Chemistry B3 and B4 Alkanes and Alkenes

How does fractional distillation work?

Fractional distillation separates hydrocarbons based on their different boiling points. Fractions condense at different temperatures due to variations in the London forces between molecules

How does branching affect boiling points?

Branched chains have less surface contact and cannot get as close together, resulting in weaker London forces. This lowers the boiling point compared to straight-chain isomers

How does molecular size affect boiling points?

Boiling point increases with an increase in molecular formula, number of carbon atoms, and chain length due to more intermolecular forces and greater surface ares

Why does incomplete combustion occur, and what are the products?

Incomplete combustion occurs due to an limited supply of oxygen, producing CO instead of carbon dioxide and water

What is the general formula for alkenes

CnH2n

How do alkenes react with bromine

1.The double bond donates electrons to Br2, forming a carbocation.

2. A bromine ion attacks the carbocation, forming a dibromo product

3. The bromine water test decolourises in the presence of alkenes

Describe the reaction of an alkene with HBr

1. The double bond donates electrons to Br and the H is now H+ (electrophile), forming a carbonation

2. The Br- attacks the carbocation, forming a bromoalkane

How do alkenes undergo addition polymerisation?

1. The double bond opens up, forming a repeating unit

2. No other products are formed

   e.g Ethene polymerises to poly(ethene)

What are the environmental concerns of polymer disposal

1.Non-biodegradable - Plastics persist in the environment

2 .Combustion produces toxic fumes - E.g burning PVC releases HCl gas

3 . Recycling challenges - Seperation and processing are difficult

Define structural isomerism

Compounds with the same molecular formula but different structural arrangements of atoms

What is a nucleophile?

A species that donates a pair of electrons to form a covalent bond

What is an electrophile?

A species that accepts a pair of electrons to form a covalent bond