Alkenes (F6examonly)

Question 1

What are alkenes?

Answer 1

• unsaturated hydrocarbons containing a C=C bond
• the double bond is a π bond

Question 2

What is double bond between the two carbons comprising of?

Answer 2

• two covalent bonds
• bond 1 = sigma (σ) covalent bond - electrons are between the two carbon atoms
• bond 2 = pi (π) covalent bond - electrons are above and below the plane of the 6 central atoms of carbon. this is formed from the overlap of p-orbitals on the C atoms)
• there is restricted rotation around the pi bond

Question 3

What is the shape around the double bond? Why?

Answer 3

• trigonal planar
• the 3 areas of electron density around the carbon atom repel, resulting in the trigonal planar shape, as the they arrange themselves as far apart as possible to minimise repulsion
• bond angle = 120°

Question 4

What is an electrophile?

Answer 4

an atom or group of atoms which is attracted to an area of high electron density, where it accepts a pair of electrons to form a covalent bond

Question 5

What is Markownikoff’s rule?

Answer 5

the major product from addition of a hydrogen halide (HX) to an unsymmetrical alkene is the one where the hydrogen adds to the carbon with the most hydrogens already attached

Question 6

Explain Markownikoff’s rule:

Answer 6

• the major product is formed from the most stable carbocation
• the more carbons a carbocation is bonded to, the more stable it is
• relative stability: primary carbocation < secondary carbocation < tertiary carbocation

Question 7

How can bromine detect C=C bonds?

Answer 7

• bromine reacts with alkenes by electrophilic addition to form dibromoalkanes
• bromine solution is originally orange, but the dibromoalkanes formed are colourless
• therefore if there is a colour change from orange to colourless, C=C bonds are present