[13.1] properties of alkenes

Question 1

are alkenes saturated or unsaturated?

Answer 1

unsaturated. they contain at least one carbon to carbon double bond in their structure

Question 2

what is the general formula of alkenes?

Answer 2

CₙH₂ₙ

Question 3

the nature of the double bond

Answer 3

• each carbon has 4 electrons in its outermost shell
• for each C atom, 3 of these are used in sigma bonds: one to another C and two to other atoms (C or H)

Question 4

what happens to the one electron on each carbon atom of the double bond which is left?

Answer 4

• it is not involved in sigma bonds
• this electron is in a p-orbital
• a π-bond is formed

Question 5

what is a π-bond?

Answer 5

the bond formed by the sideways overlap of two p-orbitals, one from each carbon atom of the double bond

Question 6

what is the double bond in an alkene?

Answer 6

sigma bond + pi bond (not two bonds between carbons)

Question 7

what is the relative strength of pi and sigma bonds?

Answer 7

a pi bond is weaker than a sigma bond. therefore, when an alkene reacts, the pi bond breaks

Question 8

draw and label a diagram to explain how p-orbitals are involved in the formation of a pi bond

Answer 8

• labels: sigma bond, pi bond, p-orbital
• this is why when drawing the displayed formula of alkenes you do it so all the extra carbons are surround the central double bond, rather than being separate

Question 9

the shape around a double bond

Answer 9

• trigonal planar because
• there are three regions of electron density around each of the carbon atoms
• the three regions repel each other as far apart as possible, so the bond angle around each carbon atom is 120°
• all of the atoms are in the same plane