Chapter 13 - Alkenes

Question 1

What is the general formula of alkenes?

Answer 1

CnH2n

Question 2

Are alkenes unsaturated or saturated hydrocarbons?

Answer 2

Unsaturated

Contain at least one double carbon to carbon bond

Question 3

Describe bonding in an alkene

Answer 3

In one C=C bond:
- 1 sigma bond formed direct overlap of orbitals. Head on overlap - giving highest possible electron density
- 1 Pi bond formed by sideways overlap of two p-orbitals.
Form two parts - one below and one above molecule’s axis.
Pi bonds cannot rotate

Question 4

What kind of bond is a C-H bond?

Answer 4

Sigma

Question 5

What kind of bond is a C-C bond?

Answer 5

Pi

Question 6

Which one has lower enthalpy? Sigma or Pi bond?

Answer 6

Pi bonds have a lower enthalpy as electron density is spread out above and below the nuclei. Meaning electrostatic attraction between the nuclei and shared pair of electrons is weaker.

Question 7

What is the shape around each carbon atom in a double bond (C=C)?

Answer 7

-3 regions of electron density around each of the carbon atoms.
repel each other as far apart as possible, so the bond bond angle = 120 degrees
giving trigonal planar shape

Question 8

What are stereoisomers?

Answer 8

Stereoisomers have the same structural formula but a different arrangement of atoms in space.

Question 9

What is an E isomer?

Answer 9

When the 2 highest priority groups are on OPPOSITE sides of a C=C bond

Question 10

What is a Z isomer?

Answer 10

When the 2 highest priority groups are on the SAME side of the C=C bond

Question 11

What are the requirements for a molecule to have E/Z isomerism?

Answer 11

• A C=C bond - as this cannot rotate

- 2 different groups attached to each carbon atom of the double bond

Question 12

What is cis/trans isomerism?

Answer 12

Special case of E/Z isomerism where one of the attached groups on each carbon atom of the double bond must be the same.

Question 13

Which one of cis and trans isomers are E and Z isomers?

Answer 13

Cis - Z isomer

trans - E isomer

Question 14

What are the CIP rules used to find?

Answer 14

Priority rules used to find E and Z stereoisomers

Question 15

How are the CIP rules used to assign priority?

Answer 15

1. ) look at the atoms directly attached to carbon atoms in the double bond and decide which 2 atoms have the highest priority - the higher atomic number = higher priority.
2. ) If 2 atoms assessed are the same then find the first point of difference - and the higher the atomic number = higher priority.

Question 16

What is the name - CIP rules short for?

Answer 16

Cahn - Ingold - Prelog rules

Question 17

Why are alkenes more reactive than alkanes?

Answer 17

Pi bond electrons are more exposed than the electrons in the sigma bond.

Question 18

Give examples of what alkenes undergo addition reactions with.

Answer 18

• Hydrogen
• Halogens
• Hydrogen halides
• Steam in the presence of an acid catalyst

Question 19

What is hydrogenation of an alkene?

Answer 19

Alkene mixed with hydrogen and passed over nickel catalyst at 423K. Addition reaction where hydrogen is added across a double bond

Question 20

Draw hydrogenation of buta-1,3-diene

Answer 20

Look at paper flashcards

Question 21

What is halogenation of alkenes?

Answer 21

Rapid addition reaction with halogen. The halogen adds across the double bond.

Question 22

What test is used to detect C=C bond (unsaturation)?

Answer 22

Bromine test

bromine adds across C=C bond and orange colour disappears.

Question 23

What addition reaction occurs with alkenes that forms a haloalkane?

Answer 23

(gaseous) hydrogen halide addition

Question 24

What does hydration of alkenes with steam in presence of a phosphoric acid catalyst form?

Answer 24

Alcohol

Question 25

Draw hydration of propene

Answer 25

Look at paper flashcards

Question 26

What is electrophilic addition?

Answer 26

Double bond in alkenes open up and atoms are added to the carbon atoms

Question 27

What is an electrophile?

Answer 27

Atom or group of atoms that is attracted to an electron-rich centre and accepts an electron pair Usually a positive ion or has slight positive charge

Question 28

Describe electrophilic addition mechanism of but-2-ene and hydrogen bromide.

Answer 28

1. )Bromine is more electronegative than hydrogen, so hydrogen bromide is polar and contains the dipole H + - Br - 2. ) The electron pair in the pi bond is attracted to the partially positive hydrogen atom, causing the double bond to break. 3. ) A bond forms between the hydrogen atom of H-Br molecule and a carbon atom that was part of the double bond. 4. ) The H-Br bond breaks by hetrolytic fission, with the electron pair going to the bromine atom 5. )Bromide ion (Br-) and a carbocation are formed. A carbocation contains positively charged carbon atoms. 6. ) The br- ion reacts with the carbocation to form the addition product.

Question 29

Draw mechanism of electrophillic addition of hydrogen bromide to but-2-ene

Answer 29

Look at paper flashcards

Question 30

Whats the difference when a hydrogen halide is added to a symmetrical alkene to when it's added to an unsymmetrical alkene?

Answer 30

Two possible products when added to a unsymmetrical one.

Question 31

What does the amount of product formed in a electrophilic addition reaction depend on?

Answer 31

How stable the carbocation formed in the middle of the reaction is

Question 32

Are carbocations more stable with more or less alkyl groups? And why?

Answer 32

More alkyl groups | Alkyl groups feed electrons towards the positive charge

Question 33

State Markownikoff's rule.

Answer 33

The major product from addition of a hydrogen halide (HX) to an unsymmetrical alkene is one where hydrogen adds to the carbon with the most hydrogens already attached.

Question 34

What is addition polymerisation?

Answer 34

Joining of monomers to form chain - called polymers

Question 35

How do you write polyethene?

Answer 35

Monomer in brackets. With 'n' outside representing the repeating unit.

Question 36

How can we reduce environmental impacts of waste polymers?

Answer 36

- Recycling - Using waste polymers as fuel - Feedstock recycling

Question 37

How does recycling decrease environmental impact of waste polymers?

Answer 37

Conserves finite fossil fuels and decreases amount of waste going into landfill

Question 38

What are the downsides to recycling waste polymers?

Answer 38

Pointless if polymers are not sorted by type and are mixed.

Question 39

How does feedstock recycling reduce environmental impacts?

Answer 39

- Reassemble products produced from crude oil. Can be used as raw materials for production of new polymers - Able to handle unsorted or unwashed polymers.

Question 40

How can waste polymers be used as a fuel?

Answer 40

Incinerated to produce heat, generating steam to drive a turbine producing electricity.

Question 41

What are 3 developments that can be made to benefit the environment in polymers?

Answer 41

- Bioplastics - Biodegradable polymers - Photodegradable polymers