Organic Chemistry Study Notes: Acids, Bases, Alkenes, and Alkene Reactions

Chapter 4 – Acids and Bases

Brønsted Acids and Bases

Brønsted acids are substances that donate protons (H+), while Brønsted bases accept protons. Understanding their behavior is fundamental to predicting reaction outcomes in organic chemistry.

• Brønsted Acid: Proton donor.

• Brønsted Base: Proton acceptor.

• Lewis Acids and Bases: Lewis acids accept electron pairs; Lewis bases donate electron pairs.

Factors Affecting Acidity/Basicity:

• Acidity of carboxylic acids, alcohols, and phenols is influenced by resonance stabilization and inductive effects.

• Basicity of amines, pyridines, and anilines depends on the availability of the lone pair and resonance effects.

• Conjugate acids and bases: The strength of an acid is inversely related to the strength of its conjugate base.

Key Concepts:

• pKa values are used to compare acid strengths. Lower pKa indicates a stronger acid.

• Ionization states at a given pH can be predicted using pKa values.

• Equilibrium position of acid-base reactions favors formation of the weaker acid/base pair.

Example: Acetic acid (CH3COOH) has a lower pKa than ethanol, making it a stronger acid.

Chapter 5 – Alkenes

Nomenclature and Structure

Alkenes are hydrocarbons containing at least one carbon-carbon double bond. Their nomenclature follows IUPAC rules, prioritizing the double bond in numbering.

• Index of Hydrogen Deficiency (IHD): Indicates the degree of unsaturation in a molecule. Each double bond or ring increases IHD by one.

Example: Cyclohexene has an IHD of 2 (one ring, one double bond).

Chapter 6 – Reactions of Alkenes

Electrophilic Addition Reactions

Alkenes undergo addition reactions due to the high electron density of the double bond, which acts as a nucleophile. Electrophiles are attracted to this electron-rich region.

• Electrophile: Species seeking electrons (Lewis acids).

• Nucleophile: Species donating electrons (Lewis bases).

Example: Addition of HBr to propene forms 2-bromopropane.

Addition of HX (Hydrohalic Acids)

Alkenes react with hydrohalic acids (HX) to form alkyl halides. The reaction proceeds via a carbocation intermediate, and the regioselectivity is governed by Markovnikov's rule.

• Direction (Regioselectivity): The hydrogen atom adds to the carbon with more hydrogens (less substituted), while the halide adds to the more substituted carbon.

Markovnikov's Rule:

• In the addition of HX to an alkene, the hydrogen attaches to the carbon with the greater number of hydrogen atoms, and the halide attaches to the more substituted carbon.

Equation:

$\ce{CH2=CH2} + \ce{HBr} \rightarrow \ce{CH3-CH2Br}$

Carbocation Rearrangement: 1,2-hydride or alkyl shifts can occur if a more stable carbocation can be formed.

Nature of X– in HX

• X– must be a moderately nucleophilic species.

• HCl, HBr, HI add readily; H2SO4 adds with greater difficulty.

• Nucleophilic solvents: H2SO4, CH3OH, CH3CH2OH.

Representative Anti Addition

Anti addition involves the addition of substituents to opposite sides of the double bond, often via a three-membered, bridged intermediate.

• Halogenation (e.g., Br2, Cl2):

• Example: $\ce{CH2=CH2} + \ce{Br2} \rightarrow \ce{Br-CH2-CH2-Br}$

• Stereochemistry: Anti addition via a bridged intermediate.

Representative Syn Addition

Syn addition involves the addition of substituents to the same side of the double bond.

• Catalytic hydrogenation: Addition of H2 across the double bond using a metal catalyst (e.g., Pd/C).

• Hydroboration/oxidation: Addition of BH3 followed by oxidation to form alcohols.

Halohydrin Formation

• Reaction of alkene with X2 and H2O (or OH– or BrOH) forms halohydrins.

• Example: $\ce{CH2=CH2} + \ce{Br2} + \ce{H2O} \rightarrow \ce{Br-CH2-CH2-OH}$

Oxymercuration-Reduction

• Oxymercuration with Hg(OAc)2 followed by reduction with NaBH4 adds water across the double bond without carbocation rearrangement.

Oxidation of Alkenes

• Oxidation with OsO4 or KMnO4 produces diols (syn addition).

• Ozonolysis (reductive or oxidative workup) cleaves the double bond to form carbonyl compounds.

Summary Table: Alkene Addition Reactions

Reaction Type	Reagents	Product	Regioselectivity	Stereochemistry
Hydrohalogenation	HX (HCl, HBr, HI)	Alkyl halide	Markovnikov	None
Halogenation	Br2, Cl2	Dihalide	None	Anti
Hydration	H2O/H2SO4	Alcohol	Markovnikov	None
Hydroboration-Oxidation	BH3, H2O2	Alcohol	Anti-Markovnikov	Syn
Ozonolysis	O3, (Zn/H2O or H2O2)	Aldehyde/Ketone	Cleavage	None

Additional info: Academic context and examples have been expanded for clarity and completeness.