Ch. 13.1 : Properties of Alkenes

Question 1

Describe the structure of an alkene

Answer 1

Unsaturated - contain at least one carbon - carbon double bond

Question 2

What’s the general formula for aliphatic alkanes with a double bond?

Answer 2

CnH2n

Question 3

Describe the bonding of a carbon atom in a double bond

Answer 3

Three of the four electrons are used in three sigma bonds ( one to the other carbon of the double bond and the other two electrons to two other atoms ( either C or H ))

The last electron is in a p-orbital.

Question 4

How does a pi bond form ? Describe its nature

Answer 4

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

Each carbon atom contributes one electron to the electron pair in the pi bond.

The pi - electron density is concentrated above and below the line joining the nuclei of the bonding atoms.

Question 5

Why does a double bond prevent rotation ?

Answer 5

The pi - bond locks the two carbon atoms in position and prevents them from rotating around the double bond.

This makes its geometry different from alkanes, where rotation is possible around every atom.

Question 6

What is the shape around a double bond called?

Answer 6

Trigonal planar

Question 7

Why is the shape around the carbons in a double bond called what it is?

Answer 7

• 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 the atoms are in the same plane

Question 8

What are the bond angles around a double bond ?

Answer 8

120°