Alkenes

Question 1

What is the functional group of alkenes?

Answer 1

C=C (carbon-carbon double bond)

Question 2

Compared to alkanes, how reactive are alkenes?

Answer 2

Considerably more reactive

Question 3

Why are C=C more reactive than C-C?

Answer 3

• The π bond formed is weaker
• The π bond attracts electrophiles

Question 4

How are σ (sigma) bonds formed?

Answer 4

From the direct overlap of two atomic orbitals

Question 5

Where is the electron density in a σ (sigma) bond?

Answer 5

Mostly between the two nuclei

Question 6

How are π bonds formed?

Answer 6

From the sideways overlap of two p atomic orbitals

Question 7

Do π bonds have a higher electron density than σ (sigma) bonds?

Answer 7

Yes

Question 8

Why do π bonds have a higher electron density than σ (sigma) bonds?

Answer 8

There are twice as many electrons in a smaller space

Question 9

Are π bonds weaker than σ (sigma) bonds?

Answer 9

Yes

Question 10

In a π bond, where are the electrons located?

Answer 10

Above and below the plane of the carbon atoms

Question 11

What are bond angles measured between?

Answer 11

BONDS not atoms

Question 12

Why are π bonds weaker than σ (sigma) bonds?

Answer 12

• the electron density of a π bond is above and below the plane of the molecule
• whilst the electron density is directly between two carbon atoms in the σ (sigma) bond

Question 13

What does the higher electron density of a π bond attract?

Answer 13

Electrophiles

Question 14

Define electrophile

Answer 14

A species capable of accepting a pair of electrons

Question 15

Define stereoisomer

Answer 15

Molecules that have the same structural formula but a different arrangement of atoms in space

Question 16

What type of stereoisomerism do alkenes exhibit?

Answer 16

E/Z isomerism

Question 17

Under what conditions does E/Z isomerism occur?

Answer 17

• no rotation about a bond (as in a C=C)
• each of the carbon in the C=C is attached to two different groups

Question 18

What is cis/trans isomerism?

Answer 18

A special case of E/Z isomerism in which one of the attached groups on each carbon atom of the double bond must be hydrogen

Question 19

Are Cis isomers E or Z isomers?

Answer 19

Z

Question 20

Are Trans isomers E or Z isomers?

Answer 20

E

Question 21

Explain why cyclohexene is more reactive than cyclohexane

Answer 21

• cyclohexene has a double bond which is made up of a σ and a π bond
• cyclohexane contains only σ bonds
• the π bond has a higher electron density than the σ bond and so can attract electrophiles
• π bonds are also weaker than σ bonds so π bonds are broken more easily

Question 22

How can the presence of an alkene be tested?

What is observed if an alkene is present?

Answer 22

• bromine water is added
• the orange solution rapidly decolourises

Question 23

Where must curly arrows originate from?

Answer 23

Either from a bond or a lone pair

Question 24

What are the reaction conditions for an alkene and water?

Answer 24

Water must be steam

Concentrated phosphoric acid catalyst

Question 25

What are the reaction conditions for an alkene and hydrogen?

Answer 25

150°^C

Nickel catalyst

Question 26

What are the reaction conditions for an alkene and a halogen?

Answer 26

Room temperature and pressure

Question 27

What are the reaction conditions for an alkene and a hydrogen halide?

Answer 27

Room temperature and pressure

Question 28

Define addition reaction

Answer 28

A reaction in which a reactant is added to an unstaurated molecule to make one saturated molecule

Question 29

Define heterolytic fission

Answer 29

The breaking of a covalent bond forming a cation and an anion

Question 30

Why does the halogen/water/hydrogen/hydrogen halide become polarised during the reaction with an alkene?

Answer 30

The high electron density of the C=C polarises the molecule

Question 31

Why can the H-Br molecules be considered an electrophile in the 1st step of electrophilic addition?

Answer 31

The H-Br molecule accepts a pair of electrons from the π bond to form a new C-H bond

Question 32

Why does one of the carbon atoms have a positive charge during the intermediate of electrophilic addition?

Answer 32

One of the electrons in the π bond orginally belonged to this Carbon atom and this electron has now been lost as it is part of the new C-H bond