Alkenes - Organic Chem I Flashcards
What is the general formula for alkene:
CnH2n
Why are alkenes unsaturated?
Contain C=C double bond
How many bonds does ethene form?
Each C atom forms 3 sigma bonds, one to the C atom and two to H atoms. This gives ethene its sigma framework.
What then happens to the p orbitals on each C atom?
The p orbitals, on each C atom, that contain the unpaired e- lie perpendicular to the plane of the sigma framework
What does the overlap of p orbitals create?
Create e- density above and below the sigma framework creating the π bond
What is geometric isomerism?
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
Do geometric isomers have the same chemical properties?
Same chemical properties but have different physical properties
When does a dehydration reaction occur?
If ethanol vapour is passed over heated aluminium oxide, a catalyst, a dehydration occurs
Why are alkenes so much more reactive than alkanes?
- Firstly, the C=C is e- rich therefore it attracts species with a positive charge or a δ+ charge
- 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.
What does a dehydration reaction look like and what are the conditions?
Ethanol -> ethene + H2O
Conditions: Al2O3 catalyst, heat
What does hydrogenation reaction look like?
Alkene + H2 -> Alkane
Conditions: Ni catalyst, 150 C
What does alkene reacting with halogens look like? And what is the test for an alkene?
Alkene + Br2 –> CH2BrCH2Br
Conditions: room temp, Br2 (aq) or Br2 (l)
Test: shake with bromine water, orange to colourless
What does alkene reacting with hydrogen halides look like?
C6H10 + HBr -> C6H11Br
Conditions needed: room temp, HBr (g) or HBr (org)
What does a hydration look like?
C4H8 (alkene) + H2O -> CH3CH2CHOHCH3 (alcohol)
Conditions needed: 300C, 60-70atm, conc. H3PO4 catalyst
What does a reaction with potassium manganate (VII) look like?
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)
What are electrophiles?
A species capable of accepting a pair of electrons to form a new dative covalent bond
Define electrophile:
A species capable of accepting a pair of electrons to form a new dative covalent bond
What is the reason behind the major and minor products?
The stability of the carbocation intermediate
What are alkyl groups?
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.
What happens if there is a greater number of alkyl groups?
The greater the number of alkyl groups attached to the carbon with the positive charge, the greater the stability of the carbocation
What are polymers?
Large molecules made from lots of small molecules called monomers joined together
What is polymerisation?
Process in which monomers join together
What are the two different types of polymerisation?
Addition
Condensation
Define biodegradation:
Breakdown of organic matter by microorganisms
What is the breakdown of a different type of plastic called?
Photocatalysed - this means that it needs to be exposed to sunlight for quite length periods of time
Why are addition polymers difficult to dispose?
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.
What is incineration?
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
How does recycling work?
Saves on raw material - nearly all polymers are formed from compounds sourced/produced from crude oil. Recycling saves precious resources.
How can cracking from livestock help save raw materials?
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
