Ch11 - Alkenes

Define terminal alkene.

A terminal alkene contains the =CH₂ structural unit (i.e. the C=C bond is located at one end of the carbon chain.)

Define cycloalkene.

A cycloalkene contains one or more C=C bond in a ring structure.

What are some common examples of alkenes?

• limonene

• ethene

• α-farnesene

• β-carotene

Explain why the π-bond is weaker than the σ-bond.

The side-on overlap of p-orbital is less effective than the head-on overlap of orbitals.

Explain the existence of cis-trans isomerism in certain alkenes.

• restricted rotation about C=C bond as rotation about C=C bond requires breaking of the π-bond

• give rise to cis-trans isomerism if each double C atom is joined to two different atoms or groups

State and explain the stability of a cis-alkene relative to its stereoisomeric trans-alkene.

The cis-alkene is less stable than its stereoisomeric trans-alkene, due to the steric strain (repulsion due to electron clouds) arising from crowding between the two alkyl groups in the cis-isomer.

Generally state the physical properties of alkenes.

• insoluble in water, but quite soluble in non-polar solvents (e.g. CCl₄)

• less dense than water

• boiling point increases with increasing number of carbon atoms

State and explain the difference in boiling points between the cis-alkene and trans-alkene.

The boiling point of cis-but-2-ene is higher than that of trans-but-2-ene.

• Cis-but-2-ene is slightly polar and hence there exists instantaneous dipole-induced dipole and permanent dipole-permanent dipole interactions between cis-but-2-ene molecules.

• Trans-but-2-ene is non-polar and hence there exists only instantaneous dipole-induced dipole interactions between trans-but-2-ene molecules.

• More energy is required to overcome the stronger intermolecular forces present in cis-but-2-ene than in trans-but-2ene, thus cis-but-2-ene has a higher boiling point.

State and explain the difference in melting points between the cis-alkene and trans-alkene.

The melting point of cis-but-2-ene has a lower melting point than that of trans-but-2-ene.

• Molecules of cis-but-2-ene are packed poorly in the solid lattice due to the two bulky methyl groups located on the same side in the molecule, resulting in longer distances between molecules

• This leads to weaker intermolecular forces.

• Lower energy required to overcome weaker intermolecular forces present in cis-but-2-ene than in trans-but-2-ene, thus cis-but-2-ene has a lower melting point.

State and explain the general reactivity of alkenes.

Alkenes are highly reactive compounds, due to the presence of electron-rich C=C bonds that attracts electrophiles.

Define an electrophile.

An electrophile is an electron-pair acceptor which is attracted to an electron-rich site. It is electron deficient. (positive charge; partial positive charge; incomplete octet)

State the types of reaction alkenes typically undergo.

Electrophilic addition

State the reagents and conditions for the type(s) of electrophilic addition reaction(s) that alkenes undergo.

• Addition of HX(g)
  Reagents: HX(g)
  Conditions: Ni catalyst, room temperature

• Addition of steam
  Reagents: H₂O(g)
  Conditions: conc. H₃PO₄ catalyst, heat

• Addition of X₂ in CCl₄
  Reagents: X₂
  Conditions: in CCl₄, dark, room temperature

• Addition of X₂ in H₂O
  Reagents: X₂
  Conditions: in H₂O, dark, room temperature

State the steps in the electrophilic addition mechanism.

• Step 1: Addition of electrophile

• Step 2: Nucleophilic attack

State the factor(s) that affect stability of carbocations.

The stability of a carbocation is increased by dispersal of the charge.

State and explain how the stability of the carbocation affects rate of formation.

The more stable a carbocation is, the lower the activation energy required for its formation and hence faster rate of formation.

Define Markonikov’s Rule.

In an electrophilic addition reaction, the electrophile adds to an unsymmetrical alkene to produce the more stable carbocation intermediate. Being formed faster in the slow step, the more stable carbocation is more available to participate in the fast step to give the major product.

Explain why the product obtained is a racemic mixture.

In step 2 (nucleophilic attack), the X^- ion can attack the positively charged C of the trigonal planar carbocation from either side of the trigonal plane with equal likelihood, producing a racemic mixture of the two enantiomers.

State the reagents, conditions and observations for mild oxidation of alkenes.

Reagents: KMnO₄(aq), NaOH(aq)
Conditions: cold
Observations: Purple KMnO₄ decolourised; Brown-black precipitate of MnO₂ observed.

Note: the reaction occurs without the complete cleavage of the C=C bond and is usually regarded as mild oxidation of an alkene.

Mild oxidation could occur in acidic medium as well but less common.

State the reagents, conditions and observations for strong oxidation of alkenes.

1. Reagents: KMnO₄(aq), H₂SO₄(aq)
   Conditions: heat/heat under reflux
   Observations: Purple KMnO₄ decolourised; (for terminal alkenes) a colourless gas which forms white precipitate with limewater evolved.

2. Reagents: KMnO₄(aq), NaOH
   Conditions: heat/heat under reflux
   Observations: Purple KMnO₄ decolourised; Brown-black precipitate of MnO₂ observed

Define Saytzeff’s rule.

For elimination reactions of alcohols and halogenoalkanes to alkenes, the preferred product is the alkene with the greater number of alkyl groups attached to the doubly bonded carbon atoms i.e. the more substituted alkene.

State the reagents, conditions and observations of reduction of alkenes.

Reagents: H₂(g)
Conditions: Ni catalyst or Pt or Pd
Observations: Gas that extinguishes a lighted splint with a ‘pop’ sound evolved.

State all the reactions alkenes can undergo.

• Electrophilic addition

• Mild oxidation

• Strong oxidation

• Reduction

• Combustion

• Addition Polymerisation

State the type(s) of distinguishing test(s) for distinguishing alkenes.

1. Br₂ in CCl₄, dark, room temperature

2. Br₂(aq) at room temperature

State the different type(s) of reaction(s) to prepare alkenes, as well as reagents, conditions and observations.

1. Elimination of HX from halogenoalkanes
   Reagents: KOH in ethanol
   Conditions: heat/heat under reflux
   Observations: NIL

2. Elimination of H₂O from alcohols
   Reagents: excess conc. H₂SO₄ or Al₂O₃ or H₃PO₄
   Conditions: heat
   Observations: NIL

State the reagents and conditions for addition of water to alkenes.

Reagents & conditions: 1) cold conc. H₂SO₄; 2) H₂O, heat