Properties

Physical Properties

Alkenes share many physical characteristics with alkanes, though they differ in isomerism and polarity. The first three alkenes — ethene, propene, and butene — are gases at room temperature. The next fourteen members are liquids, and the higher ones are solids. Ethene itself is a colourless gas with a faint sweet smell; all other alkenes are colourless and odourless.

Alkenes are insoluble in water but dissolve readily in non-polar solvents such as benzene and petroleum ether. Their boiling points increase regularly with molecular size — each additional $-\mathrm{CH}_2-$ group raises the boiling point by roughly $10\text{–}20\ \mathrm{K}$. As with alkanes, straight-chain alkenes boil at higher temperatures than their branched-chain isomers.

Chemical Properties

The carbon-carbon double bond contains a $\pi$ bond — a region of loosely held electrons. This makes alkenes electron-rich and highly reactive toward electrophiles. The characteristic reaction of alkenes is electrophilic addition, where an electrophile attacks the $\pi$ bond and two new $\sigma$ bonds form across the double bond. Some reagents add via a free-radical mechanism, and under special conditions alkenes can also undergo free-radical substitution. Oxidation and ozonolysis are also prominent reactions.

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1. Addition of Dihydrogen

Alkenes add one molecule of hydrogen gas in the presence of a finely divided metal catalyst (nickel, palladium, or platinum) to form alkanes. This is the same hydrogenation reaction described in Section 9.2.2.

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2. Addition of Halogens

Bromine or chlorine adds across the double bond to give vicinal dihalides (halogens on adjacent carbons). Iodine does not add under normal conditions.

The reddish-orange colour of bromine in carbon tetrachloride is discharged as bromine adds to the double bond. This colour change is used as a test for unsaturation.

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3. Addition of Hydrogen Halides

Hydrogen halides ($\mathrm{HCl}$, $\mathrm{HBr}$, $\mathrm{HI}$) add to alkenes to form alkyl halides. The order of reactivity is:

Like halogen addition, this is an electrophilic addition reaction.

Addition to Symmetrical Alkenes

When both carbons of the double bond have identical (or similar) groups, the addition follows a straightforward electrophilic mechanism:

Addition to Unsymmetrical Alkenes — Markovnikov's Rule

For an unsymmetrical alkene like propene, two products are possible:

• Product I: $\mathrm{CH_3CHBrCH_3}$ (2-bromopropane)
• Product II: $\mathrm{CH_3CH_2CH_2Br}$ (1-bromopropane)

In 1869, the Russian chemist Vladimir Markovnikov formulated a rule based on his study of many such reactions:

According to this rule, product I (2-bromopropane) is expected, and it is indeed the principal product.

›Proof

Mechanism of Markovnikov Addition

The reaction proceeds through a carbocation intermediate:

Step 1: $\mathrm{HBr}$ provides an electrophile, $\mathrm{H^+}$, which attacks the double bond. The proton can add to either carbon, producing two possible carbocations:

$\mathrm{CH_3CH=CH_2 + H^+ \longrightarrow}$

$\mathrm{CH_3\overset{+}{C}HCH_3 \quad (secondary\ carbocation,\ more\ stable)}$

$\mathrm{CH_3CH_2\overset{+}{C}H_2 \quad (primary\ carbocation,\ less\ stable)}$

The secondary carbocation (b) is more stable than the primary carbocation (a) because alkyl groups stabilise the positive charge through hyperconjugation and inductive effects. The more stable carbocation forms at a faster rate and therefore predominates.

Step 2: The carbocation is attacked by the $\mathrm{Br^-}$ ion:

$\mathrm{CH_3\overset{+}{C}HCH_3 + Br^- \longrightarrow CH_3CHBrCH_3}$

This gives 2-bromopropane as the major product.

Anti-Markovnikov Addition — The Peroxide Effect (Kharash Effect)

In the presence of organic peroxides (such as benzoyl peroxide, $(\mathrm{C_6H_5CO})_2\mathrm{O_2}$), the addition of $\mathrm{HBr}$ to unsymmetrical alkenes occurs contrary to Markovnikov's rule. This is called the peroxide effect or Kharash effect, discovered by M.S. Kharash and F.R. Mayo in 1933.

The product is 1-bromopropane, not 2-bromopropane.

›Proof

Mechanism of the Peroxide Effect

The reaction proceeds via a free-radical chain mechanism:

Initiation:

$(\mathrm{C_6H_5CO})_2\mathrm{O_2} \xrightarrow{\text{homolysis}} 2\ \mathrm{C_6H_5COO^\bullet}$

$\mathrm{C_6H_5COO^\bullet + HBr \longrightarrow C_6H_5COOH + Br^\bullet}$

Propagation:

The bromine radical adds to the double bond. It can add in two ways, but the more stable free radical is formed preferentially:

$\mathrm{CH_3CH=CH_2 + Br^\bullet \longrightarrow CH_3\overset{\bullet}{C}HCH_2Br \quad (secondary\ radical,\ more\ stable)}$

$\mathrm{CH_3CH=CH_2 + Br^\bullet \longrightarrow CH_3CHCH_2\overset{\bullet}{B}r \quad (primary\ radical,\ less\ stable)}$

The secondary free radical is more stable and therefore predominates. This radical then abstracts a hydrogen atom from another $\mathrm{HBr}$ molecule:

$\mathrm{CH_3\overset{\bullet}{C}HCH_2Br + HBr \longrightarrow CH_3CH_2CH_2Br + Br^\bullet}$

The $\mathrm{Br^\bullet}$ radical is regenerated, continuing the chain. The net result is the formation of 1-bromopropane as the major product.

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4. Addition of Sulphuric Acid

Cold concentrated sulphuric acid adds to alkenes following Markovnikov's rule, forming alkyl hydrogen sulphates via electrophilic addition:

The alkyl hydrogen sulphate can be hydrolysed by heating with water to give the corresponding alcohol.

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5. Addition of Water (Hydration)

In the presence of a few drops of concentrated sulphuric acid (which acts as a catalyst), alkenes react with water to form alcohols. The addition follows Markovnikov's rule:

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6. Oxidation

(a) With Cold, Dilute Alkaline $\mathrm{KMnO_4}$ (Baeyer's Reagent)

Alkenes react with cold, dilute, aqueous potassium permanganate to form vicinal glycols (diols with hydroxyl groups on adjacent carbons). The purple colour of $\mathrm{KMnO_4}$ is discharged, and a brown precipitate of $\mathrm{MnO_2}$ forms. This decolourisation is used as a test for unsaturation.

(b) With Acidic Potassium Permanganate or Dichromate

Under more vigorous conditions (acidic medium, heat), alkenes are oxidised to ketones and/or carboxylic acids, depending on the structure of the alkene and the experimental conditions.

For example:

$\mathrm{CH_3CH=CHCH_3 \xrightarrow{KMnO_4/H^+} 2\ CH_3COOH}$ …