Alkenes — Revision Notes

⚡ 30-Second Revision

General Formula: Acyclic alkenes $C_nH_{2n}$.\n- Double Bond: One $\sigma$ and one $\pi$ bond. Restricted rotation.\n- Hybridization: $sp^2$ for double-bonded carbons, trigonal planar geometry ($120^\circ$).\n- Nomenclature: '-ene' suffix, lowest locant for double bond.\n- Isomerism: Structural (chain, position, functional with cycloalkanes), Geometrical (cis-trans, requires two different groups on each C of $C=C$).\n- Preparation:\n - Dehydration of alcohols ($H_2SO_4/\Delta$): Saytzeff's rule.\n - Dehydrohalogenation of alkyl halides (Alc. KOH/$\Delta$): Saytzeff's rule.\n - Dehalogenation of vicinal dihalides ($Zn/Alcohol$).\n - Partial hydrogenation of alkynes:\n - Lindlar's catalyst ($H_2/Pd/CaCO_3$, quinoline/sulfur): *cis*-alkene (syn).\n - $Na/liq.\,NH_3$: *trans*-alkene (anti).\n- Reactions (Electrophilic Addition):\n - Hydrogenation ($H_2/Ni, Pt, Pd$): Syn addition, forms alkane.\n - Halogenation ($X_2/CCl_4$): Anti addition, forms vicinal dihalide. Decolorizes $Br_2$ water.\n - Hydrohalogenation ($HX$): Markovnikov's rule (H to C with more H, X to C with fewer H). Carbocation intermediate.\n - Anti-Markovnikov ($HBr/Peroxide$): Free radical mechanism, H to C with fewer H, Br to C with more H.\n - Hydration ($H_2O/H^+$): Markovnikov's rule, forms alcohol.\n- Oxidation:\n - Baeyer's test (Cold, dil., alk. $KMnO_4$): Syn addition, forms vicinal diol (glycol). Decolorizes $KMnO_4$, brown $MnO_2$ ppt.\n - Ozonolysis ($O_3$ then $Zn/H_2O$ or $H_2O_2$): Cleaves $C=C$ bond, forms aldehydes/ketones (reductive) or carboxylic acids/ketones (oxidative).\n- Polymerization: Addition polymerization (e.g., polyethylene).

2-Minute Revision

Alkenes are unsaturated hydrocarbons defined by at least one carbon-carbon double bond ($C=C$), following the general formula $C_nH_{2n}$. The double bond comprises a sigma ($\sigma$) and a pi ($\pi$) bond, with the carbons being $sp^2$ hybridized, resulting in a trigonal planar geometry and restricted rotation, which allows for cis-trans isomerism.

\n\nNomenclature involves replacing the alkane suffix with '-ene' and numbering the double bond with the lowest possible locant. \n\nKey preparation methods include elimination reactions like dehydration of alcohols and dehydrohalogenation of alkyl halides, often guided by Saytzeff's rule for major product formation.

Partial hydrogenation of alkynes is also crucial, with Lindlar's catalyst yielding cis-alkenes and $Na/liq.\,NH_3$ yielding trans-alkenes. \n\nAlkenes are highly reactive, primarily undergoing electrophilic addition reactions.

Important examples include hydrogenation, halogenation (anti-addition), hydrohalogenation (Markovnikov's rule via carbocations, or anti-Markovnikov with HBr/peroxides via free radicals), and hydration.

Oxidation reactions like Baeyer's test (for unsaturation) and ozonolysis (for double bond position) are also vital. Understanding these reactions, their mechanisms, and stereochemical outcomes is paramount for NEET.

5-Minute Revision

Alkenes are unsaturated hydrocarbons characterized by a carbon-carbon double bond ($C=C$), with a general formula of $C_nH_{2n}$. This double bond consists of a sigma ($\sigma$) bond and a weaker, more exposed pi ($\pi$) bond.

The carbons involved are $sp^2$ hybridized, leading to a trigonal planar geometry ($120^\circ$ bond angles) and, critically, restricted rotation around the double bond, which gives rise to geometrical (cis-trans) isomerism.

Remember, for cis-trans isomerism, each carbon of the double bond must be attached to two *different* groups. \n\nNomenclature: Identify the longest chain containing the double bond, number to give the double bond the lowest possible number, and use the '-ene' suffix.

\n\nPreparation: \n1. Dehydration of Alcohols: $R-CH_2-CH_2-OH \xrightarrow{Conc.\,H_2SO_4, \Delta} R-CH=CH_2$. Follows Saytzeff's rule (more substituted alkene is major). \n2. Dehydrohalogenation of Alkyl Halides: $R-CH_2-CH_2-X \xrightarrow{Alc.

\,KOH, \Delta} R-CH=CH_2$. Also follows Saytzeff's rule. \n3. **Partial Hydrogenation of Alkynes:** \n * To *cis*-alkene:$R-C\equiv C-R' \xrightarrow{H_2, \,Lindlar's \,Catalyst} cis-R-CH=CH-R'$ (syn addition).

\n * To *trans*-alkene: $R-C\equiv C-R' \xrightarrow{Na, \,liq.\,NH_3} trans-R-CH=CH-R'$ (anti addition). \n\nReactions (Electrophilic Addition): \n1. Hydrogenation: $R-CH=CH-R' + H_2 \xrightarrow{Ni/Pt/Pd} R-CH_2-CH_2-R'$.

Syn addition. \n2. Halogenation: $R-CH=CH-R' + Br_2 \xrightarrow{CCl_4} R-CHBr-CHBr-R'$. Anti addition (via halonium ion). Decolorizes bromine water (test for unsaturation). \n3. Hydrohalogenation: $R-CH=CH_2 + HX \longrightarrow$ Markovnikov's product.

H adds to C with more H, X to C with fewer H (via stable carbocation). \n * Peroxide Effect (Anti-Markovnikov): Only for $HBr$ with peroxides. $R-CH=CH_2 + HBr \xrightarrow{Peroxide} R-CH_2-CH_2Br$.

Free radical mechanism. \n4. Hydration: $R-CH=CH_2 + H_2O \xrightarrow{H^+} R-CH(OH)-CH_3$. Markovnikov's rule, forms alcohol. \n\nOxidation Reactions: \n1. Baeyer's Test: Cold, dilute, alkaline $KMnO_4$.

$R-CH=CH-R' \longrightarrow R-CH(OH)-CH(OH)-R'$. Syn addition, forms vicinal diol. Decolorizes purple $KMnO_4$ (test for unsaturation). \n2. Ozonolysis: $R_2C=CR_2 \xrightarrow{1.\,O_3; \,2.\,Zn/H_2O} R_2C=O + O=CR_2$.

Cleaves $C=C$ bond, forms aldehydes/ketones (reductive workup). Oxidative workup ($H_2O_2$) converts aldehydes to carboxylic acids. \n\nKey Concepts to Master: Markovnikov's rule, anti-Markovnikov's rule, Saytzeff's rule, carbocation stability, syn/anti addition, and the specific reagents for each reaction.

Practice predicting products and working backward from products to reactants.

Prelims Revision Notes

Definition: — Unsaturated hydrocarbons with $C=C$ double bond. General formula $C_nH_{2n}$.\n- Structure: $sp^2$ hybridization of double-bonded carbons. Trigonal planar geometry ($120^\circ$). One $\sigma$ and one $\pi$ bond. Restricted rotation around $C=C$.\n- Isomerism:\n - Structural: Chain, position, functional (with cycloalkanes).\n - Geometrical (cis-trans): Requires each $C$ of $C=C$ to have two different groups. Trans isomers generally more stable.\n- Nomenclature: Longest chain containing $C=C$, lowest locant for $C=C$, '-ene' suffix.\n- Preparation Methods:\n - Dehydration of Alcohols: $R-CH(OH)-CH_2-R' \xrightarrow{Conc.\,H_2SO_4, \Delta}$ alkene. Saytzeff's rule for major product.\n - Dehydrohalogenation of Alkyl Halides: $R-CHX-CH_2-R' \xrightarrow{Alc.\,KOH, \Delta}$ alkene. Saytzeff's rule.\n - Dehalogenation of Vicinal Dihalides: $R-CHX-CHX-R' \xrightarrow{Zn, \,Alcohol}$ alkene.\n - Partial Hydrogenation of Alkynes:\n - *cis*-alkene: Lindlar's catalyst ($H_2/Pd/CaCO_3$, quinoline/sulfur).\n - *trans*-alkene: Birch reduction ($Na/liq.\,NH_3$).\n- Reactions (Electrophilic Addition - Characteristic):\n - Hydrogenation: $C=C + H_2 \xrightarrow{Ni/Pt/Pd}$ alkane. Syn addition.\n - Halogenation: $C=C + X_2 \xrightarrow{CCl_4}$ vicinal dihalide. Anti addition (via halonium ion). Decolorizes $Br_2$ water (test for unsaturation).\n - Hydrohalogenation: $C=C + HX \longrightarrow$ alkyl halide. Follows Markovnikov's rule (H to C with more H, X to C with fewer H). Carbocation intermediate. Possible rearrangements.\n - Anti-Markovnikov Addition (Peroxide Effect): Only for $HBr$ with peroxides. Free radical mechanism. $H$ to C with fewer H, $Br$ to C with more H.\n - Hydration: $C=C + H_2O \xrightarrow{H^+}$ alcohol. Follows Markovnikov's rule.\n- Oxidation Reactions:\n - Baeyer's Test: Cold, dilute, alkaline $KMnO_4$. $C=C \longrightarrow$ vicinal diol. Syn addition. Decolorizes purple $KMnO_4$, forms brown $MnO_2$ ppt (test for unsaturation).\n - Ozonolysis: $C=C \xrightarrow{1.\,O_3; \,2.\,Zn/H_2O \,or \,H_2O_2}$ aldehydes/ketones (reductive) or carboxylic acids/ketones (oxidative). Used to locate $C=C$ bond.\n- Polymerization: Addition polymerization (e.g., polyethylene from ethene).

Vyyuha Quick Recall

Markovnikov Has More Hydrogens, Anti-Markovnikov Has Less Hydrogens (for HBr/peroxide). \n\nThis mnemonic helps remember the regioselectivity of HBr addition to unsymmetrical alkenes.

'Markovnikov Has More Hydrogens' means the hydrogen of HBr adds to the carbon of the double bond that already has more hydrogen atoms. 'Anti-Markovnikov Has Less Hydrogens' means, in the presence of peroxide, the hydrogen of HBr adds to the carbon of the double bond that has fewer hydrogen atoms.