Physical and Chemical Properties — Revision Notes

⚡ 30-Second Revision

Physical State: — $\text{C}_{2-4}$ gases, $\text{C}_{5-17}$ liquids, $\text{C}_{18+}$ solids.
Boiling Point: — $\uparrow$ with MW, $\downarrow$ with branching. Cis > Trans (due to dipole).
Melting Point: — $\uparrow$ with MW. Trans > Cis (due to packing).
Solubility: — Insoluble in $\text{H}_2\text{O}$, soluble in organic solvents.
Density: — Less dense than $\text{H}_2\text{O}$.
Electrophilic Addition: — Characteristic reaction.

- Hydrogenation: $\text{R-CH=CH-R'} + \text{H}_2 \xrightarrow{\text{Ni/Pt/Pd}} \text{R-CH}_2\text{-CH}_2\text{-R'}$ (Syn-addition) - Halogenation: $\text{R-CH=CH-R'} + \text{X}_2 \xrightarrow{\text{CCl}_4} \text{R-CH(X)-CH(X)-R'}$ (Anti-addition) - Hydrohalogenation: $\text{R-CH=CH}_2 + \text{HX} \rightarrow \text{Markovnikov product}$ - Anti-Markovnikov (HBr/Peroxide): $\text{R-CH=CH}_2 + \text{HBr} \xrightarrow{\text{Peroxide}} \text{R-CH}_2\text{-CH}_2\text{Br}$ - Hydration: $\text{R-CH=CH}_2 + \text{H}_2\text{O} \xrightarrow{\text{H}^+} \text{Markovnikov alcohol}$

Oxidation:

- Baeyer's Test: Cold, dil, alk $\text{KMnO}_4 \rightarrow$ Vicinal diol (Syn-dihydroxylation, purple to brown ppt) - Ozonolysis: $\text{R}_2\text{C=CR}_2 \xrightarrow{\text{1. O}_3 \text{ 2. Zn/H}_2\text{O}} \text{R}_2\text{C=O} + \text{O=CR}_2$ - **Hot $\text{KMnO}_4$:** Cleavage to ketones/carboxylic acids/$\text{CO}_2$.

Polymerization: — Addition polymerization to form long chains.
Combustion: — $\text{C}_n\text{H}_{2n} + \frac{3n}{2}\text{O}_2 \rightarrow n\text{CO}_2 + n\text{H}_2\text{O}$

2-Minute Revision

Alkenes are unsaturated hydrocarbons with a C=C double bond. Their physical properties are governed by molecular size, branching, and geometric isomerism. Small alkenes are gases, larger ones are liquids or solids.

Boiling points increase with molecular mass but decrease with branching. Cis isomers generally have higher boiling points due to dipole moments, while trans isomers have higher melting points due to better crystal packing.

Alkenes are nonpolar, hence insoluble in water but soluble in organic solvents.

Chemically, the electron-rich $\pi$-bond makes alkenes highly reactive towards electrophiles. The most important reactions are electrophilic additions: hydrogenation (adding $\text{H}_2$ to form alkanes, syn-addition), halogenation (adding $\text{X}_2$ to form dihalides, anti-addition), hydrohalogenation (adding $\text{HX}$ to form alkyl halides, following Markovnikov's rule, or anti-Markovnikov with HBr/peroxides), and hydration (adding $\text{H}_2\text{O}$ to form alcohols, Markovnikov's rule).

Oxidation reactions include Baeyer's test (cold, dilute $\text{KMnO}_4$ for diols, syn-dihydroxylation) and ozonolysis (cleavage to aldehydes/ketones for structure determination). Alkenes also undergo addition polymerization and complete combustion.

5-Minute Revision

Alkenes, characterized by their carbon-carbon double bond, exhibit distinct physical and chemical behaviors. Physically, their state progresses from gas (C2-C4) to liquid (C5-C17) to solid (C18+) as molecular mass increases, due to strengthening London dispersion forces.

Boiling points follow this trend, but branching reduces them. Cis-trans isomerism is crucial: cis isomers often have higher boiling points due to a net dipole moment, while trans isomers, being more symmetrical, pack better in a crystal lattice, leading to higher melting points.

Alkenes are nonpolar, making them insoluble in water but soluble in nonpolar organic solvents, and they are less dense than water.

Chemically, the reactive $\pi$-bond is the site for most reactions, primarily electrophilic addition. Key reactions include:

1
Hydrogenation: — Addition of $\text{H}_2$ over Ni, Pt, or Pd catalysts to form alkanes. This is a syn-addition.

*Example:* $\text{CH}_2\text{=CH}_2 + \text{H}_2 \xrightarrow{\text{Ni}} \text{CH}_3\text{-CH}_3$

1
Halogenation: — Addition of $\text{X}_2$ (e.g., $\text{Br}_2$) to form vicinal dihalides. This is an anti-addition, used as a test for unsaturation (decolorizes bromine water).

*Example:* $\text{CH}_2\text{=CH}_2 + \text{Br}_2 \xrightarrow{\text{CCl}_4} \text{Br-CH}_2\text{-CH}_2\text{-Br}$

1
Hydrohalogenation: — Addition of $\text{HX}$ (HCl, HBr, HI). Follows Markovnikov's Rule: H adds to the carbon with more hydrogens, X to the carbon with fewer hydrogens, forming the more stable carbocation.

*Example:* $\text{CH}_3\text{-CH=CH}_2 + \text{HCl} \rightarrow \text{CH}_3\text{-CH(Cl)-CH}_3$ (2-chloropropane) *Peroxide Effect:* With HBr and peroxides, anti-Markovnikov addition occurs (Br adds to the carbon with more hydrogens) via a free radical mechanism. *Example:* $\text{CH}_3\text{-CH=CH}_2 + \text{HBr} \xrightarrow{\text{Peroxide}} \text{CH}_3\text{-CH}_2\text{-CH}_2\text{Br}$ (1-bromopropane)

1
Hydration: — Addition of $\text{H}_2\text{O}$ in the presence of acid catalyst ($\text{H}^+$) to form alcohols, following Markovnikov's Rule.

*Example:* $\text{CH}_3\text{-CH=CH}_2 + \text{H}_2\text{O} \xrightarrow{\text{H}^+} \text{CH}_3\text{-CH(OH)-CH}_3$

1
Oxidation:

* Baeyer's Test: Cold, dilute, alkaline $\text{KMnO}_4$ oxidizes alkenes to vicinal diols (syn-dihydroxylation). The purple $\text{KMnO}_4$ decolorizes, and brown $\text{MnO}_2$ precipitates, indicating unsaturation.

*Example:* $\text{CH}_2\text{=CH}_2 + \text{KMnO}_4 \rightarrow \text{CH}_2\text{(OH)-CH}_2\text{(OH)}$ * Ozonolysis: Cleavage of the double bond with $\text{O}_3$ followed by reductive workup (Zn/H$_2$O or (CH$_3$)$_2$S) yields aldehydes and/or ketones.

Useful for determining double bond position. *Example:* $\text{CH}_3\text{-CH=CH-CH}_3 \xrightarrow{\text{1. O}_3 \text{ 2. Zn/H}_2\text{O}} 2\text{CH}_3\text{CHO}$ * **Hot, acidic $\text{KMnO}_4$:** Strong oxidation leads to complete cleavage, forming ketones, carboxylic acids, or $\text{CO}_2$ (from terminal $\text{CH}_2$ groups).

1
Polymerization: — Alkenes undergo addition polymerization to form long-chain polymers (e.g., polyethylene from ethene).
2
Combustion: — Burn in excess oxygen to produce $\text{CO}_2$ and $\text{H}_2\text{O}$.

Prelims Revision Notes

Physical Properties

Physical State: — $\text{C}_2\text{-C}_4$ alkenes are gases (ethene, propene, butene). $\text{C}_5\text{-C}_{17}$ are liquids. $\text{C}_{18+}$ are solids. This is due to increasing van der Waals forces with molecular mass.
Boiling Points (BP): — Generally increase with molecular mass. Decrease with branching (less surface area). Cis isomers usually have higher BPs than trans isomers due to a net dipole moment (stronger dipole-dipole interactions).
Melting Points (MP): — Generally increase with molecular mass. Trans isomers usually have higher MPs than cis isomers due to better packing efficiency in the crystal lattice (more symmetrical).
Solubility: — Nonpolar molecules. Insoluble in water (polar solvent). Soluble in nonpolar organic solvents (e.g., benzene, ether, $\text{CCl}_4$). 'Like dissolves like' principle.
Density: — Less dense than water (typically $0.6-0.8,\text{g/cm}^3$).

Chemical Properties (Reactions)

Electrophilic Addition Reactions (Characteristic): — Due to electron-rich $\pi$-bond.

* **Hydrogenation (Addition of $\text{H}_2$):** Converts alkenes to alkanes. Reagents: $\text{H}_2$ with Ni, Pt, or Pd catalyst. Syn-addition. * $\text{R-CH=CH-R'} + \text{H}_2 \xrightarrow{\text{Ni/Pt/Pd}} \text{R-CH}_2\text{-CH}_2\text{-R'}$ * **Halogenation (Addition of $\text{X}_2$):** Converts alkenes to vicinal dihalides.

Reagents: $\text{Br}_2$ or $\text{Cl}_2$ in $\text{CCl}_4$. Anti-addition. Decolorizes bromine water (test for unsaturation). * $\text{R-CH=CH-R'} + \text{Br}_2 \xrightarrow{\text{CCl}_4} \text{R-CH(Br)-CH(Br)-R'}$ * **Hydrohalogenation (Addition of $\text{HX}$):** Converts alkenes to alkyl halides.

* Markovnikov's Rule: For unsymmetrical alkenes, H adds to the carbon with more H's, X adds to the carbon with fewer H's. Forms more stable carbocation. * $\text{CH}_3\text{-CH=CH}_2 + \text{HBr} \rightarrow \text{CH}_3\text{-CH(Br)-CH}_3$ * Anti-Markovnikov's Rule (Peroxide Effect): Only for HBr in presence of peroxides.

Br adds to the carbon with more H's. Free radical mechanism. * $\text{CH}_3\text{-CH=CH}_2 + \text{HBr} \xrightarrow{\text{Peroxide}} \text{CH}_3\text{-CH}_2\text{-CH}_2\text{Br}$ * **Hydration (Addition of $\text{H}_2\text{O}$):** Converts alkenes to alcohols.

Reagents: $\text{H}_2\text{O}$ in presence of acid catalyst (e.g., dilute $\text{H}_2\text{SO}_4$). Follows Markovnikov's Rule.

Oxidation Reactions:

* Baeyer's Test (Hydroxylation): Reagent: Cold, dilute, alkaline $\text{KMnO}_4$. Forms vicinal diols. Syn-dihydroxylation. Purple $\text{KMnO}_4$ decolorizes, brown $\text{MnO}_2$ ppt forms (test for unsaturation).

* $\text{R-CH=CH-R'} + \text{KMnO}_4 (\text{cold, dil, alk}) \rightarrow \text{R-CH(OH)-CH(OH)-R'}$ * Ozonolysis: Reagents: 1. $\text{O}_3$ 2. $\text{Zn/H}_2\text{O}$ or $(\text{CH}_3)_2\text{S}$.

Cleaves double bond to form aldehydes and/or ketones. Used for structure elucidation. * $\text{R}_2\text{C=CR}_2 \xrightarrow{\text{1. O}_3 \text{ 2. Zn/H}_2\text{O}} \text{R}_2\text{C=O} + \text{O=CR}_2$ * **Hot, Acidic $\text{KMnO}_4$:** Strong oxidation.

Cleaves double bond to form ketones, carboxylic acids, or $\text{CO}_2$ (from terminal $\text{CH}_2$ groups).

Polymerization: — Alkenes undergo addition polymerization under heat, pressure, and catalysts to form long-chain polymers.
Combustion: — Alkenes burn in excess oxygen to produce $\text{CO}_2$ and $\text{H}_2\text{O}$. $\text{C}_n\text{H}_{2n} + \frac{3n}{2}\text{O}_2 \rightarrow n\text{CO}_2 + n\text{H}_2\text{O}$.

Vyyuha Quick Recall

Markovnikov's Rule: Hydrogen to the Rich (more H's). Anti-Markovnikov: Bromine to the Rich (more H's, with Peroxides).