Alkenes (OCR A Level Chemistry A): Flashcards

Cis/trans naming fails when all four substituents on the C=C bond are different, requiring the E/Z system based on Cahn-Ingold-Prelog priority rules instead.

True.

Z comes from the German "zusammen" meaning together, so the higher-priority groups are on the same side of the double bond.

A Hydrogenation is the addition of H₂ across the C=C double bond of an alkene, producing an alkane, using a nickel catalyst at 200 °C and 1000 kPa.

The unknown compound is shaken with bromine water; if a C=C bond is present, an addition reaction occurs and the orange/yellow solution decolourises.

True.

The rate of reaction follows the order HI > HBr > HCl because the H-I bond is weakest and therefore easiest to break.

A Halogenation is the electrophilic addition of a halogen (X₂) across the C=C double bond of an alkene, producing a dihaloalkane, and occurs readily at room temperature.

The π bond has a bond enthalpy of approximately 265 kJ mol^-1, which is lower than the C-C single bond value of 347 kJ mol^-1, so less energy is needed to break it.

True.

Hydrogenation is an addition reaction where H₂ adds across the C=C bond of ethene to give ethane as the sole product.

A carbocation is a positively charged carbon atom with only three covalent bonds; carbocations are classified as primary, secondary or tertiary depending on the number of attached alkyl groups.

Step 1: the electrophile (H⁺) attacks the π bond of the C=C, forming a carbocation intermediate as the H-Br bond breaks heterolytically. Step 2: the Br^- nucleophile attacks the carbocation to form the halogenoalkane product.

True.

Three alkyl groups donate electron density towards the positive carbon via the inductive effect, spreading the charge more effectively than two groups in a secondary carbocation.

Markovnikov's rule states that in electrophilic addition of a hydrogen halide to an alkene, the halogen attaches to the most substituted carbon and the hydrogen attaches to the least substituted carbon, giving the major product.

Alkyl groups are electron-donating, pushing electron density towards the positively charged carbon and stabilising the carbocation by reducing the positive charge.

True.

Heterolytic fission gives H⁺ (the electrophile) and Br^- (the nucleophile), both of which are needed for the two-step addition mechanism.

Markovnikov's rule states that in electrophilic addition of HX to an unsymmetrical alkene, the halogen bonds to the most substituted carbon, producing the major product via the most stable carbocation intermediate.

An Addition polymerisation is the reaction in which many monomers containing at least one C=C double bond join together to form a long-chain polymer as the only product, with no by-products.

The repeat unit is identical to the monomer except that the C=C double bond is replaced by a C-C single bond; the repeat unit is shown inside square brackets with n outside.

True.

Unlike condensation polymerisation, addition polymerisation gives only the polymer chain and no small-molecule by-products.

Feedstock recycling breaks waste polymers down into monomers, gases and oils using chemical and thermal processes.

A key advantage of feedstock recycling is that it works with unsorted and unwashed polymers.

Polyalkenes consist entirely of C-C single bonds along the backbone, which microorganisms cannot break down, unlike the ester or amide bonds found in biodegradable condensation polymers.

A bioplastic is a polymer made from plant-based materials such as starch, cellulose or plant oils, providing a renewable and sustainable alternative to crude-oil-derived polymers.

True.

The ester and amide bonds in these condensation polymers can be broken by hydrolysis, unlike polyalkenes whose C-C backbone cannot be broken down by microorganisms.