Question 1

Why do alkenes undergo addition reactions instead of substitution?

Accepted Answer

Alkenes have a carbon-carbon double bond that contains a pi bond. The pi bond is weaker and more exposed than the sigma bond, making it a region of high electron density. This attracts electrophiles, which break the pi bond and add across the double bond. Substitution would require breaking a strong C-H bond, which is less energetically favourable. Therefore, addition is the characteristic reaction of alkenes.

Question 2

What is Markovnikov's rule and when does it apply?

Accepted Answer

Markovnikov's rule states that when an asymmetric alkene reacts with a hydrogen halide (like HBr), the hydrogen atom adds to the carbon of the double bond that already has more hydrogen atoms. This is because the carbocation intermediate formed is more stable when the positive charge is on the more substituted carbon (tertiary > secondary > primary). The rule applies to electrophilic addition reactions where the electrophile is a proton (H⁺). For other electrophiles like Br₂, the addition is typically anti-Markovnikov under radical conditions (e.g., with peroxides).

Question 3

How do you distinguish between E and Z isomers?

Accepted Answer

To assign E or Z configuration, you use the Cahn-Ingold-Prelog priority rules. For each carbon of the double bond, assign priority to the two substituents based on atomic number (higher atomic number = higher priority). If the two highest priority groups are on the same side of the double bond, the isomer is Z (from German 'zusammen', meaning together). If they are on opposite sides, it is E ('entgegen', meaning opposite). For example, in but-2-ene, the two methyl groups are on the same side in the Z isomer and opposite in the E isomer.

Question 4

What is the product when ethene reacts with steam?

Accepted Answer

Ethene reacts with steam (H₂O) in the presence of a phosphoric acid catalyst at 300°C and 60 atm to form ethanol. This is an electrophilic addition reaction where the double bond is protonated to form a carbocation, which then reacts with water. The overall equation is: C₂H₄ + H₂O → C₂H₅OH. This is an important industrial process for producing ethanol.

Question 5

Why are alkenes more reactive than alkanes?

Accepted Answer

Alkenes are more reactive than alkanes because they contain a carbon-carbon double bond with a pi bond. The pi bond is formed by sideways overlap of p orbitals and is weaker than the sigma bond. It is also more exposed, making it a region of high electron density that is readily attacked by electrophiles. In contrast, alkanes have only strong sigma bonds and are relatively inert, requiring harsh conditions (e.g., UV light for halogenation) to react.

Question 6

How do you test for an alkene?

Accepted Answer

A common test for an alkene is to add bromine water (an orange solution of Br₂ in water). If the compound is an alkene, the bromine water decolourises (turns colourless) as bromine adds across the double bond to form a colourless dibromoalkane. This is an electrophilic addition reaction. Another test is using cold, dilute potassium manganate(VII) (KMnO₄), which turns from purple to colourless and forms a diol. However, the bromine test is more specific for unsaturation.

Alkenes

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Related Topics in AQA A-Level Chemistry

Acids and bases (A-level only)

Alcohols

Aldehydes and ketones (A-level only)

Alkanes