Hydrocarbons: alkanes and alkenes (AS)

Chemistry 9701 Study Notes: Hydrocarbons

Welcome to the world of Hydrocarbons! This is the foundational chapter for all of organic chemistry, the chemistry of carbon compounds. Understanding hydrocarbons—compounds made solely of carbon and hydrogen—is crucial because they are the building blocks for fuels, plastics, and countless other organic molecules. Don't worry if the reaction mechanisms seem tricky; we’ll break them down step-by-step!

These notes cover both the AS Level content (Alkanes and Alkenes, Topic 14) and the A Level extension (Arenes, Topic 30).

Section 1: Alkanes (Saturated Hydrocarbons)

1.1 Structure and General Properties

Alkanes are the simplest family of hydrocarbons. They are classified as saturated because they contain only single bonds (C-C and C-H bonds). They have the general formula \(C_n H_{2n+2}\).

Why are Alkanes Generally Unreactive?

Alkanes are chemically inert (unreactive) towards most polar reagents (like acids or bases) because:

• C-C and C-H bonds are strong: A lot of energy is required to break these sigma (\(\sigma\)) bonds.
• C-H bonds have low polarity: The electronegativity difference between C (2.5) and H (2.1) is very small. This means there are no strong partial positive or negative charges on the molecule to attract charged species like nucleophiles or electrophiles.

Analogy: Alkanes are like smooth, non-magnetic billiard balls—nothing sticks to them easily!

1.2 Production of Alkanes

Alkanes are mainly obtained from crude oil. We need specific reactions to produce smaller, more useful alkanes:

• Cracking: Breaking down long-chain alkanes (high \(M_r\), less useful crude oil fractions) into smaller, more useful alkanes and alkenes (lower \(M_r\)).
  

  Conditions: High heat with an \(\mathrm{Al}_2\mathrm{O}_3\) catalyst.

  Example: \(\mathrm{C}_{10}\mathrm{H}_{22} \longrightarrow \mathrm{C}_8\mathrm{H}_{18} + \mathrm{C}_2\mathrm{H}_4\) (decane \(\longrightarrow\) octane + ethene)

• Hydrogenation of Alkenes: Adding hydrogen across a C=C double bond (covered in Section 2, but also a method for alkane production).
  

  Conditions: \(\mathrm{H}_2\) gas, \(\mathrm{Pt}\) or \(\mathrm{Ni}\) catalyst, and heat.

1.3 Reactions of Alkanes

1. Combustion (Burning)

Alkanes are important fuels. This is an exothermic reaction.

• Complete Combustion: Sufficient oxygen available.
  \(C_n H_{2n+2} + \left(\frac{3n+1}{2}\right) \mathrm{O}_2 \longrightarrow n \mathrm{CO}_2 + (n+1) \mathrm{H}_2\mathrm{O}\)
• Incomplete Combustion: Limited oxygen. Produces highly toxic carbon monoxide (\(\mathrm{CO}\)) and/or solid carbon (soot).

Environmental Consequences of Combustion:

Internal combustion engines produce harmful gases:

1. Carbon Monoxide (\(\mathrm{CO}\)): Toxic gas from incomplete combustion.
2. Oxides of Nitrogen (\(\mathrm{NO}\) and \(\mathrm{NO}_2\)): Formed when \(\mathrm{N}_2\) and \(\mathrm{O}_2\) in the air react at high temperatures inside the engine.
3. Unburnt Hydrocarbons: Contribute to photochemical smog.

Catalytic Removal: Catalytic converters use transition metals (like Platinum, Palladium, Rhodium) to convert these harmful pollutants into less harmful products:

• \(2\mathrm{CO} + 2\mathrm{NO} \longrightarrow 2\mathrm{CO}_2 + \mathrm{N}_2\)

2. Free-Radical Substitution (Halogenation)

Since alkanes are unreactive towards polar reagents, they react with halogens (\(\mathrm{Cl}_2\) or \(\mathrm{Br}_2\)) using high-energy free radicals, typically induced by Ultraviolet (UV) light or heat.

Reagents and Conditions: Halogen (\(\mathrm{Cl}_2\) or \(\mathrm{Br}_2\)), UV light.

Product: A halogenoalkane (substitution occurs, forming chloroalkanes or bromoalkanes). This reaction is non-specific and can lead to a mixture of products.

Mechanism: Free-Radical Substitution (using methane/ethane and chlorine)

This occurs in three stages:

i) Initiation

The UV light provides energy to break the halogen bond homolytically (evenly splitting the electron pair), forming two highly reactive free radicals.

\(\mathrm{Cl}_2 \xrightarrow{\text{UV}} 2\mathrm{Cl}\cdot\)

ii) Propagation (The chain reaction)

This is where the product is formed. Free radicals react with stable molecules to form new radicals, keeping the chain going.

For methane:

\(\mathrm{CH}_4 + \mathrm{Cl}\cdot \longrightarrow \mathrm{HCl} + \mathrm{CH}_3\cdot\)
\(\mathrm{CH}_3\cdot + \mathrm{Cl}_2 \longrightarrow \mathrm{CH}_3\mathrm{Cl} + \mathrm{Cl}\cdot\)

For ethane (forming chloroethane):

\(\mathrm{C}_2\mathrm{H}_6 + \mathrm{Cl}\cdot \longrightarrow \mathrm{HCl} + \mathrm{C}_2\mathrm{H}_5\cdot\)
\(\mathrm{C}_2\mathrm{H}_5\cdot + \mathrm{Cl}_2 \longrightarrow \mathrm{C}_2\mathrm{H}_5\mathrm{Cl} + \mathrm{Cl}\cdot\)

iii) Termination

Two radicals combine to form a stable, neutral molecule, ending the chain.

\(\mathrm{Cl}\cdot + \mathrm{Cl}\cdot \longrightarrow \mathrm{Cl}_2\)
\(\mathrm{CH}_3\cdot + \mathrm{CH}_3\cdot \longrightarrow \mathrm{C}_2\mathrm{H}_6\)
\(\mathrm{CH}_3\cdot + \mathrm{Cl}\cdot \longrightarrow \mathrm{CH}_3\mathrm{Cl}\)

Key Takeaway (Section 1 Alkanes)

Alkanes are saturated, unreactive compounds. Their key reactions are combustion (which causes pollution) and free-radical substitution (which requires UV light and proceeds via three distinct steps: Initiation, Propagation, Termination).

Section 2: Alkenes (Unsaturated Hydrocarbons)

2.1 Structure and Bonding

Alkenes are unsaturated hydrocarbons containing at least one C=C double bond. They have the general formula \(C_n H_{2n}\).

The double bond consists of:

• One sigma (\(\sigma\)) bond (formed by the direct, head-on overlap of \(\mathrm{sp}^2\) orbitals).
• One pi (\(\pi\)) bond (formed by the sideways overlap of adjacent p orbitals, lying above and below the \(\sigma\) bond).

The presence of the \(\pi\) bond causes three key features:

1. Planar Shape: The atoms around the double bond are arranged in a trigonal planar shape with bond angles of approximately \(120^\circ\).
2. Restricted Rotation: The \(\pi\) bond locks the molecule in position, preventing free rotation around the C=C axis. This leads to geometrical (cis/trans) isomerism.
3. High Reactivity: The \(\pi\) bond electrons are exposed and easily attacked by electron-loving species called electrophiles.

2.2 Test for Unsaturation

The presence of a C=C double bond can be detected using aqueous bromine (\(\mathrm{Br}_2(\mathrm{aq})\)).

Observation: The red/brown colour of the aqueous bromine water is rapidly decolourised as the bromine adds across the double bond.

2.3 Production of Alkenes

Alkenes are mainly produced via cracking (see 1.2) or by synthetic routes:

• Dehydration of an Alcohol (Elimination): Removing water from an alcohol.
  Reagents/Conditions: Heated catalyst (e.g., \(\mathrm{Al}_2\mathrm{O}_3\)) or concentrated acid (e.g., concentrated \(\mathrm{H}_2\mathrm{SO}_4\)).
• Elimination of HX from a Halogenoalkane: Removing a hydrogen halide (HX).
  Reagents/Conditions: Ethanolic \(\mathrm{NaOH}\) and heat. (Note: aqueous \(\mathrm{NaOH}\) causes substitution, not elimination!)

2.4 Reactions of Alkenes: Electrophilic Addition

The characteristic reaction of alkenes is the breaking of the weak \(\pi\) bond and the formation of two new \(\sigma\) bonds. This process is driven by electrophiles (species that love electrons, usually positively charged or electron-deficient).

General Electrophilic Addition Reactions:

1. Hydrogenation (Addition of \(\mathrm{H}_2\)): Forms an alkane.
   Reagents/Conditions: \(\mathrm{H}_2(\mathrm{g})\), \(\mathrm{Pt}\) or \(\mathrm{Ni}\) catalyst, heat.
2. Halogenation (Addition of \(\mathrm{X}_2\)): Forms a dihalogenoalkane.
   Reagents/Conditions: Halogen (\(\mathrm{X}_2\), e.g., \(\mathrm{Cl}_2\)) at room temperature.
3. Addition of Hydrogen Halide (Addition of HX): Forms a halogenoalkane.
   Reagents/Conditions: Hydrogen halide (\(\mathrm{HX}(\mathrm{g})\)) at room temperature.
4. Hydration (Addition of Steam): Forms an alcohol.
   Reagents/Conditions: \(\mathrm{H}_2\mathrm{O}(\mathrm{g})\) (steam), \(\mathrm{H}_3\mathrm{PO}_4\) catalyst.

The Electrophilic Addition Mechanism

We use the reaction of propene with hydrogen bromide (\(\mathrm{HBr}\)) as an example. The curly arrows show the movement of a pair of electrons (starting at a bond or lone pair and pointing towards where the bond forms).

Step 1: Electrophilic Attack and Carbocation Formation

The \(\pi\) bond electrons attack the partially positive H atom in \(\mathrm{HBr}\). The \(\mathrm{H}-\mathrm{Br}\) bond breaks heterolytically, forming an intermediate carbocation (a carbon atom with a positive charge) and a bromide ion (\(\mathrm{Br}^-\)).

Step 2: Nucleophilic Attack

The \(\mathrm{Br}^-\) ion (which is a nucleophile, an electron pair donor) rapidly attacks the positively charged carbocation to form the final product.

Markovnikov's Rule and Carbocation Stability

When an asymmetrical alkene (like propene) reacts with an asymmetrical reagent (like \(\mathrm{HBr}\)), two different products are possible. For example, propene + \(\mathrm{HBr}\) can give 1-bromopropane or 2-bromopropane.

The Main Product is determined by Carbocation Stability:

The reaction proceeds via the intermediate carbocation that is the most stable.

• Stability Order: Tertiary (\(3^\circ\)) > Secondary (\(2^\circ\)) > Primary (\(1^\circ\)).

Why? Inductive Effects: Alkyl groups (R-groups) are electron-releasing (they have a positive inductive effect, \((+I)\)). They donate electron density towards the positive charge on the carbon, spreading the charge out and making the ion more stable.

In the reaction of propene and \(\mathrm{HBr}\):

Option A forms a secondary carbocation (\(2^\circ\)) which is stabilised by two alkyl groups. Option B forms a primary carbocation (\(1^\circ\)) which is stabilised by only one alkyl group. Therefore, the secondary carbocation forms faster, leading predominantly to 2-bromopropane.

This is known as Markovnikov addition: The hydrogen atom adds to the carbon atom in the double bond that already has the greater number of hydrogen atoms.

2.5 Oxidation Reactions of Alkenes

Alkenes are easily oxidised by acidified potassium manganate(VII), \(\mathrm{KMnO}_4\).

• Mild Oxidation (Cold, Dilute Acidified \(\mathrm{KMnO}_4\)): Forms a diol (a molecule with two \(\mathrm{OH}\) groups).
  Reagents: Cold, dilute acidified \(\mathrm{KMnO}_4\).
  Observation: The purple \(\mathrm{KMnO}_4\) is decolourised.
• Vigorous Oxidation (Hot, Concentrated Acidified \(\mathrm{KMnO}_4\)): This is a powerful reaction that ruptures (breaks) the entire C=C double bond.
  Use: This reaction is used to determine the original position of the alkene linkage in larger molecules.

Products depend on the groups originally attached to the C=C:

1. If a C atom in the double bond has two H atoms (\(=\mathrm{CH}_2\)), it is oxidised to \(\mathrm{CO}_2\) and \(\mathrm{H}_2\mathrm{O}\).
2. If a C atom has one H atom (\(=\mathrm{CHR}\)), it is oxidised to a carboxylic acid.
3. If a C atom has two alkyl groups (\(=\mathrm{CR}_2\)), it is oxidised to a ketone.

2.6 Addition Polymerisation

Small alkene molecules (monomers, e.g., ethene or propene) join together end-to-end to form a large molecule (a polymer, e.g., poly(ethene)). The \(\pi\) bond breaks, and the two carbons form single bonds with the adjacent monomers.

Key Takeaway (Section 2 Alkenes)

Alkenes are unsaturated due to the reactive \(\pi\) bond. Their characteristic reaction is Electrophilic Addition, governed by the formation and stability of carbocations (Markovnikov's rule). They are also readily oxidised by \(\mathrm{KMnO}_4\).

Section 3: Arenes (Aromatic Hydrocarbons) (A Level Content)

3.1 The Structure of Benzene

Arenes, such as benzene (\(\mathrm{C}_6\mathrm{H}_6\)), are cyclic hydrocarbons with unique stability.

• Shape: Benzene is a planar, regular hexagonal molecule.
• Bonding: All six carbon atoms are \(\mathrm{sp}^2\) hybridised. Each carbon forms three \(\sigma\) bonds (to two adjacent C atoms and one H atom).
• Delocalised \(\pi\) System: The remaining six unhybridised p orbitals (one from each C atom) overlap sideways, forming a continuous ring of electron density (the \(\pi\) cloud) both above and below the plane of the ring.

Aromatic Stabilisation

The delocalisation of these six electrons makes benzene extremely stable. This high stability means that the characteristic reaction is Substitution, not the addition reaction typical of alkenes. Addition would destroy the stable delocalised \(\pi\) system, requiring a high input of energy.

3.2 Characteristic Reaction: Electrophilic Substitution

In this reaction, a hydrogen atom on the benzene ring is replaced by an electrophile (\(\mathrm{E}^+\)), but the stable \(\pi\) system is regenerated afterwards.

Mechanism (General Steps)

Step 1: Generation of the Electrophile

A catalyst (usually a Lewis acid like \(\mathrm{AlCl}_3\), \(\mathrm{FeBr}_3\), or concentrated \(\mathrm{H}_2\mathrm{SO}_4\)) is used to generate a powerful electrophile (\(\mathrm{E}^+\)).

Step 2: Electrophilic Attack

The delocalised \(\pi\) electrons attack the electrophile, breaking the delocalisation and forming a positively charged intermediate (a carbocation/arenium ion).

Step 3: Regeneration of Aromaticity

The intermediate loses a proton (\(\mathrm{H}^+\)) to reform the stable delocalised ring structure. The \(\mathrm{H}^+\) reacts with the conjugate base of the acid, regenerating the catalyst.

Drawing the full curved arrow mechanism for bromobenzene or nitrobenzene is required, but the concept is always the same: Attack, form intermediate, lose \(\mathrm{H}^+\) to regain stability.

3.3 Specific Electrophilic Substitution Reactions

All these reactions require a powerful electrophile, often generated using a catalyst.

1. Nitration (Addition of \(-\mathrm{NO}_2\)): Forms nitrobenzene.
   Reagents/Conditions: Concentrated \(\mathrm{HNO}_3\) and concentrated \(\mathrm{H}_2\mathrm{SO}_4\) (catalyst), temperature controlled between \(25^\circ\mathrm{C}\) and \(60^\circ\mathrm{C}\).
   Electrophile generated: The nitronium ion, \(\mathrm{NO}_2^+\).
2. Halogenation (Addition of \(-\mathrm{Cl}\) or \(-\mathrm{Br}\)): Forms halogenoarenes.
   Reagents/Conditions: \(\mathrm{Cl}_2\) or \(\mathrm{Br}_2\), with a halogen carrier catalyst (\(\mathrm{AlCl}_3\), \(\mathrm{AlBr}_3\), or \(\mathrm{FeBr}_3\)).
   Electrophile generated: \(\mathrm{Br}^+\) or \(\mathrm{Cl}^+\) (or a highly polarised halogen complex).
3. Friedel-Crafts Alkylation: Adding an alkyl group (\(-\mathrm{R}\)).
   Reagents/Conditions: Chloroalkane (\(\mathrm{R}-\mathrm{Cl}\)), \(\mathrm{AlCl}_3\) catalyst, heat.
4. Friedel-Crafts Acylation: Adding an acyl group (\(-\mathrm{COR}\)).
   Reagents/Conditions: Acyl chloride (\(\mathrm{RCOCl}\)), \(\mathrm{AlCl}_3\) catalyst, heat.

3.4 Reactions of Methylbenzene

Methylbenzene (toluene) has an alkyl side-chain, leading to two types of reactivity:

1. Side-chain Halogenation (Free Radical)

If the reaction occurs in the presence of UV light or heat without a catalyst, the reaction targets the side chain, similar to alkanes.

• Reaction: Free-radical substitution on the methyl group.
• Conditions: \(\mathrm{Cl}_2\) or \(\mathrm{Br}_2\), UV light.

2. Ring Halogenation (Electrophilic Substitution)

If a halogen carrier catalyst is used, the reaction targets the ring (aromatic system).

• Reaction: Electrophilic substitution on the ring.
• Conditions: \(\mathrm{Cl}_2\) or \(\mathrm{Br}_2\), \(\mathrm{AlCl}_3\) or \(\mathrm{AlBr}_3\) catalyst.

3. Complete Side-chain Oxidation

All alkyl groups attached to the benzene ring can be vigorously oxidised to form a carboxylic acid group.

• Reagents/Conditions: Hot alkaline \(\mathrm{KMnO}_4\), followed by dilute acid (acidification).
• Example: Methylbenzene \(\longrightarrow\) Benzoic acid.

3.5 Directing Effects of Substituents

If a substituent (a functional group) is already attached to the benzene ring, it affects where the next electrophilic substitution will occur. Groups are either 2, 4, 6-directing (Ortho and Para) or 3, 5-directing (Meta).

• 2, 4, 6-directing Groups: Direct incoming electrophiles to positions 2, 4, and 6.
  Activating examples: \(\mathbf{-NH_2}\), \(\mathbf{-OH}\), \(\mathbf{-R}\) (alkyl groups).
  Deactivating exception: Halogens (\(-\mathrm{Cl}\), \(-\mathrm{Br}\)) are electron-withdrawing/deactivating, but are 2,4-directing.
• 3, 5-directing Groups (Deactivating): These groups withdraw electron density from the ring, making substitution harder and directing the substitution to carbons 3 and 5.
  Examples: \(\mathbf{-NO_2}\), \(\mathbf{-COOH}\), \(\mathbf{-COR}\).