alkenes Flashcards
What are alkenes?
Unsaturated hydrocarbons with one or more C=C double bonds.
Alkenes are produced during the thermal cracking of crude oil.
What is the general formula for alkenes?
C_nH_{2n}
What is the shape of alkenes?
Planar
What makes alkenes more reactive?
The high electron density between the two carbon atoms in the double bond.
What type of forces exist between alkene molecules?
Only van der Waals forces.
How do melting and boiling points of alkenes change?
They increase with chain length.
Are alkenes soluble in water?
No, they are not soluble in water.
What is the role of the double bond in alkenes during addition reactions?
It forms an electron-rich area that can be attacked by electrophiles.
What are electrophiles?
Positively charged reagents that accept electron pairs.
Give an example of a good electrophile.
H+ ion.
What happens during electrophilic addition?
Electrophile is attracted to the double bond, forming a carbocation.
What is formed when Br2 undergoes electrophilic addition with an alkene?
A dihaloalkane.
How does HBr react with alkenes?
H+ forms a bond, creating a carbocation, and Br- bonds to it, forming a haloalkane.
What is created when H2SO4 reacts with an alkene?
A halidehydrogensulfate.
What is the significance of major and minor products in electrophilic addition?
Major products are formed on more stable carbocations (secondary or tertiary); minor products on less stable (primary).
What test can be used to identify the presence of a double bond?
Addition of bromine solution, which decolourises if a double bond is present.
Define polymers.
Long chain molecules made from many small monomers joined together.
What are monomers?
Short chain molecules that form polymers when bonded together.
What is addition polymerisation?
Formation of long chain molecules from smaller molecules with no other products.
What happens to the double bond during addition polymerisation?
The double bond opens up, allowing monomers to join.
What is the repeating unit in a polymer based on ethene?
Two carbons with no double bond.
What is the difference between high-density and low-density polythene?
High-density has less branching and is stronger; low-density has more branching and is weaker.
What are plasticisers?
Molecules that keep polymer chains apart, weakening van der Waals forces.
What is poly(chloroethene) commonly known as?
PVC (poly(vinylchloride)).
What effect does adding a plasticiser have on PVC?
Makes it more flexible.
