Structure

Alkanes have the general formula CₙH₂ₙ₊₂. Each carbon atom is bonded to four other atoms by single covalent bonds (σ-bonds). This saturation makes alkanes relatively unreactive compared to unsaturated hydrocarbons.

Reactions of Alkanes

1. Combustion

Alkanes burn in excess oxygen to produce carbon dioxide and water, releasing heat energy.

$CH_4 + 2O_2 → CO_2 + 2H_2O$

In limited oxygen, incomplete combustion occurs, producing carbon monoxide or carbon (soot).

2. Substitution Reactions with Halogens

Under ultraviolet light, alkanes react with halogens (chlorine or bromine) where hydrogen atoms are replaced by halogen atoms.

$CH_4 + Cl_2 → CH_3Cl + HCl$

The reaction proceeds through a free radical mechanism. Multiple substitution can occur, producing mixtures such as CH₃Cl, CH₂Cl₂, CHCl₃, and CCl₄.

Structure

Alkenes have the general formula CₙH₂ₙ and contain at least one carbon-carbon double bond (C=C). This double bond consists of one strong σ-bond and one weaker π-bond, making alkenes chemically reactive.

Reactions of Alkenes

1. Addition Reactions

The π-bond breaks easily, allowing atoms to add across the double bond.

• Hydrogenation: Addition of hydrogen gas in the presence of a nickel catalyst. $CH_2=CH_2 + H_2 → CH_3-CH_3$

• Halogenation: Addition of halogen (Cl₂ or Br₂). The product is a dihalogenated compound. $CH_2=CH_2 + Br_2 → CH_2Br-CH_2Br$

• Hydration: Addition of water in the presence of acid catalyst to form alcohols. $CH_2=CH_2 + H_2O → CH_3-CH_2OH$

• Hydrohalogenation: Addition of hydrogen halides (HCl, HBr, HI). $CH_2=CH_2 + HCl → CH_3-CH_2Cl$

2. Polymerization

Alkenes can join together under high pressure and temperature to form polymers.

$n(CH_2=CH_2) → (CH_2-CH_2)_n$

Polyethene (polythene) is produced from ethene monomers.

Structure

Alkynes have the general formula CₙH₂ₙ₋₂ and contain a carbon-carbon triple bond (C≡C). The triple bond consists of one σ-bond and two π-bonds, making alkynes even more reactive than alkenes.

Reactions of Alkynes

1. Addition Reactions

Like alkenes, alkynes undergo addition reactions. Because of the triple bond, two molecules of reagent can add.

• Hydrogenation: With excess hydrogen and nickel catalyst, alkyne forms alkane. $HC≡CH + 2H_2 → CH_3-CH_3$

• Halogenation: Addition of two molecules of halogen. $HC≡CH + 2Br_2 → CHBr_2-CHBr_2$

• Hydration: With acid and mercury(II) catalyst, alkyne forms aldehyde or ketone (not alcohol directly). $HC≡CH + H_2O → CH_3CHO$

Structure

Alcohols contain the –OH (hydroxyl) functional group. The general formula is R–OH, where R is an alkyl group. The oxygen-hydrogen bond is polar, giving alcohols distinctive physical properties.

Reactions of Alcohols

1. Reaction with Carboxylic Acids (Esterification)

In the presence of concentrated sulfuric acid, alcohols react with carboxylic acids to form esters and water.

$C_2H_5OH + CH_3COOH → CH_3COOC_2H_5 + H_2O$

Ethanol + Ethanoic acid → Ethyl ethanoate + Water

2. Reaction with Active Metals

Alcohols react with metals like sodium, potassium, and magnesium to produce metal alkoxides and hydrogen gas.

$2C_2H_5OH + 2Na → 2C_2H_5ONa + H_2$

3. Oxidation

Primary alcohols oxidize first to aldehydes, then to carboxylic acids. Secondary alcohols oxidize to ketones.

$C_2H_5OH → CH_3COOH$

Ethanol oxidized to ethanoic acid (using acidified potassium dichromate or alkaline potassium permanganate).

4. Dehydration

With concentrated sulfuric acid at 170°C, alcohols lose water to form alkenes.

$C_2H_5OH → CH_2=CH_2 + H_2O$

5. Reaction with Phosphorus Halides

Alcohols react with PCl₅, PCl₃, or SOCl₂ to form alkyl halides.

$C_2H_5OH + PCl_5 → C_2H_5Cl + POCl_3 + HCl$

The key principle connecting structure to reactivity is bond strength and unsaturation:

The presence of π-bonds (double or triple) provides sites for addition reactions. The polar –OH group in alcohols enables reactions with acids, metals, and oxidation.

Question: Predict the product when ethene reacts with hydrogen chloride gas.

Solution:

• Identify the functional group: Ethene (C₂H₄) is an alkene containing a C=C double bond.
• Type of reaction: Addition across the double bond.
• HCl adds across the C=C, with H attaching to one carbon and Cl to the other.

$CH_2=CH_2 + HCl → CH_3-CH_2Cl$

The product is chloroethane (ethyl chloride).

In Tanzania, petroleum (containing various aliphatic hydrocarbons) is refined to produce fuels like petrol and diesel. The cracking process used in refineries converts larger alkanes into smaller, more useful alkenes like ethene, which is then used to manufacture polythene plastics. Additionally, ethanol produced from fermentation of cassava or maize sugars is used as a biofuel additive and in hand sanitizers — demonstrating both alcohol chemistry and its industrial applications in local contexts.