Organic Chemistry Study Notes: Acids/Bases and Alkenes (Chapters 4-6)

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Understanding acid and base strength is fundamental in organic chemistry, as it influences reaction mechanisms and product formation.

Ranking Acids and Bases: Acids and bases can be ranked by their strength, typically using pKa values for acids and pKb for bases. Lower pKa indicates a stronger acid; higher pKa means a weaker acid.
Factors Affecting Strength: Consider electronegativity, resonance stabilization, inductive effects, and atom size.
Example: Acetic acid (pKa ≈ 4.8) is a stronger acid than ethanol (pKa ≈ 16).

Every acid-base reaction involves two conjugate pairs.

Acid: Donates a proton (H+).
Base: Accepts a proton.
Conjugate Base: The species formed after an acid donates a proton.
Conjugate Acid: The species formed after a base accepts a proton.
Example: In the reaction of acetic acid with water: - Acetic acid = acid - Water = base - Acetate ion = conjugate base - Hydronium ion = conjugate acid

Resonance delocalizes electrons, stabilizing ions and molecules.

Drawing Resonance: Use curved arrows to show electron movement. Only move electrons, not atoms.
Example: The acetate ion has two resonance forms, with the negative charge delocalized over both oxygen atoms.

Curved arrows are used to indicate the movement of electron pairs during reactions.

Single-headed arrow (fishhook): Movement of a single electron (radical reactions).
Double-headed arrow: Movement of an electron pair (most common in acid/base and polar reactions).
Example: In deprotonation, the base's lone pair attacks the hydrogen, and the bond electrons move to the conjugate base.

Alkenes are hydrocarbons containing at least one carbon-carbon double bond. Proper naming is essential for clear communication.

Longest Chain: Identify the longest carbon chain containing the double bond.
Numbering: Number the chain from the end nearest the double bond.
Suffix: Use the suffix -ene to indicate the presence of a double bond.
Example: CH2=CHCH2CH3 is named 1-butene.

Alkenes can exhibit geometric isomerism due to restricted rotation around the double bond.

Cis/Trans: Used for disubstituted alkenes. Cis = same side; Trans = opposite sides.
E/Z System: Used for alkenes with more than two different substituents. Assign priorities using Cahn-Ingold-Prelog rules. - Z (zusammen): Higher priority groups on the same side. - E (entgegen): Higher priority groups on opposite sides.
Example: 2-butene: CH3CH=CHCH3 can be cis or trans.

Alkene stability is influenced by substitution and geometry.

More Substituted = More Stable: Tetrasubstituted > Trisubstituted > Disubstituted > Monosubstituted.
Trans Alkenes: Generally more stable than cis due to less steric strain.
Example: 2-methyl-2-butene (tetrasubstituted) is more stable than 1-butene (monosubstituted).

The degree of unsaturation indicates the number of rings and/or multiple bonds in a molecule.

Formula: Where C = number of carbons, N = number of nitrogens, H = number of hydrogens, X = number of halogens.
Interpretation: Each degree corresponds to a ring or a double bond; a triple bond counts as two degrees.
Example: C4H6 has two degrees of unsaturation (could be two double bonds, a triple bond, or a ring and a double bond).

Alkenes undergo a variety of addition reactions, often following Markovnikov or anti-Markovnikov rules.

Hydration (Addition of Water): Converts alkenes to alcohols, typically using acid catalysis.
Ozonolysis: Cleaves double bonds to form carbonyl compounds. (reductive workup)
Epoxidation: Formation of epoxides (three-membered cyclic ethers) using peracids.
Hydrohalogenation: Addition of HX (X = Cl, Br, I) to form alkyl halides.
Halogenation in Presence of Alcohol: Forms halohydrins (addition of X and OH across the double bond).
Oxymercuration-Demercuration: Hydration of alkenes without carbocation rearrangement.

Reaction intermediates such as carbocations and radicals are stabilized by alkyl substitution and resonance.

Carbocation Stability: Tertiary > Secondary > Primary > Methyl.
Radical Stability: Follows the same trend as carbocations.
Example: The intermediate in Markovnikov addition is a more substituted carbocation.

Understanding and drawing mechanisms is essential for predicting products and rationalizing reactivity.

Curved Arrows: Show the movement of electron pairs during bond formation and breaking.
Mechanism Steps: Identify nucleophile and electrophile, show electron flow, and indicate intermediates.
Example: In acid-catalyzed hydration, the alkene attacks a proton, forming a carbocation, which is then attacked by water.