Alkenes - Organic Chem I

Question 1

What is the general formula for alkene:

Answer 1

CnH2n

Question 2

Why are alkenes unsaturated?

Answer 2

Contain C=C double bond

Question 3

How many bonds does ethene form?

Answer 3

Each C atom forms 3 sigma bonds, one to the C atom and two to H atoms. This gives ethene its sigma framework.

Question 4

What then happens to the p orbitals on each C atom?

Answer 4

The p orbitals, on each C atom, that contain the unpaired e- lie perpendicular to the plane of the sigma framework

Question 5

What does the overlap of p orbitals create?

Answer 5

Create e- density above and below the sigma framework creating the π bond

Question 6

What is geometric isomerism?

Answer 6

Geometric isomerism arises because there is restricted rotation around the C=C but only occurs if there are two different groups attached to each carbon in the C=C

Question 7

Do geometric isomers have the same chemical properties?

Answer 7

Same chemical properties but have different physical properties

Question 8

When does a dehydration reaction occur?

Answer 8

If ethanol vapour is passed over heated aluminium oxide, a catalyst, a dehydration occurs

Question 9

Why are alkenes so much more reactive than alkanes?

Answer 9

1. Firstly, the C=C is e- rich therefore it attracts species with a positive charge or a δ+ charge
2. The C=C is composed of a sigma bond and a pi bond. The pi bond is weaker therefore easily bond, and it is replaced with two sigma bonds in the product, so the reaction will be energetically favourable.

Question 10

What does a dehydration reaction look like and what are the conditions?

Answer 10

Ethanol -> ethene + H2O
Conditions: Al2O3 catalyst, heat

Question 11

What does hydrogenation reaction look like?

Answer 11

Alkene + H2 -> Alkane
Conditions: Ni catalyst, 150 C

Question 12

What does alkene reacting with halogens look like? And what is the test for an alkene?

Answer 12

Alkene + Br2 –> CH2BrCH2Br
Conditions: room temp, Br2 (aq) or Br2 (l)
Test: shake with bromine water, orange to colourless

Question 13

What does alkene reacting with hydrogen halides look like?

Answer 13

C6H10 + HBr -> C6H11Br
Conditions needed: room temp, HBr (g) or HBr (org)

Question 14

What does a hydration look like?

Answer 14

C4H8 (alkene) + H2O -> CH3CH2CHOHCH3 (alcohol)
Conditions needed: 300C, 60-70atm, conc. H3PO4 catalyst

Question 15

What does a reaction with potassium manganate (VII) look like?

Answer 15

C3H6 (alkene) –> CH2OHCHOHCH3 (diol)
Conditions needed: room temp, in acid (i.e KMnO4 in dil. H2SO4)
Observations: purple to colourless (could also be used as a test for an alkene)

Question 16

What are electrophiles?

Answer 16

A species capable of accepting a pair of electrons to form a new dative covalent bond

Question 17

Define electrophile:

Answer 17

A species capable of accepting a pair of electrons to form a new dative covalent bond

Question 18

What is the reason behind the major and minor products?

Answer 18

The stability of the carbocation intermediate

Question 19

What are alkyl groups?

Answer 19

They’re electron donating (aka electron releasing). This means they can push electron density towards a positive charge, which therefore reduces the positive charge to some extent.

Question 20

What happens if there is a greater number of alkyl groups?

Answer 20

The greater the number of alkyl groups attached to the carbon with the positive charge, the greater the stability of the carbocation

Question 21

What are polymers?

Answer 21

Large molecules made from lots of small molecules called monomers joined together

Question 22

What is polymerisation?

Answer 22

Process in which monomers join together

Question 23

What are the two different types of polymerisation?

Answer 23

Addition
Condensation

Question 24

Define biodegradation:

Answer 24

Breakdown of organic matter by microorganisms

Question 25

What is the breakdown of a different type of plastic called?

Answer 25

Photocatalysed - this means that it needs to be exposed to sunlight for quite length periods of time

Question 26

Why are addition polymers difficult to dispose?

Answer 26

Because the C-C bonds are inert (unreactive and non-polar) so they cannot be broken down by microorganisms in the environment (do not biodegrade). Therefore, these plastics will remain for hundreds of years in landfill sites.

Question 27

What is incineration?

Answer 27

Rubbish is burnt and energy produced is used to generate electricity. Some toxins are produced on incineration. Modern incinerators can burn plastics more efficiently and most toxins and pollutants can be removed before entering the atmosphere. Greenhouse gases will still be emitted though

Question 28

How does recycling work?

Answer 28

Saves on raw material - nearly all polymers are formed from compounds sourced/produced from crude oil. Recycling saves precious resources.

Question 29

How can cracking from livestock help save raw materials?

Answer 29

Polymers can be cracked into small molecules which can be used to make other chemicals and new polymers. This saves raw materials. Waste gases from the incinerator are scrubbed/reacted with a base/carbonate. Neutralising it