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Alkenes formulas

6 formulas and 5 common traps for MHT-CET Chemistry Alkenes, grouped by subtopic.

Full notes with worked examples

Alkenes: IUPAC Names, Isomers, Hybridisation and Stability

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Naming a Substituted Alkene

Name pattern

(locant-substituent)alpha + parent-n-ene,n as low as possible\text{(locant-substituent)}_{\text{alpha}}\ +\ \text{parent}\text{-}n\text{-ene},\quad n\ \text{as low as possible}

Stability of Substituted Alkenes

Stability order

R2C=CR2>R2C=CHR>R2C=CH2>RCH=CH2>CH2=CH2\mathrm{R_2C{=}CR_2 > R_2C{=}CHR > R_2C{=}CH_2 > RCH{=}CH_2 > CH_2{=}CH_2}

Common traps

Taking the longest chain that skips the double bond

Options like '4-bromo-4-ethyl-3-methylbut-3-ene' come from a short chain through the C=C with an ethyl branch left over. The parent must contain the C=C AND be as long as possible — hex-3-ene here.

Preparing Alkenes: Dehydrohalogenation and Saytzeff's Rule

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Dehydrohalogenation with Alcoholic KOH

Dehydrohalogenation

R−CH2−CHX−R′→alc. KOH, ΔR−CH=CH−R′+KX+H2O\mathrm{R{-}CH_2{-}CHX{-}R' \xrightarrow{\text{alc. KOH},\ \Delta} R{-}CH{=}CH{-}R' + KX + H_2O}

Common traps

Picking the alcohol

2-Methylpentan-3-ol is offered for alcoholic KOH. That is the AQUEOUS-KOH (substitution) product; alcoholic KOH eliminates.

Reactions of Alkenes: Addition and Its Direction, KMnO₄ Oxidation, and Ozonolysis

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Addition to Alkenes: Markovnikov, Peroxide Effect and Hydroboration

Markovnikov's rule

CH3CH=CH2+HBr→CH3CHBrCH3(with peroxide: CH3CH2CH2Br)\mathrm{CH_3CH{=}CH_2 + HBr \rightarrow CH_3CHBrCH_3}\quad(\text{with peroxide: } \mathrm{CH_3CH_2CH_2Br})

Oxidation by KMnO₄: Glycols or Cleavage

Ring cleavage

cyclohexene→KMnO4/H+, ΔHOOC(CH2)4COOH\text{cyclohexene} \xrightarrow{\mathrm{KMnO_4/H^+},\ \Delta} \mathrm{HOOC(CH_2)_4COOH}

Ozonolysis

R2C=CR2′→O3, Zn/H2OR2C=O+O=CR2′\mathrm{R_2C{=}CR'_2 \xrightarrow{O_3,\ Zn/H_2O} R_2C{=}O + O{=}CR'_2}

Common traps

Applying the peroxide effect to HCl

Only HBr reverses with peroxide. 2-Methylbut-2-ene + HCl gives the Markovnikov product, 2-chloro-2-methylbutane, peroxide or not.

'3-Bromopropane' is the paper's name for allyl bromide

The 2024 paper prints '3-Bromopropane' as the answer for propene + Br₂ at high temperature. It means CH₂=CH–CH₂Br (3-bromoprop-1-ene): allylic substitution, not addition — pick it over 1,2-dibromopropane.

Stopping at cyclohexanol or cyclohexanone

Hot acidic KMnO₄ does not stop inside the ring — it cleaves the C=C and opens it. Cyclohexanol and cyclohexanone are offered to catch a student who forgets that.

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