Chapter 13 - Alkenes

Question 1

What are alkenes?

Answer 1

Unsaturated hydrocarbons
Contain at least one C=C

Question 2

What is the general formula of alkenes?

Answer 2

CnH2n

Question 3

Describe the nature of the double bond

Answer 3

Each carbon atom has 4 electrons to form bonds
Three of the four electrons are used in three σ-bonds

Question 4

What happens to the one electron on each carbon atom not involved in σ-bonds?

Answer 4

The electron is in a p-orbital
Sideways overlap of two p-orbitals creates a π-bond

Question 5

What does a C=C double bond consist of?

Answer 5

1x π-bond
1x σ-bond

Question 6

What does the π-bond specifically do around the C=C double bond?

Answer 6

Restricts rotation

Question 7

What is a π-bond?

Answer 7

Sideways overlap of p-orbitals above and below the C-C σ-bond

Question 8

How many regions of electron density are around each carbon atom of a double bond?

Answer 8

3 regions of electron density
These repel one another as far apart as possbile

Question 9

Describe the shape around the C=C double bond:

Answer 9

The 3D shape is trigonal planar
Bond angle in 120*

Question 10

What are stereoisomers?

Answer 10

Same structural formula but a different arrangement of atoms in 3D space

Question 11

When does E/Z isomerism occur?

Answer 11

A C=C bond
Two different groups attached to each C of the C=C double bond

Question 12

Name this molecule

Answer 12

(E)-but-2-ene

Question 13

Name this molecule

Answer 13

(Z)-but-2-ene

Question 14

What atoms attached to the carbons take priority?

Answer 14

The atom with the highest atomic number

Question 14

What are the requirement for cis-trans isomerism?

Answer 14

Must have C=C double bond
Two different groups must be attached to each carbon
One of the attached groups on each carbon must be the same

Question 15

Name this molecule

Answer 15

Cis-but-2-ene
(on this side of double bond)

Question 15

Name this molecule

Answer 15

Trans-but-2-ene
(across the double bond)

Question 16

Distinguish between these two molecules in terms of E/Z and cis-trans isomerism:

Answer 16

1. Highest priority groups are on the same side of the double bond (Z)
   Both Cl atoms are across the double bond (trans-isomer)
2. Highest priority groups are on the opposite sides of the double bond (E)
   Both Cl atoms are on the same side of the double bond (cis-isomer)

Question 17

Compare the reactivity of alkenes and alkanes:

Answer 17

Alkenes are much more reactive than alkanes because of the presence of the π-bond
The π-electrons are more exposed than the σ-electrons as they are on the outside of the double bond
The π-bond readily breaks and alkenes under go addition reactions relatively easily

Question 18

What is meant by an addition reaction?

Answer 18

Two reactant molecules join together to form one product

Question 19

What are some examples of addition reactions which alkenes may undergo?

Answer 19

Hydrogen in the presence of a nickel catalyst
Halogens
Hydrogen halides
Steam in the presence of an acid catalyst

Question 20

Describe the hydrogenation of alkenes

Answer 20

Alkenes react with hydrogen in the presence of a nickel catalyst to form alkanes
Hydrogen molecule adds across the C=C double bond

Question 21

Describe the halogenation of alkenes

Answer 21

Alkenes react with halogens, chlorine or bromine, at room temperature to form dihaloalkanes

Question 22

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

Answer 22

Add bromine water dropwise
The orange colour disappears (the bromine is decolourised)

Question 23

Describe the reaction of alkenes reacting with hydrogen halides:

Answer 23

Alkenes react with hydrogen halides at room temperature to form haloalkanes Alkenes also react with conc. HCl or hydrobromic acid. which are solutions of the hydrogen halides in water

Question 24

What happens if the alkene which is reacting with a hydrogen halide is unsymmetrical?

Answer 24

Two possible products are formed e.g.

Question 25

Describe the hydration reaction of alkenes:

Answer 25

Alkenes react with steam, H2O(g) in the presence of a phosphoric acid catalyst, H3PO4, to form alcohols

Question 26

What is an electrophile?

Answer 26

Is an atom or group of atoms that is attracted to an electron rich centre and accepts an electron pair Usually a positive ion or a molecule containing an atom with a partial positve (delta+) charge

Question 26

What happens if the alkene undergoing hydration is unsymmetrical?

Answer 26

Two possible products are formed e.g.

Question 27

What is special about the double bond in an alkene in terms of electron density?

Answer 27

Represents a high region of electron density

Question 28

What does the high electron density of the π-electron do?

Answer 28

Attracts electrophiles

Question 29

Describe the electrophilic addition mechanism between an alkene and a hydrogen halide

Answer 29

Bromine is more electronegative than hydrogen, so hydrogen bromide is polar The electron pair in the π-bond is attracted to the partially positive hydrogen atom, causing the double bond to break The H-Br bond breaks by heterolytic fission, with the electron pair going to the bromine atom A bromide ion and a carbocation intermediate form The bromide ion reacts with the carbocation to form the addition product

Question 30

What is Markownikoff's rule?

Answer 30

The major product forms via the most stable carbocation intermediate

Question 31

What happens to the stability of the carbocation intermediate with an increasing amount of alkyl groups attached?

Answer 31

Increases

Question 32

Example of Markownikoff's rule:

Answer 32

Question 33

Describe the halogenation reaction mechanism between an alkene and a halogen

Answer 33

Bromine is a non polar molecule When bromine approaches the alkene, the π-electrons induce a dipole in the bromine molecule The electron pair in the π-bond is attracted to the partially positive bromine atom, causing the double bond to break The Br-Br bond breaks by heterolytic fission, with the electron pair going to the delta -ve-Br atom A broide ion and a carbocation intermediate form The bromide ion reacts with the carbocation to form the addition product

Question 34

What is a polymer?

Answer 34

Consists of many repeating units (monomers) joined together to make a long chain

Question 35

What reaction do unsaturated alkene molecules undergo to form long saturated polymer chains containing no double bonds?

Answer 35

Addition polymerisation

Question 36

How is polyethene formed?

Answer 36

From ethene monomer units Under high temperature and pressure

Question 37

What are some environmental concerns of using polymers?

Answer 37

Conservers finite fossil fuels and decreases amount of waste going into landfill Costly to sort by hand Mixed polymers renders the product unstable

Question 38

What are bioplastics?

Answer 38

Made from plant based materials Renewable and sustainable

Question 39

How are biodegradable polymers broken down?

Answer 39

By bacteria into water, carbon dioxide, and biological compounds