🚗 Hydrocarbons: Alkanes, Alkenes, Alkynes & Arenes

Hydrocarbons constitute the foundational framework of organic synthetic chemistry. Advanced examination topics include free-radical selectivity ratios ( $C l_{2}$ vs $B r_{2}$ ), regio- and stereoselective alkene electrophilic additions, reductive vs oxidative ozonolysis, terminal alkyne acidity, and Electrophilic Aromatic Substitution (EAS) directing effects.

1. 🛢️ Alkanes & Free-Radical Halogenation

1.1 Halogenation Selectivity ( $F_{2} > C l_{2} > B r_{2} > I_{2}$ )

Free radical substitution mechanism:

Initiation: $X_{2} \overset{h ν}{\to} 2 X^{∙}$ (Homolytic fission).
Propagation: $R - H + X^{∙} \to R^{∙} + H X$ ; $R^{∙} + X_{2} \to R - X + X^{∙}$ .
Termination: $2 R^{∙} \to R - R$ ; $R^{∙} + X^{∙} \to R - X$ .

Relative Reactivity Rates per Hydrogen at 25^{\circ} C :

Chlorination: 3^{\circ} : 2^{\circ} : 1^{\circ} = 5.0 : 3.8 : 1.0 (Low selectivity, high reactivity)

Bromination: 3^{\circ} : 2^{\circ} : 1^{\circ} = 1600 : 82 : 1 (Extremely high selectivity for 3^{\circ} position!)

Yield Ratio = \frac{(Number of equivalent H’s) \times (Relative Reactivity)}{\sum [(Number of H’s) \times (Relative Reactivity)]}

2. ⚡ Alkenes Electrophilic Additions Master Matrix

Reagent & Conditions	Intermediate Involved	Regioselectivity	Stereoselectivity	Rearrangement Possible?
$H X$ ( $H C l, H B r, H I$ )	Classical Open Carbocation ( $C^{+}$ )	Markovnikov ( $H$ to more hydrogenated C)	Non-stereospecific (Racemic mixture if chiral)	YES (1,2-Hydride / Methyl shifts)
$H B r + Peroxide (R_{2} O_{2})$ (Kharasch Effect)	Secondary Free Radical ( $C^{∙}$ )	Anti-Markovnikov	Non-stereospecific	NO
$X_{2}$ in $C C l_{4}$ ( $B r_{2} / C C l_{4}$ )	Cyclic Halonium Ion ( $B r^{+}$ ring)	Markovnikov attack on more substituted C	Strictly Anti-Addition (Trans product)	NO
$X_{2} / H_{2} O$ (Halohydrin)	Cyclic Halonium Ion	$- O H$ goes to more substituted C; $- X$ to less	Strictly Anti-Addition	NO
Oxymercuration-Demercuration ( $H g (O A c)_{2}, H_{2} O \to N a B H_{4}$ )	Cyclic Mercurinium Ion	Markovnikov Addition of $- O H$	Anti-Addition in step 1	NO (Clean product without rearrangement)
Hydroboration-Oxidation ( $B H_{3} \cdot T H F \to H_{2} O_{2} / O H^{-}$ )	4-Centered Cyclic Transition State	Anti-Markovnikov Addition of $- O H$	Strictly Syn-Addition ( $H$ and $O H$ on same face)	NO
Cold Dilute Alkaline $K M n O_{4}$ (Baeyer's Reagent)	Cyclic Manganate Ester	cis-1,2-Diol (Glycol)	Strictly Syn-Addition	NO
$O s O_{4} \to N a H S O_{3}$	Cyclic Osmate Ester	cis-1,2-Diol	Strictly Syn-Addition	NO

3. ✂️ Ozonolysis ( $O_{3}$ ) Diagnostic Cleavage

R_{1} R_{2} C = C R_{3} R_{4} + O_{3} \overset{- 78^{\circ} C}{\to} Molozonide \to Ozonide

Reductive Ozonolysis ( $O_{3} / Z n - H_{2} O$ or $O_{3} / (C H_{3})_{2} S$ ):
- Cleaves double bond cleanly into aldehydes and ketones: $> C = C < \to > C = O + O = C <$
- Aldehydes ( $- C H O$ ) are preserved without oxidation to carboxylic acids.
Oxidative Ozonolysis ( $O_{3} / H_{2} O_{2}$ or $O_{3} / H_{2} O$ ):
- Aldehydes are further oxidized to Carboxylic Acids ( $- C O O H$ ); terminal $= C H_{2}$ is converted into $C O_{2} + H_{2} O$ .

4. 🏰 Electrophilic Aromatic Substitution (EAS)

Mechanism: Benzene + E^{+} \overset{Slow (RDS)}{\to} Arenium Ion / σ -Complex (Wheland Intermediate) \overset{- H^{+}, Fast}{\to} Substituted Benzene

4.1 Activating & Directing Effects Matrix

Group Class	Substituents	Electronic Nature	Directing Nature	Rate vs Benzene
Strongly Activating	$- N H_{2}, - N H R, - N R_{2}, - O H, - O^{-}$	Strong $+ M$ via non-bonded lone pair	$o r t h o / p a r a$	$10^{3} - 10^{8} \times$ faster
Moderately Activating	$- N H C O C H_{3}, - O C H_{3}, - O C_{2} H_{5}$	$+ M$ (moderated by adjacent carbonyl)	$o r t h o / p a r a$	Faster
Weakly Activating	$- C H_{3}, - C_{2} H_{5}, - R, - P h$	$+ H$ (Hyperconjugation) and $+ I$	$o r t h o / p a r a$	Faster
Deactivating (Halogens)	$- F, - C l, - B r, - I$	$- I > + M$ (Overall electron-withdrawing)	$o r t h o / p a r a$	Slower (Unique exception: Deactivating but $o / p$ -directing!)
Moderately Deactivating	$- C H O, - C O R, - C O O H, - C O O R, - S O_{3} H, - C N$	$- M$ and $- I$	$m e t a$	Slower
Strongly Deactivating	$- N O_{2}, - C F_{3}, - C C l_{3}, - N R_{3}^{+}, - N H_{3}^{+}$	Powerful $- M$ / $- I$ (Positive or partial positive on benzylic atom)	$m e t a$	$10^{4} - 10^{7} \times$ slower

🚗 Hydrocarbons: Alkanes, Alkenes, Alkynes & Arenes ​

1. 🛢️ Alkanes & Free-Radical Halogenation ​

1.1 Halogenation Selectivity (F2>Cl2>Br2>I2) ​

2. ⚡ Alkenes Electrophilic Additions Master Matrix ​

3. ✂️ Ozonolysis (O3) Diagnostic Cleavage ​

4. 🏰 Electrophilic Aromatic Substitution (EAS) ​

4.1 Activating & Directing Effects Matrix ​