Hydrocarbons — Revision Notes

⚡ 30-Second Revision

Alkanes — $C_nH_{2n+2}$, only C-C single bonds, $sp^3$ hybridization. Reactions: Free radical halogenation ($h

u$), combustion, pyrolysis.

Alkenes — $C_nH_{2n}$, at least one C=C double bond, $sp^2$ hybridization. Reactions: Electrophilic addition (H2, X2, HX, H2O), Markovnikov's rule, anti-Markovnikov's (HBr/peroxides), ozonolysis, Baeyer's test.
Alkynes — $C_nH_{2n-2}$, at least one C≡C triple bond, $sp$ hybridization. Reactions: Electrophilic addition (2 steps), acidity of terminal alkynes ($R-C equiv C-H$), Tollens' test, Fehling's test.
Aromatic Hydrocarbons — Cyclic, planar, conjugated, $(4n+2)pi$ electrons (Huckel's rule). Reactions: Electrophilic substitution (nitration, halogenation, sulfonation, Friedel-Crafts alkylation/acylation). Directive influence of substituents.
Key Reagents — $Br_2/CCl_4$ (unsaturation), Baeyer's reagent (unsaturation), Tollens' reagent (terminal alkynes), $H_2/Pd/C$ (alkene/alkyne to alkane), Lindlar's catalyst (alkyne to cis-alkene), Na/liq $NH_3$ (alkyne to trans-alkene), $AlCl_3$ (Friedel-Crafts).

2-Minute Revision

Hydrocarbons are carbon-hydrogen compounds, categorized as saturated (alkanes) or unsaturated (alkenes, alkynes), and aromatic. Alkanes ($C_nH_{2n+2}$) are stable, undergoing free radical substitution (e.

g., halogenation with UV light) and combustion. Alkenes ($C_nH_{2n}$) and alkynes ($C_nH_{2n-2}$) are reactive due to pi bonds, primarily undergoing electrophilic addition. Key alkene reactions include hydrogenation, halogenation, hydrohalogenation (Markovnikov's rule, with anti-Markovnikov for HBr in peroxides), hydration, and ozonolysis.

Alkynes undergo similar additions, often in two steps. Terminal alkynes are weakly acidic due to sp-hybridization and react with Tollens' or Fehling's reagent. Aromatic hydrocarbons, like benzene, are characterized by Huckel's rule ($(4n+2)pi$ electrons) and undergo electrophilic substitution (nitration, halogenation, Friedel-Crafts).

Substituents on benzene influence reactivity and regioselectivity (ortho-para vs. meta directing). Distinguishing tests like bromine water, Baeyer's reagent, and Tollens' reagent are crucial for identifying different hydrocarbon types.

5-Minute Revision

Hydrocarbons form the backbone of organic chemistry, classified into aliphatic (alkanes, alkenes, alkynes) and aromatic. Alkanes are saturated, with only C-C single bonds ($sp^3$ carbons). They are relatively unreactive, undergoing free radical substitution (e.

g., $CH_4 + Cl_2 xrightarrow{h u} CH_3Cl + HCl$) and complete combustion to $CO_2$ and $H_2O$. Preparation methods include Wurtz reaction ($2RX + 2Na \rightarrow R-R$), decarboxylation, and hydrogenation of unsaturated compounds.

Alkenes contain at least one C=C double bond ($sp^2$ carbons) and are more reactive. Their characteristic reactions are electrophilic additions: hydrogenation ($H_2/Ni$), halogenation ($Br_2/CCl_4$, decolorizes), hydrohalogenation (HX, Markovnikov's rule: H to C with more H's; anti-Markovnikov for HBr/peroxides), and hydration ($H_2O/H_2SO_4$).

Ozonolysis ($O_3$ then $Zn/H_2O$) cleaves the double bond to form carbonyl compounds, useful for structure determination. Baeyer's test (cold, dilute, alkaline $KMnO_4$) is a test for unsaturation. Alkynes have at least one C≡C triple bond ($sp$ carbons) and are highly reactive.

They undergo electrophilic additions in two steps. Partial hydrogenation with Lindlar's catalyst yields cis-alkenes, while Na/liq $NH_3$ yields trans-alkenes. Terminal alkynes ($R-C equiv C-H$) are weakly acidic due to the high s-character of sp-hybridized carbon, reacting with Tollens' reagent (white ppt) or ammoniacal cuprous chloride (red ppt).

Aromatic Hydrocarbons, exemplified by benzene, are cyclic, planar, fully conjugated, and follow Huckel's rule ($(4n+2)pi$ electrons). They exhibit special stability and undergo electrophilic substitution reactions: nitration ($HNO_3/H_2SO_4$), halogenation ($X_2/FeX_3$), sulfonation ($H_2SO_4$), and Friedel-Crafts alkylation ($R-X/AlCl_3$) or acylation ($RCO-X/AlCl_3$).

Substituents on the benzene ring influence the rate and regioselectivity (ortho-para vs. meta directing) of further substitutions. For example, a methyl group (in toluene) is ortho-para directing and activating, while a nitro group (in nitrobenzene) is meta-directing and deactivating.

Mastering these reactions, rules, and distinguishing tests is key for NEET.

Prelims Revision Notes

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Alkanes ($C_nH_{2n+2}$): — Saturated, $sp^3$ carbons.

* Preparation: Wurtz reaction (alkyl halides + Na/dry ether $ightarrow$ symmetrical alkanes), Decarboxylation (RCOONa + NaOH/CaO $ightarrow$ R-H), Hydrogenation of alkenes/alkynes ($H_2/Ni, Pd, Pt$). * Reactions: Free radical halogenation ($CH_4 + Cl_2 xrightarrow{h u} CH_3Cl$), Combustion ($CO_2 + H_2O$), Pyrolysis (cracking).

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Alkenes ($C_nH_{2n}$): — Unsaturated, C=C double bond, $sp^2$ carbons.

* Preparation: Dehydration of alcohols ($R-CH_2-CH_2OH xrightarrow{conc. H_2SO_4} R-CH=CH_2$), Dehydrohalogenation of alkyl halides (R-CHX-CH2-R' $xrightarrow{alc. KOH} R-CH=CH-R'$), Partial hydrogenation of alkynes (Lindlar's catalyst $ightarrow$ cis-alkene).

* Reactions (Electrophilic Addition): * Hydrogenation: $H_2/Ni, Pd, Pt \rightarrow$ Alkane. * Halogenation: $X_2/CCl_4 \rightarrow$ Vicinal dihalide (decolorizes $Br_2$ water). * Hydrohalogenation: HX.

Markovnikov's rule (H to C with more H's). Anti-Markovnikov's (Peroxide effect): *only* for HBr in presence of peroxides. * Hydration: $H_2O/H_2SO_4 \rightarrow$ Alcohol (Markovnikov's). * Ozonolysis: $O_3$ then $Zn/H_2O \rightarrow$ Aldehydes/Ketones.

* Oxidation: Baeyer's test (cold, dil, alk $KMnO_4$) $ightarrow$ Vicinal diol (decolorizes $KMnO_4$).

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Alkynes ($C_nH_{2n-2}$): — Unsaturated, C≡C triple bond, $sp$ carbons.

* Preparation: From $CaC_2$ (for ethyne), Dehydrohalogenation of vicinal/geminal dihalides (strong base like $NaNH_2$). * Reactions (Electrophilic Addition): Similar to alkenes, but two moles of reagent add.

Partial hydrogenation: Lindlar's catalyst $ightarrow$ cis-alkene; Na/liq $NH_3 \rightarrow$ trans-alkene. * Acidity of Terminal Alkynes: $R-C equiv C-H$ is acidic. Reacts with Tollens' reagent ($[Ag(NH_3)_2]OH$) $ightarrow$ white ppt of silver acetylide.

Reacts with ammoniacal $Cu_2Cl_2 \rightarrow$ red ppt of copper acetylide.

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Aromatic Hydrocarbons (Benzene $C_6H_6$): — Cyclic, planar, conjugated, $(4n+2)pi$ electrons (Huckel's rule).

* Preparation: Cyclic polymerization of ethyne (red hot iron tube), Decarboxylation of sodium benzoate, Reduction of phenol (Zn dust). * Reactions (Electrophilic Substitution): * Nitration: $HNO_3/H_2SO_4 \rightarrow$ Nitrobenzene ($NO_2^+$ electrophile).

* Halogenation: $X_2/FeX_3 \rightarrow$ Halobenzene ($X^+$ electrophile). * Sulfonation: $H_2SO_4/SO_3 \rightarrow$ Benzenesulfonic acid ($SO_3$ electrophile). * Friedel-Crafts Alkylation: $R-X/AlCl_3 \rightarrow$ Alkylbenzene (carbocation $R^+$ electrophile, prone to rearrangement/polyalkylation).

* Friedel-Crafts Acylation: $RCO-X/AlCl_3 \rightarrow$ Acylbenzene (acylium ion $RCO^+$ electrophile, no rearrangement/polyacylation). * Directive Influence: Activating groups (e.g., $-CH_3, -OH, -NH_2$) are ortho-para directing.

Deactivating groups (e.g., $-NO_2, -COOH, -CHO$) are meta-directing. Halogens are deactivating but ortho-para directing.

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Distinguishing Tests:

* Alkanes vs. Alkenes/Alkynes: Bromine water test (decolorizes for unsaturation), Baeyer's test (decolorizes for unsaturation). * Terminal Alkynes vs. Alkenes/Internal Alkynes: Tollens' reagent or ammoniacal $Cu_2Cl_2$ (precipitate for terminal alkynes only).

Vyyuha Quick Recall

All Always Add And Aromatics Substitute:

Alkanes: Always Substitute (Free radical substitution).
Alkenes & Alkynes: Always Add (Electrophilic addition).
Aromatics: Always Substitute (Electrophilic aromatic substitution).