3.4 Alkenes

Question 1

General formula of alkenes

Answer 1

C₂H₂ₙ

Question 2

Functional group of alkenes

Answer 2

C=C

Question 3

Shape around C atom in a C=C double bond in alkenes

Answer 3

Trigonal planar

Question 4

What bonds are in a C=C bond?

Answer 4

σ-bond and a π-bond

Question 5

Diene

Answer 5

2 C=C bonds

Question 6

Triene

Answer 6

3 C=C bonds

Question 7

General formula of cycloalkenes

Answer 7

CₙH₂ₙ₋₂

Question 8

What causes E/Z isomerism in alkenes

Answer 8

Restricted rotation about the planar C=C bond

Question 9

What condition has to be met for an alkene to have E/Z isomerism?

Answer 9

Each carbon atom in the C=C bond must have 2 different atoms/groups bonded to it

Question 10

Which rules determine priority in E/Z isomerism?

Answer 10

The Cahn-Ingold-Prelog Priority Rules (CIP rules)

Question 11

What are the CIP rules?

Answer 11

• The atom with the highest atomic number has priority
• If atoms have the same atomic number, the next atoms along the chain should be compared

Question 12

Why are alkenes reactive?

Answer 12

The C=C has a high electron density

Question 13

Electrophile

Answer 13

An electron pair accepter

Question 14

Why are electrophiles attracted to alkenes?

Answer 14

The high electron density in the C=C

Question 15

Curly arrow

Answer 15

Shows the movement of a pair of electrons

Question 16

Mechanism when alkenes react with halogens/hydrogen halides

Answer 16

Electrophilic addition

Question 17

What is the reactive intermediate in electrophilic addition? (alkenes)

Answer 17

A carbocation

Question 18

Heterolytic fission

Answer 18

Where both electrons remain with one atom when a covalent bond is broken

Question 19

Why are halogen molecules attracted to alkenes?

Answer 19

The high electron density in the C=C induces a temporary dipole in the halogen molecule

Question 20

Chemical test for C=C group

Answer 20

Bromine water
orange —> colourless

Question 21

Conditions for electrophilic addition

Answer 21

Room temperature

Question 22

Hydrolysis

Answer 22

The breaking of bonds using water

Question 23

What is formed when a hydrogensulfate is hydrolysed with warm water?

Answer 23

An alcohol

Question 24

What is the catalyst for :
alkene → hydrogensulfate (+water) → alcohol

Answer 24

Sulfuric acid (reacts with alkene, formed with alcohol)

Question 25

How are carbocations classified?

Answer 25

By the number of alkyl groups attached directly to the carbon with the positive charge. Either primary (1°), secondary (2°), or tertiary (3°)

Question 26

Positive inductive effect

Answer 26

Alkyl groups releasing electrons (this helps to stabilise the positive charge of carbocations).

Question 27

Which carbocations are the most stable?

Answer 27

Tertiary (3°)

Question 28

What determines which is the major product in electrophilic addition of asymmetrical alkenes?

Answer 28

The stability of the carbocation intermediate.

Question 29

Major product Minor product

Answer 29

Produced in the largest amount Produced in the smallest amount

Question 30

Hydrogenation

Answer 30

Addition of hydrogen, therefor a reduction reaction

Question 31

What is formed in the hydrogenation of alkenes?

Answer 31

Alkanes

Question 32

Catalyst for the hydrogenation of alkenes

Answer 32

Nickel

Question 33

Use of hydrogenation of alkenes

Answer 33

Producing margarine from long chain fats or oils

Question 34

Repeat unit (polymers)

Answer 34

A specific arrangement of atoms that occurs in the structure over and over

Question 35

What determines the properties of poly(alkenes)?

Answer 35

Structure and intermolecular force

Question 36

Is LDPE branched?

Answer 36

Yes

Question 37

Is HDPE branched?

Answer 37

No

Question 38

What are plasticisers?

Answer 38

Small molecules used to increase flexibility in polymers

Question 39

How do plasticisers work?

Answer 39

They force neighboring chains apart and reduce intermolecular forces.