Exam Master Review Sheet - Alkenes

CHEMCA

EXAM MASTER FORMULA SHEET

Hydrocarbons: Alkenes (Olefins)

Structure, Preparation & Electrophilic Additions

1. General Properties & Stability

Unsaturated hydrocarbons containing at least one $C=C$ double bond (Hybridization: $sp^2$, Geometry: Trigonal Planar).

Stability Rule

Stability $\propto$ Number of $\alpha$-hydrogens (Hyperconjugation / Baker-Nathan effect).

Tetrasubstituted > Trisubstituted > Disubstituted > Monosubstituted

Heat of Hydrogenation ($\Delta H_{hydro}$)

Energy released when 1 mole of alkene is catalytically hydrogenated.

\[ |\Delta H_{hydro}| \propto \frac{1}{\text{Stability of Alkene}} \]

2. Important Preparations

From Alcohols & Halides ($\beta$-Elimination)

1. Acidic Dehydration of Alcohols ($E1$):

\[ \ce{R-CH(OH)-CH3 ->[\text{Conc. } H2SO4][\Delta, 170^\circ C] R-CH=CH2 + H2O} \]

Carbocation intermediate $\implies$ Rearrangements are possible! Follows Saytzeff rule.

2. Dehydrohalogenation of Alkyl Halides ($E2$):

\[ \ce{R-CH(Br)-CH3 ->[\text{Alc. } KOH][\Delta] R-CH=CH2 + KBr + H2O} \]

Requires Anti-periplanar geometry. Major product is Saytzeff (unless bulky base like t-BuOK is used $\to$ Hofmann).

Stereoselective Reduction of Alkynes

1. Lindlar's Catalyst ($\to$ CIS Alkene):

\[ \ce{R-C\equiv C-R' ->[H2][Pd/BaSO4, \text{Quinoline}] \text{Cis-Alkene}} \]

Syn-addition of Hydrogen.

2. Birch Reduction ($\to$ TRANS Alkene):

\[ \ce{R-C\equiv C-R' ->[Na \text{ or } Li][\text{Liq. } NH3] \text{Trans-Alkene}} \]

Anti-addition via radical anion mechanism. (Note: Terminal alkynes form acetylide ions instead).

Dehalogenation of Vicinal Dihalides:

\[ \ce{R-CH(Br)-CH(Br)-R' ->[Zn \text{ dust}][\Delta] R-CH=CH-R' + ZnBr2} \]

Mechanism is strictly Anti-Elimination.

3. Electrophilic Additions ($Ad_E$)

Markovnikov's Rule (The Core Principle)

"During addition across an unsymmetrical alkene, the negative part of the addendum attaches to the carbon with fewer hydrogen atoms."

Mechanistic Truth: The reaction proceeds via the most stable Carbocation. Rearrangements (Hydride/Alkyl shifts) WILL occur if a more stable carbocation can form!

1. Addition of HX

\[ \ce{R-CH=CH2 + HX -> R-CH(X)-CH3} \]

• Reactivity: $\ce{HI > HBr > HCl > HF}$
• Intermediate: Carbocation (Rearrangements happen).
• Kharasch / Peroxide Effect: Only for $\ce{HBr + Peroxide}$. Proceeds via Free Radical mechanism leading to Anti-Markovnikov product (No rearrangements).

2. Halogenation ($\ce{X2 / CCl4}$)

\[ \ce{R-CH=CH2 + Br2 -> R-CH(Br)-CH2(Br)} \]

• Intermediate: Cyclic Halonium Ion (No Carbocation $\to$ No Rearrangements).
• Stereochemistry: Strictly ANTI-Addition.
• Test for unsaturation: Reddish-brown color of $\ce{Br2}$ discharges.

4. Hydration of Alkenes (Master Comparison)

Reaction Type	Reagents	Regioselectivity	Rearrangement?	Stereochemistry
Acid Catalyzed Hydration	$\ce{H+ / H2O}$ or dil. $\ce{H2SO4}$	Markovnikov	YES	Mixed (Syn + Anti)
Oxymercuration Demercuration (OMDM)	1. $\ce{Hg(OAc)2, H2O}$ 2. $\ce{NaBH4, OH^-}$	Markovnikov	NO	ANTI Addition
Hydroboration Oxidation (HBO)	1. $\ce{B2H6 / THF}$ 2. $\ce{H2O2 / OH^-}$	Anti-Markovnikov	NO	SYN Addition

5. Oxidation, Hydroxylation & Ozonolysis

Di-hydroxylation (Formation of Diols)

1. Baeyer's Reagent (Cold, dil. alk. $\ce{KMnO4}$):

\[ \ce{>C=C< ->[\text{Cold } KMnO4] >C(OH)-C(OH)<} \]

Stereochem: SYN Addition. (Pink color discharges $\to$ Brown ppt of $\ce{MnO2}$).

2. Osmium Tetroxide ($\ce{OsO4 / NaHSO3}$):

Also gives SYN Addition diol. Very high yield.

3. Peroxy Acids ($\ce{RCO3H}$ or MCPBA) + $\ce{H3O^+}$:

Forms epoxide first, then opens to give ANTI Addition diol.

Oxidative Cleavage

Hot, Conc. $\ce{KMnO4}$ or $\ce{K2Cr2O7 / H^+}$:

Breaks the double bond completely.

• Terminal $\ce{=CH2} \to \ce{CO2 + H2O}$
• Monosubstituted $\ce{=CH-R} \to \ce{R-COOH}$ (Acids)
• Disubstituted $\ce{=C(R)R'} \to \ce{R-CO-R'}$ (Ketones)

Ozonolysis ($\ce{O3}$ cleavage) Rules

Reductive Ozonolysis

Reagent: $\ce{O3}$ followed by $\ce{Zn / H2O}$ or $\ce{Me2S}$.

Products: Aldehydes & Ketones

The Zn prevents further oxidation of aldehydes.

Oxidative Ozonolysis

Reagent: $\ce{O3}$ followed by $\ce{H2O2}$ or $\ce{H2O}$ (without Zn).

Products: Carboxylic Acids & Ketones

Aldehydes formed are immediately oxidized to Acids.

6. Addition Stereochemistry (High-Yield Mnemonics)

// The CAR-TAM Rule

Used for ANTI additions (like Halogenation $X_2$):

• Cis Alkene + Anti Add $\rightarrow$ Racemic Mixture

• Trans Alkene + Anti Add $\rightarrow$ Meso Compound

// The CSM-TSR Rule

Used for SYN additions (like Baeyer's reagent, $D_2/Pd$, HBO):

• Cis Alkene + Syn Add $\rightarrow$ Meso Compound

• Trans Alkene + Syn Add $\rightarrow$ Racemic Mixture

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