Chapter 13

Question 1

What is a pi bond?

Answer 1

A sideways overlap of two p-orbitals above and below the C-C sigma bond

Question 2

What kind of bonds does a double bond consist of?

Answer 2

A C=C double bond consists of 1 pi bond and one sigma bond

Question 3

How does a pi bond restrict the shape of the molecule

Answer 3

pi bonds restrict rotation around the C=C double bond

Question 4

What is a E/Z isomerism, what are the conditions it needs

Answer 4

Atoms attached to the carbon atoms of the C=C double bond are given a priority based on their atomic number ( Z=Zusammen E=Entgegen)
Only occurs in compounds with:
A double C=C bond
Two different groups attached to each C of the C=C double bond

Question 5

What is a Cis-trans isomerism?

Answer 5

Must have a C=C double bond
Two different groups must be attached to each carbon of the double bonds
One of the attached groups on each carbon must be the same
Cis -> same side of molecule
Trans-> opposite sides of the molecules

Question 6

Why are Alkenes more reactive than alkanes

Answer 6

Because of the presence of the pi-bond
-The pi electrons are more exposed than the sigma electrons as they are on the outside of the double bond
-The pi bond readily breaks and alkenes undergo addition reactions relatively easily

Question 7

Describe the hydrogenation of alkenes

Answer 7

Alkenes react with hydrogen in the presence of a nickel catalyst to form alkanes
Ethene + Hydrogen -(NI
i)-> Ethane

Question 8

Describe the Halogenation of alkenes

Answer 8

Alkenes react with the halogens, chlorine or bromine at room temperature to form dihaloalkanes.
Ethene + bromine -> 1,2-dibromoethane

Question 9

How do you test for the presence of a C=C double bond

Answer 9

Halogenation:
Add bromine water dropwise
if C=C double bonds are present, the orange colour disappears

Question 10

Describe the reactions of alkenes with hydrogen halides

Answer 10

Alkenes react with gaseous hydrogen halides at room temperature to form haloalkanes
ethene + hydrogen bromide -> bromoethane
(Also react with concentrated HCL or HBr acid)

Question 11

Describe the Hydration reactions of alkenes

Answer 11

Alkenes react with steam, in the presence of a phosphoric acid catalyst, H3PO4, to form alcohols.
ethene + steam -> ethanol
(If the alkene is unsymmetrical, then two possible products are formed

Question 12

What is an electrophile

Answer 12

An electrophile is an atom, or group of atoms that is attracted to an electron rich centre and accepts an electron pair.
It is usually a positive ion or a molecule containing an atom with a partial positive charge.
(The high electron density of the pi-electrons attracts electrophiles

Question 13

What is the Electrophilic addition mechanism?

Answer 13

T

Question 14

What is a Carbocation

Answer 14

Contains a positively charged carbon atom