Reaction of Alkenes

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Reaction with halogens (Br₂)

Reagent : Br₂

Conditions : aqueous

What happens : C=C opens up; added onto the two C atoms of the C=C double bond: Br & Br

Overall equation : CH₂=CHCH₃+ Br₂ ➔ CH₂BrCH₂Br

Mechanism : electrophilic addition

<p><strong>Reagent : </strong>Br<sub>2</sub></p><p><strong>Conditions : </strong>aqueous</p><p><strong>What happens : </strong>C=C opens up; added onto the two C atoms of the C=C double bond: Br & Br</p><p><strong>Overall equation : </strong>CH<sub>2</sub>=CHCH<sub>3 </sub>+ Br<sub>2</sub> <strong>➔</strong> CH<sub>2</sub>BrCH<sub>2</sub>Br</p><p><strong>Mechanism : </strong>electrophilic addition</p>

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Addition of hydrogen halides (HBr)

Reagent : HBr

Conditions :

What happens : C=C opens up; and added onto the two C atoms of the C=C double bond: H & Br

Overall equation : CH₃CH=CH₂ + HBr ➔ CH₃CH₂CH₂Br (1-bromopropane)

CH₃CH=CH₂ + HBr ➔ CH₃CHBrCH₃ (2-bromopropane) - major product because secondary carbocations are more stable

Mechanism : electrophilic addition

<p><strong>Reagent : </strong>HBr</p><p><strong>Conditions : </strong></p><p><strong>What happens : </strong>C=C opens up; and added onto the two C atoms of the C=C double bond: H & Br</p><p><strong>Overall equation : </strong>CH<sub>3</sub>CH=CH<sub>2</sub> + HBr ➔ CH<sub>3</sub>CH<sub>2</sub>CH<sub>2</sub>Br (1-bromopropane)</p><p>CH<sub>3</sub>CH=CH<sub>2</sub> + HBr ➔ CH<sub>3</sub>CHBrCH<sub>3</sub> (2-bromopropane) - major product because secondary carbocations are more stable</p><p><strong>Mechanism : </strong>electrophilic addition</p>

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Addition of concentrated H₂SO₄

Reagent : H₂SO₄

Conditions : Cold, Concentrated H₂SO₄

What happens : C=C opens up; and added onto the two C atoms of the C=C double bond: H & O-SO₂OH

Overall equation : CH₃CH₂OSO₃H + H₂O ➔ CH₃CH₂OH + H₂SO₄

Mechanism : electrophilic addition

<p><strong>Reagent : </strong>H<sub>2</sub>SO<sub>4</sub></p><p><strong>Conditions : </strong>Cold, Concentrated H<sub>2</sub>SO<sub>4</sub></p><p><strong>What happens : </strong>C=C opens up; and added onto the two C atoms of the C=C double bond: H & O-SO<sub>2</sub>OH</p><p><strong>Overall equation : </strong><span>CH</span><sub>3</sub><span>CH</span><sub>2</sub><span>OSO</span><sub>3</sub><span>H + H</span><sub>2</sub><span>O ➔ CH</span><sub>3</sub><span>CH</span><sub>2</sub><span>OH + H</span><sub>2</sub><span>SO</span><sub>4</sub></p><p><strong>Mechanism : </strong>electrophilic addition</p>

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Hydration to form alcohols

Reagent : H₂O with strong acid (e.g.H₂SO₄, H₃PO₄)

Conditions : Steam, 300° C, conc. phosphoric acid catalyst, 6.5 ×10³KPa

What happens : C=C opens up; and added onto the two C atoms of the C=C double bond: H & OH

Overall equation : CH₃CH₂OSO₃H + H₂O ➔ CH₃CH₂OH + H₂SO₄

Mechanism : electrophilic addition

<p><strong>Reagent : </strong>H<sub>2</sub>O with strong acid (e.g.H<sub>2</sub>SO<sub>4</sub>, H<sub>3</sub>PO<sub>4</sub>)</p><p><strong>Conditions : </strong>Steam, 300° C, conc. phosphoric acid catalyst, 6.5 ×10<sup>3 </sup>KPa</p><p><strong>What happens : </strong>C=C opens up; and added onto the two C atoms of the C=C double bond: H & OH</p><p><strong>Overall equation : </strong>CH<sub>3</sub>CH<sub>2</sub>OSO<sub>3</sub>H + H<sub>2</sub>O ➔ CH<sub>3</sub>CH<sub>2</sub>OH + H<sub>2</sub>SO<sub>4</sub></p><p><strong>Mechanism : </strong>electrophilic addition</p>

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Dehydrating alcohols to form alkenes

Reagent : concentrated H₂SO₄

Conditions : heated acid catalyst such as concentrated H₂SO₄

What happens : protonation of the alcohol oxygen, formation of a carbocation intermediate via the loss of water, formation of an alkene via the loss of a proton.

Overall equation : CH₃CH₂OH ➔ CH₂=CH₂ + H₂O

Mechanism : elimination

<p><strong>Reagent : </strong>concentrated H<sub>2</sub>SO<sub>4</sub></p><p><strong>Conditions :</strong> heated acid catalyst such as concentrated H<sub>2</sub>SO<sub>4</sub></p><p><strong>What happens : </strong>protonation of the alcohol oxygen, formation of a carbocation intermediate via the loss of water, formation of an alkene via the loss of a proton.</p><p><strong>Overall equation : </strong><span>CH</span><sub>3</sub><span>CH</span><sub>2</sub><span>OH ➔ CH</span><sub>2</sub><span>=CH</span><sub>2</sub><span> + H</span><sub>2</sub><span>O</span></p><p><strong>Mechanism : </strong>elimination</p>