons next to a double bond

1. Vinylic - direct carbons

2. Allylic - indirect carbons

What electrons allow for electrons to take place

Delocalized electrons

Bronsted Lowry definition of acid and bases

Acid - proton donor

Base - proton acceptor

In a bronsted Lowry acid base reaction, where does the flow of electron density always originate

From an electron rich site on the base like a low pair or pi bond

In an acid base reaction what does equilibrium favour

Weaker acid

How to compare how much larger a pKa is (how much more prod to reactant or reactant to prod)

Larger pKa - smaller pKa

• take that number and put it as a base 10

Ex. Acid has pKa 14, conj acid has pKa 50, thus 50-14 =36

• Conj acid is 10^ 36 x more product formation than the acid

Lower the pKa…

Greater the acidity, greater the stability

Lewis acid definition of acid and base

Acid - accepts pair of electrons

Base - donates pair of electrons

Saturated hydrocarbons are also known as

Alkanes

What makes a compound more stable

Has to be lowest energy thus

• more branching

• More acidic

• Staggered

• Largest groups are ANTI to one another in a Newman projection

• Chair conformation

• Largest molecule is equatorial and trans

Stereoisomer

Same molecular formula and collectivity but different spatial arrangement of atoms

Enantiomers

2 molecular that are mirror images but are not superimposable (opposite R and S configurations)

• only a chiral compound can have an enantiomer

• Extremely similar physical properties

Diastereomers

Can be chiral or achiral

• some chiral centers are inverted but not all

• NOT mirror images

• Different physical properties

Optical active

Only chiral compounds are optically active, and can rotate plane polarized light in a positive or negative direction

• + = dextrorotatory

• - = levrorotatory

Maximum number of possible stereoisomers can be calculated as

2^N

• N is the number of chiral centers

Meso compounds

Compound with chiral centers but is achiral due to symmetry

Racemic mixture

50/50 mixture of 2 enantiomers with a 0 degree rotation

Distillation

Seperates compounds with different boiling points

Recrystallization

Seperates compounds with different solubility’s

Successful seperation of enantiomers

• resolution of enantiomers using race mix mixture

• Chiral resolving agent

• Affinity chromatography

Atropisomer

Stereoisomers that would be interchangeable through the rotation of a sigma bond but because the bond is unable to rotate, the conformations are stuck and interchangeable

Alllenes

Compounds that possess adjacent carbon double bonds that may or may not be chiral depending on the substituent

Enthalpy vs entropy

enthalpy - Is the heat energy exchange between the reaction and its surroundings

Entropy - molecular disorder

Conditions for spontaneous and non spontaneous reactions

Spon - +ve entropy (S), -ve free energy (G), exergonic favoured and favours products

Non spontaneous - -ve entropy (S), +ve free energy (G), endergonic favours reactants

5 factors that affect reaction rate

1. Reactant concentration

2. Activation energy

3. Temperature

4. Geometry and sterics

5. Catalysts

Rate law for first, second and third order

1) K(A)

2) K(A)(B)

3) K(A)²(B)

Transition state

• high energy

• Fleeting and cant be observed

• Energy maxima

Intermediate

• low energy

• Energy minima

• Observable

Nucleophile

Electron rich that can donate electrons

• Lewis bases

• Are stronger when more polar

Electrophile

Electron deficient and can accept pair of electrons

• Lewis acids

• Carbocations and partially positive atoms

When can a reaction be reversible

1. If the attacking nuclephile is also a good leaving group (ex. H2O)

2. Loss of a leaving group

3. Proton transfers (unless pKa difference is greater than 10 units)

• everything else is typically irreversible including rearrangement

2 main reasons why alkyl halides undergo substitution and elimination reactions

1. Halogen is electron withdrawing, creates a partial positive charge on the adjacent carbon making it susceptible to nuclephilic attacks

2. Presence of good leaving groups

SN2 rate law

K(alkyl halide)(nuclephile)

E2 elimination rate law

Follows second order

K(alkyl halide)(base)

What’s the rate determining step in SN1

Formation of carbocation because its the highest energy transition state (thus has highest Ea)

SN1 rate law

Follows first order reaction

K(substrate)

Which solvents are best for SN1 and SN2 reactions

SN1 - Protic solvents bc it stabilizes the carbocation intermediate → lowers Ea and causes faster reaction

SN2 - aprotic solvent bc it raises energy of the Nu → lowers Ea and causes faster reaction

• nucleophiles are more reactive in aprotic solvents

At what temps are enthalpy and entropy dominated

Low temps → enthalpy dominates and addition reactions are favoured

High temps→ entropy dominates and elimination reactions are favoured

What 2 factors increase entropy

• more Mols products

• Acyclic