Products of Reactions Involving Hydrocarbons

 

This is part of HSC Chemistry course under the topic of Hydrocarbons

HSC Chemistry Syllabus

• investigate, write equations and construct models to represent the reactions of unsaturated hydrocarbons when added to a range of chemicals, including but not limited to: – hydrogen (H₂) – halogens (X₂) – hydrogen halides (HX) – water (H₂O) (ACSCH136)

• investigate, write equations and construct models to represent the reactions of saturated hydrocarbons when substituted with halogens

 Addition Reactions with Alkenes

 

• Alkenes are good nucleophiles
• Nucleophiles are molecules which can initiate an ‘attack’ on a nearby molecule and hence start a reaction. Nucleophiles are molecules which commonly exhibit one or all of the following properties:
• Negatively charged (either formal or partial charge)
• Electron dense centres
• Reactive bonds e.g. pi-bonds

 

Hydrohalogenation of Alkene

• Addition of a hydrogen halide molecule (e.g. HBr) to an alkene forms an alkane with the same number of carbon atoms, but with a newly added halogen.

Ethene (ethylene) reacts with hydrogen bromide	Intermediate is an unstable, primary carbocation	Bromoethane is formed

Hydrogen halide are considered good electrophiles because the electronegative halogen e.g. Br causes the adjacent hydrogen atom to acquire a partial positive charge (d+).

• Halogens are also good ‘leaving groups’ due to their high electronegativity and stability as anions.

Markovnikov’s Rule & Major and Minor Products

• As a result of carbocation rearrangement, isomers may be produced from a single reaction. The product resulted from the more stable carbocation is always the major product. For example, the hydrohalogenation of propene with HBr.

2-bromopropane is the major product because the bromine atom is added to the middle carbon atom which has two other carbon atoms already bonded to it, as compared to the first carbon (which only has one)

 

The outcome of carbocation rearrangement is described (and simplified) by Markovnikov’s rule

 

The rule states that in an alkene addition reaction with hydrogen halide (HX), the proton (H) is attached to the carbon atom with more hydrogen substituents while the halogen (X) is attached to the carbon atom with the greatest number of carbon substituents.

 

Halogenation of Alkene

• Halogenation of an alkene adds two halogen atoms across the double bond. This creates an alkane molecule with the same number of carbon atoms, but two new halogen atoms.

 

 

• Since elemental halogen molecules are non-polar, halogenation is carried out in a non-polar solvent, most commonly CCl₄which is also inert so it does not interfere with the desired halogenation of an alkene.

 

Hydrogenation of Alkene

• Hydrogenation of an alkene adds two hydrogen atoms across the double bond. This creates an alkane molecule with the same number of carbon atoms.

 

• Since hydrogen gas is diatomic and non-polar, a transition metal catalyst is required to initiate the reaction. The catalyst causes hydrogen to become polarised and a better electrophile. Catalysts include:
• Nickel (Ni)
• Palladium on carbon (written as Pd/C)
• Platinum (Pt)

 

 

• Markovnikov’s rule does not apply to hydrogenation of alkenes because the two added atoms are both hydrogens.

 

Hydration of Alkene

• Hydration of alkene adds an –OH substituent across the double bond. This transforms an alkene into an alcohol

 

• Reagent: dilute acid (e.g. dilute H₂SO₄)
  • Hydration uses water as a reactant. The reaction requires a large amount of water which dilutes the acid (catalyst)
  • Hydration requires acid (H⁺) as a catalyst.

 

• H⁺ ions act as electrophiles to initiate the first step of the reaction. Water molecules will then be added onto the positive carbocation. The conjugate base of the acid will then remove a proton from the newly added water to produce an alcohol. 

 

• Common acid catalysts for hydration include:
• Sulfuric acid (H₂SO₄)
• Phosphoric acid (H₃PO₄)
• Nitric acid (HNO₃)

 

Note that dilute HCl will not initiate a hydration reaction but instead hydrohalogenation, so it cannot be used as an acid catalyst to produce alcohol from alkene.

 

• Markovnikov’s rule applies to hydration reactions.
• When water attacks a tertiary carbocation, a tertiary alcohol is produced
• When water attacks a secondary carbocation, a secondary alcohol is produced
• When water attacks a primary alcohol, a primary alcohol is produced.

 

The rule states that in an alkene addition reaction with water, the proton (H) is attached to the carbon atom with more hydrogen substituents while the alcohol (–OH) is attached to the carbon atom with the greatest number of carbon substituents.

For example, the hydration of 2-methylpropene produces two position isomers.

	Tert-butyl alcohol (tertiary alcohol) is formed from a tertiary carbocation.
	2-methyl-1-propanol (primary carbocation) is formed from a primary carbocation.

Addition Reactions Involving Alkynes

Reaction with Alkynes

• Alkynes are more reactive than alkenes due to the presence of two pi-bonds, both of which are strong nucleophiles.

 

Hydrohalogenation & Halogenation of Alkynes

• Hydrohalogenation: addition of hydrogen halide and halogen works via a similar mechanism (not identical) as it would with alkenes. After first addition reaction, an alkyne would form an alkene, which can further react (if reagents are supplied in sufficient quantities) to form an alkane.

 

 

• Halogenation of alkynes produces alkanes with tetra-substituted halogens across the triple bond

 

 

• Hydrohalogenation of alkynes produces di-substituted halogens where the major product is one where both halogen atoms are attached to the same carbon atom (Markovnikov’s rule).
• (extension): This is because a halogen atom can stabilise an adjacent carbocation by donating one of its electron lone pairs to that carbocation. This overcomes the halogen’s electron-withdrawing effect / electronegativity.

 

Hydrochlorination
Hydrobromination
Bromination

 

 

Hydrogenation of Alkynes

• Hydrogenation of an alkyne produces an alkene which in turn forms an alkane. Therefore, the final product of an alkyne would be the same as that of its corresponding alkene.
• Reagent: hydrogen gas (H₂) with Pd/C as catalyst

 

 

 

Summary of Addition Reactions of Alkenes and Alkynes

Table: products of different reactions involving ethene and ethyne. Note that only one product is formed for each reaction because ethene and ethyne both only have two carbon atoms, so the effect of carbocation rearrangement is negligible.

 

Reaction	Alkene	Alkyne
Hydrohalogenation
	bromoethane	1,1-dibromoethane
Halogenation
	1,2-dibromoethane	1,1,2,2-tetrabromoethane
Hydrogenation
	ethane	ethane
Hydration
	ethanol	ethanal (acetaldehyde)

 

 

Addition Reactions Involving Alkanes

 

• Alkanes are saturated hydrocarbons, so they do not undergo addition reactions.

 

Catalytic Cracking

• Cracking is the process when long-chain alkanes are broken into smaller hydrocarbons including alkenes which can be used for a diverse range of reactions.

 

• C–C bonds are broken during cracking which requires a large amount of energy. Therefore, metal-based catalysts are required to lower the activation energy of cracking.

 

Combustion

• Alkanes are volatile and flammable
• Combustion reactions are irreversible and exothermic
• Complete combustion: reaction with excess oxygen in the presence of heat to produce carbon dioxide and water.

$$2C_8H_{18}(l) + 25O_2(g) \rightarrow 16CO_2(g) + 18H_2O(g)$$

 

• When oxygen is limited, incomplete combustion occurs, producing carbon monoxide (odourless, toxic gas) and soot which is carcinogenic.

                                         

$$2C_8H_{18}(l) +17O_2(g) \rightarrow 8CO_2(g) + 8C(s) + 18H_2O(g)$$

 

Substitution Reaction with Alkanes

• Alkanes cannot undergo addition reactions because they are saturated; all carbon atoms are already connected to four atoms each.
• However, alkanes can undergo substitution reactions with hydrogen halides or halogens in the presence of UV light. UV light provides the necessary energy to break covalent bonds and allow the reaction to proceed via a radical-based mechanism.
• Note: UV radiation is not a catalyst as it does not reduce the activation energy but simply provides the energy to meet the activation energy

 

 

If a halogen is supplied in excess, alkane molecules will become completely substituted. Hydrogen atoms will be replaced by halogens.   For example, when Cl2 is in excess, methane will undergo four stepwise substitution reactions until it is transformed into tetrachloromethane

 

• Halogenation of alkanes are selective in that:

Bromine Water Test

• Reaction with bromine water distinguishes an alkene from an alkane. Alkane and alkene are nonpolar molecules which can dissolve in bromine water which is also nonpolar.

 

• Safety Considerations
  • Cyclohexane and cyclohexene are typically used in schools as they are less volatile than smaller alkanes and alkenes due to their stronger dispersion forces.
  • Bromine water is also safer to use than bromine gas.

 

• Method and experimental condition:
  • A few drops of orange/brown coloured bromine water are added to a solution of cyclohexane and cyclohexene in the absence of UV light
  • Record changes in the solution’s appearance

 

• Observation: the reactive C=C bond in alkene undergoes addition reaction with bromine (Br₂) to form an haloalkane. This decreases [Br₂] and hence decolorises the solution. For example, bromination of ethene:

$$C_2H_4(aq) + Br_2(aq) \rightarrow C_2H_5Br(aq) + HBr(aq) $$

 

• If the bromine water containing alkane is exposed to UV light, the alkane will undergo substitution reaction to produce haloalkane. This will also decolorise the solution, but at a much slower rate.

 

$$C_2H_6 + Br_2 \rightarrow C_2H_5Br + HBr$$

 

$$C_2H_5Br + HBr \rightarrow C_2H_4Br_2 + H_2$$