Alkenes Revision Notes

Alkenes are a family of hydrocarbons characterised by having at least one double bond between two carbon atoms. This double bond consists of one sigma (σ) bond and one pi (π) bond, which together hold the two carbon atoms more tightly than a single bond would.

The general formula for alkenes with one double bond is CnH2n, reflecting the reduced number of hydrogen atoms compared to alkanes (which have the formula CnH2n+2). For example, ethene (C2H4) has two carbon atoms connected by a double bond and four hydrogen atoms.

The double bond is a region of higher electron density, making alkenes more reactive than alkanes. This bond also restricts rotation around the bond axis, affecting the molecule's geometry and possible isomerism.

• Alkenes contain at least one carbon–carbon double bond.
• The double bond consists of one sigma and one pi bond.
• General formula for alkenes with one double bond is CnH2n.

Hydrocarbons are compounds made exclusively of carbon and hydrogen atoms. They are classified as saturated or unsaturated based on the types of bonds between carbon atoms.

Saturated hydrocarbons, such as alkanes, contain only single bonds between carbon atoms, meaning they have the maximum number of hydrogen atoms bonded to carbon. In contrast, alkenes have one or more double bonds, which reduce the number of hydrogen atoms attached.

Because alkenes have fewer hydrogen atoms than the corresponding alkanes, they are called unsaturated hydrocarbons. This unsaturation allows alkenes to undergo addition reactions, where atoms can add across the double bond, increasing the hydrogen content temporarily during reactions.

• Saturated hydrocarbons have only single bonds; unsaturated have double or triple bonds.
• Alkenes are unsaturated due to their carbon–carbon double bond.
• Unsaturation affects chemical reactivity and hydrogen content.

Ethene (C2H4) is the simplest alkene, consisting of two carbon atoms joined by a double bond and four hydrogen atoms. It is a colourless gas used industrially to produce plastics like polyethylene.

Propene (C3H6) has three carbon atoms with one double bond and six hydrogen atoms. It is also a gas at room temperature and is used to make polypropylene, another important plastic.

These examples illustrate the general formula CnH2n and the presence of the double bond that defines alkenes.

• Ethene (C2H4) – simplest alkene, used in plastics manufacture.
• Propene (C3H6) – another common alkene, used industrially.
• Both have one double bond and follow the formula CnH2n.

The carbon–carbon double bond in alkenes is more reactive than the single bonds in alkanes. This reactivity is due to the pi bond, which is weaker and more exposed than the sigma bond.

Because of this, alkenes readily participate in addition reactions, where atoms or groups add across the double bond, converting it into a single bond temporarily during the reaction.

This property is fundamental to many industrial processes and distinguishes alkenes from saturated hydrocarbons in both structure and chemical behaviour.

• Double bond contains a reactive pi bond.
• Alkenes undergo addition reactions at the double bond.
• This reactivity is a key difference from saturated hydrocarbons.

Hydrocarbons are organic compounds made up of carbon and hydrogen atoms. Saturated hydrocarbons, known as alkanes, contain only single bonds between carbon atoms. This means each carbon atom forms four single covalent bonds, either with hydrogen or other carbon atoms, resulting in a stable structure.

Unsaturated hydrocarbons include alkenes and alkynes, which contain at least one double or triple bond between carbon atoms, respectively. The presence of these multiple bonds introduces regions of higher electron density and reactivity compared to single bonds. For example, ethane (C2H6) is a saturated hydrocarbon with single bonds, while ethene (C2H4) is an unsaturated hydrocarbon with a carbon-carbon double bond.

• Saturated hydrocarbons have only single C–C bonds.
• Unsaturated hydrocarbons have one or more double (alkenes) or triple (alkynes) C–C bonds.
• Multiple bonds increase chemical reactivity.

Bromine water is a brownish-orange aqueous solution of bromine (Br2). It acts as an electrophile and can add across multiple bonds in unsaturated hydrocarbons through an addition reaction.

In unsaturated hydrocarbons, the bromine molecule reacts with the carbon-carbon double bond, breaking it and forming a colourless dibromo-compound. This reaction removes bromine from the solution, causing the brown colour to disappear.

In contrast, saturated hydrocarbons lack double or triple bonds and do not undergo this addition reaction with bromine water under normal conditions. Therefore, the bromine water retains its original colour when mixed with saturated hydrocarbons.

• Bromine adds across double bonds in unsaturated hydrocarbons.
• This addition reaction decolourises bromine water from brown to colourless.
• Saturated hydrocarbons do not react, so bromine water remains brown.

To perform the bromine water test, place a small amount of the hydrocarbon sample in a test tube. Add a few drops of bromine water to the sample and gently shake or stir the mixture.

Observe any colour change carefully. If the brown colour of bromine water fades or disappears, the hydrocarbon is unsaturated. If the colour remains unchanged, the hydrocarbon is saturated.

It is important to conduct the test in a well-ventilated area and wear gloves and eye protection, as bromine is toxic and corrosive. Repeat the test to confirm results and avoid contamination between samples by using clean apparatus.

• Add bromine water to hydrocarbon sample in a test tube.
• Shake gently and observe colour change.
• Brown to colourless indicates unsaturation; no change indicates saturation.
• Use safety precautions due to bromine toxicity.
• Repeat for reliability.

For example, when bromine water is added to ethene (C2H4), the solution rapidly changes from brown to colourless as bromine adds across the double bond, forming 1,2-dibromoethane.

Conversely, adding bromine water to ethane (C2H6) results in no colour change because ethane is saturated and does not react with bromine under these conditions.

This test is a quick and effective qualitative method to identify the presence of carbon-carbon multiple bonds in hydrocarbons.

• Ethene + bromine water → colourless solution (addition reaction).
• Ethane + bromine water → no colour change (no reaction).
• Colour change confirms unsaturation.

While the bromine water test is useful, it has limitations. Some compounds other than alkenes can decolourise bromine water, such as phenols or certain aromatic compounds, which may cause false positives.

Also, the test does not distinguish between alkenes and alkynes, as both are unsaturated and react similarly with bromine water.

To improve accuracy, the test should be combined with other analytical methods or chemical tests when identifying unknown hydrocarbons.

• Other compounds may also decolourise bromine water, causing false positives.
• Test does not differentiate between alkenes and alkynes.
• Use alongside other tests for confirmation.

Cracking is a process used to break down large alkane molecules, which are saturated hydrocarbons with single carbon-carbon bonds, into smaller, more useful molecules. This is important because large alkanes, such as those found in heavy fractions of crude oil, are less useful as fuels or chemical feedstocks. By breaking them into smaller molecules, cracking produces alkenes, which contain carbon-carbon double bonds and are valuable in chemical manufacturing, and hydrogen gas, which has various industrial uses.

• Large alkanes have high molecular weights and are less reactive.
• Smaller alkenes are more reactive and useful as starting materials.
• Hydrogen gas produced can be used in other chemical processes.

Cracking requires specific conditions to break the strong carbon-carbon bonds in alkanes. The process typically occurs at high temperatures ranging from 500°C to 700°C. Additionally, a catalyst such as silica or alumina is used to increase the efficiency and selectivity of the reaction. The catalyst provides a surface that weakens bonds in the alkane molecules, allowing them to break more easily at lower temperatures than thermal cracking alone.

• Temperature range: 500°C to 700°C.
• Catalysts used: silica or alumina.
• Catalytic cracking is more efficient than thermal cracking.

There are two main types of cracking: thermal and catalytic. Thermal cracking uses only high temperatures and sometimes high pressure to break down alkanes. It tends to produce a mixture of alkenes and hydrogen but requires more energy and can produce unwanted by-products. Catalytic cracking uses both heat and a catalyst, which lowers the energy needed and improves the yield of desirable products like alkenes. Catalytic cracking is more common in industry due to its efficiency and control over product distribution.

• Thermal cracking: high temperature, no catalyst, less selective.
• Catalytic cracking: high temperature plus catalyst, more selective, lower energy.
• Catalytic cracking favours production of branched alkanes and alkenes.

During cracking, the long alkane molecules break at carbon-carbon bonds, forming smaller molecules. For example, a large alkane like decane (C10H22) can break into an alkene such as ethene (C2H4) and a smaller alkane or hydrogen gas. The general reaction involves breaking a single bond and forming a double bond in the alkene product, while releasing hydrogen atoms that combine to form hydrogen gas (H2). This process increases the proportion of unsaturated hydrocarbons in the product mixture.

• Breaking C–C single bonds in alkanes.
• Formation of C=C double bonds in alkenes.
• Release of hydrogen gas (H2).
• Example: C10H22 → C2H4 + C8H18 + H2 (simplified).

Consider the cracking of decane (C10H22) at 600°C with a silica catalyst. The molecule breaks to form ethene (C2H4), octane (C8H18), and hydrogen gas (H2). This can be represented by the equation:

C10H22 → C2H4 + C8H18 + H2 (simplified). This shows how a large alkane splits into a smaller alkene, a smaller alkane, and hydrogen gas. The exact products vary depending on conditions and catalyst used.

• Decane cracking produces ethene (alkene), octane (alkane), and hydrogen.
• High temperature and catalyst enable bond breaking.
• Products are useful for fuels and chemical synthesis.

Alkenes are hydrocarbons containing at least one carbon-carbon double bond (C=C). This double bond consists of one sigma (σ) bond and one pi (π) bond. The π bond is less stable and more reactive than a single bond, making alkenes unsaturated and chemically reactive.

The presence of the double bond allows alkenes to undergo addition reactions, where atoms add across the double bond, converting it into a single bond and saturating the molecule. This contrasts with alkanes, which are saturated hydrocarbons and generally undergo substitution reactions rather than addition.

• Alkenes contain C=C double bonds with a reactive π bond.
• The double bond makes alkenes unsaturated and reactive.
• Addition reactions involve breaking the double bond and adding atoms across it.

When bromine (Br₂) is added to an alkene, the bromine molecules add across the double bond, breaking it and forming a dibromoalkane. This reaction is rapid and causes the characteristic reddish-brown colour of bromine to disappear, turning the solution colourless.

This decolourisation is a practical test for unsaturation: alkanes do not react with bromine under normal conditions, so the colour remains. For example, ethene (C₂H₄) reacts with bromine to form 1,2-dibromoethane (C₂H₄Br₂).

Equation: C₂H₄ + Br₂ → C₂H₄Br₂ (colour change from brown to colourless)

• Bromine adds across the C=C double bond forming dibromoalkanes.
• The reaction decolourises bromine solution, indicating unsaturation.
• Used as a simple test to distinguish alkenes from alkanes.

Hydrogenation is the addition of hydrogen (H₂) to alkenes, converting the double bond into a single bond and forming alkanes. This reaction requires a catalyst, commonly finely divided nickel, and elevated temperatures (around 150°C).

For example, ethene reacts with hydrogen in the presence of nickel to form ethane: C₂H₄ + H₂ → C₂H₆. The catalyst provides a surface for the reaction, lowering activation energy and allowing hydrogen atoms to add across the double bond.

Hydrogenation is industrially important, for example, in converting unsaturated vegetable oils into saturated fats (margarine).

• Hydrogen adds across the double bond to form alkanes.
• Requires a nickel catalyst and heat to proceed.
• Used industrially to harden vegetable oils.

Hydration involves the addition of steam (H₂O) to alkenes in the presence of an acid catalyst, typically phosphoric acid (H₃PO₄), at high temperature and pressure.

The reaction converts the alkene into an alcohol by adding an -OH group and a hydrogen atom across the double bond. For example, ethene reacts with steam to form ethanol: C₂H₄ + H₂O → C₂H₅OH.

This process is important industrially for producing ethanol from ethene, which can be used as a solvent, fuel, or in alcoholic beverages. The reaction follows Markovnikov’s rule, where the -OH group attaches to the carbon with more alkyl substituents, but this detail is beyond the current scope.

• Steam adds across the double bond forming alcohols.
• Requires an acid catalyst and elevated temperature and pressure.
• Used industrially to produce ethanol from ethene.

Alkenes undergo addition reactions that convert their double bonds into single bonds, increasing saturation. The three key reactions are: bromine addition (test for unsaturation), hydrogenation (forming alkanes), and hydration (forming alcohols).

Each reaction involves the breaking of the π bond and addition of atoms or groups across the carbons formerly involved in the double bond.

These reactions demonstrate the chemical versatility of alkenes and their importance in both laboratory identification and industrial synthesis.

• Addition reactions saturate the alkene’s double bond.
• Bromine addition tests for unsaturation.
• Hydrogenation produces alkanes using nickel catalyst.
• Hydration produces alcohols using steam and acid catalyst.