14.1

Question 1

alkenes are _______________ hydrocarbons (1)

Answer 1

unsaturated

Question 2

Ethene is a planar molecule. This makes the angles between each bond roughly ____ degrees. (1)

Answer 2

120

Question 3

Unlike the C–C bonds in alkanes, there’s no rotation around the double bond in alkenes. Why is that? (3)

Answer 3

(1) in C–C bonds, there is a p-orbital on each carbon.
(2) these 2 orbitals overlap to form an orbital with a cloud of electron density above & below the single bond. this is a pi orbital
(3) the presence of this orbital means the bond can’t rotate or in other words, it causes restricted rotation

Question 4

What are the 2 types of isomers can alkenes form? (2)

Answer 4

(1) positional
(2) geometrical

Question 5

Define stereoisomerism. (2)

Answer 5

(1) same structural formula
(2) different arrangement of atoms/bonds in space

Question 6

In alkenes, the H–C–H angle is slightly less than 120. Why? (1)

Answer 6

(1) the group of 4 electrons in the C=C dbl bond repels more strongly than the groups of 2 in the C-H single bonds

Question 7

What is the naming system of the Cahn-Ingold-Prelog (CIP) notation? What does it entail? (2)

Answer 7

(1) If the 2 atoms with the greatest atomic number are on the same side of the double bond, the name has the prefix Z
(2) If not, it has the prefix E

Question 8

Explain the factor that affects the alkenes’ boiling and melting points. (2)

Answer 8

(1) because van der Waals forces are the only forces that act between the alkene molecules ,
(2) the MP or BP increases with the number of carbon atoms present.

Question 9

The bond enthalpy for C–C is 347 and for C=C it’s 612. This makes alkenes more reactive. Why? (3)

Answer 9

(1) the C=C bond forms an electron rich area in the molecule
(2) so they can easily be attacked by electrophiles or positively charged reagents
(3) This occurs in electrophilic addition reactions

Question 10

Define an electrophile. (1)

Answer 10

an electron pair acceptor