Alkenes

Question 1

What are alkenes?

Answer 1

• Alkenes are unsaturated hydrocarbons.

Question 2

Why are alkenes fairly reactive?

Answer 2

• Alkenes have double bonds which have a HIGH ELECTRON DENSITY, making alkenes fairly reactive.

Question 3

What are the double bonds in alkenes described as the centre of?

Answer 3

• Double bonds in alkenes = centre of high electron density.

Question 4

How many hydrogens do cycloALKENES have compared to their straight-chain alkene counterparts?

Answer 4

• 2 fewer hydrogens than straight-chain alkene counterparts.

Question 5

What is an “electrophile?”

Answer 5

• An electrophile is an electron-defficient species which can accept a pair of electrons, forming a dative covalent bond.

Question 6

What attacks alkenes in addition reactions?

Answer 6

• Electrophiles.
• The electrophiles are then added to the molecule.

Question 7

Outline the mechanism for all addition reactions of alkenes using electrophiles.

Answer 7

• Curly arrow that starts at double bond and ends at the electrophile.
• Pair of electrons move from a double bond to the electrophile, electrophile gains two electrons.

Question 8

Give examples of electrophiles.

Answer 8

• Positively-charged ions (ie. H⁺)
• Polar molecules (with S+ charge): ie. HBr/ H₂SO₄.

Question 9

What is the test for alkenes/ unsaturation? What acts as an elecrophile in this addition reaction/ what actually happens?

Answer 9

• Add bromine water to sample to test for unsaturation.
• If alkene is present, colour change = from brown-orange to colourless.
• Br₂ acts as electrophile.
• Br₂ is added to alkene to form dibromoalkane = colourless.

Question 10

How does Br₂ become polarised as it approaches the C=C double bond in addition reaction of an alkene?

Answer 10

• Br₂ is polarised as the molecule comes near to C=C double bond as the electrons in the C=C double bond will repel the electrons in bond in Br- Br to one side of molecule, induced dipoles formed S+/ S-

Question 11

Outline the mechanism of addition reaction of ethene (as example: could be any alkene!) using Br₂

Diagram for this (which you need to know!) is in the pink hand folder!!

Answer 11

• Br₂ is polarised as electrons in double bond will repel the electrons in Br₂.
• Pair of electrons in C=C double bond move to to S+ in bromine and forms a bond between C- Br.
• Pair of electrons move from Br-Br bond to Br, breaking Br- Br bond.
• Carbocation intermediate is formed, Br- is attracted to C+. Pair of electrons move from Br- to C+, bond is formed between C and other Br.
• Colourless 1,2-dibromoethane is formed.

Question 12

What is the intermediate formed in additon reaction of alkenes using Br₂ called?

Answer 12

• Carbocation intermediate.