Alkenes

Question 1

Introduction to Alkenes

What kind of hydrocarbons are Alkenes?

Answer 1

Alkenes are a homologous series of unsaturated hydrocarbons that contain one or more C=C double bond.

Question 2

Introduction to Alkenese

What is the general formula for Alkenes?

Answer 2

The general formula is CnH2n.

Question 3

Introduction to Alkenes

What are some ways to systematically draw isomers?

Answer 3

1. Start with the longest continuous carbon chain and generate all possible positional isomers. (Position the functional group at different carbon atoms, but make sure they are not the flipside of each other. )
2. Shorten the longest continuous carbon chain by one Carbon atom and place the side chain in the middle of the carbon chain. (never place them at the ends)
3. Repeat Step 2

Question 4

Physical Properties of Alkenes

What is the change in melting and boiling points as the number of Carbon atoms increases down the group?

Answer 4

Melting and Boiling Points increases. As the number of Caarbon atoms increases, size of electron cloud increases and hence polarisability increases. Hence, more energy is required to overcome the stronger dispersion forces between molecules, resulting in an increase in melting and boiling points.

Question 5

Physical Properties of Alkenes

What is the change in viscosity of Alkenes as the number of Carbon atoms increases?

Answer 5

Like alkanes, alkenes are more viscous as the number of Carbon atoms increases.

Question 6

Physical Properties of Alkenes

State the solubility of Alkenes.

Answer 6

Alkenes are non-polar and are insoluble in water but dissolve in organic solvent such as hexane or CCl4.

Question 7

Sources of Alkenes

How does the process of cracking produce alkenes?

Answer 7

The cracking of a long-chain alkane can produce

1) A shorter chain alkane and a short chain alkene.
CH3(CH2)10CH3 –> CH3(CH2)4CH3 + 3CH2=CH2

2) Two short chain alkenes and hydrogen gas (under certain conditions)

C18H28 ––> C8H16 + C10H20 + H2

Question 8

Alkenes from other Organic Compounds

Describe the process of elimination for alkenes.

Answer 8

In elimination, two atoms attached to C in alkanes are removed, after which an alkene and another product are formed.

CH3CH2(OH) —> CH2=CH2 + H2O

The H and OH are removed such that a C=C double bond may be formed, along with the product H2O.

Question 9

Chemical Properties

Explain why Alkenes are more reactive than Alkanes.

Answer 9

Alkenes are extremely reactive due to the presence of the C=C bond, where a C=C bond consists of a pi bond and a sigma bond.

Question 10

Chemical Properties

Explain what the reactivity of the C=C is due to.

Answer 10

The reactivity of alkenes is due to the presence of the pi bond. Unlike the sigma bond, the electron cloud that makes up the pi bond is foudn above and below the plane of the C=C bond. Hence, the pi electrons are loosely held, making them accessible, and quite attractive to electrophiles.

Question 11

Chemical Properties

State the combustion equation of alkenes.

Answer 11

CnH2n + (3n/2) O2 ––> nCO2 + nH2O

Question 12

Chemical Properties

Why do alkenes produce smokier flame than alkanes?

Answer 12

Compared to alkanes, the percentage by mass of carbon in alkene molecules is higher. Hence, alkenes are more likely to undergo incomplete combustion, thereby producing carbon monoxide and carbon (soot). Thus, alkenes tend to burn with a smoky flame compared to alkanes.

Question 13

Addition Reaction

What is the addition reaction for Alkenes?

Answer 13

Alkenes undergo addition reaction due to the presence of C=C bond. During the addition reaction, the weaker pi bond of the C=C bond is broken and 2 new single sigma bonds.
| |
C=C + X – Y —-> ––C–C–
| |
X. Y

The alkene is converted into a saturated organic compound - in this case, it has been converted to an alkane.

Question 14

Addition Reaction of Hydrogen

Explain what are the conditions for the addition of hydrogen to alkenes (hydrogenation).

Answer 14

Alkenes react with hydrogen gas at high temperature and pressure and in the presence of catalyst such as nickel.

C=C + H-H —> C–C
| |
H H

Question 15

Addition Reaction of Bromine

What are the conditions for addition reaction of bromine to occur. (bromination)

Answer 15

Alkenes react with bromine at r.t.p

C=C + Br-Br –> CBr-CBr

Question 16

Addition Reaction of Bromine

Why is bromination also known as electrophilic addition?

Answer 16

This is due to the alkenes being electron rich and is attracted to electron-deficient species.

Question 17

Addition of Bromine

Explain why the process of bromination is so important.

Answer 17

Bromination is used as a test to see whether an alkene is present.

Question 18

Addition of Bromine

What are observations made if liquid bromine is used to test whether an alkene is present.

Answer 18

Inital Color of liquid bromine: Reddish-Brown

Observation if alkene is pr. esent: Reddish-brown solution decolorises

Question 19

Addition of Bromine

What are observations made if bromine dissolved in CCl4 is used to test whether an alkene is present.

Answer 19

Initial Color: Reddish-Brown

Observations if alkene is present: Reddish-Brown Solution decolorises.

Question 20

Addition of Bromine

What are observations made if aqueous bromine is used to test whether an alkene is present.

Answer 20

Initial Color: Yellow or Orange

Observation if alkene is present: Yellow/Orange solution decolorises.

Question 21

Addition with steam

Describe the addition reaction of Alkenes with steam and and state the conditions for such a reaction to occur.

Answer 21

Alkenes react with steam to form alcohols, under high temperature and pressure and in the presence of phosphoric acid, H3PO4, a catalyst for the reaction.

C=C + H-OH –> CHC(OH)

Question 22

Addition of Hydrogen Halides

Describe the process of addition of hydrogen halide.

Answer 22

Alkenes can be converted to halogenoalkane (compound where one or more H atoms of alkane are replaced by a halogen atom) by reaction with a hydrogen halide.

C=C + H-X —> C-H C-X

Reagent: Gaseous HX, where X = Cl, Br or I (Halogen atom)

Condition: R.t.p and pressure

Question 23

Addition Polymerisation

What is polymerisation?

Answer 23

Polymerisation is a process whereby two or more simple molecules link together to form a much large molecule. The simple molecules are known as monomers, and larger molecule formed by the linking of the monomers is known as the polymer.

Question 24

Addition Polymerisation

How do alkenes undergo addition polymerisation?

Answer 24

Alkenes break apart their pi bond, hence C atoms are able to form 1 more bond each, forming four bonds, becoming an alkane in the process.
H H H H
| | | |
C=C –> – C-C –
| | | |
H. H H H
As can be seen from the diagram, each Carbon atom can now form 1 more bond with another monomer instead of having a double bond in the centre.

Question 25

# Addition Polymerisation Give some examples of addition polymerisation for alkenes.

Answer 25

Poly(ethene): Monomer: Ethene Poly(propene): Monomer: Propene Poly(chloroethane): Monomer: chloroethane

Question 26

# Addition Polymerisation What are the conditions for addition polymerisation to take place?

Answer 26

1. Add trace amounts of O2/radical initators 2. High pressure

Question 27

# Fats and Oils Why are polyunsaturated fats or oils healthier?

Answer 27

Polyunsaturated fats or oils contain more than one C=C double bond in the long hydrocarbon chain. . They are found in vegetable oil, salmon, nuts and seeds,etc.

Question 28

# Fats and Oils Why are not all polyunsaturated fats healthy?

Answer 28

Partially hydrogenated oil contains trans fats, which are wose than saturated fats. Trans fats raise LDL cholesterol and increases the risk of heart disease.

Question 29

# Fats and Oils How are trans fats formed?

Answer 29

Trans fats are formed during the hydrogenation of vegetable oil. The addition of hydrogen across the C=C bond causes the vegetable oil to solidify to form solid fats. During the process, not all C=C bonds are hydrogenated. Some of the natural cis C=C double bond in the unsaturated fats may be converted to trans C=C double bonds.

Question 30

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