Chemistry (9701) | Hydrocarbons

Welcome to the World of Hydrocarbons!

Hello future chemists! This chapter is the foundation of much of organic chemistry. Hydrocarbons—molecules made only of carbon and hydrogen—are essential. They are the building blocks of fuels, plastics, and countless pharmaceutical products.

We will dive deep into three main families: the "lazy" Alkanes (AS Level), the "reactive" Alkenes (AS Level), and the "stable aromatic" Arenes (A Level). Don't worry if the mechanisms seem complicated; we'll break them down step-by-step!

Section 1: Alkanes (Saturated Hydrocarbons) (AS Level 14.1)

1.1 Structure, Bonding, and Unreactivity

Alkanes are the simplest family, represented by the general formula \(C_nH_{2n+2}\). They are classified as saturated because they contain only single bonds, meaning all carbon atoms are bonded to the maximum possible number of hydrogen atoms.

• Bonding: All bonds in alkanes (C-C and C-H) are sigma ($\sigma$) bonds, formed by the head-on overlap of orbitals. Carbon atoms exhibit $sp^3$ hybridisation, leading to a tetrahedral shape and bond angles of approximately \(109.5^\circ\).
• Unreactivity Explained: Alkanes are often called "paraffins" (meaning little affinity). Their low reactivity is due to two key factors (14.1.5):
  
  1. Strong Bonds: Both C-C and C-H bonds have high bond energies, requiring significant energy to break.
  2. Lack of Polarity: The electronegativity difference between C (2.5) and H (2.1) is very small. This means the bonds are virtually non-polar, so they are not easily attacked by polar reagents like acids (electrophiles) or bases (nucleophiles).

1.2 Reactions of Alkanes

Complete and Incomplete Combustion (14.1.2a)

Alkanes are primarily used as fuels because they burn readily.

• Complete Combustion (Excess Oxygen): Produces carbon dioxide and water.
  Example: Methane
  \(CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)\)
• Incomplete Combustion (Limited Oxygen): Produces highly toxic carbon monoxide (CO) and/or unburnt carbon (soot, C), as well as water.
  Example:
  \(2CH_4(g) + 3O_2(g) \rightarrow 2CO(g) + 4H_2O(l)\)

Environmental Consequences (14.1.6): Combustion in car engines produces three major pollutants:

1. Carbon Monoxide (CO): Highly toxic, produced by incomplete combustion.
2. Oxides of Nitrogen ($NO_x$): Formed when atmospheric nitrogen and oxygen react at high temperatures inside the engine.
3. Unburnt Hydrocarbons: Contribute to smog and ground-level ozone formation.

Did you know? Catalytic converters use transition metals (like Palladium, Platinum, Rhodium) to convert these harmful gases into less harmful ones ($CO_2$, $N_2$, $H_2O$).

Free-Radical Substitution (Halogenation) (14.1.2b, 14.1.3)

This is the characteristic reaction of alkanes, typically using $Cl_2$ or $Br_2$ in the presence of UV light (ultraviolet light) or high heat. A hydrogen atom is replaced (substituted) by a halogen atom.

The mechanism proceeds via three crucial steps involving free radicals (species with an unpaired electron, highly reactive).

Step-by-Step Mechanism (Example: Ethane and Chlorine):

1. Initiation (Starting the Chain)
The UV light provides energy for homolytic fission of the halogen molecule (e.g., $Cl_2$).
\(Cl_2 \xrightarrow{UV} 2Cl\cdot\)

2. Propagation (The Chain Reaction)
This is where the product is formed, and the radical is regenerated, allowing the chain to continue.
(a) Attack on Alkane: The chlorine radical abstracts a hydrogen atom from ethane, creating an ethyl radical and HCl.
\(C_2H_6 + Cl\cdot \rightarrow C_2H_5\cdot + HCl\)
(b) Attack on Halogen: The ethyl radical attacks a chlorine molecule, forming the product (chloroethane) and regenerating the highly reactive chlorine radical.
\(C_2H_5\cdot + Cl_2 \rightarrow C_2H_5Cl + Cl\cdot\)

3. Termination (Stopping the Chain)
The chain stops when any two radicals collide and combine to form a stable molecule. This removes the reactive radicals from the system.
\(Cl\cdot + Cl\cdot \rightarrow Cl_2\)
\(C_2H_5\cdot + Cl\cdot \rightarrow C_2H_5Cl\)
\(C_2H_5\cdot + C_2H_5\cdot \rightarrow C_4H_{10}\) (forming unwanted by-products)

Cracking (Production of Alkenes and Smaller Alkanes) (14.1.1b, 14.1.4)

Cracking is the process of breaking down large, less useful hydrocarbon molecules (from crude oil) into smaller, more valuable alkanes and alkenes.

• Reagents & Conditions: Heat (high temperature) with an \(Al_2O_3\) catalyst (Alumina) or $SiO_2$ (Silica).
• Why it's Useful: Crude oil fractions often have an excess of heavy fractions (like fuel oil). Cracking converts these into light fractions (like petrol/gasoline) and valuable alkenes (for plastics).
• General Equation:
  \(\text{Long alkane} \xrightarrow{\text{Heat, } Al_2O_3} \text{Shorter alkane} + \text{Alkene(s)}\)

Section 2: Alkenes (Unsaturated Hydrocarbons) (AS Level 14.2)

2.1 Structure and Bonding in Alkenes

Alkenes are defined by the presence of a carbon-carbon double bond (\(C=C\)). They are unsaturated, meaning they could potentially accept more hydrogen atoms. Their general formula is \(C_nH_{2n}\).

• The Double Bond: The double bond consists of two types of bonds (13.3.3):
  1. One sigma ($\sigma$) bond (strong, formed by head-on overlap).
  2. One pi ($\pi$) bond (weaker, formed by sideways overlap of p-orbitals, located above and below the $\sigma$ bond plane).
• Shape: The carbon atoms involved in the double bond are $sp^2$ hybridised, leading to a trigonal planar geometry around each carbon, with bond angles of approximately \(120^\circ\).
• Reactivity: The presence of the weak $\pi$ bond makes alkenes highly reactive. The $\pi$ bond electrons are accessible and exposed, acting as a site of high electron density, making them prime targets for electrophiles (electron-loving species).

2.2 Isomerism in Alkenes: Geometrical (Cis/Trans or E/Z)

The $\pi$ bond causes restricted rotation around the \(C=C\) axis (13.4.3). If each carbon in the double bond is attached to two different groups, geometrical isomerism occurs.

• Cis/Z isomer: Identical groups are on the same side of the double bond.
• Trans/E isomer: Identical groups are on opposite sides of the double bond.

2.3 Production of Alkenes (14.2.1)

Alkenes can be produced from alcohols and halogenoalkanes.

1. Dehydration of an Alcohol (Elimination of Water):
   An alcohol loses water across two adjacent carbon atoms.
   Reagents & Conditions: Heated catalyst (e.g., \(Al_2O_3\)) OR concentrated acid (e.g., concentrated \(H_2SO_4\)).
2. Elimination of Hydrogen Halide (HX) from Halogenoalkane:
   A hydrogen halide is lost from a halogenoalkane.
   Reagents & Conditions: Ethanolic sodium hydroxide (NaOH) solution and heat.
   (Note the use of "ethanolic" NaOH—aqueous NaOH causes substitution to form an alcohol!)
3. Cracking: As described in Section 1 (breaking large alkanes).

2.4 Characteristic Reactions of Alkenes: Electrophilic Addition (14.2.2a)

The double bond breaks, and the molecule adds two atoms/groups across the carbons.

1. Hydrogenation (Addition of \(H_2\)):
   Product: Alkane (saturated).
   Reagents & Conditions: \(H_2(g)\) and Pt/Ni catalyst, heat.
2. Addition of Halogen (Halogenation):
   Product: Dihaloalkane.
   Reagents & Conditions: Halogen ($X_2$, e.g., $Br_2$) in an organic solvent (e.g., \(CCl_4\)) or aqueous.
   This is used as a test for unsaturation (14.2.3): Bromine water (brown/orange) is instantly decolourised in the presence of a \(C=C\) bond.
3. Addition of Hydrogen Halide (Addition of HX):
   Product: Halogenoalkane.
   Reagents & Conditions: Hydrogen halide (\(HX(g)\)) at room temperature.
4. Hydration (Addition of Steam): (Production of alcohol, 14.2.2a(ii))
   Product: Alcohol.
   Reagents & Conditions: Steam (\(H_2O(g)\)) and \(H_3PO_4\) catalyst (phosphoric acid).

Mechanism of Electrophilic Addition (14.2.4)

This is a key mechanism. An electrophile (E$^+$) attacks the electron-rich $\pi$ bond.

Step 1: Electrophilic Attack & Carbocation Formation

• The electron pair from the $\pi$ bond moves to attack the electrophile, breaking the $\pi$ bond.
• The electrophile bonds to one carbon, leaving the other carbon positively charged. This intermediate is called a carbocation.
• (Curly arrow starts at the C=C bond and points towards the electrophile.)

Step 2: Nucleophilic Attack

• The carbocation (positive centre) is immediately attacked by the negative ion (nucleophile).
• (Curly arrow starts at the lone pair/negative charge of the nucleophile and points towards the positive carbon.)

Analogy: Imagine the $\pi$ bond is a "tasty electron snack." The electrophile (which "loves" electrons) rushes in and grabs the snack first. The carbocation that remains is then immediately stabilised by the negative partner.

Markovnikov's Rule and Carbocation Stability (14.2.5)

When a hydrogen halide (HX) adds to an unsymmetrical alkene (like propene), two products are possible, but one usually predominates.

Markovnikov's Rule (Simplified): The hydrogen atom adds to the carbon atom of the double bond that already has the most hydrogen atoms attached.

Why this happens: Stability of Carbocations (14.2.5)

The intermediate formed in Step 1 determines the main product. Carbocations are classified by the number of alkyl groups attached to the positive carbon:

Stability Order: Tertiary (3 alkyl groups) > Secondary (2 alkyl groups) > Primary (1 alkyl group)

Alkyl groups (like $\text{-CH}_3$) have an inductive effect, meaning they "push" electron density towards the positive centre, helping to spread out and stabilise the positive charge. The more alkyl groups present, the more stable the carbocation.

• During addition to propene, the formation of the secondary carbocation is favoured over the primary carbocation because it is more stable.
• This stability determines which product is formed fastest and in the greatest quantity.

Memory Aid: "The rich get richer." The carbon atom richer in hydrogen atoms gets the incoming hydrogen atom (from HX).

Oxidation of Alkenes (14.2.2b, 14.2.2c)

Alkenes react with acidified potassium manganate(VII), \(KMnO_4\).

1. Mild Oxidation (Cold, Dilute, Acidified \(KMnO_4\)):
   Product: A diol (a molecule with two -OH groups). The purple manganate(VII) solution decolourises and forms a brown precipitate.
2. Vigorous Oxidation (Hot, Concentrated, Acidified \(KMnO_4\)):
   The double bond is completely ruptured (broken). The products depend on the structure of the alkene:
   
   • Carbon atoms attached to two H atoms ($\text{=CH}_2$) produce \(CO_2\) and \(H_2O\).
   • Carbon atoms attached to one H atom ($\text{=CHR}$) produce a carboxylic acid.
   • Carbon atoms attached to two alkyl groups ($\text{=CR}_2$) produce a ketone.

   This reaction is vital for determining the position of the double bond linkage in larger molecules.

Addition Polymerisation (14.2.2d)

Many alkene molecules (monomers) join together by opening their double bonds to form a single long chain (polymer).

Example: Ethene monomers polymerise to form poly(ethene).
$n(\text{monomer}) \rightarrow \text{Polymer}$ (The double bond becomes a single bond linking the repeating units).

Section 3: Arenes (Aromatic Hydrocarbons) (A Level 30.1)

Arenes, epitomized by benzene (\(C_6H_6\)), are cyclic hydrocarbons with a unique stability derived from delocalised electrons.

3.1 Structure, Stability, and Bonding (29.3.1)

• Shape and Hybridisation: Benzene is a planar, hexagonal ring structure. All six carbon atoms are $sp^2$ hybridised (120$^\circ$ bond angles).
• Delocalisation: The $p$ orbitals on all six carbon atoms overlap sideways to form a continuous ring of electron density (the delocalised $\pi$ system) above and below the plane of the ring. This delocalisation spreads the electrons evenly, making the molecule highly stable.
• Stabilisation: Benzene does not undergo typical alkene addition reactions because that would destroy the stable delocalised $\pi$ system, requiring a high input of energy. Instead, it prefers Electrophilic Substitution, which maintains the ring structure.

3.2 Characteristic Reaction: Electrophilic Substitution (30.1.2)

Because the delocalised $\pi$ system is electron-rich, it attracts electrophiles. However, unlike alkenes, substitution occurs rather than addition.

The General Mechanism:

1. Generation of the Electrophile ($E^+$): The catalyst reacts with the attacking reagent to form a very powerful electrophile.
Example (Nitration): \(HNO_3 + 2H_2SO_4 \rightarrow NO_2^+ + 2HSO_4^- + H_3O^+\)

2. Attack on Benzene: The delocalised $\pi$ system attacks the electrophile, forming a temporary, unstable, positive intermediate (a non-aromatic carbocation).

3. Loss of Proton and Regeneration of Aromaticity: The intermediate loses an $H^+$ ion, which is removed by the base ($HSO_4^-$ or $AlCl_4^-$), regenerating the stable delocalised system and the catalyst.

3.3 Reactions of Benzene and Methylbenzene (30.1.1)

1. Halogenation (e.g., Bromination): (30.1.1a)
   Reagents & Conditions: $Cl_2$ or $Br_2$ with a Lewis Acid catalyst ($AlCl_3$ or $AlBr_3$).
   (Note: A catalyst is necessary. Unlike alkanes, UV light is NOT used for ring substitution.)
2. Nitration: (30.1.1b)
   Reagents & Conditions: Mixture of concentrated $HNO_3$ and concentrated $H_2SO_4$ (producing $NO_2^+$ electrophile) between \(25^\circ C\) and \(60^\circ C\).
   (Too high a temperature increases the risk of multiple substitutions and side reactions.)
3. Friedel-Crafts Reactions (Alkylation/Acylation): (30.1.1c, d)
   These reactions add an alkyl ($\text{-R}$) or an acyl ($\text{-COR}$) group to the ring.
   Reagents & Conditions: Alkyl halide ($\text{RCl}$) or Acyl chloride ($\text{RCOCl}$) with $AlCl_3$ catalyst and heat.
4. Hydrogenation: (30.1.1f)
   Destroys aromaticity by complete addition.
   Reagents & Conditions: \(H_2\) and Pt/Ni catalyst, heat (requires much higher temperature/pressure than alkene hydrogenation).
   Product: Cyclohexane (an alkane).
5. Side-Chain Oxidation (Methylbenzene only): (30.1.1e)
   Alkyl groups attached to the benzene ring can be oxidized to a carboxylic acid group, while the ring itself remains intact.
   Reagents & Conditions: Hot alkaline \(KMnO_4\), followed by dilute acid ($\text{H}^+$).
   Product: Benzoic acid.

3.4 Directing Effects of Substituents (30.1.4)

When a benzene ring already has a substituent ($\text{-Y}$), this group influences where the second incoming electrophile will attack. Groups are classified as 2,4,6-directing (ortho/para) or 3,5-directing (meta).

1. 2,4,6-Directing Groups (Ortho/Para):

• These groups increase the electron density in the ring (they are generally activating).
• The new substituent enters at positions 2, 4, or 6 (next to or opposite the existing group).
• Syllabus Examples: $\text{-NH}_2$, $\text{-OH}$, $\text{-R}$ (alkyl groups).

2. 3,5-Directing Groups (Meta):

• These groups decrease the electron density in the ring (they are generally deactivating).
• The new substituent enters at positions 3 or 5.
• Syllabus Examples: $\text{-NO}_2$, $\text{-COOH}$, $\text{-COR}$.

Trick for Remembering: Generally, groups with a double or triple bond connected directly to the ring (e.g., $NO_2$, $COOH$) are usually meta-directors and deactivating. Groups with lone pairs or simple alkyl chains are usually ortho/para-directors and activating.