PHOrgChem (Lecture) | Module 6 (Part 3: ORGANIC REACTIONS ONLY)

Organic Reactions

They are chemical reactions involving organic compounds which primarily consist of Carbon and Hydrogen, often with Oxygen, Nitrogen, Sulfur, and other elements.

Organic Reactions

These reactions plays a crucial role in biochemistry, pharmaceutical, material sciences and industrial chemistry

Organic Reactions

These are essential in creating drugs, plastics dyes, and many other materials.

Organic Reactions

Understanding their mechanism helps in designing efficient synthetic pathways.

Nature of reactants, Catalyst, Temperature and Pressure, and Solvent Used

FACTORS THAT INFLUENCES ORGANIC REACTIONS

Nature of reactants

FACTORS THAT INFLUENCES ORGANIC REACTIONS: affects stability and reactivity of molecules

Catalyst

FACTORS THAT INFLUENCES ORGANIC REACTIONS: Used to speed up reactions without being consumed

Temperature and Pressure

FACTORS THAT INFLUENCES ORGANIC REACTIONS: Affects reaction rate and equilibrium

Solvent Used

FACTORS THAT INFLUENCES ORGANIC REACTIONS: Stabilizes intermediate or influence reaction pathways

Substitution Reactions, Addition and Elimination Reactions, Oxidation-Reduction (Redox) Reactions, and Rearrangement Reactions

TYPES OF ORGANIC REACTIONS

Substitution Reactions

One functional group in a molecule is replaced by another.

Substitution Reactions

Example: Nucleophilic Substitution (SN1 & SN2) – Halogenoalkanes react with nucleophiles.

Substitution Reactions

Nucleophile

SUBSTITUTION REACTIONS:
EXAMPLE REACTION

• In this reaction bromo methane under undergoes nucleophilic substitution with hydroxide ion

• The hydroxide ion is a strong ________ and attacks the carbon bonded to bromine, since bromine is more electronegative it takes the bonding electrons and leave as a bromide ion

• The hydroxide ion replace bromine forming methanol

Electronegative

SUBSTITUTION REACTIONS:
EXAMPLE REACTION

• In this reaction bromo methane under undergoes nucleophilic substitution with hydroxide ion

• The hydroxide ion is a strong nucleophile and attacks the carbon bonded to bromine, since bromine is more ______________ it takes the bonding electrons and leave as a bromide ion

• The hydroxide ion replace bromine forming methanol

SN1 (Substitution Nucleophilic Unimolecular) Reaction, and SN2 (Substitution Nucleophilic Bimolecular) Reaction

TWO TYPES OF SUBSTITUTION REACTION

Substitution Nucleophilic Unimolecular

SN1 stands for?

Substitution Nucleophilic Bimolecular

SN2 stands for?

SN1 (Substitution Nucleophilic Unimolecular) Reaction

A two-step reaction where the rate determining step (slow step) involves the unimolecular departure of the leaving group, forming a carbocation intermediate.

SN1 (Substitution Nucleophilic Unimolecular) Reaction

One step at a time

SN1 (Substitution Nucleophilic Unimolecular) Reaction

First, the leaving group like Chlorine, Bromine, or Iodine leaves, forming a carbon cation then the nucleophile attacks.

SN1 (Substitution Nucleophilic Unimolecular) Reaction

The rate depends on one molecule.

SN1 (Substitution Nucleophilic Unimolecular) Reaction

The reaction speed depends only on the concentration of the substrate not the nucleophile.

SN1 (Substitution Nucleophilic Unimolecular) Reaction

Prefers stable cations works best with tertiary carbons because they stabilize the carbon cation

Tertiary Carbons

SN1 prefers stable cations works best with ___________ because they stabilize the carbon cation

SN1 (Substitution Nucleophilic Unimolecular) Reaction

Forms a racemic mixture, since the nucleophile can attack from either side, so a mix of products forms

Racemic Mixture

SN1 forms a ______________, since the nucleophile can attack from either side, so a mix of products forms

SN1 (Substitution Nucleophilic Unimolecular) Reaction

Common in polar protic solvents, like solvents with hydrogen bonding such as water and alcohol

SN1 (Substitution Nucleophilic Unimolecular) Reaction

If it follows this mechanism, the Carbon Bromine bonds breaks first forming a carbon cation then the hydroxyl attaches in the second steps

SN2 (Substitution Nucleophilic Bimolecular) Reaction

A one-step reaction where the nucleophile attacks the substrate at the same time as the leaving group departs, resulting in a concerted mechanism

SN2 (Substitution Nucleophilic Bimolecular) Reaction

A one step reaction, where the nucleophile directly attacks the carbon, forcing the leaving group out at the same time

SN2 (Substitution Nucleophilic Bimolecular) Reaction

Rate depends on the two molecules

SN2 (Substitution Nucleophilic Bimolecular) Reaction

Both the substrate and the nucleophile affects the reaction speed

SN2 (Substitution Nucleophilic Bimolecular) Reaction

Prefer less hindered carbons works best with primary or secondary carbon because the static hindrance slows it down, causes inversion or a backside attack

SN2 (Substitution Nucleophilic Bimolecular) Reaction

The product has opposite configuration

SN2 (Substitution Nucleophilic Bimolecular) Reaction

Common in aprotic solvents, works well in solvents like acetone or DMSO which don't form hydrogen bonds

SN2 (Substitution Nucleophilic Bimolecular) Reaction

If it follows this mechanism, the hydroxide attacks from the opposite side of the leaving bromine leading to a one step reaction with inversion of configuration

SN1 (Substitution Nucleophilic Unimolecular) Reaction

It is equals to stepwise and stable carbocation

SN2 (Substitution Nucleophilic Bimolecular) Reaction

It is single step and steric hindrance matters

Addition Reactions

Atoms or groups are added to a molecule, typically occurring in compounds with double or triple bonds.

Addition Reactions

Addition Reactions

Example: Hydrogenation of Alkenes – Ethene reacts with hydrogen to form ethane. ○

• Ethene has a carbon carbon double bond meaning there are two pairs of electrons between the carbon atoms

Hydrogenation of Alkenes

In the presence of a catalyst like platinum, palladium, or nickel, hydrogen gas is added to the molecule, so the double bond breaks and each carbon gains a hydrogen atom, converting ethene into ethane which has only single bonds

Hydrogenation

This is called ___________ - commonly used in the food industry, ex. Converting vegetable oils into margarine

Elimination Reactions

The removal of atoms or groups from a molecule, often forming a double bond.

Elimination Reactions

Elimination Reactions

Example: Dehydration of Alcohols – Ethanol loses water to form ethene.

• Ethanol undergoes dehydration or loss of water in the presence of an acid catalyst . Example: concentrated sulfuric acid

• A hydrogen atom and the OH group is removed from adjacent carbon atoms

• This forms ethene which has a carbon-carbon double bond and water as a by product

• This is an elimination reaction because atoms are removed from the molecule rather than added

Addition Reactions

Example: Hydrogenation of Alkenes

Elimination Reactions

Example: Dehydration of Alcohols

Oxidation

It involves the loss of electrons (or gain of oxygen)

Reduction

It involves the gain of electrons (or loss of oxygen).

Oxidation-Reduction (Redox) Reactions

Example: Oxidation of Alcohols – Ethanol oxidizes to ethanoic acid.

• Ethanol is oxidized using an oxidizing agent such as potassium dichromate and acid

• It first forms ethanal, as an intermediate, further oxidation leads to ethanoic acid, commonly known as acetic acid or vinegar

• The oxygen atoms supplied by the oxidizing agent

• Alcohols oxidize to aldehyde and then to carboxylic acids

Potassium dichromate, and Acid

Oxidizing Agent

Ethanal

Example: Oxidation of Alcohols – Ethanol oxidizes to ethanoic acid.

• Ethanol is oxidized using an oxidizing agent such as potassium dichromate and acid

• It first forms ______, as an intermediate, further oxidation leads to ethanoic acid, commonly known as acetic acid or vinegar

Ethanoic Acid

Example: Oxidation of Alcohols – Ethanol oxidizes to ethanoic acid.

• Ethanol is oxidized using an oxidizing agent such as potassium dichromate and acid

• It first forms ethanal, as an intermediate, further oxidation leads to __________, commonly known as acetic acid or vinegar

Oxidation-Reduction (Redox) Reactions

Example: Oxidation of Alcohols

Rearrangement Reactions

The structure of a molecule is rearranged to form an isomer.

Rearrangement Reactions

Rearrangement Reactions

Example: Keto Enol Tautomerism – Interconversion of ketones and enols.

• Propanone exist in equilibrium with its enol form

• This is tautomeric shift where a hydrogen atom moves and a double bond is rearranged

• The keto form is usually more stable than the enol form, however the enol form can be important in reactions like electrophilic addition

• This type of rearrangement is crucial in biological and organic chemistry affecting stability and reactivity

Rearrangement Reactions

Example: Keto Enol Tautomerism

Substitution Reaction (Halogenation), and Combustion Reaction

REACTION OF ALKANES

Substitution Reaction (Halogenation)

Example: CH₄ + Cl₂ CH₃Cl + HCl (UV light)

• The carbon-hydrogen bond in methane is broken, a chlorine molecule is split by the by the UV light into two chlorine radicals

• One chlorine replaces a hydrogen atom in CH4 forming methyl chloride and hydrochloride

• If excess chlorine is present, further substitution can occur forming methylene chloride, chloroform, and carbon tetrachloride

Methylene chloride, Chloroform, and Carbon tetrachloride

SUBSTITUTION REACTION (HALOGENATION) IN ALKANES
Example: CH₄ + Cl₂ CH₃Cl + HCl (UV light)

• The carbon-hydrogen bond in methane is broken, a chlorine molecule is split by the by the UV light into two chlorine radicals

• One chlorine replaces a hydrogen atom in CH4 forming methyl chloride and hydrochloride

• If excess chlorine is present, further substitution can occur forming __________, __________, and ___________

Combustion Reaction

Example: CH₄ + 2O₂ CO₂ + 2H₂O

• Methane reacts with oxygen in a highly exothermic reaction

• Complete combustion produces carbon dioxide and water

• If oxygen is limited, carbon monoxide or carbon may form instead

Carbon dioxide, and Water

COMBUSTION REACTION IN ALKANES
Example: CH₄ + 2O₂ CO₂ + 2H₂O

• Methane reacts with oxygen in a highly exothermic reaction

• Complete combustion produces __________, and ________

• If oxygen is limited, carbon monoxide or carbon may form instead

Carbon monoxide, or Carbon

Example: CH₄ + 2O₂ CO₂ + 2H₂O

• Methane reacts with oxygen in a highly exothermic reaction

• Complete combustion produces carbon dioxide and water

• If oxygen is limited, ________, or _______ may form instead

Hydrogenation, Halogenation, Hydrohalogenation, and Hydration

REACTION OF ALKENES

Hydrogenation

C₂H₄ + H₂ C₂H₆ (Ni catalyst)

• The double bond in ethene is broken, two hydrogen atoms are added across the bond converting ethene into ethane, a saturated hydrocarbon

Halogenation

C₂H₄ + Br₂ C₂H₄Br₂

• The pi bond in the double bond is broken, two bromine atoms are added to carbon atoms forming 1,2 dibromoethane

• The reddish brown bromine solution becomes colorless which is a test for unsaturation

Hydrohalogenation

C₂H₄ + HCl C₂H₅Cl

• The double bond breaks and the hydrogen from hydrochloride, bonds with one carbon and chlorine attaches to another carbon forming chloroethane

• In Markovnikov's rule applies, the hydrogen attaches to the carbon that already has more hydrogens

Markovnikov's Rule

What rule applies on Hydrohalogenation (an organic reaction on ALKENES) the hydrogen attaches to the carbon that already has more hydrogens

Hydration

C₂H₄ + H₂O C₂H₅OH (H₂SO₄ catalyst)

• The double bond is broken and hydrogen attaches to one carbon while hydroxide is attaches to other

• Ethanol is formed, which is a key industrial alcohol

Ethanol

This is formed in Hydration (an organic reaction in ALKENES) which is a key industrial alcohol

Addition of Hydrogen, and Oxidation (Combustion)

REACTIONS OF ALKYNES

Addition of Hydrogen

C₂H₂ + 2H₂ C₂H₆

• The hydrogenation of alkynes, the triple bond in ethyne is completely hydrogenated converted to single bonds, two molecules of hydrogen are added, converting ethyne to ethene to ethane

Oxidation (Combustion)

C₂H₂ + O₂ CO₂ + H₂O

• Acetylene burns in oxygen releasing the intense heat forming carbon dioxide and water

• This is used in oxyacetylene welding torches due to high temperature

Oxyacetylene Welding Torches

Oxidation (Combustion) is used in?

Substitution

Aromatic hydrocarbons undergo ________ instead of addition due to benzene stable structure

Halogenation, Nitration, and Friedel-Crafts Alkylation

REACTIONS OF AROMATIC HYDROCARBONS (BENZENES)

Halogenation

C₆H₆ + Cl₂ C₆H₅Cl + HCl (FeCl₃ catalyst)

• One halogen from benzene is replace by a chlorine from chloride

• Ferric chloride catalyst helps generates chlorine that attacks the benzene

Nitration

C₆H₆ + HNO₃ C₆H₅NO₂ + H₂O (H₂SO₄ catalyst)

• A nitronium ion is generated by nitric acid and and sulfuric acid

• Nitrogen dioxide replaces a hydrogen, forming nitrobenzene

Friedel-Crafts Alkylation

C₆H₆ + CH₃Cl C₆H₅CH₃ + HCl (AlCl₃ catalyst)

• The methyl cation is generated by the chloromethane and aluminum trichloride

• Methyl replaces a hydrogen in benzene forming toluene

Nucleophilic Substitution (SN1 and SN2), and Elimination (E1 and E2)

REACTIONS OF ALKYL HALIDES

Nucleophilic Substitution (SN1 and SN2)

CH₃Br + OH⁻ CH₃OH + Br⁻

• The bromine is replace by OH, converting methyl bromide into methanol

SN1 Mechanism

This mechanism is a two step process with a carbocation intermediate

SN2 Mechanism

This mechanism is a one step direct attack by hydroxyl

Elimination (E1 and E2)

CH₃CH₂Br + NaOH (heat) CH₂=CH₂ + HBr

• Instead of substitution, hydrogen and bromine are remove from adjacent carbon atoms, which forms an alkene which is ethene, by an elimination of hydrogen halide

Alkanes

They react by a substitution due to strong carbon and hydrogen bonds

Alkenes, and Alkynes

They react by a addition due to reactive double and triple bonds

Benzenes

They undergo electrophilic substitution due to resonance stability

Alkyl Halides

They undergo nucleophilic substitution or elimination depending on conditions