Organic Chemistry I: Alkenes, Alkyl Halides, and Stereochemistry Study Notes

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Alkyl Halides: Substitution and Elimination Mechanisms

SN1 and SN2 Reaction Mechanisms

Alkyl halides undergo nucleophilic substitution reactions via two main mechanisms: SN1 (unimolecular) and SN2 (bimolecular). The mechanism depends on substrate structure, nucleophile strength, solvent, and leaving group quality.

SN1 Mechanism: Occurs in two steps: ionization to form a carbocation, followed by nucleophilic attack. Favored by weak nucleophiles, good ionizing solvents, and substrates that can stabilize carbocations (3° > 2°).
SN2 Mechanism: Occurs in a single concerted step. Favored by strong nucleophiles, wide variety of solvents, and unhindered substrates (CH3X > 1° > 2°).

Promoting Factor	SN1	SN2
Nucleophile	Weak nucleophiles are OK	Strong nucleophile needed
Substrate (RX)	3° > 2°	CH3X > 1° > 2°
Solvent	Good ionizing solvent needed	Wide variety of solvents
Leaving group	Good one required	Good one required

Characteristic	SN1	SN2
Kinetics	First order,	Second order,
Stereochemistry	Mixture of inversion and retention	Complete inversion
Rearrangements	Common	Impossible

Substrate Reactivity

Substrate	SN1 (weak nucleophile)	SN2 (strong nucleophile)
Methyl halides CH3X	No reaction (unstable cation)	SN2 unhindered and favored
Primary halides RCH2X	Rarely reacts unless resonance-stabilized	SN2 favored unless R group is bulky
Secondary halides R2CHX	SN1 often with rearrangement in good solvent	SN2 can occur unless alkyl/nucleophile are bulky
Tertiary halides R3CX	SN1 occurs readily in good solvent	SN2 cannot occur (steric hindrance)

Bond Lengths and Angles in Alkenes

Hybridization and Pi Bonding

Alkenes feature sp2 hybridized carbons, resulting in shorter C–C bonds compared to alkanes. The pi () bond arises from the sideways overlap of unhybridized p orbitals.

sp2 hybrid orbitals have greater s-character and are shorter than sp3 orbitals.
Pi overlap shortens the C–C bond in alkenes to about 1.33 Å.
Each carbon in ethylene has one unpaired electron in a p orbital, requiring the molecule to be coplanar for effective overlap.

Elements of Unsaturation and Calculation

Index of Hydrogen Deficiency

An element of unsaturation is a structural feature (double bond or ring) that reduces the number of hydrogens in a molecule by two. The index of hydrogen deficiency quantifies this.

Double bonds and rings each count as one element of unsaturation.
To calculate: Find the number of hydrogens if saturated, subtract actual hydrogens, divide by 2.

Example: Propene (C3H6) and cyclopropane (C3H6) each have one element of unsaturation.

Nomenclature of Alkenes

Systematic Naming Rules

Alkene nomenclature follows IUPAC rules, prioritizing the longest chain containing the double bond and assigning the lowest possible number to the double bond.

Change the ending to -ene.
Number the chain so the double bond has the lowest possible locant.
Examples: 1-butene (but-1-ene), 2-butene (but-2-ene), 1-pentene (pent-1-ene), 2-pentene (pent-2-ene).

E/Z Nomenclature for Alkenes

Ingold-Prelog Priority Rules

When alkenes have different substituents, the E/Z system is used to describe stereochemistry.

Assign priority to substituents based on atomic number.
If highest priority groups are on the same side, the isomer is Z (zusammen).
If on opposite sides, the isomer is E (entgegen).

Example: 1-chloropropene: Assign priorities, then determine E or Z configuration.

Alkene Stereoisomers

Geometric Isomerism

Alkenes can exhibit cis-trans (E/Z) isomerism due to restricted rotation around the double bond. Multiple double bonds require specification of each bond's stereochemistry.

Double bonds outside rings can show E/Z isomerism.
For compounds with more than one double bond, specify each (e.g., (3Z,5E)-3-bromoocta-3,5-diene).

Elimination Reactions: E1 and E2 Mechanisms

E1 and E2 Elimination

Alkyl halides can undergo elimination to form alkenes via E1 (unimolecular) or E2 (bimolecular) mechanisms.

E1 Mechanism: Two-step process: ionization to form carbocation, then base abstracts a proton to form alkene.
E2 Mechanism: Single-step process: base abstracts proton and leaving group departs simultaneously.

General E1 Equation:

General E2 Equation:

Zaitsev's Rule and Hofmann Product

Zaitsev's Rule: The most substituted alkene (most stable) is the major product in elimination reactions.
Hofmann Product: Bulky bases favor formation of the less substituted alkene.

Alcohols: Dehydration and Addition Reactions

Dehydration of Alcohols

Alcohols can be dehydrated to form alkenes using an acid catalyst and heat. The reaction follows Zaitsev's rule and may involve carbocation rearrangements.

Common acids: concentrated H2SO4 or H3PO4.
Removal of low-boiling alkene shifts equilibrium toward product.

Addition of HX to Alkenes

Alkenes react with hydrogen halides (HX) via electrophilic addition, following Markovnikov's rule.

Protonation of the double bond forms a carbocation.
Halide ion attacks the carbocation to yield the alkyl halide.

General Equation:

Hydration of Alkenes

Alkenes can be converted to alcohols by addition of water in the presence of acid, following Markovnikov's rule.

Uses dilute H2SO4 or H3PO4.
Reverse of alcohol dehydration.

Example: Hydration of 2,3-dimethylbut-1-ene yields a tertiary alcohol via carbocation rearrangement.

Additional info: These notes cover key concepts from chapters on alkyl halides, alkenes, stereochemistry, and alcohols, including reaction mechanisms, nomenclature, and product prediction rules essential for Organic Chemistry I.